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Lte Module-5 Notes - Radio Resource Management And
Mobility Management
Wireless Communication and 4G LTE Networks (Visvesvaraya Technological University)
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Radio Resource Management and Mobility Management
Module-5
Radio Resource Management and Mobility Management
Packet Data Convergence Protocol (PDCP)
•
A PDCP entity is associated with the control plane or with the user plane depending
on which radio bearer it is carrying data for.
•
Each radio bearer is associated with one PDCP entity and each PDCP entity is
associated with one or two Radio Link Control (RLC) entities depending on the radio
bearer characteristic and the RLC mode.
•
PDCP is used only for radio bearers mapped on Dedicated Control Channel (DCCH)
and Dedicated Traffic Channel (DTCH) types of logical channels.
User Plane
•
Header Compression and decompression of IP data flows with the RObust Header
Compression (ROHC) Protocol.
•
Ciphering and deciphering of user plane data.
•
In-sequence delivery and reordering of upper layer PDUs at handover.
•
Buffering and forwarding of upper layer Protocol Data Units (PDUs) from the serving
eNode-B to the target eNode-B during handover.
•
Timer based discarding of Service Data Units (SDUs) in the uplink.
Tilak, Dept. of ECE, GAT
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Control Plane
•
Ciphering and Deciphering of control plane data.
•
Integrity protection and integrity verification of control plane data.
•
Transfer of control plane data.
PDCP PDUs are divided into 2 categories.
 PDCP data PDU
 PDCP control PDU
PDCP data PDU
•
It is used in both control and user plane to transport higher layer packets.
•
It is used to convey either user plane data containing a compressed/uncompressed IP
packet or control plane data containing one Radio Resource Control (RRC) message
and a Message Authentication Code for Integrity (MAC-I) field for integrity
protection.
PDCP control PDU
•
It is used only within the user plane to convey a PDCP status report during handover
and feedback information for header compression.
•
It does not carry any higher layer SDU but rather is used for peer to peer signaling
between PDCP entities at two ends.
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There are 3 different types of PDCP data PDUs distinguished by the length of
Sequence Number (SN).
•
The PDCP SN is used to provide robustness against packet loss and to guarantee
sequential delivery at the receiver.
•
The PDCP data PDU with the long SN is used for the Unacknowledge Mode (UM)
and Acknowledge Mode (AM) and the PDCP data PDU with the short SN is used for
the Transparent Mode (TM).
•
The PDCP data PDU for the user plane contains a „D/C‟ field which distinguishes
data and control PDUs.
Header Compression
•
The header compression protocol in LTE is based on the Robust Header
Compression (ROHC) framework defined by Internet Engineering Task Force
(IETF).
•
PDCP entities are configured by upper layers to use header compression which is only
performed on user plane data.
•
These protocols bring a significant amount of header overhead at the network layer
(Internet Protocol), transport layer (Transmission Control Protocol, User Datagram
Protocol) and application layer (Real Time Transport Protocol) which contains
redundant and repetitive information and consumes precious radio resources.
•
Multiple header compression algorithms called profiles are defined for the ROHC
framework and each profile is specific to the particular network layer, transport layer
or upper layer protocol combination.
Tilak, Dept. of ECE, GAT
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Integrity and Ciphering
•
The PDCP PDU counter denoted by the parameter COUNT is maintained and used as
an input to the security algorithm.
•
The format of COUNT has a length of 32 bits and consists of 2 parts: Hyper Frame
Number (HFN) and PDCP SN.
•
The SN is used for recording and duplicate detection of RLC packets at the receive
end.
•
The data unit that is integrity protected is the PDU header and data part of the PDU
before ciphering.
•
The integrity protection is provided by the field MAC-I of 32 bit length.
•
At transmission, the value of MAC-I field is calculated based on the key provided by
the RRC, the radio bearer identifier, the COUNT value and the direction of
transmission.
•
The receiver computes the expected message authentication code on the received
message using the same parameters and algorithms used by the sender.
•
If it does not match the MAC-I field, then the PDCP PDU does not pass the integrity
check and the PDCP PDU will be discarded.
•
The ciphering function includes both ciphering and deciphering and is performed on
both control plane data and user plane data.
•
For the control plane, the data unit that is ciphered is the data part of the PDCPPDU
and MAC-I.
•
For the user plane, the data unit that is ciphered is the data part of the PDCP PDU.
•
Neither integrity nor ciphering is applicable to PDCP control PDUs.
•
The ciphering function is activated by the upper layer which also configures the
ciphering algorithm and the ciphering key to be used.
•
The ciphering is done by an XOR operation of the data unit with the ciphering stream.
•
The ciphering stream is generated by the ciphering algorithm based on ciphering keys,
the radio bearer identity, the value of COUNT, the direction of transmission and the
length of key stream.
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Medium Access Control (MAC) / Radio Link Control (RLC)
•
The RLC layer performs segmentation and/or concatenation on PDCP PDUs based
on the size indicated by the MAC.
•
RLC also records the RLC PDUs once they are received out of order possibly due to
H-ARQ processes in MAC layer.
•
The RLC layer also supports an ARQ mechanism which resides on top of the MAC
layer H-ARQ and is used only when all the H-ARQ transmissions are exhausted and
the RLC PDU has not yet been received without errors.
•
At the transmitter and the receiver there is one RLC entity per radio bearer.
•
The MAC layer performs multiplexing and demultiplexing of the various logical
channels on to the transport channels.
•
At User Equipment (UE), the MAC layers only performs the task of multiplexing and
prioritizing the various radio bearers associated with UE.
•
The MAC layer provides services to RLC layer through logical channels while it
accesses the data transfer services provided by PHY layer through transport channels.
Data Transfer Modes
•
RLC layer functions are performed by RLC entities.
•
Each RLC entity is operated in either Transparent Mode (TM), Unacknowledged
Mode (UM) or Acknowledged Mode (AM).
Transparent Mode (TM)
•
The RLC entity does not add any RLC header to the PDU and no data segmentation
or concatenation is performed.
•
It is suitable for services that do not need retransmission or are not sensitive to
delivery order.
•
Only RRC messages such as broadcast system information messages and paging
messages use TM.
•
TM is not used for user plane data transmission.
•
The RLC data PDU delivered by a TM RLC entity is called TM Data (TMD) PDU.
Unacknowledged Mode (UM)
•
The UM provides in-sequence delivery of data that may be received out of sequence
due to H-ARQ process in MAC but no retransmission of lost PDU is required.
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This mode is used in delay sensitive and error tolerant real time applications such as
VoIP.
•
The DTCH logical channel can be operated in UM and the RLC data PDU delivered
by an UM RLC entity is called UM Data (UMD) PDU.
•
UMD PDU includes RLC headers.
•
At the transmit end, the UM RLC entity segments and/or concatenates the RLC SDUs
according to the total size of RLC PDUs indicated by MAC layer.
•
The receiving UM RLC entity performs duplicate, detection, recording and
reassembly of UMD PDUs.
Acknowledged Mode
•
The most complex AM requests retransmission of missing PDUs in addition to UM
functionalities.
•
It is mainly used by error sensitive and delay tolerant applications.
•
An AM RLC entity can be configured to deliver/receive RLC PDUs through DCCH
and DTCH.
•
An AM RLC entity delivers/receives the AM Data (AMD) PDU and the STATUS
PDU indicating the ACK/NAK information of the RLC PDUs.
•
When an AM RLC entity needs to retransmit a portion of an AMD PDU, which
results from ARQ process and segmentation, the transmitted PDU is called AMD
PDU segment.
•
The receiving AM RLC entity can send a STATUS PDU to inform the transmitting
RLC entity about the AMD PDUs that are received successfully and that are detected
to be lost.
Purpose of MAC and RLC Layers
The main services and functions of RLC sublayer include
•
Transferring/receiving PDUs from upper layers i.e. from RRC for the Common
Control Channel (CCCH) or from PDCP for other cases.
•
Error correction through ARQ only when RLC is operated in AM.
•
Concatenation, Segmentation and Reassembly of RLC SDUs only for UM and AM
data transfer.
•
Re-segmentation of RLC data PDUs only for AM data transfer.
•
In sequence delivery of upper layer PDUs only for UM and AM data transfer.
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Duplicate detection only for UM and AM data transfer.
•
Protocol error detection and recovery.
•
RLC SDU discards only for UM and AM data transfer.
•
RLC re-establishment.
The main services and functions of MAC sublayer include
•
Mapping between logical channels and transport channels.
•
Multiplexing/demultiplexing of MAC SDUs belonging to one or different logical
channels into/from the same transport block.
•
Scheduling for both downlink and uplink transmission.
•
Error correction through H-ARQ which has tight interaction with the ARQ in the
RLC layer.
Additional fields are available for AMD PDU and AMD PDU segments:
•
Data/Control (D/C) field which indicates whether the RLC PDU is an RLC Data
PDU or an RLC Control PDU.
•
Re-segmentation Flag (RF) field which indicates whether the RLC PDU is an AMD
PDU or an AMD PDU segment.
•
Polling bit (P) field which indicates whether the transmitting side of an AM RLC
entity requests a STATUS report from its peer AM RLC entity.
The RLC header of an AMD PDU segment contains special fields
•
Segment Offset (SO) field which indicates the position of the AMD PDU segment in
bytes within the original AMD PDU.
•
Last Segment Flag (LSF) field which indicates whether the last byte of the AMD
PDU segment corresponds to the last byte of an AMD PDU.
Status PDU
•
The Status PDU is used by the receiving AM RLC entity to indicate the missing
portions of AMD PDUs.
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It consists of the following fields:
•
Control PDU Type (CPT) field which indicates the type of the RLC control PDU.
•
Acknowledgement SN (ACK_SN) field which indicates the SN of the next not
received RLC Data PDU which is not reported as missing in the STATUS PDU.
•
Extension bit 1 (E1) field which indicates whether a set of NACK_SN, E1 and E2
follows.
•
Extension bit 2 (E2) field which indicates whether a set of Sostart and Soend
follows.
•
Negative Acknowledgement SN (NACK_SN) field which indicates the SN of the
AMD PDU (or portions of it) that has been detected as lost at the receiving side of
AM RLC entity.
•
SO start (Sostart) field and SO end (Soend) field which together indicates the
portion of the AMD PDU with SN=NACK_SN that has been detected as lost at the
receiving side of the AM RLC entity.
MAC PDU Formats
•
The MAC layer receives data from RLC as MAC SDUs and passes the MAC PDUs to
PHY.
•
A MAC PDU consists of a MAC header and a MAC payload.
•
The MAC payload consists of zero or more MAC SDUs, zero or more MAC control
elements and optional padding.
•
The MAC PDU header consists of one or more MAC PDU sub headers while each
sub header corresponds to either a MAC SDU, a MAC control element or padding.
•
Both MAC SDU and MAC header are of variable sizes.
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The format of a typical MAC sub header contains 5 different fields:
•
“R” field or Reserved field which is always set to “0”.
•
“E” field or Extension field which indicates if more fields are present in the MAC
header.
•
If it is set to “1”, another set of at least R/R/E/LCID fields follows otherwise either a
MAC SDU, a MAC control element or padding follows.
•
“LCID” field or Logical Channel ID field which identifies the logical channel
instance of the corresponding MAC SDU or the type of the corresponding MAC
control element or padding.
•
One “LCID” field is included in the MAC PDU for each MAC SDU, MAC control
element or padding.
•
“F” field indicates the size of the Length field.
•
It is set to “0” if the size of the MAC SDU or MAC control element is less than 128
bytes otherwise it is “1”.
•
There is one “F” field per MAC PDU sub header except for the last sub header and
the sub header corresponding to fixed sized MAC control elements.
•
“L” field or Length field indicates the length of the corresponding MAC PDU or
MAC control element in bytes.
•
For the last sub header in the MAC PDU, the MAC PDU sub header corresponding to
padding and sub headers for fixed size MAC control elements, there are only 4 header
fields: R/R/E/LCID.
•
The MAC control element can be used for signaling for buffer status reporting,
Discontinuous Reception (DRX) command, timing advance command, UEs power
headroom and UE contention resolution.
•
In MAC PDU for random access response, there is a MAC header that consists of one
or more MAC PDU sub headers where each sub header contains information about
the payload.
•
The MAC payload consists of one or more MAC Random Access Responses (MAC
RAR) and optional padding.
•
Each MAC RAR is of fixed size and consists of 4 fields:
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Reserved Bit which is set to “0”.
•
Timing Advance Command which indicates the index value used to control the
amount of timing adjustment that UE has to apply and it is of 11 bits.
•
UL Grant which indicates the resources to be used on uplink and it is of 20 bits.
•
Temporary C-RNTI which indicates the temporary identity that is used by the UE
during random access.
ARQ Procedures
•
A highly reliable selective repeat ARQ protocol is used in RLC layer and a fast HARQ protocol with low latency and low overhead feedback is used in the MAC layer.
•
The H-ARQ protocol is responsible for handling transmission errors by performing
retransmissions based on H-ARQ processes with incremental redundancy or chase
combining which is handled by PHY layer.
•
The ARQ protocol in the RLC layer is to correct residual H-ARQ errors mainly due to
the error in H-ARQ ACK feedback.
•
ARQ procedures are only performed in the AM by an AM RLC entity.
•
The latency associated with the RLC layer ARQ is much larger.
•
A second layer ARQ protocol is used to correct the error event due to H-ARQ
feedback errors and this additional ARQ protocol provides a much more reliable
feedback protected by a CRC.
•
ARQ retransmission is triggered by an ARQ NAK received at the transmit side of an
AM RLC entity.
•
The ARQ NAK is received either by the STATUS PDU from its peer AM RLC entity
or by H-ARQ delivery failure notification from the transmit MAC entity which can
happen when the maximum number of H-ARQ transmissions is exhausted without a
successful transmission of the transport block.
•
The STATUS reporting is triggered by setting the Polling field of the RLC data PDU
to “1” and NAK reporting is contained in the STATUS PDU.
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Upon receiving an ARQ NAK, the transmit side of an AM RLC entity will deliver the
AMD PDU if it fits within the total size of RLC PDU indicated by the lower layer
otherwise it segments the AMD PDU and forms a new PDU segment that fits the
PDU size.
•
Both H-ARQ and ARQ are terminated in the eNode-B which enables a tighter
interconnection between H-ARQ and ARQ protocols.
Radio Resource Control (RRC)
•
RRC layer takes care of RRC connection management, radio bearer control, mobility
functions and UE reporting and control.
•
It is also responsible for broadcasting system information and paging.
RRC States
•
LTE has 2 RRC states namely RRC_IDLE and RRC_CONNECTED.
•
RRC state machine handling and radio resource management controls RRC state.
•
A UE is in RRC_CONNECTED state when an RRC connection has been established
otherwise the UE is in RRC_IDLE state.
•
In the RRC_IDLE state, the UE can receive broadcasts of system information and
paging information and there is no signaling radio bearer established.
•
The mobility control is handled by the UE which performs neighboring cell
measurements and cell selection/reselection.
•
The UE also monitors a paging channel to detect incoming calls.
•
In the RRC_CONNECTED state, the UE is able to transmit and/or receive data
to/from the eNode-B network.
•
The
UE
monitors
control
channels
-
Physical
Downlink
Control
Channel(PDCCH)associated with the shared data channel to determine if data is
scheduled for it.
•
The UE also report Channel Quality Information and feedback information to the
eNode-B to assist the data transmission.
•
The network controls mobility/handover of UE while UE provide neighboring cell
measurement information.
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Signaling Radio Bearers (SRBs)
•
Radio bearers that are used only for the transmission of RRC and Non Access Stratum
(NAS) messages are called Signaling Radio Bearers.
•
Three different SRBs defined in LTE are
•
SRB0 is for RRC messages using the Common Control Channel (CCCH) logical
channel.
•
SRB1 is for RRC messages and NAS messages prior to the establishment of SRB2 all
using the Dedicated Control Channel (DCCH) logical channel.
•
SRB2 is for NAS messages using DCCH logical channel and has a lower priority than
SRB1 and is always configured by the E-UTRAN after security activation.
RRC Functions
The main functions of RRC protocol are
•
Broadcast of system information which is divided into Master Information Block
(MIB) and a number of System Information Blocks (SIBs).
•
The MIB includes a limited number of the most essential and most frequently
transmitted parameters that are needed to acquire other information from the cell and
is transmitted on the Broadcast Channel (BCH) logical channel.
•
SIBs other than SIB Type1 are carried in System Information (SI) messages.
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SIB Type1 contains parameters needed to determine if a cell is suitable for cell
selection as well as information about the time domain scheduling of other SIBs.
•
SIB Type1 and all SI messages are transmitted on Downlink Shared Channels (DLSCH).
•
RRC connection control which includes procedures related to the establishment,
modification and release of an RRC connection including paging, initial security
activation, establishment of SRBs & radio bearers carrying user data, radio
configuration control &QoS control and recovery from the radio link failure.
•
Measurement configuration and reporting which includes establishment,
modification, release of measurements, configuration, activation/deactivation of
measurement gaps and measurement reporting for intra frequency, inter frequency &
inter RAT (Radio Access Technology) mobility.
•
Other functions include transfer of dedicated NAS information and non-3GPP
dedicated information, transfer of UE radio access capability information and support
of self configuration&self optimization.
Mobility Management
Mobility Management functions are categorized into 2 groups:
•
Mobility within the LTE system (intra-LTE mobility)
•
Mobility to other systems (inter RAT mobility)
•
Intra LTE mobility can happen over either S1 interface or over X2 interface.
•
When the UE moves from one eNode-B to another anothereNode-B within the same
Radio Access Network (RAN) attached to the same MME, the mobility takes place
over X2 interface.
•
When the UE moves from one eNode-B to another that belongs to different RAN
attached to different RAN attached to different MME, then the mobility takes place
over S1 interface.
•
The inter RAT mobility essentially uses the S1 Mobility.
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S1 Mobility
1. Preparation Phase
•
Once a decision has been made for a handover and a target MME and eNode-B have
been identified, the MME sends a handover request to the target eNode-B requesting
it to set up the appropriate resources for UE.
•
Once the resources have been allocated at the target eNode-B, it sends a handover
request ACK to the MME
•
Once this message is received by MME, it sends a handover command to UE via the
source eNode-B.
2. Execution Phase
•
Once the UE receives the handover command, it responds by performing the various
RAN related procedures needed for the handover including accessing the target
eNode-B using Random Access Channel (RACH).
•
While the UE performs the handover, the source eNode-B initiates the status transfer
where the PDCP context of the UE is transferred to the target eNode-B.
•
The source eNode-B also forwards the data stored in the PDCP buffer to target
eNode-B.
•
Once the status and data have been transferred to the target eNode-B and the UE is
able to establish a Radio Access Bearer (RAB) on the target eNode-B, it sends the
handover confirm message to the target eNode-B.
3. Completion Phase
•
When the target eNode-B receives the handover confirm message, it sends a handover
notify message to the MME.
•
The MME then informs the source eNode-B to release the resources originally used
by UE.
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X2 Mobility
1. Preparation Phase
•
Once the handover decision has been made by the source eNode-B, it sends a
handover request message to the target eNode-B.
•
The target eNode-B upon receipt of this message works with the MME and S-GW to
set up the resources for the UE.
•
Resources are set up on a per RAB basis which implies that upon the completion of
the handover, the UTE will have same RABs at the target eNode-B with the same set
of QoS as it had on the source e Node-B.
•
This process makes the handover quick and seamless and the UE is not required to set
up the RAB with the target eNode-B once the handover is completed.
•
The target eNode-B responds to source eNode-B with a handover request ACK once
it is ready.
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2. Execution Phase
•
Upon receiving the handover request ACK, the source eNode-B sends a handover
command to the UE.
•
While the UE completes the various RAN related handover procedures, the source
eNode-B starts the status and data transfer to the target eNode-B.
3. Completion Phase
•
Once the UE completes the handover procedure, it sends a handoff complete message
to the target eNode-B.
•
Then the target eNode-B sends a path switch request to the MME/S-GW and S-GW
switches the GTP tunnel from the source eNode-B to the target eNode-B.
•
When the data plane in the user plane is switched, the target eNode-B sends a
message to the source eNode-B to release the resources originally used by UE.
•
In the case of X2 mobility, both the PDCP processed and PDCP unprocessed packets
are sent to the target eNode-B during the status transfer.
•
The PDCP processed packets are the data packets that have been transmitted by the
source eNode-B to the UE but UE has not yet acknowledged the receipt of such
packets.
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The PDCP unprocessed packets are the packets buffered by the PDCP layer that are
yet to be transmitted by the source eNode-B.
•
An additional feature called Selective Retransmission is enabled in LTE where the
target eNode-B may not retransmit the PDCP processed packets that were forwarded
but were acknowledged by the UE after the status transfer was initiated.
RAN Procedures for Mobility
•
RAN related mobility management procedures happen between the UE and the
eNode-B or between the UE and the MME in order to enable the UE to handover
from one eNode-B to another.
•
T prevent the ping pong effect between two eNode-Bs when the UE undergoes RRC
state transitions, the mobility management in 2 states (RRC_IDLE and
RRC_CONNECTED) is designed to be consistent.
•
UE selects the eNode-B with the best radio link quality indicated by the Radio Signal
Received Power (RSRP) for an LTE cell and by Reference Signal Code Power
(RSCP) for an UMTS cell.
•
Selecting the eNode-B/Node-B with the best quality is optimum from both an
interference management and a battery life point of view.
•
Factors such as UE capability, call type, QoS requirements are also included in
handover decision process.
•
The measurement report from the UE which contains the radio link measurement for
the neighboring eNode-B is the primary mechanism used by the network to trigger
and control a handover procedure.
•
For intra LTE handover 5 events trigger measurement reporting:
•
Event A1: The serving cell radio link quality becomes better than an absolute
threshold.
•
Event A2: The serving cell radio link quality becomes worse than an absolute
threshold.
•
Event A3: The neighbor cell radio link quality becomes better than an offset relative
to the serving cell.
•
Event A4: The neighbor cell radio link quality becomes better than an absolute
threshold.
•
Event A5: Serving cell radio link quality becomes less than an absolute threshold and
the neighbor cell radio link quality becomes better than another absolute threshold.
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For inter RAT handover 2 events trigger measurement reporting:
•
Event B1: Neighbor cell radio link quality on a different RAT becomes better than an
absolute threshold.
•
Event B2: Serving cell radio link quality becomes worse than an absolute threshold
and the neighbor link quality on different RAT becomes better than another threshold.
•
The time to Trigger parameter is chosen to prevent the UE from ping ponging
between eNode-Bs.
•
In the RRC_IDLE state, the UE decides when a handover is required and which
cell/frequency the UE should target.
•
The E-UTRAN allocates absolute priorities to the different frequencies and these
priorities are conveyed by the system information message carried over BCH.
•
The priority of each cell is determined by the UE based on the priority of the
frequency and the radio link quality of the cell.
•
In the RRC_CONNECTED state, the E-UTRAN determines the optimum cell and
frequency for the target eNode-B in order to maintain the best radio link quality.
•
The handover is initiated by the E-UTRAN based on one or more of the events that
trigger a measurement that trigger a measurement report (A1-A5) and (B1-B2).
•
The E-UTRAN may initiate a handover without any of these trigger events called a
blind handover.
•
All handovers in LTE are hard handovers i.e. UE can be connected to only one
eNode-B at a time and is usually a backward handover where the target eNode-B
controls the handover and requests the target e-NodeB to prepare for the handover and
allocate resources for the UE.
•
When the resources are allocated and the target eNode-B is ready to send an RRC
message requesting the UE to perform the handover, the UE uses the random access
procedure in the target eNode-B to establish a connection and execute the handover.
•
To obtain physical layer synchronization such as time and frequency synchronization
and initial open loop power control estimates, UE needs to use RACH.
•
The UE can use dedicated RACH resources, thus eliminating the possibility of
collision.
Tilak, Dept. of ECE, GAT
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Paging
•
Paging is a connection control function of the RRC protocol.
•
The paging message is used to inform the UEs in the RRC_IDLE or
RRC_CONNECTED state about a system information change.
•
The UE in the RRC_IDLE state monitors a paging channel to detect incoming calls.
•
The system information can be transmitted a number of times with the same content
within a modification period.
•
Upon receiving a change in notification contained in the paging message, the UE
knows that the current system information is valid until the next modification period
boundary.
•
E-UTRAN initiates the paging procedure by transmitting the paging message at the
UEs paging occasion.
•
One Paging Frame (PF) is one radio frame in which the E-UTRAN can page the UE.
•
One PF may contain one or multiple sub frame in which a Paging message can be
transmitted and each sub frame is called a Paging Occasion (PO) which is configured
by the E-UTRAN.
•
The paging information is carried on the Physical Downlink Shared Channel
(PDSCH) physical channel.
•
In a certain PO, the UE is configured to decode PDCCH with CRC scrambled by the
Paging-Radio Network Temporary Identifier (P-RNTI) and then decode the
corresponding PDSCH for the information.
•
To reduce power consumption, the UE may use Discontinuous Reception (DRX) in
the idle mode, so it needs only to monitor one PO per DRX cycle.
•
After receiving the paging message, the UE can switch off its receiver to preserve
battery power.
•
The DRX cycle is configured by the E-UTRAN.
Inter Cell Interference Coordination
•
In cellular networks, each UE suffers from Inter Cell Interference (ICI) due to
frequency reuse in other cells.
•
To meet the spectrum efficiency target, LTE will be deployed with universal
frequency reuse i.e. the same spectrum will be reused in each cell.
Tilak, Dept. of ECE, GAT
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•
The basic approaches to mitigate ICI in downlink are
 ICI randomization
 ICI cancellation
 ICI coordination/avoidance
•
The basic approaches to mitigate ICI in uplink are
 ICI randomization
 ICI cancellation
 Uplink power control
 ICI coordination/avoidance
Downlink
ICI randomization
•
It is achieved by scrambling the codeword after channel coding with a pseudo-random
sequence.
•
Without scrambling, the channel decoder might be equally matched to interfering
signals as to the desired signals on the same radio resource.
•
ICI randomization is applied in systems such as UMTS.
ICI cancellation
•
If a UE is able to decode the interfering signal, it can regenerate and then subtract
them from the desired signal which can be achieved from the multi user detector at
UE.
•
The UE needs to know its transmission format to decode the interfering signal from
neighboring cells.
•
ICI cancellation can also be performed in spatial domain with the statistical
knowledge of interference channels.
ICI coordination/avoidance
•
It is achieved by applying restrictions to the downlink resource management in a
coordinated way between neighboring cells.
•
The restrictions can be on time-frequency resources or transmit power used at each
eNode-B.
•
It requires additional inter eNode-B communication and UE measurements and
reporting.
Tilak, Dept. of ECE, GAT
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•
ICI coordination/avoidance can be either static or semi-static.
•
To assist ICI coordination, LTE system defines eNode-B power restriction signaling
in the downlink which is a bitmap termed the Relative Narrowband Transmit Power
(RNTP) indicator that can be exchanged between eNode-Bs over the X2 interface.
•
Each bit of RNTP indicator corresponds to one Physical Resource Block (PRB) and is
to indicate the maximum anticipated transmit power on that PRB.
Static ICI coordination/avoidance
•
This is mainly done during cell planning process and does not require frequent
reconfiguration.
•
It requires no or little inter eNode-B signaling.
•
An example is static Fractional Frequency Reuse (FFR).
Semi-static ICI coordination/avoidance
•
It requires reconfigurations on a time scale of the order of seconds or longer, and inter
eNode-B communication over X2 interface is needed.
•
The information exchanged between neighboring eNode-Bs can be transmission
power and/or traffic load on different resource blocks.
Uplink
ICI randomization
•
It is achieved by scrambling the encoded symbols prior to modulation.
•
Instead of cell specific scrambling as used in Downlink, UE specific scrambling is
used in Uplink as ICI comes from multiple UEs in neighboring cells.
Tilak, Dept. of ECE, GAT
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ICI cancellation
•
It is more applicable in Uplink than in Downlink as the eNode-B has higher
computational capacity and usually more antenna elements.
Uplink Power Control
•
Power Control is an efficient way to suppress ICI in the Uplink.
•
Fractional Power Control (FPC) is used in LTE.
ICI coordination/avoidance
•
Similar to Downlink, FFR technique is used in Uplink.
•
To assist Uplink ICI coordination, 2 messages are defined in LTE that can be
exchanged over X2 interface between eNode-Bs for power allocation and user
scheduling:
•
Interference Overload Indicator (OI)
•
High Interference Indicator (HII)
•
OI indicates physical layer measurements of the average Uplink interference plus
noise for each PRB based on which eNode-Bs can adjust uplink power to suppress
ICI.
•
HII indicates which PRBs will be used for cell edge UEs in a certain cell.
Coordinated Multi-Point Transmission
•
Coordinated Multi-Point (CoMP) transmission/reception is the technique developed in
LTE advanced for ICI mitigation to improve cell edge performance.
•
Downlink CoMP transmission involves dynamic coordination among multiple
geographically separated transmission points.
•
It can be deployed in the form of coordinated scheduling and/or beam forming or
multi cell joint transmission.
Coordinated Multi-Point Reception
•
CoMP reception will be developed for Uplink in LTE advanced.
•
It means coordinated reception at multiple eNode-Bs of transmitted signals from
multiple geographically separated UEs in different cells.
•
Uplink CoMP reception is expected to have very limited impact on the radio interface
specifications.
Tilak, Dept. of ECE, GAT
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