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Let us begin our journey to understand the LTE(Long Term Evolution). Here, we will be concentrating on the
LTE expecting us to have a fair bit of idea about wireless technologies.
Our main focus will be to explain you the LTE in the most simplistic way and we will be concentrating more on the network side in FDD mode.
What happens when you powers on a mobile phone ?
Whenever a user powers on a mobile , first thing it should do is to try to connect with the Network. UE has in the equipment the available frequency bands it supports. It will try to scan each frequency available and will select the network with the highest signal quality( Usually UE protocol stacks will have some algorithms to make this search faster in order to save time otherwise it will take lot of time to scan each frequency available.
Now suppose UE has selected a Network, next step it will try is to camp on it( Meaning try to synchronise with the Network and gets attached to it)
To understand the synchronisation process, we must be familiar with some basic LTE concepts,
As of now I am hoping, we all are aware of the frame structure in LTE for FDD
I hope everybody has some idea about the architecture of LTE, different entities like eNodeB. So for UE to synchronise with the network, eNodeB keeps on transmitting signals in downlink every milli sec.
We should first understand how eNodeB transmits data in frequency time domain as we know that in LTE
OFDM (orthogonal frequency division multiplexing) technology is getting used for downlink data transmission.
Please refer to the diagram for the resource grid from the link:http://paul.wad.homepage.dk/LTE/lte_resource_grid.html [http://paul.wad.homepage.dk/LTE/lte_resource_grid.html]
Please consider only first column corresponding to the subframe 0. As we know that each subframe has 2 slots of 0.5 millisec each in LTE. In the diagram,
Horizontally we have time domain , so for one subframe(1 ms) we have 2 slot( 0.5 ms each). Every slot containing 7 OFDM symbols with normal cyclic prefix(will explain later).
Vertically we have frequency domain with each carrier of 17.06 Khz.
In the diagram, each block corresponding to a subframe represents 1 RB(Resource Block).Each small block within a RB represents RE(Resource element). So to summarise,
1 RB - 2 slots ( each containing 7 OFDM symbols) Horizontally
1 RB - 12 sub carriers(204.8 Khz) vertically
1 sub carrier - 17.06 Khz
1 RB = 14 * 12 =168 RE's( For normal CP)
In LTE, we supports different channel bandwidths starting from 1.4 Mhz to 20 Mhz. Lets take one example:-
1. So for 10 Mhz: In one subframe we will have (10 * 1024) / 204.8 number of RB's. That means 50 RB's.
Please try yourself for 20 Mhz and 1.4 Mhz to get a hang on the number of RB's and RE's available with the eNodeB for different system bandwidths.
Steps followed by UE for the synchronization: -
Step 1: UE first try to find the Primary synchronization signal and Secondary synchronization signal.
So, what is PSS and SSS and where exactly eNodeB transmits those and what kind of information it contains .
The synchronisation process uses two physical signals namely primary synchronisation signal and the secondary synchronisation signal.
After successfully decoding these two UE will have:-
1. Enables synchronization in time and frequency with the eNodeB.
2. UE receives the physical Cell Id.
3. It will know whether eNodeB is using FDD or TDD.
4. UE will have the cyclic prefix length( CP ).
Where exactly eNodeB transmits them :-
EnodeB transmits the PSS in last OFDM symbol of the slot 0 and slot 11 in a frame for FDD. SSS is located in the symbol before PSS. Please refer to the link for the diagram which shows the PSS in slot 0 and slot 11 for
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[http://paul.wad.homepage.dk/LTE/lte_resource_grid.html] search
PSS and SSS are transmitted in central six RB's. As it takes RE's from 6 RB's only, so this can work for all system bandwidths from 1.4 Mhz to 20 Mhz. PSS and SSS are mapped to the central 62 sub carriers.So 5 RE's on both the extreme RB's will be left unused.
Example: - Suppose system Bandwidth is 3 Mhz. Total RB's available will be from RB number 0 to RB number
14. Total sub-carrier will be 180 for 3 Mhz. So central 62 sub carriers will be used for PSS and SSS and 5 RE's on both RB number 4 and RB number 10 will be unused.
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LTE ( Long Term Evoluti … How does the eNodeB selects the RE's on which PSS needs to be sent?
There are total 504 unique physical cell identities are available which are further categorised into 168 groups each containing 3 identities.Usually identities belonging to the same group is under the control of the same eNodeB representing different cells in control under that eNodeB.
PSS uses Zadoff Chu algorithm to select a sequence. Please find the formula to map the sequence generated with the RE's in spec 36.211 section 6.11.1.2
Important points to remember for PSS and SSS:-
1. PSS and SSS will be transmitted on the same Antenna port for a particular subFrame.
2. PSS in a cell will be same in whichever frame it is transmitted but SSS transmissions change in a specific manner.
Coherent Detection :- When we have the channel knowledge i.e. amplitude and phase information ( e.g. in case of SSS detection)
Non - Coherent :- We don’t have the phase information of the channel. (e.g. in case of PSS detection)
Step -2 :-
After the successful decoding of PSS and SSS , UE tries to decode the reference signals from the cell ID it got from PSS and SSS.
Reference Signals:-
These are used by UE for channel estimation.These pilot symbols provide an estimate of the channel conditions at given locations within a subframe. Reference signals are embedded into specific RE's such that channel conditions can be known at different frequency and time domain within a subframe. Reference signals are always QPSK modulated.
There are 5 kinds of reference signals as mentioned in specifications as:-
1. Cell specific reference signals :- These are common for all the UE's within a cell. The RE's on which reference signals will be present is given by a formula mentioned in spec 36211 sec 6.10.1.2
Cell specific reference signals supports Antenna ports 1,2 ,3 and 4. Please refer to Fig 8.3 from Theory and practice book of LTE
2. UE specific reference signals : - These are usually multiplexed with the downlink data for that corresponding
UE. It supports Antenna port 5,7 and 8.
3.
Positioning Reference Signals :- It supports Antenna port 6.
4. MBSFN reference signals
5. CSI reference signals
We are not going in much detail for reference signals as this is mainly a Physical Layer Funda and as of now we will concentrate on MAC on this blog.
Step-3:- After successfully decoding the reference signals, UE can now easily find the MIB ( Master
Information Block). In cellular systems , the basic information which allows other channels in the cell to be configured and operated is carried by broadcast channel( BCH). In LTE broadcast information is divided into two parts:-
1. Master Information Block(MIB) :- It contains the essential parameters which are needed by the UE for initial access to the cell. It is transmitted very frequently so the size of information has kept very small. It is carried by a separate channel called PBCH ( Physical Broadcast Channel).
MIB contains :i)PHICH structure
a. PHICH duration (Normal , Extended)
b. PHICH Resource (Six , half, one , two)
We will be elaborating this when we will cover the PHICH. ii)Downlink system Bandwidth( In terms of RB's)
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LTE ( Long Term Evoluti … iii) Most significant 8 bits of system frame number -
… to decode the 2nd transmission of MIB so two bits will become 01).
Example MIB snippet:search
Unique characteristics of MIB: -
1. UE does not know the system bandwidth till now, so MIB is transmitted in central 72 subcarriers, as central frequency is known to UE through PSS and SSS.
2. MIB information is just 14 bits in size which is repeated every 40 ms, so system overhead is less.
3. Even if the SIR ( signal to interference ratio) is high MIB can be decoded by UE as same information is repeated every 10 ms. So UE can combine the results of 4 MIB information transmitted in 40 ms and get the desired MIB info.
4. MIB is always QPSK modulated.
MIB is CRC scrambled with number of transmit antenna ports used by eNodeB. So UE initially does the blind decoding by checking for all the values of Antenna port.
The timing of the 40 ms transmission interval for each MIB on the PBCH is not indicated explicitly to the UE; this is determined implicitly from the scrambling and bit positions, which are re-initialised every 40 ms. The UE can therefore initially determine the 40 ms timing by performing four separate decodings of the PBCH using each of the four possible phases of the PBCH scrambling code, checking the CRC for each decoding.
Please refer diagram of Theory and Practice book as:-
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[http://2.bp.blogspot.com/-CSyOCT-YqJA/UYpa6xsfEGI/AAAAAAAAAAg/IbvdSKdC2Kk/s1600/MIB1.GIF]
MIB has 1728 bits for extended CP as compare to 1920 for normal CP.
On which RE's MIB will be transmitted by eNodeB:-
MIB will use the resource elements of first 4 OFDM symblos in slot 1.In total MIB will be transmitted in central
72 subcarriers( 6 RB's). While allocating RE's for MIB, eNodeB shall exclude the RE's for cell specific reference signals for all antenna ports 0 to 3 irrespective of the actual antenna configuration used by eNodeB.
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STEP-4:-
After UE has successfully decoded the MIB, now it will try to decode the PCFICH ( Physical control format
Indicator channel).
PCFICH:-
In a subframe we need to limit the control information transmitted for the UE's to decode the information present in that subframe. In LTE this PCFICH channel carries the number of OFDM symbols used for the control information.
Points to remember for PCFICH:-
• Its value can vary from 1 to 3. Value 4 is reserved for future use.For less system bandwidth( less than 10
RB's) number of OFDM symbols are increased for control information to ensure coverage at cell edges.
• It is of 32 bits in size mapped to 16 RE's( 4 REG). Each REG is distributed in frequency domain.It uses
QPSK modulation technique.
• All the RE's are of first OFDM symbol in first slot and on which RE's this information will be present is also pre defined such that UE can always locate this information.
• 32 bit codeword is also scrambled with cell Identity which UE has got from PSS and SSS such that UE can get the PCFICH information from the desired cell only.Even the RE's on which CFI information is present cell ID offset is applied.
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[http://lteuniversity.com/cfs-file.ashx/__key/communityserver-blogs-components-weblogfiles/00-00-00-01-37/8446.Fig_2D00_2.png]
As shown in Fig while forming REG's the refernce signals RE's are left alone which are marked with cross. 32 bits codewords are also present in the diagram for 1 to 3 OFDM symbols
1 REG (Resource Element Group) - 4 RE's
QPSK ( Quadrature Phase Shift Keying ) – 2 bits per symbol
64QAM ( Quadrature Amplitude Modulation ) - 6 bits per symbol
16QAM – 4 bits per symbol
Step- 5:-
Now UE knows the number of PDCCH symbols used for control information . Next UE will try to get the system information by decoding PDCCH for the SI - RNTI .
• How the PDCCH is formed for the system information, we will discuss at the time of PDCCH explanation.
We have already discussed about MIB which contains the most essential parameters to access a cell for UE.
SIB's carry important data for the UE to access a cell and gets attached to it and perform several other functions too. ENodeB broadcast this SIB information which is common for all the UE's.
There is a unique identity attached to SIB's which is SI-RNTI whose value is kept as 65535. So, UE's can decode the SIB's from the PDCCH by using this identity.
When does UE need to acquire the SIB information :-
According to spec 36331 section 5.2.2.2 UE need to acquire the information in following cases
• During power on scenario
• Cell Re-selection
• After Handover completion
• Entering E-UTRA from another RAT, means inter RAT handover
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• Return from out of coverage (OOC ) search
• Upon receiving a request from CDMA2000 upper layers
• Upon exceeding the maximum validity duration of SIB's( Usually 3 hours )
Important points about SIB's: -
• SIB's are transmitted on PDSCH channel.
• Physical layer imposes a limit of maximum size a SIB can take.
• For DCI format 1C = Max allowed is 1736 bits.
• For DCI format 1A = 2216 bits
• For a UE to access the eNodeB, UE atleast needs SIB1 and SIB2.
• Change of system Information :- System information can change only at specific radio frames. A concept of modification period is used. eNodeB keeps on transmitting the same system information with in a modification period. Modification period is usually in terms of number of radio frames.
When the network changes (some of the) system information, it first notifies the UEs about this change, i.e. this may be done throughout a modification period. In the next modification period, the network transmits the updated system information.
Upon receiving a change notification, the UE acquires the new system information immediately from the start of the next modification period. The UE applies the previously acquired system information until the UE acquires the new system information. System Information changes gets indicated to UE through:-
• Paging Message :- Whenever UE receives the paging message including systemInfoModification, it knows that system information will change at next modification period. This can be used for both UE's in IDLE and Connected Mode
• A value tag in SIB1 systemInfoValuetag which gets incremented at each change of system information. So UE can compare this new value received with the already stored value.Only for
Connected mode UE's.
Scheduling of SIB's :-
There are total 13 types of SIB's supported in LTE. SIB1 uses the fixed scheduling while scheduling information of all other SIB's from SIB2 - SIB 9 is present inside the SIB1 info. From SIB10 - SIB13, paging is received by
UE for incoming SI message.
• SIB1 gets scheduled in subframe 5 of the radio frame for which SFN mod 8 = 0. Its periodicity is 80 ms.
But within 80 ms repetitions are scheduled in subframe 5 of the radio frame for which SFN mod 2 = 0
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[http://3.bp.blogspot.com/-At_F2aD57m4/UG0nDeQ2M2I/AAAAAAAAES0/vwuo_eYvWew/s1600/SIB1.png]
• Scheduling information of all other SIB's is present inside SIB1 in two variables as:
• Scheduling Info List - Each node of the list contains:
• SI periodicity :- This variable is used by UE to calculate the Radio frame on which the system information will be received. Its value will be in number of radio frames.
• SI mapping info :- This maps to a particular SIB present in the current node of the List. First node of the list will always contain SIB2 info so no mapping information is required for that.
• SI window length :- It tells the UE that SI information should be present within this duration starting from the subframe of the Radio frame decided from SI periodicity. Its value is in milli second.
Lets take an Example :-
This is the snippet taken from the SIB1 info which is getting transmitted from eNodeB. In this case 6 entries are present in the scheduling info list. For each entry SI periodicity and mapping info is present.
• For first Entry in the list, there is no mapping info present because by default first entry is for SIB2 as shown in yellow.
• The periodicity value is in radio frame, In green rf64 represents 640 ms.
• Window length is common for all the entries in scheduling info list shown in pink which is 20 ms. http://nitintayal-lte-tutorials.blogspot.com/2013/05/lte-long-term-evolution.html
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Now the question is how UE will calculate the SFN and SF in which it will receive the SIB's.
1. Radio Frame on which UE will receive the SIB can be calculated as:
SFN mod T = FLOOR(x/10)
2 . Subframe on which SI window will start for the corresponding radio frame is
a = x mod 10
3 . value of x = (n -1) * w n - number which represents the order of entry in scheduling info list.
For Example :-
For SIB2, n = 1
For SIB3, n = 2 and so on w - SI window length ( 20 ms in our case)
T - SI periodicity of the corresponding SI message
Now lets calculate for SIB3:
1.
Subframe from which SI window will start for SIB3 x = (2 - 1) * 20 = 20 a = 20 mod 10 = 0 so, the SI window will start from subframe 0 for the corresponding radio frame.
2.
Radio Frame on which SIB3 will come
SFN mod T = FLOOR ( x /10 )
T = rf64 = 640 ms
SFN mod 640 = FLOOR ( 20 / 10 )
SFN mod 640 = 2
So, SIB3 will come on the radio frame for which SFN mod 640 = 2. UE will start looking for SIB3 from subframe number 0 of radio frame till the end of window length. http://nitintayal-lte-tutorials.blogspot.com/2013/05/lte-long-term-evolution.html
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Now we will be explaining what each SIB contains for the UE.
search
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1 :-
SIB1 is transmitted in RLC mode TM on logical channel BCCH.
SIB2:- It contains radio resource configuartion info which is common for all UE's.
It contains mainly:-
• Rach related parameters
• Paging configurations and SI modification period related parameters.
• Uplink bandwidth in terms of RB
• PUSCH ,PUCCH, uplink power control and sounding reference related configurations.
• Cell barring information.
We will be explaining these parameters in detail when we will cover the usage of these parameters. As of now I am pasting some information from another blog for reading purpose:-
Definition of important Parameters:-
Rach Configurations numberOfRA-Preambles: Total number of random access preambles available for contention based random access. Since there are maximum 64 preambles sequences available, others could be reserved by eNB for
Non-Contention based random access. Range of this parameter is 4 to 64 sizeOfRA-PreamblesGroupA: Total number of random access preambles sequences available within Group
A. Preambles are divided into Group A and Group B. Group A preambles are intended for sending small packets and Group B preambles are intended for sending large packets. Range of this parameter is 4 to 60 messageSizeGroupA: Message size threshold for selecting preamble Group A in term of bits (56, 144, 208 or
256 bits) messagePowerOffsetGroupB: Power offset for selecting preamble Group B (0, 5, 8, 10, 12, 15 or 18 dB) powerRampingStep: power ramping step size with possible values of 0, 2, 4 or 6 dB preambleInitialReceivedTargetPower: Preamble initial received target power with values from -120 dBm to -90 dBm with step size of 2 dBm preambleTransMax: Maximum number of preambles transmissions. Possible values are 3, 4, 5, 6, 7, 8,
10 ,20, 50, 100, 200.
ra-ResponseWindowSize: Duration of RA response window. RA response window size is in unit of subframes
(2, 3, 4, 5, 6, 7, 8, or 10 subframes) mac-ContentionResolutionTimer: Mac contention resolution timer in unit of subframes (8, 16, 24, 32, 40,
58, 56 or 64 subframes)
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LTE ( Long Term Evolution ) | Tayal's way to learn LTE Page 8 of 9 maxHARQ-Msg3Tx: Maximum number of HARQ retransmissions for message 3 of RACH process (contentionsearch
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BCCH Configurations modificationPeriodCoeff: The value (2,4,6,8) of this parameter is multiplied with default DRX cycle (e.g.
320ms, 640ms) to generate the BCCH modification period. It is the period in which the change in SI is repeated to UEs so that the change in SI is acquired by UE.
BCCH modification period = modificationPeriodCoeff x idle mode DRX cycle
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LTE ( Long Term Evoluti …
PCCH Configurations defaultPagingCycle: The default DRX cycle in idle mode in unit of radio frames (rf64 means 640ms ) nB: This parameter value is used in finding the actual paging frames and paging occasions in RRC idle mode with the following formula
SFN modT = (T/N) x (UE_ID mod N)
Where
T = Drx cycle
N = Min (T, nB) (nB is broadcasted in SIB2)
UE_ID = IMSI mod 1024
PRACH Configurations rootSequenceIndex: RA preambles are generated from Zadoff Chu sequence which consists of series of root sequences. Each root sequence can be cyclic shifted to obtain preamble sequence. Range of rootSequenceIndex is 0 to 837. prach-ConfigIndex: This parameter defines exactly when UE should send RACH in frequency/time grids
(Details TS36.211 Table 5.7.1-2) highSpeedFlag: For high speed UEs , as this can impact the correlation between cycles zeroCorrelationZoneConfig: The zero correlation zone is used to guarantee orthogonality of generated sequences. The value depends on particular condition in the cell prach-FreqOffset: With this information cell informs UE and other neighbor cells know about which PRB is available for RACH access
PDSCH Configurations referenceSignalPower: This defines the energy per resource element for the reference signal using a range from -60 to 50 dBm. p-b: It is used to calculate the power difference between PDSCH and Reference Signal. Value is from 0 to 3
PUSCH Configurations n-SB: Number of subbands (range 1 to 4) hoppingMode: Hopping mode can be inter-subframe, intra or inter-subframe pusch-HoppingOffset: Offset values range from 1 to 98 enable64QAM: if 64QAM capable UE should use it (True or False) groupHoppingEnabled: True or False groupAssignmentPUSCH: Gives sequence shift pattern for group hopping (0 to 29) sequenceHoppingEnabled: True or False cyclicShift: Frequency shift for demodulation (0 to 7)
PUCCH Config deltaPUCCH-Shift: 1,2 or 3 nRB-CQI: Number of PRBs per slot for PUCCH2 (0 to 98) nCS-AN: Cyclic shift used for PUCCH1 (0 to 7) n1PUCCH-AN: PUCCH to be used for HARQ (0 TO 2047)
Sounding Reference Signaling Configurations:
The uplink Sounding Reference Signal (SRS) is configured in terms of bandwidth and subframes
Uplink Power Control p0-NominalPUSCH: It impacts the calculation of PUSCH transmit power and applicable to non-persistent scheduling only (-126 to 24 dBm) alpha: It also impacts the calculation of PUSCH transmit power and also scales the contribution of path loss.
Possible values are 0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1 p0-NominalPUCCH: This parameter impacts the calculation of PUCCH transmit power (-127 to -96 dBm)
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… deltaFList-PUCCH: These parameters impacts the calculation of PUCCH transmit power search
It impacts the transmit power of PUSCH when responding to random access response grant (-1 to 6dB) ul-CyclicPrefixLength: To differentitate between normal (len1) OR extended (len2) cyclic prefix for uplink transmission
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LTE ( Long Term Evoluti …
UE Timers and Constants
T300: Time during which UE waits for RRC connection request message response (100, 200, 300, 400, 600,
1000, 1500, 2000 ms)
T301: Started after RRC Connection Reestablishment request message. On expiration UE will go to RRC idle
(100, 200, 300, 400, 600, 1000, 1500, 2000 ms)
T310: Started after receiving N310 out of sync indications (0, 50, 100, 200, 500, 1000, 2000 ms)
T311: Started after initiating connection re-establishment procedure. On expiration UE goes to RRC idle mode if it is unable to locate suitable cell (1, 3, 5, 10, 15, 20, 30 seconds)
N310: Consecutive out of sync indications (1, 2, 3, 4, 6, 8, 10, 20) N311: Consecutive in-sync indications (1,
2, 3, 4, 6, 8, 10, 20)
Frequency Information ul-CarrierFreq: Defined in terms of EARFCN ul-Bandwidth: Defined in terms of resource blocks additionalSpectrumEmission: This allows spectrum emission limits to be configured according to local requirements (1 to 32) timeAlignmentTimerCommon: it tells UE how long it should consider itself to be time aligned in uplink in unit of subframes. (500, 750, 1280, 1920, 2560, 5120, 10240 or infinity subframes)
I will update these terms explanation whenever we will be using them in some procedures later.
Abbreviations:-
SFBC - Space frequency block Code
Imp Questions : -
1. Why do we transmit SSS just before the symbol on which PSS is transmitted?
2. Importance of Cyclic prefix in the context of PSS and SSS.
3. MIB contains only 8 nits of SFN, so how UE gets the other 2 bits for complete SFN of 10 bits.
Posted 6th May by Nitin Tayal
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