Development, Conformance Testing, Optimization
Certification Course – Amateur Level (3PCA-RRC)
LTE – RRC
LTE Protocol Stack
Author: Surya Patar Munda
3PCA-RRC
LTE Protocol Stack- 1
suryapatar@yahoo.com
Preface:
Dedication – This book is dedicated to my family who has given me support to complete this book.
The colleagues in office have given me encouragement to start and complete this book. My hearty
thanks to all of you. The first release is printed with many terms unexplained and even sentences are
shortened but intended to cover in this book. They will gradually be expanded in next release. Please
do write me on the email given in the pages below to improve.
Who is this book for?
Over the years I have seen the telecom industry struggling to get right people with sufficient domain
knowledge in 2G or 3G or 4G. The specification is very huge and it is often horrendous to go through
the details. This book is referring most of the time with respect to LTE 3GPP specification, Rel-10.
This is an effort to consolidate information in an organised way to give a methodical way of
understanding LTE. This is a very good start for an engineer who is either going to pursue:
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LTE Protocol Stack Development
LTE ConformanceTesting
LTE Network/RF Optimization
LTE entities (UE and Network both) troubleshooting
If you need 3GNets LTE Physical Layer for Amateur Level (3PCA-RRC), you need this course.
This knowledge and level is required for the next level – Professional Level (3PCP-RRC) where you
can be trained for higher level with Hands on Projects and real implementation. Full Amateur level
courses are:



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LTE Physical Layer LTE L2 Layer - MAC, RLC, PDCP LTE RRC –
LTE NAS –
(3PCA-L1)
(3PCA-L2)
(3PCA-RRC)
(3PCA-NAS)
About Author:
Surya Patar Munda has been in Telecommunications Since 1987 and has gone through the life cycle
of Software Development, Software Testing, Network Deployments, Integration, Testing,
Troubleshooting, Handphone Testing with Specification etc.. a full round of the Telecom industry. He
has worked with Motorola, Nortel Networks, Spirent Communications, Sasken etc. companies with full
round cycle. The Software engineers midset and Testing engineers mindsets are different and so is
the mindset of an RF optimization engineer. This book will cater to all.
Author also conducted many trainings for Telecom industry and has a very good understanding of
what kind of requirement is there for engineers. The goal is not just what and how does it work, but
also the goal is how do I start implementing and how do I test.
Edition: July 2013
Price: Rs.149
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Contents
1.
Access Stratum – RRC L3 .............................................................................................................. 5
1.1.
Radio Resource Controller – Idle ............................................................................................ 5
1.1.1.
PLMN and Cell Selection ................................................................................................ 5
1.1.2.
PLMN Selection............................................................................................................... 5
1.1.3.
Cell Selection .................................................................................................................. 6
1.1.4.
Cell Reselection .............................................................................................................. 6
1.1.5.
Cell Access Restrictions .................................................................................................. 8
1.1.6.
Closed Subscriber Group (CSG) ..................................................................................... 8
1.1.7.
Neighbour Monitoring and Cell Reselection .................................................................... 8
1.1.8.
Paging ............................................................................................................................. 9
1.1.9.
RRC Messages and Controls ........................................................................................ 10
1.1.10.
System Information Broadcast (SI) ............................................................................... 10
1.2.
Cell ReSelection & PLMN Selection Design-Development .................................................. 13
1.2.1.
Cell Re-Selection (CRS) Description ............................................................................ 13
1.2.2.
CRS Actions: ................................................................................................................. 13
1.2.3.
CRS Process Inputs: ..................................................................................................... 14
1.2.4.
Internal Messages Design ............................................................................................. 14
1.2.5.
CRS Outputs Message(Internal): .................................................................................. 15
1.2.6.
IPC-Design Diagram: .................................................................................................... 15
1.2.7.
Cell_Selection_Function() ............................................................................................. 16
1.2.8.
Cell_Re-Selection_Function() ....................................................................................... 16
1.2.9.
PLMN Selection Process Design .................................................................................. 20
1.2.10.
PLMN Selection Actions ................................................................................................ 20
1.2.11.
PLMN Selection Inputs .................................................................................................. 20
1.2.12.
PLMN SelectionOutputs(Internal) ................................................................................. 20
1.2.13.
PLMN Selection IPC-Design ......................................................................................... 21
1.3.
Measurement and Reporting................................................................................................. 23
1.3.1.
LTE Measurements ....................................................................................................... 23
1.3.2.
Measurement Objects and Management ...................................................................... 23
1.3.3.
NON-LTE Measurements .............................................................................................. 24
1.3.4.
Measurement Report Triggering ................................................................................... 25
1.3.5.
Measurement Reporting ................................................................................................ 25
1.3.6.
Measurements when Camped on LTE ......................................................................... 25
1.3.7.
LTE Mobility in RRC_CONNECTED ............................................................................. 26
1.4.
Radio Resource Controller – Connected .............................................................................. 31
1.4.1.
Connection Control within LTE...................................................................................... 31
1.4.2.
Security KeyManagement ............................................................................................. 31
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1.4.3.
Connection Establishment and Release ....................................................................... 32
1.4.4.
RRC_CONNECTED Messages .................................................................................... 33
1.4.5.
Mobility Control in RRC_IDLE and RRC_CONNECTED .............................................. 34
1.4.6.
Message sequence for handover within LTE ................................................................ 35
1.4.7.
Connection Re-Establishment Procedure ..................................................................... 35
1.4.8.
Connected Mode Inter-RAT Mobility ............................................................................. 36
1.4.9.
Handover to LTE ........................................................................................................... 36
1.4.10.
Major HANDOVER Steps (Intra-LTE) ........................................................................... 37
1.4.11.
Handover to UMTS........................................................................................................ 38
1.4.12.
Handover to GSM.......................................................................................................... 38
1.4.13.
Other RRC Signalling Aspects ...................................................................................... 39
1.5.
Radio Resource Management .............................................................................................. 41
1.5.1.
UE Mobility Activities Overview ..................................................................................... 41
1.5.2.
LTE Cell Search ............................................................................................................ 41
1.5.3.
UMTS Cell Search......................................................................................................... 42
1.5.4.
GSM Cell Search........................................................................................................... 42
1.6.
MU-Scheduling & Interference Coordination ........................................................................ 45
1.6.1.
Resource Allocation Strategies ..................................................................................... 45
1.6.2.
Scheduling Algorithms .................................................................................................. 46
1.6.3.
Performance of Scheduling Strategies.......................................................................... 47
1.6.4.
Considerations for Resource Scheduling in LTE .......................................................... 47
1.6.5.
Interference Coordination and Frequency Reuse ......................................................... 47
1.7.
Sample Call Flows ................................................................................................................ 51
1.7.1.
Basic Call Flow – Attach Procedure .............................................................................. 51
1.7.2.
Basic Call Flow – Incoming Call with Handover ............................................................ 52
1.7.3.
Call Flow example from tool .......................................................................................... 53
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1. Access Stratum – RRC L3
1.1. Radio Resource Controller – Idle
Radio Resource Controller (RRC) performs Control Plane of Access Stratum (AS). AS interacts with
NAS, which handles PLMN_selection, Tracking Area update, paging, authentication, EPS bearer
establishment, modification and release. RRC is in either RRC_IDLE or RRC_CONNECTED state.
UE in RRC_IDLE performs cell selection, reselection – select best cell to camp. Consider priority of
frequency, RAT, radio link quality, cell status, speed, CSG and MBMS. An RRC_IDLE UE monitors
paging to detect incoming calls, acquires system information (SI). SI consists of parameters for cell
(re)selection, Paging.
Fig 5.1.0 – Idle Mode Processes
In RRC_CONNECTED, eNB allocates RB to UE to transfer data via shared data channels. UE
monitors PDCCH for allocated transmission resources in time and frequency. UE reports buffer status
and DL channel quality (CQI), neighbouring cell (including other freq/RAT) measurement to select
most appropriate cell for UE. UE continues to receive SI. To extend battery lifetime, UE may be
configured with a Discontinuous Reception (DRX) cycle. RRC performs security, inter and intra RAT
mobility, establishment and reconfiguration of radio bearers to carry control and user data.
1.1.1. PLMN and Cell Selection
Once UE is switched on, first it selects a PLMN, then it performs cell selection –searches for a
suitable cell to camp on with help of acquired SI parameters. Subsequently, UE registers in the
tracking area if not done in that TAI, then it can receive paging for incoming calls. UE may establish
RRC connection either to establish a call or to register. In Idle mode, UE regularly verifies if there is a
better cell (cell reselection). Detected Cells can be:
Suitable cell (normal service),
Acceptable cell ( „limited service‟, emergency calls),
Reserved cells (normal service but with special rights like for operators or AC >=11) or
Barred cells (No service at all).
1.1.2. PLMN Selection
NAS handles PLMN selection based on available PLMN list provided by AS. NAS indicates selected
PLMN together with list of equivalent PLMNs. After registration, selected PLMN becomes R-PLMN.
AS may autonomously indicate available PLMNs after full search. For all the PLMNs UE receives, it
searches for the strongest cells(PLMNs are retrieved from SI) on each carrier frequency. PLMNs are
reported as high quality or ; just reported with their quality.
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1.1.3. Cell Selection
In Cell selection, UE searches for strongest cell on all supported carriers of each supported RAT until
it finds a suitable cell. After switch on, first selection is Cell Selection. To speed up, UE may use
stored information and NAS may indicate RATs associated with selected PLMN. Cell selection
criterion is known as S-criterion and is fulfilled when Srxlev > 0 dB, where
Srxlev = Qrxlevmeas − (Qrxlevmin − Qrxlevminoffset) where
Qrxlevmeas = measured cell receive level value(RSRP),
Qrxlevmin = minimum RSRP in cell,
Qrxlevminoffset = configured offset to prevent PLMN ping-pong.
Cell selection parameters are broadcast in SIB1.
1.1.4. Cell Reselection
Once UE camps on a suitable cell, it starts Cell Reselection to go to the „best‟ cell of selected
PLMN/e- PLMNs. UE first evaluates frequencies of all RATs based on their priorities. Secondly,
compare cells on that frequency based on radio link quality, using ranking. Verify cell‟s accessibility
before comparing. UE may re-trigger cell reselection only after having camped for at least one second
on the current serving cell.
Measurement Rules
Minimize measurements required by UE. Firstly, measure intra-frequency only if S-Cell S <=
„SintraSearchP‟. Measure other frequencies/RATs of lower or equal priority only when S-Cell S <=
„SnonintraSearchP‟). Always measure frequencies/RATs of higher priority.
Frequency/RAT Evaluation
E-UTRAN configures an absolute priority for all frequencies of each RAT. Cell-specific priorities are
optionally provided by SI. eNB can assign UE-specific priorities via dedicated signalling. S Criteria
must be valid for Treselection for Re-selection. When reselecting to new freq/RAT, reselect to highestranked cell. Thresholds and priorities are configured per frequency, while Treselection is configured per
RAT.
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UE reselects higher priority freq cell if T-Cell S> ThreshX-High.
UE reselects lower-priority freq/RAT cell if S-Cell S < ThreshServing-Low & T-Cell S >
ThreshX-Low, while no higher-priority freq/RAT cell available.
From Release-8 onwards, LTE, UMTS and GERAN support same priority-based cell reselection. Any
differences are managed by different offsets.
Cell Ranking
UE ranks the intra-frequency cells and cells on other frequencies of equal priority which fulfil Scriterion with R-criterion. R-criterion generates rankings Rs and Rn.
For Serving cell: Rs = Qmeas,s + Qhyst,s
For Neighbour cells: Rn = Qmeas,n + Qoff s,n
Qmeas is measured cell RSRP & Qhyst,s is degree of hysteresis for ranking, and Qoff s,n is offset
between SCell and NCell on frequencies of equal priority (cell-specific + frequency-specific offsets).
UE reselects to the highest-ranked candidate cell if best ranked for at least Treselection. Treselection and
Qhyst, may be rescaled based on speed.
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Fig 5.1.4 – Cell Reselection process flow diagram
Accessibility Verification
If best cell is barred or reserved, exclude it from candidate list. If barred, UE may consider other cells
on same frequency unless barred cell indicates Intra-Freq-Not-Allowed in SI, except for CSG cells. If,
however the best cell is unsuitable for other specific reasons, UE should not consider any cell on
concerned frequency for 300s.
Speed Dependent Scaling
UE scales cell reselection orHandover parameters depending on its speed. UE speed is categorized
by a mobility state (high, normal or low), which is determined by number of cell
reselections/handovers within a defined period, excluding consecutive reselections/handovers
between the same two cells. The state is determined by comparing the count with thresholds for
medium and high state, while applying some hysteresis. Parameters are signalled in SIB3.
Any Cell Selection
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When no Suitable cell found at all in PLMN, perform „any cell selection‟. In this case, perform normal
idle mode operation: monitoring paging, acquiring SI, cell reselection . UE is not allowed to receive
MBMS.
1.1.5. Cell Access Restrictions
Each UE belongs to an Access Class (AC) in range 0–9. In addition it may belong to one or more
high-priority ACs in the range 11–15. AC10 is used for emergency access. UE considers access to be
barred if access
is barred for all its applicable ACs. SIB2 includes AC barring for MO calls and/or MO signalling.
This barring has a probability factor and a barring timer for AC0–9 and a list of barring bits for AC11–
15. For AC0–9, if UE initiates a MO call and relevant AC is barred, UE draws a random number. If this
number exceeds probability factor, access is not barred, otherwise access is barred for a duration
which is randomly selected centred on the broadcast barring timer value. For AC11–15, if UE initiates
a MO call, access is barred whenever the bit corresponding to all of the UE‟s ACs is set. The
behaviour is similar in the case of UE-initiated MO signalling.
For cell (re)selection, UE is expected to consider Suitable Cells.UE with AC=11–15 shall consider
reserved cell in HPLMN also suitable. UE can not make even emergency calls in barred cells.
1.1.6. Closed Subscriber Group (CSG)
UE maintains a CSG white list (CSG identities where UE is granted Access), received from NAS or
updated upon successful access of a CSG cell. UEs support „manual selection‟ request by NAS for
CSG cells(text sent) not in CSG white list.
1.1.7. Neighbour Monitoring and Cell Reselection
All LTE mobility procedures in RRC_IDLE state are performed autonomously within the UE and
extreme care is taken for sufficient UE mobility performance without sacrificing power-efficiency. By
being autonomous, UE minimizes transmission of resource in inactive UEs. In fact, over-the-air
signalling is only required for Tracking Area Update in RRC_IDLE and thus saves UE power.
In RRC_IDLE UE just camps on a serving cell where paging reception is sufficiently reliable with good
cell quality for an incoming call. UE is not required to perform any frequent neighbour cell monitoring
(cell search and measurements) unless S-Cell quality drops below a specified threshold.
Priority-Based Cell Reselection
In LTE Priority-based cell reselection is adopted to improve cell reselection in multiple RATs. This
reduces the need to monitor all available intra-system and inter-RAT carriers defined by a set of
priority rules provided to UE. Following steps are carried out in cell reselection and camps on a new
cell while in RRC_IDLE:
1. Decode broadcast information. A UE decides to camp on a cell if cell selection criteria S
are met the best. Then it will receive all relevant system information on BCH, like cell-specific
paging and random access parameters, cell bandwidth, serving cell minimum quality
threshold (S-threshold), UMTS neighbour cell list, GSM neighbour cell list etc. Neighbour cell
priority is provided to the UE through RRC dedicated signalling (not broadcast).
2. Tracking Area Update. If UE has moved to a cell belonging to a different tracking area, it will
establish a brief signalling connection with eNodeB to inform MME about its new location
before it can enters the paging reception stage.
3. Paging reception. UE determines paging DRX cycle (min(Cell default cycle, UE specific
paging cycle)) and other paging parameters broadcast. Henceforth, unless UE is being paged
or cell reselection occurs, UE will periodically wake up on every paging occasion to check for
paging messages. Same time, the UE can measure its S-cell quality.
a. When S-cell quality is poor (RSRP < S-threshold), UE risks losing S-cell, it must
attempt to identify and reselect a new suitable cell. All possible PLMNs are searched
and measured regardless of their priority, and UE camps on the highest priority (R)
PLMN detected cell which meets suitability criteria (S). The search rate will be
frequent (small multiple of the paging DRX period, min 1 second).
b. If S-cell quality (S Criteria) is good enough, then searching for lower priority layers is
not done, but still searches for higher priority cells at a reduced rate and UE must
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reselect a higher priority cell if it meets S-criteria. If S-cell quality is good, search rate
can be far less frequent to reduce UE power consumption (may be order of 60 s).
4. Cell reselection evaluation. On every paging occasion S-criterion is evaluated. If S-cell
quality is poor (RSRP < S-threshold) and N-cell meets S-criterion then cell reselection
towards N-cell is initiated and UE restarts cell-specific configuration on the new cell by
updating new cell broadcast.
Measurements in Idle Mode
N-cell search, measurement rates and S-Criteria evaluation rates are function of configured paging
DRX cycle, the layer being measured (search and measurement rates LTE intra-frequency cells >
LTE inter-frequency cells > inter-RAT cells) and S-cell quality (RSRP < S-threshold). Frequency of
measurement (rate) can be set inversely proportional to S-cell quality.
1.1.8. Paging
Paging process start with an incoming call at MME. MME sends paging to all the cells who belong to
the TAC where UE is at the moment of paging: Here is the figure explaining the flow:
Fig 5.1.8.1 – Paging distribution in relevant TAC‟s
Fif 5.1.8.2 – Paging Protocol Stack
To receive paging, UEs in idle mode monitor PDCCH for P-RNTI on specific subframes(called Paging
Occasion –PO) within specific frames (Called Paging Frames- PF). At other times, It may apply
DRX(switch off its receiver to preserve battery power). eNB configures PF and PO for paging by
broadcasting a default paging cycle for all UE‟s. Upper Layers may use dedicated signalling to a UE
for specific paging cycle. If both configured, UE applies the lowest value.
UE calculates PF and PO as follows:
SFN mod T = (T/N) × (UE_ID mod N)
i_s = (UE_ID/N) mod Ns
T = min(Tc, Tue)
N = min(T , number of paging subframes per frame × T )
Ns = max(1, number of paging subframes per frame) (3.1)
where:
Tc  cell-specific default paging cycle {32, 64, 128, 256} radio frames,
Tue  UE-specific paging cycle {32, 64, 128, 256} radio frames,
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N  number of paging frames with the paging cycle of the UE,
UE_ID  IMSI mod 4096, with IMSI being the decimal rather than the binary number,
i_s  index pointing to a pre-defined table defining the corresponding subframe,
Ns  number of „paging subframes‟ in a radio frame that is used for paging.
Below are the steps for paging processing in eNB.
Fig 5.1.8.3 – Paging buffering until PF and PO in eNB
1.1.9. RRC Messages and Controls
RRC covers following functional areas.
 System information- handles broadcasting of SI, for NAS, RRC, L2 and PHY.
 RRC connection control- covers (re)establishment, modification and release of RRC
connection, paging, initial security activation, establishment of SRBs & DRBs, inter/intra-RAT
handover including context, configuration of L1 & L2, AC barring and radio link failure (RLF).
 Network controlled inter-RAT mobility besides mobility, security activation and context
information transfer.
 Measurement configuration and reporting for intra/inter-frequency and intra/inter-RAT
mobility, with measurement gaps management.
 Miscellaneous functions – Perform transfer of dedicated NAS & access capability
information.
RRC messages are transferred across SRBs, mapped via PDCP and RLC onto logical channels –
either CCCH or DCCH. SI is mapped to BCCH and Paging is mapped to PCCH.
SRB0 is used for CCCH, SRB1 is for DCCH, and SRB2 is for NAS messages using DCCH. All DCCH
messages are integrity-protected and ciphered by PDCP (after security activation) and use ARQ for
AM RLC. CCCH messages are not integrity-protected and no ARQ in RLC. NAS independently
applies integrity protection and ciphering.
For low transfer delay parameters, MAC signalling is used when no security concerns applies.
1.1.10. System Information Broadcast (SI)
SI broadcast is structured by three types of messages: MIB, SIB1 and SI (SIB2-SIB16) messages.
Each contains function related Parameters:
• Master Information Block (MIB), most frequently Params, essential for UE‟s initial NW
access.
• System Information Block Type 1 (SIB1), cell selection, time domain scheduling of
other SIBs.
• System Information Block Type 2 (SIB2), common and shared channel information.
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•
SIB3–SIB16, control intra/inter-frequency/RAT cell reselection, inter-RAT Handover,
ETWS, MBMS etc.
Fig 5.1.10 – SIBs summary
SI message includes one or more SIBs which have the same scheduling periodicity. SIB2 is always
the first entry in SI messages.
Scheduling of System Information
MIB and SIB1 messages Transmission tine is fixed: periodicities of 40 ms and 80 ms respectively.
Scheduling of SI messages is dynamically flexible: each SI is indicated in which subframes within this
window the SI is scheduled. SI-windows are consecutive (i.e. neither overlaps nor gaps between
windows) with common length. SI-windows can include subframes in which no SI messages can be
sent. For illustration, please refer to the diagram in the physical layer processing of PBCH.
SI messages may have different periodicities. In some clusters of SI windows all SI messages are
scheduled, while in other windows only SIs with shorter repetition periods are transmitted.
SI Validity and Change Notification
SI changes only at specific SFN, where SFN mod N = 0, whereN = modification period. LTE provides
two mechanisms for indicating that system information has changed:
1. A paging with SystemInfoModification flag set.
2. A value tag in SIB1 which is incremented every time one or more SI messages
change.
UEs in RRC_IDLE use first mechanism, while in RRC_CONNECTED can use either mechanism. To
ensure reliability change notification, paging is repeated a number of times during BCCH modification
period. Modification period is expressed as multiple of cell-specific default paging cycle. UEs in
RRC_CONNECTED try paging message the same number of times per modification period as in
RRC_IDLE using default paging cycle. Connected mode UEs can utilize any of IDLE mode paging
subframes to receive change indications.
If UE receives SI change notification, it considers all SI to be invalid from the start of next modification
period. UE operations may be restricted until UE has re-acquired SI, especially in
RRC_CONNECTED. UE considers SI valid if it was received within 3 hours & value tag matches.
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Here is an example of how SI is scheduled:
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1.2. Cell ReSelection & PLMN Selection
Design-Development
This sample design process helps you how you may attempt to design various protocol stack
modules. This chapter takes you to the level of pseudo code. The overall software design design
depends on your rest of the software architecture and how the inter-process meassage and
information flow is planned. This can give you an idea how to connect your theory knowledge to
practice of software design.
1.2.1. Cell Re-Selection (CRS) Description
All the times in Idle mode, UE performs measurements for cell selection and reselection. NAS can
control RAT(s) in which the cell selection should be performed, by indicating RAT(s) associated with
the selected PLMN. UE also maintains a list of forbidden registration area(s) and a list of equivalent
PLMNs. UE selects a suitable cell based on measurements and cell selection criteria.
Stored information for several RATs are maintained in the UE to speed up the cell selection process.
When camped on a cell, UE regularly searches for a better cell according to the cell reselection
criteria. If a better cell is found, that cell is selected. The change of cell may imply a change of RAT.
NAS is informed if the cell selection and reselection results in changes in the received system
information relevant for NAS. This is an autonomous function in the UE and network may not be
congested by any message for this.
1.2.2. CRS Actions:
1. Measurement process will perform Measurement and keep it in each cell structure – Shared
Memory. CRS should be able to access measured information.
2. NAS controls RATs with PLMN selection and provides to PLMN-RAT to CRS.
3. Maintain fb_PLMN and e_HPLMN list and select PLMN from there.
4. In the selected PLMN – select suitable cell, based on measurement and CRS criteria.
5. Start the selection with Stored information.
6. Camp on a cell, and continuously select a better cell after 1000ms.
7. Once cell is changed, camped - changes in SI are informed to NAS.
a. Receive SI in the cell – TAC
b. If registered, receive paging and Notifications
c. If NAS requests – initiate Connected Mode.
8. Maintain state – Idle or Connected.
9. If state=Idle do CRS else suspend CRS until it comes back from Connected to Idle.
10. On every PLMN change, start CRC at #1 (START_CELL_RESELECTION).
11. If Store_Info available then StoredInfo_CRS else Initial_CRS.
12. Maintain
CampState
=
Normally,
Anystate,
Connected,
Nowhere.
Initially
CampState=Nowhere.
13. Maintain list of following type of cells- Barred Cells, Reserved Cells, Acceptable Cells and
SuitableCells, CSG/Hybrid cells.
14. Arrange the cells in these lists with priority application.
a. If any CSG, CSG priority.
b. Carrier Priority
c. Ranking Priority
d. All the cells within the same PLMN should be considered.
e. RAT, TDD/FDD is not considered and does not affect the priority.
15. Traverse among the Suitable cells and pick the best ranked Suitable cell. If not found Suitable
Cell, Try searching from Acceptable Cells and camp but not more than 10s on AnyCell.
16. Camp on a cell, and continuously select a better cell after 1000ms.
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17. Maintain a timer for Tsearching_for_suitable (10s required to search intra, after expiry, do
inter-freq) and Treselect1s(1s-minimum required before subsequent CRS initiated)
1.2.3. CRS Process Inputs:
a. SI parameters input – Shared Memory
1. cellReselectionPriority
2. Qoffsets,n , Qoffsetfrequency
3. Qhyst , Qqualmin, Qrxlevmin
TreselectionRAT , TreselectionEUTRA, TreselectionUTRA
4.
TreselectionGERA, TreselectionCDMA_HRPD, TreselectionCDMA_1xRTT
5.
ThreshX, HighP, ThreshX, HighQ, ThreshX, LowP
6.
ThreshX, LowQ, ThreshServing, LowP, ThreshServing, LowQ
7.
8. SIntraSearchP, SIntraSearchQ
9. SnonIntraSearchP, SnonIntraSearchQ
10. TCRmax, NCR_M, NCR_H, TCRmaxHyst
11. Speed dependent ScalingFactor for Qhyst
12. Speed dependent ScalingFactor for TreselectionEUTRA
13. Speed dependent ScalingFactor for TreselectionUTRA
14. Speed dependent ScalingFactor for TreselectionGERA
15. Speed dependent ScalingFactor for TreselectionCDMA_HRPD
16. Speed dependent ScalingFactor for TreselectionCDMA_1xRTT
ii. PLMN Shared Memory
iii. NAS Shared Memory
iv. Meas. Shared Memory
1.2.4. Internal Messages Design
v. From PLMN Message – Change the PLMN now. Either initiated by NAS or on CRS
request.
vi. From CSG Message – Any change in current available CSG list,
vii. From TAU Message – from NAS response if any PLMN, forbidden_PLMN to be
changed.
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viii. From MBMS Message – If any session is current (within start-end time), them MBMS
priority.
ix. From Meas. Message - ??
x. From SI Message – Change of SIB3 or CRS related parameter change.
xi. From RRC Message – Inform CRS of success or failure of tuning to new Cell. If failure,
stick to the same cell. If success, update the variables and lists.
xii. From RRC Message – redirectedCarreerInfo of RRCConnectionRelease
1.2.5. CRS Outputs Message(Internal):
a. Updates in Parameters
a. PLMN Message – Request for new PLMN as no suitable cell in the current PLMN for
last 10s.
b. Internal Output Messages
a. To PLMN Message – Request for new PLMN as no suitable cell in the current PLMN
for last 10s.
b. To TAU Message – There is a change in TAC and that TAC is not in the TAI list.
Subsequently TAU should update that to NAS.
c. To Meas. Message(Based on design) – Schedule measurement for the neighbour
cells as received from SI SIB4. (Add/Mod/Del for MO+MI+RC internal). It may be done
by SI itself, depending on your overall design.
d. To RRC Message – Inform RRC of New cell selection and Camp on that new Cell.
Tune as per CRS info on Freq/Cell-Id information. RRC should scan PSS, SSS, RS
and BCCH signals and channels in that new Cell.
i. If successful, inform CSR of Success.
ii. If Fail, inform CRS of failure and stick to the old existing Cell and continue
CRS.
1.2.6. IPC-Design Diagram:
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Functions:
1.2.7. Cell_Selection_Function()
Please refer to the flow diagram in 36.304 – Idle mode processing. This module excludes the design
of PLMN selection and that is assumed to be done by another process.
Cell_selection_function()
{
If stored_Info StoredInfo_CRS – Carrier freq and cell parameters from prev measured cell.
Find the strongest suitable cell and camp.
If ((no stored_Info) OR (no suitable cell from stored_Info)),
Initial_CRS - Scan all RF channels to find suitable cell. On each carrier – find
strongest cell. Among suitable cells, hook to strongest cell.
Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation
Squal = Qqualmeas – (Qqualmin + Qqualminoffset)
Cell SelectionCriteria(&Srxlev, &Squal) if (Srxlev>0 && Squal>0) S=1 else S=0;
Use offsets only for higher priority PLMN while camped normally in VPLMN.
If manually CSG cell is selected, select that cell, if suitable.
}
1.2.8. Cell_Re-Selection_Function()
Cell_reselection_function()
{
0) Keep measuring the serving cell.
1.
Measure RSRP and RSRQ of Serving cell and find S at least every DRX cycle.
2.
Filter at least 2 measurements spaced by, at least DRX cycle/2.
1) Execute CRS evaluation process when:
1.
When (Srxlev < SIntraSearchP || Squal < SIntraSearchQ,) in Nserv consecutive DRX cycles,
trigger CRS (after min 1s from last camping Normally).
2.
When CRS parameters in BCCH is modified in CRS SIBs.
2) UE internally triggers CRS, to meet performance, if anyone above happens;
3.
initiate neighbour cells measurement
4.
Evaluate CRS Criteria for cells.
5.
Find a Suitable Cell or AnyCell and Camp either Normally (preferred) or in AnyCell.
3) If for 10s, no new Suitable cell found in intra-freq, inter-freq and inter-RAT, initiate PLMN
selection.
Reselection priority handling – freq.Priority.SI and freq.Priority.ded
1. Maintain Carriers list and their priorities – from SI(freq w/o
cellReselectionPriority), release(freq with priority), or inter-RAT CRS.
2. If dedicated signalling priority, that is taken, else SI priority taken.
3. If in AnyCellState – Apply SI priority.
4. If CSG cell, consider this freq priority as Highest.
5. If Normally camped on non-CSG cell, consider this as lowest priority
freq.
6. If MBMS session is on (calculate from start and end time), consider
MBMS freq to be of highest freq.
7. Delete priorities when – RRC Connected,
8. When priority validity(T320) expires, PLMN selection is done.
9. Exclude black listed cells from priority calculations.
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Measurement rules for CRS
1. If (!(Srxlev > SIntraSearchP and Squal > SIntraSearchQ, )), perform Intra-freq
Measurement.
2. If (any Intra & Inter-RAT frequency priority > current freq), Perform
measurement.
3. If (any Intra & Inter-RAT frequency priority <= current freq)
a. If (Srxlev > SnonIntraSearchP and Squal > SnonIntraSearchQ) No
measurement.
b. Else Measure lower freq. Cell
Mobility states – scaling rules
4. If (TCRmax, NCR_H, NCR_M and TCRmaxHyst) available, mobility=1;
5. Keep counting #CRS within last TCRmax .
6. Initially lasttolast=-2, last=-1, current=current_PCI; then Store –
lasttolast=last, last=current, current=new;
7. If new == lasttolast, don‟t count.
8. If within TCRmax , ((#CRS > NCR_M ) & (#CRS < NCR_H )) Mstate=
MEDIUM.
9. If (#CRS > NCR_H ) Mstate=HIGH.
10. If (Mstate!=HIGH && Mstate != MEDIUM) during TCRmaxHyst,
Mstate=NORMAL
11. If Mstate==HIGH (Qhyst += sf-High;
TreselectionEUTRA*= sf-High)
for respective RAT.
12. If Mstate==MEDIUM, (Qhyst += sf-Medium; TreselectionEUTRA*= sfMedium) for respective RAT.
13. Roundup the above TreselectionEUTRA results to nearest seconds.
Barring, Reservation, restrictions or unsuitable filtering
1. Make a list of candidate cells – excluding restricted cells
2. Store highest rank cell. If Highest rank cell changes- remake
candidate list.
3. Check if access restricted for the best rank cell.
4. If the best rank cell is Not suitable – Forbidden TAC or not ePLMN to
RPLMN
a. Consider this cell and others in this freq Not Suitable for
300s.
b. After 300s, they may be in the candidate list and check if still
forbidden.
5. If CRSstate==Anycellselection, No restrictions apply.
6. If CSG cell and CSG-Id is not in CSGWhitelist, consider not suitable,
but other cells may be suitable of the same freq.
Inter-Freq or Inter-RAT CRS Criteria
If for 10s, no new suitable cell found intra-freq, inter-freq and inter-RAT, initiate
PLMN selection.
Scale TreselectionRAT as per Mstate= medium or high
If threshServingLowQ in SIB3, & candidate cell freqPrio > Serving, CRS if
{Candidate LTE cell Squal > Threshx,HighQ for Treselection-RAT, OR
Candidate non-LTE cell Srxlev > Threshx,HighP for Treselection-RAT.
AND current Cell camping time>1s.- Check 1s camped timer expired}
Else if
{Candidate cell Srxlev > Threshx,HighP for Treselection-RAT.
AND current Cell camping time>1s.- Check 1s camped timer expired}
If priority is equal, select on the basis of Ranking if Rn > Rs.
If threshServingLowQ in SIB3, & candidate cell freqPrio < Serving, CRS if
{Serving cell Squal < Thresh,servingLowQ && Candidate Squal >
Threshx,LowQ for Treselection-RAT, OR
Serving cell Squal < Thresh,servingLowQ && Candidate Srxlev >
Threshx,LowP for Treselection-RAT,
AND current Cell camping time>1s.- Check 1s camped timer expired}
Else if
{Serving cell Srxlev < Thresh,servingLowP && Candidate Srxlev >
Threshx,LowP for Treselection-RAT.
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AND current Cell camping time>1s.- Check 1s camped timer expired}
The Higher priority candidate Cell takes priority over Lower.
If freq priority is equal, select on the basis of Ranking if Rn > Rs.
...Current design leave out the CDMA2000 and CDMA_HRPD Cells.
Among all the above selected cells, prioritise them as follows:
First - Highest priority freq from E-UTRA cells.
Next – Highest priority freq from other RAT cells.
For Other RAT, if Squal(RSRQ) supported, CRS is based on Squal
Else CRS is based on Srxlev.
Intra-frequency and equal priority Ranking criteria
1. Rn=0;
2. if S==0(if Suitable), Rdturn(Rn=0)
3. Rs = Qmeas,s(SRRP) + Qhyst
4. Rn = Qmeas,n(RSRP) - Qoffset
5. Select CellN if
Rn > Rs. During TreselectionRAT
a.
b. More than 1s elapsed since last camp on Scell.
CRS with CSG/Hybrid cells Procedures(Rules) –
CSG is an autonomous process maintaining shared CSGav cell list, CSGwhite cell
list, including inter-RAT frequencies. Consider only current PLMN cell ids. Maintain
a shared variable for CRS to know if CSGav cell list is empty. Disable this function if
the CSGav cell list is empty; This process is expected to be designed separately
and it updates the respective list and variables.
i. With Hybrid Cells
1. if cell in CSGwhite list
a. Treat the Hybrid cell as CSG cell,
2. Else
a. Treat the Hybrid cell as Normal Cell
ii. Candidate is CSG (CSG/non-CSG -> CSG)
1. CSG cell of Current freq will be ranked higher than other freq priority.
2. If more than one CSG cell within same frequency, apply ranking
rules.
iii. CSG to NonCSG rule.
1. Apply normal CRS rules.
Actions in Camped Normally-Suitable cell
a. Monitor Paging Channel – Independent Process
b. Monitor SI - Independent Process
c. Continuously perform CRS every 1s or if CRS parameters change(from SI).
CRS at leaving connected to Idle state
i. Store status if idle-to-connected was from AnyCell or Suitable Cell.
ii. Attempt to CRS as per redirectedCarreerInfo in RRCConnectionRelease.
1. If CRS fails to directed cell – still try any suitable cell in that RAT.
2. If idle-to-connected=AnyCell, Allow to select AnyCell in that RAT.
iii. If No redirectedCarreerInfo, CRS on current or other E-UTRA carrier.
iv. If non above successful, do StoredInfo CRS to find suitable cell.
v. If still not camping on any suitable cell, Select_PLMN and search for any
Acceptable Cell.
Any Cell Selection state
Keep attempting all PLMN‟s from Higher priority PLMN to lower. Try finding
Suitable Cell.
At least remain camped on Any Acceptable Cell.
Stay in this state until at least Acceptable Cell is found and camp on it.
Camped on Any Cell
a. Monitor Paging Channel – Independent Process
b. Monitor SI - Independent Process
c. Perform necessary measurements
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d. Continuously perform CRS in all RATs and camp at least Any Acceptable
Cell.
e. If suitable Cell found, move to Suitable Cell and Camp normally.
f. If UE supports voice call and current cell doesn‟t support emergency, perform
CRS to any RAT regardless of priority and move to voice supporting cell.
g. If Cell does not support IMS emergency call, do not do CRS on that cell.
Access Control
Check for CellStatus, Barred and Reserved in SIB1
a. cellBarred – common for all PLMN
b. cellReservedForOperatorUse - (per PLMN).
c. If !cellBarred and ! cellReservedForOperatorUse, treat cell
for CRS.
d. If cellBarred
i. No CRS allowed, even for emergency NO. Select
Another cell
ii. If CSG cell,
1. select another suitable cell in same freq
iii. else
1. if
SIB1->cellAccessRelatedInfo>intraFreqReselection = “allowed”, select
another suitable cell (including same
frequency cells and any RAT);
2. Bar only for 300s and recheck after 300s if it
is still barred.
3. if
SIB1->cellAccessRelatedInfo>intraFreqReselection = “Not allowed”,
select another suitable cell (excluding same
frequency) in any RAT;
4. Bar only for 300s and recheck after 300s if it
is still barred.
Check for AC 0-15 where it belongs. It may have one or many AC’s.
a. Depending on AC‟s
b. reserved cell may be marked as barred or acceptable for a
UE.
c. If !cellBarred && cellReservedForOperatorUse
i. If AC ==11 or 15 in H/eH-PLMN, treat cell as
Suitable for CRS.
ii. If AC ==0-9 or 12-14 in H/eH-PLMN, treat cell as
barred.
1. AC 12-14 valid only in home country.
Emergency Call
ac-BarringForEmergency in SI indicates emergency restriction.
1. If cell-AC[10]=barred
a. If UE-AC[0-9]=barred or without IMSI - Ecall not allowed.
b. Else if UE-AC-11-15]=barred && cell-AC[10]=barred and cellAC[11-15] barred – Ecall not allowed
c. Else Ecall are allowed.
2. Else Ecall are allowed.
}
Exercises:
Data structures – Student should try designing data structure.
C Coding – Student should try writing code as per the design.
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1.2.9. PLMN Selection Process Design
In UE, the AS reports available PLMNs to the NAS on request from the NAS or autonomously. During
PLMN selection, based on PLMN identities list in priority order, a PLMN may be selected either
automatically or manually. Each PLMN in the list of PLMN identities is identified by a 'PLMN identity'.
In the system information on the broadcast channel, the UE can receive one or multiple 'PLMN
identity' (upto 6) in a given cell.
1.2.10. PLMN Selection Actions
18.
19.
20.
21.
22.
23.
24.
25.
On NAS request, provide PLMN list or the selected PLMN to NAS and/or to CRS.
Accept manual selection and provide selection.
Receive the PLMN identities of the current cell and maintain this cell_PLMN;
Maintain Lists - avPLMN-list, eHPLMN list, cell-PLMN list and fb_PLMN list.
Maintain PLMN list in priority order – eHPLMN, fb_PLMN, av_PLMN list.
Maintain RPLMN and VPLMN.
Return the highest priority PLMN.
For LTE Cell:
a. Provision for NAS to stop PLMN selection anytime.
b. Receive/Use (make e/HPLMN and fb_PLMN list) the initial SIM PLMN info.
c. Check if CSG list is provided. If yes, return the highest priority PLMN with suitable
cell. If no Suitable CSG cell, no PLMN from the CSG cell.
1.2.11. PLMN Selection Inputs
1. Request – NAS, CRS – reply is expected.
a. NAS to PLMN – Request to change PLMN/RPLMN
b. TAU to PLMN – Whether there is any update of the PLMN list from NAS.
c. CSG to PLMN – If any HeNB or network is of higher priority.
1.2.12. PLMN SelectionOutputs(Internal)
1. PLMN to NAS – reply any PLMN selection request to NAS. NAS should initiate Meas with
this PLMN.
2. PLMN to CRS – Give out the selected PLMN to CRS.
3. eHPLMN update
4. fbPLMN update
5. cellPLMN update
6. avPLMN update
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1.2.13. PLMN Selection IPC-Design
Flow Diagram – Student to try doing this.
Data structures – Student to try doing this.
C Coding – Student to try doing this.
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1.3. Measurement and Reporting
1.3.1. LTE Measurements
1. LTE Reference Signal Received Power (RSRP)
a. RSRP measures cell-specific signal strength of Cell-specific Reference Signal(CRS).
It is used to rank candidate cells according to RSRP and is used as an input for
handover and cell reselection decisions. RSRP for a specific cell is linear average
over the power contributions (in Watts) of Resource Elements (REs) which
carry CRS within considered bandwidth. Normally the first antenna port RS are used
for RSRP determination, but second antenna port can also be used if UE can
determine that they are available. If receive diversity is in use, the reported value is
linear average of values of all diversity branches.
2. LTE Carrier Received Signal Strength Indicator (RSSI)
a. RSSI is defined as total received wideband power by UE from all sources,
including co-channel S-cells and Non-Serving cells, adjacent channel
interference and thermal noise within measurement bandwidth. RSSI is not
reported but used to derive RSRQ measurement.
3. LTE Reference Signal Received Quality (RSRQ)
a. RSRQ is used to rank different LTE candidate cells according to their signal quality,
as input for handover and cell reselection decisions when RSRPs alone do not
provide reliable mobility decisions.
b. RSRQ = N · RSRP/(LTE carrier RSSI) where N = RBs of used for RSSI
measurement bandwidth.
c. RSRQ enables the combined effect of signal strength and interference in an efficient
way.
1.3.2. Measurement Objects and Management
E-UTRAN
provides
measurement
config
to
UE
in
RRC_CONNECTED
RRCConnectionReconfiguration. The measurement configuration parameters are:
by
Fig 5.3.7 – Measurement ID for Measurement Objects and Report Configs
1. Measurement objects(MO): Objects on which UE performs measurements. It may be
a. a single E-UTRA carrier frequency.
b. a list of cell specific offsets
c. list of 'blacklisted' cells.
d. a set of cells on a single UTRA carrier frequency.
e. a set of GERAN carrier frequencies.
2. Reporting configurations(RC): RC consists of :
a. Reporting criterion: triggers (periodic or once) UE to send a measurement report.
b. Reporting format and
c. quantities.
3. Measurement identities (M-Id):
a. each ID links one MO with one RC.
b. possible to link many MO to same RC, as well as many RC to same MO.
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4. Quantity configurations:
a. Defines quantities and associated filtering,
b. One filter per measurement quantity.
5. Measurement gaps:
a. Periods that the UE may use to perform measurements,
b. No (UL, DL) transmissions are scheduled in between.
The following table is maintained for each measurement ID:
1.3.3. NON-LTE Measurements
UMTS Measurements
1. UMTS FDD CPICH Received Signal Code Power (RSCP)
a. CPICH RSCP is equivalent to LTE RSRP, used to rank different UMTS FDD
candidate cells according to their signal strength for decisions on handover and cell
reselection to UMTS. It is received power measured on the P-CPICH. If transmit
diversity is there for P-CPICH, received code power from each antenna is measured
and summed together in Watts as total received code power.
2. UMTS FDD Carrier RSSI
a. RSSI is the received power including thermal+receiver noise, for the carrier,
within considered bandwidth.
3.
UMTS FDD CPICH Ec/N0
a. CPICH Ec/N0 is the received energy per chip (Ec) on P-CPICH of a cell divided by
total noise power density (N0) on UMTS. CPICH Ec/N0 is used to rank different
candidate cells according to signal quality for handover and cell reselection decisions.
b. If no diversity used by UE, CPICH Ec/N0 = CPICH RSCP / RSSI.
c. If transmit diversity is used on P-CPICH, received Ec from each antenna is summed
together (inWatts) to total received energy per chip on P-CPICH, before calculating
Ec/N0.
UMTS TDD RSI is the received wideband power, including thermal+receiver noise, within bandwidth
for UMTS TDD within a specified timeslot.
P-CCPCH RSCP is defined as the received power on the P-CCPCH of a UMTS TDD cell, used to
rank different UMTS TDD candidate cells for handover and cell reselection decisions.
GSM Measurements
GSM Carrier RSSI
GSM RSSI is wideband received power within bandwidth of BCCH carrier.
CDMA2000 Measurements
CDMA2000 1x RTT Pilot Strength
Pilot Strength is equivalent to RSRP, used to rank different CDMA2000 1x candidate cells for
handover and cell reselection decisions.
CDMA2000 HRPD Pilot Strength
This also is equivalent to LTE RSRP, used to rank different CDMA2000 HRPD candidate cells for
handover and cell reselection decisions.Measurement Configuration
eNB configures UE to report measurements for UE mobility via RRCConnectionReconfiguration:
1. Measurement objects (MO). MO defines what should UE measure – like carrier frequency,
list of cells (white-list or black-list), offsets etc.
2. Reporting Configurations (RC). Periodic or Event-triggered RC defines criteria for UE to
send a measurement report and details of what UE is expected to report (quantities,
RSCP/RSRP & number of cells etc.).
3. Measurement identities(MID). M-ID identify a measurement and defines MO and RC
attached.
4. Quantity configurations. It defines filtering used on each measurement.
5. Measurement gaps(Meas Gaps). It defines time periods when no UL/DL transmissions will
be scheduled, so that UE may perform measurements.
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Above details vary depending on LTE, UMTS, GERAN or CDMA2000 RAT/frequency. eNB configures
single MO for a given frequency, but more than one M-ID may use same MO.
In LTE it is possible to configure the quantity which triggers the report (RSCP or RSRP) for each
reporting configuration. The UE may be configured to report either the trigger quantity or both
quantities.
1.3.4. Measurement Report Triggering
Depending on measurement type, UE may measure and report any of the following:
1. Serving cell;
2. Listed cells (i.e. Part of MO);
3. Detected cells on a listed frequency (i.e. unlisted cells but detected by UE).
For some RATs, UE measures and reports listed cells only (white-list), while for other RATs UE also
reports detected cells.
Following event-triggered reporting criteria are specified for intra-RAT:
1. Event A1. SCell > absolute threshold.
2. Event A2. SCell < absolute threshold.
3. Event A3. NCell > offset relative to SCell.
4. Event A4. Ncell > absolute threshold.
5. Event A5. SCell < absolute threshold1 and NCell > absolute threshold2.
For inter-RAT mobility, following event-triggered reporting criteria are specified:
6. Event B1. NCell > absolute threshold1.
7. Event B2. SCell < absolute threshold1 and NCell > absolute threshold1.
UE triggers event when one or more cells meets a specified „entry condition‟. eNB can influence entry
and exit condition by setting of thresholds, offset, and/or a hysteresis. Entry condition must be met for
at least „timeToTrigger‟ parameter. UE scales timeToTrigger by speed state.
UE may be configured to provide periodic reports after it triggered an event. „event-triggered periodic
reporting‟ is configured by „reportAmount‟(number of periodic reports) and „reportInterval‟(time period
between reports). Whenever a new cell meets the entry condition, count of number of reports is reset
to „0‟. Same cell cannot then trigger a new set of periodic reports unless it first meets „leaving
condition‟.
UE may be configured for periodic reporting. Same parameters may be configured as for eventtriggered reporting, except that UE starts reporting immediately rather than after occurrence of an
event.
1.3.5. Measurement Reporting
6. In MeasurementReport message, UE includes measurement results related to a single
measurement –not combined by multiple RC. If multiple cells triggered report, UE
includes cells in order of decreasing value of reporting quantity – i.e. best cell is
reported first. Max number of cells in a MeasurementReport may be ‘maxReportCells’.
1.3.6. Measurements when Camped on LTE
Majority of UE measurements require coherent demodulation and processing, hence it measures only
after synchronization with Tcell and knows parameters (slot timing, frame timing and scrambling
codes) required to perform coherent processing. However LTE RSSI, UMTS RSSI and GSM RSSI
can be measured non-coherently.
All UE reported measurements are obtained by averaging uniformly distributed samples over
measurement period over the measurement bandwidth.Measurement model contains four different
reference points.
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Fig 5.3.5 – Measurement filtering layers
• Reference point ‘A’ represents physical layer („Layer 1‟) measurements like a single aggregated
measurement sample of LTE RSRP in 1 ms. Actual measurements procedure is not specified, but it
may be the measurement of all the RS received power averaged during 1 subframe.
• Reference point ‘B’ represents measurements after L1 filtering reported to RRC („Layer 3‟).
Sampling rate/periodicity is not defined, but performance objective, bandwidth and measurement
period may be defined. The reporting rate at point B should be sufficient to meet the specified
performance objectives.
• Reference point ‘C’ represents a measurement after L3 filtering in RRC. Reporting rate is again not
defined but should meet the performance objective (depends on measurement type). Layer 3 filters is
standardized and configuration is provided by RRC signalling. So, result at point C is a filtered
(averaged) version of the samples available at point B.
• Reference point ‘D’ contains measurement reports by UE to eNodeB. Evaluation means checking if
RRC measurement reporting is necessary at point D, which may be based on multiple flow of
measurements (C, C‟) after L3 filtering (for example after comparing different measurements). UE
maintains reporting configuration triggers, set by the network RRC by Measurement configuration
message.
1.3.7. LTE Mobility in RRC_CONNECTED
During RRC_CONNECTED UE is actively transmitting and receiving user data. Every effort is made
to maintain the radio link, and N-cell monitoring is given priority over power saving. In
RRC_CONNECTED, UE cell search and measurements controlled and configured by eNodeB.
When a better cell than current one is identified, eNodeB will trigger handovers to other cells.
Handover is requested by eNodeB to other cells either on same carrier (intra-frequency), to LTE cells
on other carriers (inter-frequency) and to other cells of a different RAT (inter-RAT).
Monitoring Gap Pattern Characteristics
During RRC_CONNECTED, if eNodeB decides UE needs to perform LTE inter-frequency and interRAT monitoring, it will provide UE with a monitoring gap pattern sequence. Same purpose is achieved
by „Compressed Mode gaps‟ and „FACH Measurement Occasions‟ in UMTS and by Idle frames in
Dedicated and Packet Transfer Mode states in GSM.
Fig 5.3.9.1 – Measurement Gap concept
During monitoring gaps, UE reception and transmission activities with S-cell are interrupted. How
does monitoring gap patterns work and help?
1. Same LTE receiver is used both to perform intra-frequency monitoring and to receive data
when there is no transmission gap.
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2. Monitoring gaps allows the receiver to be used to receive data and to perform inter- RAT
activity, but not simultaneously.
3. Even if UE has multiple receivers for inter-RAT monitoring (e.g. one LTE receiver, one UMTS
receiver and one GSM receiver), for some band, monitoring gaps are still required in UL,
specially when UL carrier used for transmission is adjacent to monitored band. There may be
significant power difference between inter-RAT signal measured and UE signal transmitted.
The receive filter may not be sufficient to filter out the transmitted signal at receiver front end,
so the transmit signal leaks into the receiver band creating interference which saturates the
radio front end stages. This interference desensitizes the receiver being used to detect interRAT cells. Uplink gaps in LTE are configured for all such scenarios.
LTE monitoring gap patterns contain gaps every N LTE frames (gap periodicity is multiple of 10 ms)
and these gaps have 6 ms duration. Single monitoring gap pattern is used to monitor all possible
RATs.
Different gap periodicities are used to trade off between monitoring performance, data throughput and
efficient utilization of resources. Cell identification performance increases as the monitoring gap
density increases, UE throughput decreases as monitoring gap density increases.
Most RATs broadcast sufficient pilot and synch information to enable a UE to synchronize and start
measurements within a useful period slightly in excess of 5 ms, as most RATs transmit DL synch
signals with a periodicity no lower than 5 ms.
In LTE, PSS and SSS symbols are transmitted every 5 ms. Therefore 6 ms gap provides sufficient
additional headroom to retune the receiver to inter-frequency LTE carrier and back to S-cell and still to
cope with the worst-case relative alignment between both cells. GSM requires special treatment
because synch information is organized differently in the time domain.
Fig 5.3.9.2 – Measurement Gap configuration
With these restrictions the following diagram explains DCI and PHICH restrictions during
Measurement Gap.
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Fig 5.3.9.3 – PDCCH and PHICH restrictions due to Measurement Gap
LTE Intra-FrequencyMonitoring
LTE intra-frequency monitoring perform measurements both on Scell and Ncells which use same
carrier frequency. For RSRP and RSRQ measurements, UE must first synchronize to find cell ID of Ncells. LTE UE has to be able to perform search without an explicit NCL being provided.
The intra-frequency measurement period is defined to be 200 ms.
Even when monitoring gap patterns are activated, vast majority of time is available to perform intrafrequency monitoring. When DRX is enabled, UE can use opportunities to save power between
subsequent DRX „On periods‟. Intra-frequency monitoring performance relaxations will only be
defined for cases when „On period‟ periodicy > 40 ms.
LTE Inter-FrequencyMonitoring
LTE inter-frequency monitoring is similar to intra-frequency except that it performs in monitoring gaps.
For a 6ms gap pattern only 5ms is available for inter-frequency monitoring once the switching time
has been removed. If monitoring gaps repeat every 40 ms only 5/40 = 12.5% is available for interfrequency monitoring. So, LTE inter-frequency maximum cell identification time and measurement
periods need to be longer than for intra-frequency case.
Within one monitoring gap, PSS and SSS symbols is guaranteed and there are also sufficient RSs to
perform power accumulation and obtain RSRP, RSSI and derive RSRQ. Normal measurement
bandwidth are 6 central RBs of an LTE carrier (i.e. 1.08MHz), which include PSS and SSS. An
optional 50 RB configuration is also defined.
GSM Monitoring from LTE
GSM is the only RAT where synch information with a single 6 ms monitoring gap may not be
sufficient. A monitoring gap pattern used for GSM monitoring must do: GSM RSSI measurements,
initial BSIC identification and BSIC reconfirmation, by allocating every third monitoring gap to each
one of the above three.
Careful selection of gap repetition period to a period which is a factor of 240 ms (30, 40, 80, 120 and
240 ms) can detect synchronization burst(SB) containing BSIC and SFN within a guaranteed maxtime. A control channel multiframe is 51 frames (=51*577*8µs=240ms).
How/Why a gap pattern of 6 ms gaps repeating every 240 ms, guarantees initial BSIC identification
and BSIC reconfirmation time, under good reception conditions? Maximum time will always involves
multiple gaps. Any gap duration exceeding nine timeslots (9*577µs) = 5.19ms is guaranteed to
contain a timeslot 0 regardless, which contains FB or SB. Once receive switching overhead is added,
6ms gap is sufficiently large. Moreover, a gap pattern repeating every 240 ms will be guaranteed to
observe FB or SB in at most 11 consecutive gaps because there is a shift of one GSM frame with
respect to the GSM 51-frame control multiframe between two adjacent monitoring gaps. Thus, a
single step BSIC reconfirmation is guaranteed not to require more than 11 consecutive gaps since all
it requires is decoding the SB. For the same reason, both the FB and the SB can be observed in no
more than 12 consecutive gaps (12*240ms = 2880 ms). Therefore, two step initial BSIC identification
single attempt is guaranteed not to require more than 2880ms. Single-step initial BSIC reconfirmation
requires the same time as BSIC reconfirmation.
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More complicated analysis must be performed in order to determine the worst-case initial BSIC
identification and BSIC reconfirmation times when using other monitoring gap periodicities (i.e. 40, 80
and 120ms etc..).
UMTS Monitoring from LTE
UMTS monitoring is performed within the monitoring gaps. UE needs to read P-SCH, S-SCH and
CPICH for cell identification and measurements (RSCP and Ec/No), and are guaranteed to be present
within a 6 ms gap.
Measurement Reporting
Two types of measurement reporting are specified by Measurement Configuration:
1. Periodic reporting: Measurement reports are configured to be reported periodically.
2. Event-triggered measurement reporting: Measurement reporting can be configured to
trigger when some conditions are met by measurements by UE. Reporting conditions are
criteria to start N-cell measurements or trigger handovers due to poor cell coverage or poor
quality. Once criteria is met, UE can be configured to report additional measurements even
unrelated to the event condition. This report is used by eNodeB RRM algorithms to determine
the best handover command.
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1.4. Radio Resource Controller –
Connected
Radio Resource Controller (RRC) performs Control Plane of Access Stratum (AS). AS interacts with
NAS, which handles PLMN_selection, Tracking Area update, paging, authentication, EPS bearer
establishment, modification and release. RRC is in either RRC_IDLE or RRC_CONNECTED state.
In RRC_CONNECTED, eNB allocates RB to UE to transfer data via shared data channels. UE
monitors PDCCH for allocated transmission resources in time and frequency. UE reports buffer status
and DL channel quality (CQI), neighbouring cell (including other freq/RAT) measurement to select
most appropriate cell for UE. UE continues to receive SI. To extend battery lifetime, UE may be
configured with a Discontinuous Reception (DRX) cycle. RRC performs security, inter and intra RAT
mobility, establishment and reconfiguration of radio bearers to carry control and user data.
RRC covers following functional areas.
 System information- handles broadcasting of SI, for NAS, RRC, L2 and PHY.
 RRC connection control- covers (re)establishment, modification and release of RRC
connection, paging, initial security activation, establishment of SRBs & DRBs, inter/intra-RAT
handover including context, configuration of L1 & L2, AC barring and radio link failure (RLF).
 Network controlled inter-RAT mobility besides mobility, security activation and context
information transfer.
 Measurement configuration and reporting for intra/inter-frequency and intra/inter-RAT
mobility, with measurement gaps management.
 Miscellaneous functions – Perform transfer of dedicated NAS & access capability
information.
RRC messages are transferred across SRBs, mapped via PDCP and RLC onto logical channels –
either CCCH or DCCH. SI is mapped to BCCH and Paging is mapped to PCCH.
SRB0 is used for CCCH, SRB1 is for DCCH, and SRB2 is for NAS messages using DCCH. All DCCH
messages are integrity-protected and ciphered by PDCP (after security activation) and use ARQ for
AM RLC. CCCH messages are not integrity-protected and no ARQ in RLC. NAS independently
applies integrity protection and ciphering.
For low transfer delay parameters, MAC signalling is used when no security concerns applies.
1.4.1. Connection Control within LTE
Connection control involves:
1. Security activation;
2. Connection establishment, modification and release;
3. DRB establishment, modification and release;
4. Mobility within LTE.
1.4.2. Security KeyManagement
Two functions provided for security: ciphering of both control and user plane data and integrity
protection of control plane (RRC) only. Ciphering is used to protect data, while integrity protection
detects packet insertion or replacement. RRC always activates both functions together, either
following connection establishment or after handover to LTE.
The key „K‟ is always in Authentication Centre (AuC) in Home Subscriber Server (HSS). At MME‟s
request, HSS with RAND number, generates vector (IK,CK, RES,AUTN) and subsequently KASME
(Access Security Management Entity). Generated KASME, checksums(RES) and random number
(RAND) are transferred to MME, which passes AUTN and RAND to UE. „K‟ is also in a secure part of
the Universal Subscriber Identity Module (USIM) in the UE. USIM in UE then computes the same set
of keys using the RAND with secret key. Verify if AUTN matches with received one.
Upon connection establishment, AS derives eNB specific KeNB from KASME. KeNB is used to generate
RRCINT RRCENC and UPENC. In handover within E-UTRAN, a new AS base-key and AS derived-keys
are computed from AS base-key used in Scell. The use of security keys is handled by PDCP layer.
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Security functions are never deactivated, although „NULL‟ algorithm may be used.
1.4.3. Connection Establishment and Release
UE may be in NAS states EMM-DEREGISTERED or EMM-REGISTERED. Within REGISTERED, it
may be in EPS Connection Management (ECM) state (ECM-IDLE or ECM-CONNECTED).
ECM-IDLE to ECM-CONNECTED involves RRC connection and S1-connection establishment. NAS
initiates RRC connection establishment prior to S1-connection establishment. Connectivity in
RRC_CONNECTED is initially limited to exchange of control information between UE and E-UTRAN.
UEs move to ECM-CONNECTED when becoming active within 100ms.
RRC connection release is initiated by eNodeB following S1 connection release between eNodeB and
CN.
Connection establishment message sequence. RRC connection establishment establishes SRB1
and transfers initial UL NAS message. This NAS message triggers S1 connection, Security,
establishes SRB2 and one or more DRBs (for default and optionally dedicated EPS bearers).
Step 1: Connection establishment
•
•
•
Upper layers in UE trigger connection establishment, which may be in response to
paging. Lower layers in the UE perform a contention-based random access(RA)
procedure, UE starts T300 and sends RRCConnectionRequest to Cell with initial identity
(S-TMSI or a random number) and an establishment cause.
If E-UTRAN accepts connection, it returns RRCConnectionSetup with initial radio
resource configuration with SRB1. E-UTRAN may order default configuration as per
specification.
Fig 5.2.3.1 – rrcConnectionSetup IE
UE returns RRCConnectionSetupComplete with NAS message, selected PLMN and
registered MME code. Based on this, eNodeB decides CN node for S1-connection.
Step 2: Initial security activation and radio bearer establishment
•
•
•
•
•
eNB sends SecurityModeCommand to activate integrity protection and ciphering. Itself, it
is integrity-protected but not ciphered, indicates which algorithms to be used.
UE verifies integrity protection of SecurityModeCommand and, start applying integrity
protection and ciphering to all subsequent messages (SecurityModeComplete (or
SecurityModeFailure) onwards).
eNB sends RRCConnectionReconfiguration with RB configuration, to establish SRB2 and
one or more DRBs, possible piggybacked NAS message or a measurement
configuration. It may be sent prior to receiving SecurityModeComplete. eNB should
release the connection when one or both (Security and Configuration) procedures fail.
UE finally returns RRCConnectionReconfigurationComplete.
A connection establishment may fail for many reasons, such as:
o Access may be barred.
o Cell re-selection occurs during connection establishment, UE aborts procedure.
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o
o
Wait timer expires.
NAS may abort on NAS timer expiry.
Fig 5.2.3.2 – rrcConnectionSetupComplete IE
Step 3: DRB Establishment
• RRCconnectionReconfiguration commands is sent to UE to establish, modify or release
DRBs.
Fig 5.2.3.3 – rrcConnectionReconfiguration IE
•
For DRB, eNB decides RB interface for EPS bearer which is mapped (1-to-1) to a DRB
which is mapped (1-to-1) to a DTCH, further mapped/multiplexed (n-to-1) to DL-SCH or
UL-SCH, which are mapped (1-to-1) to PDSCH or PUSCH. RB configuration covers
PDCP, RLC, MAC and PHY layers. The main configuration parameters / options include
the following:
o PDCP may be configured for header compression to reduce signalling overhead.
o RLC Mode (AM, UM or TM) is selected. Normally RLC-AM is applicable for reliable
transmission.
o eNB assigns priorities and PBRs to control how resources and data rate.
o UE may be configured with a DRX cycle.
o For VoIP, semi-persistent scheduling (SPS) may be configured to reduce signalling
overhead.
o Delays may be configured with a Hybrid ARQ (HARQ) profile.
1.4.4. RRC_CONNECTED Messages
Here is the list of all RRC Messages:
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Fig 5.2.4 – List of all RRC messages in UL and DL.
1.4.5. Mobility Control in RRC_IDLE and RRC_CONNECTED
Cell-reselection in RRC_IDLE is UE-controlled, while Handover, CCO, Redirection in
RRC_CONNECTED is E-UTRAN controlled. Mobility mechanisms are designed to support a wide
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variety like network sharing, MCC borders, HeNB and Macro/Micro/Pico/Femto cells and varying
subscriber densities. Radio link quality is the primary criterion for selecting a cell on an LTE
frequency. When choosing between cells, consider frequencies, RATs, UE capability, subscriber
type, call type etc. Voice centric call request can be retained (or forwarded to) GSM and data centric
calls may be forwarded to LTE.
eNB provides neighbouring frequencies and cells for cell reselection and measurements. In general,
white-list are considered and used for selection and black-list are forbidden. eNB is not required to
indicate all neighbour cells that UE shall consider. UE can detect itself what cells it can move to.
Mobility in idle mode. Cell re-selection between frequencies is based on absolute priorities, provided
by SI. eNB may assign UE-specific values upon release, based on UE capability or subscriber type.
Among equal priority freq cells, cells are ranked based on radio link quality. Equal priorities are not
applicable between frequencies of different RATs. Cells without frequency priority are not considered.
Mobility in connected mode. In RRC_CONNECTED, eNB decides target cell to maintain the radio
link, taking into account UE capability, subscriber type and access restrictions. Although eNB may
trigger blind handover without measurement report, normally it configures UE to report measurements
of target cells.
In LTE, handover from a Scell to Tcell is a hard handover. eNB which controls Scell requests target
eNB to prepare for handover. T-eNB generates RRC message to S-eNB to order UE for handover,
and message is forwarded by S-eNB to UE. In case S-Cell Radio link fails during preparation, UE by
itself decides to connect to T-cell as connection reestablishment. This succeeds only if T-cell was
prepared in advance for handover.
UE may be redirected to another freq/RAT on release. Redirection may also be performed if Ssecurity
not activated. Redirection during connection establishment is not supported, before and after is
supported.
1.4.6. Message sequence for handover within LTE
In RRC_CONNECTED, eNB controls mobility – to intra or inter frequency cells. Inter-frequency
measurements may require measurement gaps depending on UE dual receiver capabilities.
Handover may be used for change of security keys or to perform a „synchronized reconfiguration‟.
The message sequence for itra-freq handover is as follows:
1. UE may send a MeasurementReport to eNB.
2. Before handover command to UE, S-eNB sends „handover preparation request‟ to one or
more T-cells. S-eNB provides UE context about UE capabilities, current AS-configuration and
UE-RRM information. T-eNB sends „handover command‟to S-eNB, who will forward this
transparently to UE in RRCConnectionReconfiguration message.
3. RRCConnectionReconfiguration orders UE to perform handover with mobility control
information (target cell id, frequency) and common RRM information(SI with RA parameters,
dedicated Resource configuration, security, C-RNTI) in T-cell. Optionally it may include
measurement configuration. If no measurement configuration is included for inter-frequency
handover, UE stops any inter-freq/RAT measurements and deactivates measurement gap.
4. If UE can comply with RRCConnectionReconfiguration, UE starts T304, and initiates a
random access (RA) using received RACH configuration, to target cell. Note that UE does not
need to acquire SI in T-cell prior to RA and resuming data communication. UE may be unable
to use SPS, PUCCH and SRS from very start. UE derives new security keys and applies
received configuration in T-cell.
5. On successful RA, UE stops T304. Now UL and DL, RRC, NAS communications continue.
1.4.7. Connection Re-Establishment Procedure
In many failure like RLF, handover failure, RLC error, reconfiguration compliance failure - UE initiates
RRCconnection reestablishment, if security is active. If security is not active, UE moves to RRC_IDLE
instead.
UE starts T311 and performs cell selection, prioritize searching on LTE frequencies. Upon finding a
suitable
cell,
UE
stops
T311,
starts
T301
and
initiates
RA
to
send
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RRCConnectionReestablishmentRequest including UE-Id used in failed cell, failed cell-id, MAC and a
cause.
Re-establishment procedure reuses/continues SRB1 and reactivates security without changing
algorithms. A subsequent RRCconnectionreconfiguration is used to resume operation on radio
bearers and measurements. If re-establishing T-cell is not prepared (i.e. does not have UE context),
eNB will reject and UE should move to RRC_IDLE.
1.4.8. Connected Mode Inter-RAT Mobility
Handover to LTE
Handover to LTE is largely same as handover within LTE. Main difference is that entire ASconfiguration needs to be signalled, whereas within LTE „delta signalling‟, changes to the
configuration are signalled. If ciphering not activated in previous RAT, E-UTRAN activates ciphering,
possibly NULL algorithm. eNB establishes SRB1, SRB2 and one or more DRBs (i.e. at least for
default EPS bearer).
Mobility from LTE
Mobility from LTE to another RAT supports both handover and Cell Change Order (CCO or even
NACC – Only GERAN), but only after security is activated. Here is the brief procedure:
1. UE may send MeasurementReport message.
2. In handover (not CCO), S-eNB sends „handover preparation request‟ to T-RAN, with
applicable inter-RAT UE capabilities and established bearers. In response, T-RAN sends
„handover command‟ to SeNB.
3. S-eNB sends MobilityFromEUTRACommand to UE including either inter-RAT message from
T-RAN (handover case), or T-cell/frequency and inter-RAT parameters (CCO case).
4. On MobilityFromEUTRACommand, UE starts T304 and connects to T-node, either by
handover or CCOas per applicable specifications of T-RAT.
CDMA2000
For CDMA2000, additional procedures are defined to transfer dedicated information from UE
CDMA2000 layers, used to register UE‟s presence in target core network before handover
(preregistration) using SRB1.
1.4.9. Handover to LTE
When eNB-RRC decides to initiate a handover it sends a „MOBILITY FROM E-UTRA COMMAND‟
RRC message to UE, including target RAT, frequency and relevant parameters required for UE to
establish a radio link with target cell. Handovers reasons can be many:
1. Quality: Measurement report shows that UE can communicate better with a N-cell than
current S-cell.
2. Coverage: Inter-RAT Handover may be initiated because UE is losing coverage for current
RAT. UE could be moving away from LTE coverage and eNB hands over the connection to
next preferred RAT which UE has detected, such as UMTS or GSM.
3. Load-Balancing: To spread the load more evenly between different cells or even RATs
belonging to same operator, when current cell is overloaded. If LTE cell is congested then
some users may be moved to nearby LTE or UMTS or GSM cells.
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1.4.10. Major HANDOVER Steps (Intra-LTE)
The steps are as follows:
Fig 5.2.14 – Intra RAT Handover Ladder diagram
1. UE generates and transmits measurement report to current S-Cell. At least in one
measurement, there must be one target cell(T-Cell) with higher RSRP level than the current
S-cell.
2. eNB controlling S-Cell decides that a handover is necessary, identifies a suitable T-cell
(assumed LTE cell) and requests access to eNodeB controlling T-cell.
3. T-eNB accepts handover request and provides S-eNB with the parameters required for UE to
access T-cell, including cell ID, frequency and UL (PRACH) resources.
4. S-cell sends „RRCConnectionReconfig‟ RRC message to UE.
5. UE receives message, interrupts radio link (stops data transmission) with S-eNB and initiates
establishment of new radio link with T-eNB. There are a number of steps involved:
a. DL synchronization establishment. UE will have to perform LTE PSS and SSS
synchronization steps. DL synchronization is only considered established when DL
RS quality is sufficiently good.
b. Random Access: By Random access procedure, it receives uplink resource, its
identity and Timing advance command. From this point onwards data reception in DL
from T-eNB may take place.
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c.
Timing advance. It is provided by T-eNb based on received delay measured on
PRACH.
d. Data transmission. UE starts UL/DL data transmitting towards T-eNB.
S-eNB may forward UE data to T-eNB pending in the S-eNB.
6. Moment UL data transmission is established with T-eNB, RRC message is sent to S-eNB to
notify that handover has been completed.
7. T-eNB also notifies the MME that UE has handed over to T-Cell and MME reroutes the DL
data to T-eNB. S-GW Notifies S-eNB that DL data is switched to T-eNB. S-eNB then forwards
rest of the pending data to T-eNB and clears the UE-Context.
8. UE now just continues the communication with T-Cell in T-eNB..
The above steps are just representative of best-case scenario and many deviations/failures are
possible. Failure cases can necessitate recovery procedures. Preferred T-eNB may have no spare
resources to grant to UE, DL synchronization might fail, UL RACH process might fail etc.
Differences between LTE Intra- and Inter-Frequency Handover
In LTE all handovers are hard handovers. The steps for inter and intra-frequency handover within LTE
are very similar.
Handover interruption time= time between the end of last TTI UE has received handover command on
PDCCH/PDSCH and the time UE is ready to start a PRACH transmission to new uplink.
Tinterrupt = Tsearch + TIU + 20 ms
where Tsearch = time required to find T-cell when not already known, else if known, Tsearch
=0ms.
TIU = interruption uncertainty to locate first available PRACH occasion in new cell.
20 ms is added to allow for UE processing time to execute handover.
1.4.11. Handover to UMTS
This procedure can be decomposed into two stages, each of which relates to an identical stage in a
single-mode handover procedure:
1. Handover initiation. In UE case, it receives “MobilityFromEUtraCommand”, within which it
has the “UMTS handover command” what it would have received if it was in UMTS network.
eNB through MME would have received all the information required for Target UMTS T-NB
from T-RNC/SGSN.
2. Radio link establishment to UMTS T-cell. This stage is identical to UMTS inter-frequency
handover. Handover execution delay requirements are very similar to intra-UMTS hard
handover requirements.
Handover to UMTS may be blind or guided depending on whether or not the UE has been able to
synchronize to T-cell prior to receiving „MOBILITY FROM E-UTRACOMMAND‟ RRC message.
1.4.12. Handover to GSM
Similar to UMTS, Handover to GSM can also be decomposed into two separate stages:
1. Handover
initiation.
In
UE
case,
similar
for
UMTS,
it
receives
“MobilityFromEUtraCommand”, within which it has the “GSM handover command” what it
would have received if it was in GSM network. eNB through MME would have received all the
information required for Target GSM BTS from T-BSC/SGSN.
2. Radio link establishment to GSM T-cell. This stage is identical to a GSM intra-system
unsynchronized handover.
Handover to GSM interruption time can be decomposed into three main contributions:
Tinterrupt GSM = Tprocessing time + Tsync,blind + Tinterruption,guided , where
Tprocessing time = 50 ms (different implementations to reconfigure modem for GSM)
Tsync,blind =time to enable UE to synchronize to T-cell during a blind handover (100 ms).
Tsync,guided = 0 ms).
Tinterruption, guided = 40 ms (UE has been able to synchronize to T-cell command)
Seamless mobility is ensured in LTE, to deliver uninterrupted mobile user experience. A wide range of
measurements and signalling are defined to support different handover scenarios.
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1.4.13. Other RRC Signalling Aspects
UE Capability Transfer
Core Network stores AS capabilities when UE gets registered as EMM-REGISTERED and not each
transition from RRC_IDLE to RRC_CONNECTED. Upon S1 connection establishment, CN provides
capabilities to eNB. If eNB does not receive (required) capabilities (e.g. due to UE in EMMDEREGISTERED), it may requests UE by UEcapabilityEnquiry, may be for each (LTE, UMTS,
GERAN). UE responds with UECapabilityInformation.
Uplink/Downlink Information Transfer
UL/DL Information transfer is used to transfer only NAS/OtherRAT messages. NAS information may
be included in RRCConnection-SetupComplete and RRCConnectionReconfiguration also. This
applies
for
EPS
bearer
establishment,
modification
and
release
also.
HandoverFromEUTRAPreparationRequest and ulHandoverPreparationTransfer are defined for
CDMA2000 for preregistration.
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1.5. Radio Resource Management
Radio Resource Management (RRM) provide the user with mobility whereby UE and NW take care of
mobility seamlessly, without much user intervention. There is a trade-off between additional UE
complexity (e.g. cost, power consumption, processing power), network complexity (e.g. radio interface
resource, network topology) and achievable performance.
The main procedures are cell search, measurements, cell reselection and handover. RRM includes
protocols for handling mobility, synchronization, cell search, mobility with LTE and other Radio Access
Technologies (RATs) cells.
1.5.1. UE Mobility Activities Overview
To maintain service continuity as a user moves, UEs must be connected to a S-cell and monitor
neighbour cells continuously, since propagation conditions(interference) to different eNodeBs
changes rapidly. UE and NW will always be directed to a preferred RAT according to preference
criterion based on QoS, cost or operator.
Generally UE will be requested to perform mobility decisions (handover or cell reselection) towards
other cells of same RAT. But, whenever preferred network is not available (poor coverage, congestion
etc), NW and UE must cooperate to identify fallback options to other NW or RATs for continuity. A
fallback RAT may result in some degradation in terms of services provided, but at least continuity can
be preserved.
In all mobility cases UE must meet minimum performance requirements, for cell search, measurement
accuracy and periodicity and handover execution delay. UE power consumption and cost are
important factors. The performance requirements consists of:
1. Current serving S-Cell RAT.
2. Layer 3 state of S-Cell, e.g. LTE states RRC_CONNECTED and RRC_IDLE.
3. Monitored RAT. Camped on an LTE cell, monitor other cells on same LTE frequency
(ntra-frequency) and cells on other LTE frequencies (inter-frequency). It should
monitor one or more other RATs, such as GSM, UMTS, WiMAX, TD-SCDMA, CDMA
HRPD, or CDMA2000. Criteria in each RAT may be different.
Among 3GPP RATs (LTE, UMTS and GERAN1), minimum-effort are broadly the same regardless of
RAT involved:
1. Serving cell quality monitoring and evaluation: S-cell quality is evaluated periodically. If Scell quality is satisfactory (above a configured threshold), then no further action is required
and stay in step 1. However, if S-Cell quality is below threshold, the next step 2 is executed.
2. Initiate periodic cell search for candidate neighbour cells: Candidate neighbour cells can
be intra-frequency, inter-frequency and inter- RAT cells, and search is performed in a defined
order of priority. Cell search is repeated periodically. Even if a UE has identified a N-cell
(neighbour), it will continue cell search until either S-cell quality becomes satisfactory again,
or UE moves to another S-cell through handover, cell reselection, redirection or cell change
order. If some N-cells are identified as candidate, then the following step is performed or else
continue step 1 or 2.
3. Neighbour cell measurement: Signal Strength for N-cells in step 2 is measured periodically
until either S-cell quality becomes satisfactory again, or UE moves to another S-cell.
a. To avoid measurement fluctuations, measurement is obtained by averaging over a
number of evenly spaced samples within a measurement period (Intra-frequency
RSRP meas-period is 200 ms) and then next step 4 is performed.
4. Mobility evaluation: Next, decision is made UE should move to another S-cell. The eNodeB
decides on handover and UE redirection/cell change orders, and UE decides cell reselection.
If mobility criteria are fulfilled then mobility procedure to move towards Target T-cell is
executed. The T-cell may be in same RAT (intra- and inter-frequency handover and cell
reselection) or in different RAT (for inter-RAT handover and cell reselection).
1.5.2. LTE Cell Search
UE‟s detects N-cells, which may belong to LTE (intra or inter-frequency) or to other RATs (inter-RAT).
The cell search in LTE in summary consists of:
1. Primary Synchronization Signal (PSS) detection to obtain physical cell ID (within a
group of three) and slot synchronization.
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2. Secondary Synchronization Signal (SSS) detection to obtain Cyclic Prefix (CP)
length, physical cell group ID and frame synchronization.
3. Physical Broadcast CHannel (PBCH) decoding to obtain critical system information
(MIB(40ms periodicity), SIB1 and SIB2).
4. For new cell identification, PBCH may not be required but Reference Signal (RS) may
be decoded to measure RSRP and RSRQ to be reported to NW.
1.5.3. UMTS Cell Search
UE is synchronized to UMTS cell when it knows cell‟s frame boundaries timing and cell‟s primary
scrambling code which distinguishes the cell‟s transmissions from other cells. UMTS synchronization
process stages are:
1. Primary-Synchronization CHannel (P-SCH) search. Only one UMTS P-SCH code exists,
and repeated on the first 256 chips of every slot (0.666 ms). UE performs matched filter
correlation between the received signal and P-SCH sequence for all possible timing offsets
within one slot and correlation peaks can be observed in those locations where a P-SCH
sequence is present. This gives the slot boundary timing for each detected P-SCH. For one or
more of the strongest detected peaks, next step is performed.
2. Secondary-SCH decoding. The S-SCH code sequence is one of 15 codewords present on
the first 256 chips of every slot (same time as P-SCH). One S-SCH code sequence is defined
for all cells belonging to the same „code group‟. Each S-SCH code sequence identifies
uniquely a code group and 10 ms frame boundary position. In good signal conditions the
information contained within three slots is sufficient to identify uniquely both the frame timing
and the code group, but in order to reliably decode the S-SCH in difficult reception conditions
longer decoding periods are required.
3. Primary scrambling code identification. The code group of S-SCH indicates a group of
eight primary scrambling codes. A given cell uses one code from this group as the scrambling
code for all DL channels, including Primary Common PIlot CHannel (P-CPICH). UE performs
a correlation against the eight scrambling sequences, looking for known CPICH sequence
(which is the same in all UMTS cells), to determine which code is being used.
4. System Frame Number (SFN) detection. Primary Common Control Physical CHannel (PCCPCH) carries the Broadcast CHannel (BCH), encoded over a 20 ms TTI. Earlier
synchronization stages only provide timing information up to a 10 ms period (one frame). The
location of the even frame boundaries can be found, by trial and error by performing decoding
attempts on the two possible TTI boundaries. Only the correct boundary will ensure
successful channel decoding and return a correct CRC. BCH carries the SFN. This step is
only required on cell reselection from LTE to UMTS and on handover to UMTS after handover
initiation.
Once a UE has camped on an LTE cell it will receive a UMTS neighbour cell list containing up to 32
primary scrambling codes per UMTS carrier, used by the UE to speed up the UMTS cell search
process.
1.5.4. GSM Cell Search
GSM Synchronization includes: GSM RSSI measurements, initial BSIC identification and BSIC
reconfirmation.
When camped on LTE cell, UE will be provided Neighbour Cell List (NCL) with at least 32 GSM carrier
numbers (ARFCNs) indicating neighbouring cells frequencies, and optionally an associated BSIC for
each GSM carrier in the NCL.
GSM RSSI Measurements
For GSM monitoring, UE will measure GSM RSSI (averaged over at least 3 samples) for all carriers in
the NCL in every measurement period (usually 480ms). Once measurements for all cells in NCL are
available, the strongest N cells are passed to the next step.
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Initial BSIC Identification
BSIC is within GSM Synchronization Burst (SB), carrying GSM Synch CHannel (SCH). The SCH also
carries cell SFN. The BSIC (3 bits Base station Colour Code (BCC) and 3 bits Network Colour Code
(NCC)) allows UE to distinguish two different cells which share the same beacon frequency. BCC is
also used to identify Training Sequence Code (TSC) used while reading BCCH. NCC is used to
differentiate between operators utilizing same frequencies, (e.g. on border when both NW have same
frequency or frequencies).
Initial BSIC identification is performed in for N = 8 strongest GSM carriers as follows:
a. Frequency Control Channel (FCCH) detection. To detect a Frequency Burst (FB) by
FCCH, UE tunes to a beacon frequency and performs a continuous correlation against the
signal contained within FB. The FB is transmitted on timeslot 0 of frames 0, 10, 20, 30 and 40
of 51-frame control multiframe. When a correlation peak is detected, coarse frame timing and
coarse frequency synchronization can be acquired. If continuous correlation is performed,
FCCH is surely detected in no more than 11 frames. One GSM frame duration = 60/13ms =
4.61 ms.
b. GSM SCH detection. There is always one SB (carried by SCH) exactly one frame after the
FB. If initial BSIC identification is being performed within a gap in LTE signal specially created
for inter-RAT monitoring and this gap is too short, then GSM SCH can be decoded later. Then
decoding SCH becomes more complicated for the presence of the GSM idle frame in 51frame control multiframe. Idle frame introduces a N/(N + 1) frame ambiguity forcing UE to
perform decoding attempts at two adjacent locations after FB detection separated by one
GSM frame: 10/11 frames, 20/21 frames, etc. Since SCH contains CRC, CRC check outcome
determines which of the two options is the correct outcome. Once SCH is decoded, both
BSIC and frame number are obtained, and position of SB can be predicted.
BSIC Reconfirmation
BSIC Reconfirmation decodes the SB periodically on GSM carriers where BSIC has already been
detected. Then SB position within a cell can be predicted exactly as neighbour cell SFN is acquired
earlier. UE needs to check periodically that the carrier with this BSIC was previously identified with
same BSIC. After many unsuccessful BSIC reconfirmation attempts, a carrier must be moved back to
initial BSIC identification.
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1.6. MU-Scheduling & Interference
Coordination
The eNodeB is responsible for managing resource scheduling for UL/DL channels to fulfil the
expectations of as many users as possible as per Quality-of-Service (QoS) requirements of their
respective applications.
A single-cell with K UEs communicates with one eNodeB over a fixed total bandwidth B. Each UE has
several data queues for different UL channel groups, each with different delay and rate constraints. In
DL also, eNodeB maintains several buffers per UE with dedicated data traffic with different QoS and
broadcast services. Total BW B is divided into M RBs. Data is split into blocks of duration T = 1
subframe (1 ms, 1 TTI). Channel can be assumed stationary for the duration of each subframe, but
vary from subframe to subframe and channel is assumed constant over the subcarriers in one RB, but
channel gain of a user may change from one RB to another.
Resource scheduling algorithm in eNodeB allocates RBs and powers for each subframe to optimize
performance metrics, for example max/min/avg throughput, delay, spectral efficiency or outage
probability. In DL, allocation is constrained by total transmission power of eNodeB, while in UL,
constraint of power in different RBs are due to multicell inter-cell interference.
Resource allocation algorithms considers orthogonal design of multiple access schemes, where only
one user is allocated a particular RB in any subframe.
1.6.1. Resource Allocation Strategies
Scheduling algorithms uses channel-state information (CSI) and traffic measurements (volume and
priority) via feedback signalling channels. Algorithms aim is to maximize the data rate in one direction
at the expense of more overhead in the other. In TDD, amplitude coherence between UL and DL may
be used to assist scheduling algorithm.
It is tightly coupled with adaptive coding and modulation scheme based on channel measurement and
HARQ. Secondly, the queue dynamics, impacting throughput and delay, depend heavily on HARQ
protocol and TB sizes. Combination of channel coding and HARQ retransmission enables the spectral
efficiency. eNodeB resource scheduler manages differing requirements of all UEs in the cells to
ensure sufficient RBs are allocated to each UE within acceptable latencies to meet their QoS
requirements in a spectrally-efficient way. There are mainly two approaches to scheduling:
Opportunistic scheduling and fair scheduling are two methods.
Opportunistic Scheduling is designed to maximize total data rates to all users by exploiting channel
conditions at different times and frequencies. For a multiuser system, more information can be
transmitted across a fading channel than a non-fading channel for the same average signal power at
the receiver, it is known as multi-user diversity. Allocating channel only to the user with the best
channel condition can increase the total throughput for large active users, if UE is able to adapt the
power dynamically according to the channel state. This allows some simplification and is well suited to
DL where transmitted power in a given subframe is limited by dynamic range of UE receivers and the
need to transmit wideband RS for channel estimation. Opportunistic Scheduling doesn‟t ensure
fairness and QoS, users‟ data cannot always wait until the channel conditions are sufficiently
favourable for transmission. It is important to provide reliable wide area coverage, including to
stationary users near the cell edge – not just to the users which happen to experience good channel
conditions by virtue of their proximity to the eNodeB.
Fair scheduling, pays more attention to latency for each user than to total data rate achieved,
particularly important for real-time applications like VoIP or video-conferencing, where a minimum rate
must be guaranteed independently of the channel state.
In practice scheduling algorithms fall between the two extremes to deliver the required mix of QoS. A
Cumulative Density Function (CDF) metrics of throughput of all users is used. Ensure that the CDF of
the throughput lies to the right-hand side of a particular threshold. This saves penalizing the celledge
users to give high throughputs to the users with good channel conditions. In a network, individual cells
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cannot be considered in isolation –eNodeBs should consider interference generated by co-channel
cells.
1.6.2. Scheduling Algorithms
Multi-user scheduling performs capacity-maximizing resource allocation. A capacity metric is first
formulated and then optimized across all possible resource allocation solutions with predetermined
constraints, may be based on bandwidth and total power or QoS.
Ergodic Capacity
The ergodic capacity (Shannon capacity) is defined as the maximum data rate possible over the
channel with asymptotically small error probability, averaged over fading process. When CSI is
available, Tx power and CSI can be varied depending on the fading state to maximize the average
rates. Ergodic capacity metric considers average data rate which can be delivered to a user when the
user does not have any latency constraints.
Maximum Rate Scheduling
The maximum sum rate is achieved by orthogonal multiplexing where in each subchannel (each RB in
LTE) user with the best channel gain is scheduled. Water-filling formulae in both frequency and time
is used where we allocate more power to a scheduled user when his channel gain is high and less
power when it is low.
A variant of this resource allocation strategy with no power control is called „maximum-rate constantpower‟ scheduling, where only the user with the best channel gain is scheduled in each RB, but with
no adaptation of the transmit power. Most of the performance gains offered by the maximum rate
allocation are due to multi-user diversity and not to power control, so an on–off power allocation can
achieve comparable performance to maximum-rate scheduling.
Proportional Fair Scheduling
The ergodic sum rate gives optimal rate for traffic without delay constraint. This is unfair sharing and
when the QoS required by the application includes latency, this strategy is not suitable.
A fair approach is Proportional Fair Scheduling (PFS) algorithm. PFS schedules a user when its
instantaneous channel quality is high relative to its own average channel condition over time. PFS
takes into account link adaptation dynamic range, power and code resources, convergence settings,
signalling overhead and code multiplexing.
A large time window tends to maximize total average throughput; PFS and maximum-rate constantpower scheduling result in the same allocation of resources. For small window, the PFS tends
towards a round-robin scheduling.
Delay-Limited Capacity
The fairness of PFS may not be sufficient for very tight latency constraint. A different capacity metric
is needed like „delay-limited capacity‟ (zero-outage capacity), where transmission rate is guaranteed
in all fading states under finite long-term power constraints. It is relevant to traffic classes of
guaranteed throughout the connection time, regardless of the fading dips. Guaranteeing a delaylimited rate incurs only a small throughput loss in high SINR conditions when number of users is
large, but requires non-orthogonal scheduling of the users in each RB, which is unsuitable for LTE.
Orthogonal Delay-limited Rate Allocation: It is possible to combine orthogonal multiple access with
hard QoS requirements. It finds the allocation of users to RBs which maximizes the number of served
users for a given total transmit power while achieving a target rate-tuple R = (R1,R2,.. .. ..,Rk) through
an orthogonal multiplexing of the users. Solution uses power adaptation across the RBs.
Max-Min allocation: At any instant, minimum channel gain of any of the allocated users is the highest
possible among all possible allocations and thus maximizes the minimum allocated rate when an
equal and fixed power is used for all users. It is useful where no power control (like in DL) can be
used.
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1.6.3. Performance of Scheduling Strategies
The per-user average throughput increases with the number of users, hence even under delay-limited
requirements, high multi-user diversity gains can be achieved. Even under hard fairness constraints it
can achieve performance very close to the optimal unfair policy; thus hard fairness constraints do not
necessarily introduce a significant throughput degradation, even with orthogonal resource allocation,
provided that the number of users and BW are large like VoIP users, with a latency constraint typically
requiring each packet to be successfully delivered within 50 ms. For a high SINR scenario, PFS does
not provide any significant gain and may even perform worse than the optimal non-orthogonal delaylimited scheduling; even if imposed fairness constraint is less stringent. For low to moderate SINR,
the stricter hard-fairness constraint incurs a large throughput penalty for delay-limited scheduling with
respect to PFS.
Summarily, strategies assumes that all users have an equal and infinite queue length (full-buffer traffic
model). For real-time services, users‟ queue lengths is necessary to guarantee system stability. If a
scheduling algorithm keeps the average queue length bounded, the system is said to be stabilized. To
achieve this, use the queue length to set the priority order in the allocation of RBs. This generally
works for lightly-loaded systems. In wideband frequency-selective channels, low average packet
delay can be achieved even if the fading is very slow.
1.6.4. Considerations for Resource Scheduling in LTE
Each logical channel has a QoS description influencing eNodeB resource scheduling algorithm. QoS
could potentially be updated for each service in a long-term fashion. Mapping between QoS
descriptions of different services and resource scheduling algorithm in the eNodeB makes a
difference.
Availability and accuracy of CSI for the active UEs to the scheduler in cell is a limitation. CSI reporting
differs for UL and DL. For DL, CSI is provided through CQIs by UEs, while for UL, eNodeB may use
SRSs or other signals by UEs to estimate CQI. The CQI reports and SRS frequency is configurable
by eNodeB, and signalling overhead is controlled to get up-to-date CSI, which if received too long
ago, may degrade the decision.
To perform frequency-domain scheduling, CSI needs to be frequency-specific. eNodeB may configure
CQI reports to relate to specific subbands to assist DL scheduling. UL frequency-domain scheduling
can be facilitated by configuring SRS to be transmitted over a large bandwidth. For cell-edge UEs,
wider the transmitted BW, lower the available power per RB; this means, accurate frequency-domain
scheduling is difficult for UEs near cell edge. Limiting SRS to a subset of system BW will improve CQI
estimation on these RBs but restrict the ability of the scheduler to find an optimal scheduling solution
for all users. In general, if CQI estimated for scheduling is greater than the intended scheduling
bandwidth, a useful element of multi-user diversity gain may still be achievable. In order to support
QoS and queue-aware scheduling, scheduler must have CQI and queue status, both. In DL, eNB
MAC BSR to each UE is available;
1.6.5. Interference Coordination and Frequency Reuse
LTE is designed to operate with a frequency reuse factor of one, which requires inter-cell interference
coordination among adjacent cells to increase the data rates for users at the cell edge. This implies
imposing restrictions on specific RBs available to the scheduler, or what transmit power may be used
in certain RBs.
If a user k is experiencing no interference, then its achievable rate in a RB m of subframe f =
2
R(k).no-Int(m,f) = B/M log [1 + (Ps(m,f) (Hs(m, f )) )/ N0]
where Hs(m, f ) = channel gain from serving cell s,
Ps(m, f ) = transmit power from cell s and
N0 is the noise power.
Neighbouring cells are transmitting in the same time-frequency resources, then the achievable rate
reduces because of these interfering cells to:
2
i
i
2
R(k).Int(m,f) = B/M log [1 + (Ps(m,f) (Hs(m, f )) )/ (N0+ Σ(i!=s)p (m,f)(H k(m,f)) ]
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Where „i‟ is the interfering cell.
The loss of rate = R(k).no-Int(m,f) - R(k).Int(m,f).
For a level of interference = desired signal level, use k experiences a rate loss of approximately 40%.
To demonstrate further the significance of interference and power allocation depending on the system
configuration we consider two examples of a cellular system with two cells (s1 and s2) and one active
user per cell (k1 and k2 respectively). Each user receives the wanted signal from its serving cell, while
the inter-cell interference comes from the other cell.
In the first example, each user is located near its respective eNodeB. The channel gain from the
interfering cell is small compared to the channel gain from the serving cell. Maximum throughput is
achieved when both eNodeBs transmit at maximum power.
In the second example, we consider the same scenario but with the users now located close to the
edge of their respective cells. Channel gain from the serving cell and the interfering cell are
comparable. Maximum capacity is reached by allowing only one eNodeB to transmit.
Optimal power allocation for maximum capacity with two base stations is binary this means, either
both base stations should be operating at maximum power in a given RB, or one of them should be
turned off completely in that RB. This result is exploited in the eNodeB scheduler by treating users in
different ways depending on whether they are cell-centre or cell-edge users.
Each cell can be divided into two parts – inner and outer. In the inner part (low interference) require
less power to communicate with S-cell, frequency reuse factor of 1 can be adopted. For outer part,
scheduling restrictions are applied: when cell schedules a user in a RB, system capacity is optimized
if the neighbouring cells do not transmit at all; alternatively, they may transmit only at low power (to
users in the inner parts of neighbour cells) to avoid creating strong interference to the scheduled user
in the first cell. This effectively results in a higher frequency reuse factor at cell-edge; it is often known
as „partial frequency reuse‟.
To coordinate scheduling in different cells, communication between neighbours is required. If
neighbours are managed by same eNodeB, a coordinated scheduling strategy can be followed
without standardized signalling.
When neighbouring cells are controlled by different eNodeBs, Inter-Cell Interference Coordination
(ICIC) is managed in frequency domain (Not time domain, as it will affect HARQ processes).
For DL transmissions, a bitmap Relative Narrowband Transmit Power (RNTP) indicator is exchanged
between eNodeBs over X2. Each RNTP bit indicator corresponds to one RB in frequency domain and
is used to inform neighbouring eNodeBs if a cell is planning to keep the transmit power for the RB
below a certain upper limit or not. The upper limit, and validity period, are configurable and this helps
the neighbouring cells to minimize interference in each RB when scheduling UEs in their own cells.
The reaction is implementation dependent, but avoid scheduling cell-edge UEs in such RBs. In RNTP
indicator, transmit power per antenna port is normalized by maximum output power of a base station
or cell, because a cell with a smaller maximum output power, corresponding to smaller cell size, can
create as much interference as a cell with a larger maximum output power corresponding to a larger
cell size.
For UL, two messages may be exchanged between eNodeBs (X2) for transmit powers and
scheduling of users:
1. A reactive indicator, „Overload Indicator’ (OI), to indicate physical layer measurements of
average uplink interference plus thermal noise for each RB. The OI may be = low, medium, or
high levels of interference + noise. To avoid excessive signalling load, update frequency is
not more than every 20 ms.
2. A proactive indicator, „High Interference Indicator’ (HII), to inform neighbour that it will, in
near future, schedule UL by one or more cell-edge UEs in certain RB, and high interference
might occur in those RBs. Neighbouring cells will then avoid scheduling their own users to
limit the interference impact. The HII is a bitmap with one bit per RB, and, is not sent more
often than every 20 ms. The HII bitmap is addressed to specific neighbour eNodeBs.
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In addition to RB scheduling in UL, eNodeB also controls UE compensation for the path-loss when
setting its UL power. This enables the eNodeB to trade off fairness for cell-edge UEs against inter-cell
interference generated towards other cells, and maximize system capacity.
1. Static interference coordination: Coordination is done with cell planning and
reconfigurations are rare. This avoids signalling on X2, but has performance limitation since it
cannot adaptively use cell loading and user distribution informations.
2. Semi-static interference coordination: Reconfigurations are carried out in the order of
seconds or longer. X2 interface is used. Traffic load is shared.
A scheduling algorithms will depend on the optimization criteria, such as traffic classes, throughput
maximization for delay-tolerant apps, QoS for delay limited apps etc. Multi-user diversity is important
when user density is high. System optimization requires coordination between cells and eNodeBs, to
avoid inter-cell interference. Best results are realized by simple „on–off‟ allocation of RBs, where some
eNodeBs avoid scheduling in certain RBs used by neighbouring eNodeBs for cell-edge users.
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1.7. Sample Call Flows
In this section we will list some sample representative call flows. This will make the understanding
easier:
1.7.1. Basic Call Flow – Attach Procedure
Fig 5.5.1 – Attach Procedure including L1,L2,L3 Access Messages
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1.7.2. Basic Call Flow – Incoming Call with Handover
Fig 5.5.2 – Call flow for incoming call and Handover
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1.7.3. Call Flow example from tool
Fig 5.5.3 – A TEMS tools output showing the message sequence with Protocol and Latency
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1.7.4. Sequence of inter-cell handover
In general, the Inter-Cell handover is done without activation time, i.e. the timing information for
configuration of the SS and sending of the RRCConnectionReconfiguration is „Now'.
1. Transfer of the PDCP Count for AM DRBs from source to target cell:
a) Source Cell: Get PDCP COUNT.
b) Target Cell: Set PDCP COUNT.
There shall be no further sending/receiving of AM DRB data before the HO has been done.
2. Target Cell:
Inform the SS about the HO and about the source cell id.
3. Target Cell:
Configure RACH procedure either dedicated or C-RNTI based.
4. Target Cell:
Activate security.
For AM DRBs the PDCP count is maintained (for SRBs and UM DRBs the PDCP count is
reset).
5. Target Cell:
configure DRX and measurement gap configuration (if necessary).
As long as the DRX configuration is not modified by the RRCConnectionReconfiguration the
target cell gets the same DRX configuration as the source cell.
Measurement gap configuration is released at the UE due to the handover, therefore nothing
needs to be configured at the target cell regarding measurement gaps unless a new
measurement gap configuration is explicitly given in the
RRCConnnectionReconfiguration.
6. Source Cell:
Stop periodic TA.
Unless explicitly specified UL grant configuration keeps configured as per default at the
source cell.
7. Target Cell:
Configure UL grant configuration ("OnSR", periodic TA is not started).
8. Source Cell:
Send RRCConnectionReconfiguration.
9. Target Cell:
Receive RRCConnectionReconfigurationComplete.
10. Target Cell:
Start periodic TA.
11. Target Cell:
Inform the SS about completion of the HO (e.g. to trigger PDCP STATUS
PDU).
12. Target Cell:
Re-configure RACH procedure as for initial access.
13. Source Cell:
Reset SRBs and DRBs.
14. Source Cell:
Release DRX and MeasGapConfig configuration.
1.7.5. Sequence of intra-cell handover
For Intra-Cell handover dedicated timing information is used: the sequence starts at time T with
sending of the RRCConnectionReconfiguration. T is set to 300 ms in advance of the handover.
0. Before T:
Get PDCP count for AM DRBs.
1. At T:
Send RRCConnectionReconfiguration.
2. At T + 5ms:
Release SRBs and DRBs.
3. At T + 5ms:
Configure RACH procedure either dedicated or C-RNTI based.
NOTE 1: Since the RACH procedure may require a new C-RNTI to be used it cannot be
configured before sending out the RRCConnectionReconfiguration.
3A At T + 5ms:
Release MeasGapConfig configuration.
NOTE 2: According to TS 36.331, clause 5.5.6.1 the measurement gap configuration is released
at the UE due to the handover, therefore MeasGapConfig is released unless a new
measurement gap configuration is explicitly given in the RRCConnectionReconfiguration.
4. At T + 10ms: (Re-) configure SRBs and DRBs.
5. At T + 10ms: Reestablish security, disable TA transmission.
NOTE 3: For AM DRBs the PDCP count is maintained while for SRBs and UM DRBs the PDCP
count is reset.
6. (after step 5) Receive RRCConnectionReconfigurationComplete.
7. (after step 6) Re-configure RACH procedure as for initial access, enable TA transmissions.
8. (after step 7) Restore the PDCP count for AM DRBs.
1.2.14. UL Grants used in RA procedure during handover
In the Random Access Procedure a grant is assigned to the UE by the Random Access Response
and another grant, as initial grant, is assigned for contention resolution.
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When UL data is pending, the UE will try to put as much data into given grants as possible, i.e. it will
segment the user data and send it e.g. with the initial grant if possible. To avoid this segmentation of
user data, the grants assigned during handover will be set in TTCN to:
Grant assigned by Random Access Response: 56 bits.
Initial grant:
104 bits.
56 bits are the minimum grant which can be assigned by the Random Access Response. That is
sufficient to convey C-RNTI (3 bytes) and short BSR (2 bytes) or long BSR (4 bytes) but
even with short BSR the remaining 2 bytes are not sufficient to convey any segment of
the RRCConnectionReconfigurationComplete (at least 4 bytes).
The RRCConnectionReconfigurationComplete (9 bits) shall completely be conveyed in the initial
grant of RA procedure. This requires a minimum of 10 bytes (1 byte MAC header + 2
bytes RLC header + 5 bytes PDCP header + 2 bytes payload). Additionally an optional
PHR MAC element (2 bytes) needs to be considered since the PHR has higher priority
than the MAC SDU. Any further user data would require a minimum of 5 additional bytes
(2 bytes MAC header + 2 bytes RLC header + 1 byte payload).
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