Saturday, May 18, 2013

Dedicated Bearer Setup Procedure

Dedicated Bearer Setup Procedure

UE Service Request Procedure

UE Service Request Procedure

UE Context Release Requested by eNodeB Procedure

UE Context Release Requested by eNodeB Procedure

S1AP: Initial Attach Procedure

S1AP : Initial Attach Procedure.

RNTI (Radio Network Temporary Identifier)

RNTIs are always used to identify information dedicated to a particular subscriber on the radio interface, especially if common or shared channels are used for data transmission.

FDD RNTI Range Values FDD RNTI Type
0000 - 0009 RA-RNTI
000A - FFF2 C-RNTI, semi-persistent scheduling C-RNTI, temporary C-RNTI, TPC-PUCCH-RNTI and TPC-PUSCH-RNTI
FFF3 - FFFC Reserved

P-RNTI is the paging RNTI. It does not refer to a particular UE, but to a group of UEs. The P-RNTI is derived from the IMSI of the subscriber to be paged and constructed by the eNB. For this reason the IMSI is transmitted in a S1AP paging message from the MME to eNB, although in other S1AP signaling only the GUTI is used to mask the true identity of the subscriber.

The RA-RNTI is assigned by the eNB to a particular UE after this UE has sent a random access preamble on the Physical Random Access Channel (PRACH).

The C-RNTI format and encoding are specified in 36.321 (MAC). The C-RNTI is part of the MAC Logical Channel Group ID field (LCG ID). It defines which data sent in a DL direction.
C-RNTI comes in three different flavors: temp C-RNTI, semi-persistent scheduling C-RNTI, and permanent CRNTI.
The temp CRNTI is allocated to the UE during random access procedure (with a RRC Connection Setup message) and may turn into a permanent CRNTI depending on the result of a subsequently performed contention resolution procedure or in the case of contention free random access.
The semi-persistent scheduling C-RNTI is used if the UE is running services with a predictable unchanging QoS profile. A typical example is VOPI for which the required bit rate will not change during the entire session.

The SI-RNTI is send on the PDCCH. It signals to all mobiles in a cell where the broadcast System Information Blocks (SIBs) are found on the Physical Downlink Shared Channel (PDSCH).

GUTI (Global Unique Temporary Identity)

The GUTI is assigned only by the MME during initial attach of a UE to the E-UTRAN. The purpose of the GUTI is to provide an unambiguous identification of the UE that does not reveal the UE or the user's permanent identity in the E-UTRAN. It also allows identification of the MME and network to which the UE attaches. The GUTI can be used by the network to identify each UE unambiguously during signaling connections.

GUTI  =  MCC (3 bit) + MNC (3 bit) + MME Group ID (16 bit) + MME code (8 bit) + M-TMSI(32 bit)

GUMMEI = MCC (3 bit) + MNC (3 bit) + MME Group ID (16 bit) + MME code (8 bit)

MME Identifier (MMEI) = MME Group ID (16-bit) + MME Code (8-bit)

S-TMSI = MME Code (8 bit) + M-TMSI (32 bit)

For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and the M-TMSI.

The operator needs to ensure that the MMEC is unique with the MME Pool area and, if overlapping pool areas are in use, unique within the area of overlapping MME pools.

It is important to understand that on the S1 interface the IMSI is typically not seen, just like the GUTI. Exceptions are initial attach to the network when no old GUTI is stored on the USIM card or the true subscriber's identity is checked using NAS signaling, which  regularly happens when roaming subscribers attach.




LTE MAC Presentation from EventHelix

LAWFUL Interception Architecture for LTE

LTE RLC presentation from EventHelix

RLC Presentation from EvenHelix.

MIMO technologies in 3GPP LTE and LTE Advanced

I have come across good article on the MIMO Technologies in LTE. 

PDF Document:

LTE UE Classes

Article on Timing advancement in LTE

LTE Physical Hybrid ARQ Indicator Channel (PHICH)

PHICH is the physical channel that carrier the Hybrid ARQ Indicator (HI).  The HI contains the Acknowledgement/Negative Acknowledgement (ACK/NACK) feedback to the UE for the uplink blocks received by the eNB.

PHICH group is the group of multiple PHICHs mapped to same set of resource elements, where PHICHs within the same PHICH group are separated through different orthogonal sequences.

 A PHICH resource is identified by the index pair [ n(group,PHICH), n(seq, PHICH)] , where  n(group, PHICH) is the PHICH group number and n(seq, PHICH) is the orthogonal sequence index within the group for frame structure type 1, the number of PHICH groups n(group, PHICH) is

constant in all subframes and given by where Ng is {1/6,1/2,1,2}   is provided by higher layers. The index n(group, PHICH) ranges from 0  to N(group, PHICH) - 1 For Normal Cyclic Prefix

Ng, NRB 1/6 1/2 1 2
6 1 1 1 2
15 1 1 2 4
25 1 2 4 7
50 2 4 7 13
75 2 5 10 19
100 3 7 13 25

The HARQ Indicator undergoes repetition coding to create a HARQ indicator codeword made up of three bits, 

HARQ Indicator HARQ Indicator Codeword
0 – Negative acknowledgement <0,0,0,>
1 – Positive acknowledgement <1,1,1>

The HARQ Indicator codeword undergoes BPSK modulation. The block of modulated symbols is bit-wise multiplied with an orthogonal sequence and a cell-specific scrambling sequence.  The three modulated symbols  are repeated   times and scrambled to create a sequence of six or twelve symbols depending on whether a normal or extended cyclic prefix is used. As resource element groups (REGs) contain four resource elements (each able to contain one symbol) the blocks of scrambled symbols are aligned to create blocks of four symbol.


The physical control format indicator channel (PCFICH) carries information about the number of OFDM symbol used for transmission of PDCCHs in a subframe. PCFICH uses cell specific scrambling code.  2 bit CFI is block coded into 32 bits and mapped into 16 sub carriers with QPSK Modulation. 

PCFICH is located at OFDM symbol #0 of every sub frame and the assignment to the sub carriers is determined by Cell Id Information.The PCFICH is mapped in terms of Resource Element Groups (REGs) and is always mapped onto the first OFDM symbol. The number of REGs allocated to the PCFICH transmission is fixed to 4 i.e. 16 Resource Elements (REs). 

A PCFICH is only transmitted when the number of OFDM symbols for PDCCH is greater than zero. The 32 bit coded CFI block undergoes a bit-wise XOR operation with a cell specific scrambling sequence. The scrambling sequence is a pseudo-random sequence created using a length-31 Gold sequence generator. Scrambling with a cell specific sequence serves the purpose of inter-cell interference rejection.
When a UE descrambles a received bit stream with a known cell specific scrambling sequence, interference from other cells will be descrambled incorrectly and will only appear as uncorrelated noise. The scrambled bits are then QPSK modulated to create a block of complex-valued modulation symbols. 

Subframe Number of OFDM symbols for PDCCH when Number of OFDM symbols for PDCCH when
Subframe 1 and 6 for frame structure type 2 1, 2 2
MBSFN subframes on a carrier supporting both PMCH and PDSCH for 1 or 2 cell specificc antenna ports 1, 2 2
MBSFN subframes on a carrier supporting both PMCH and PDSCH for 4 cell specific antenna ports 2 2
MBSFN subframes on a carrier not supporting PDSCH 0 0
All other cases 1, 2, 3 2, 3, 4


BCH Transport Channel is used to carry BCCH. In LTE, BCH carries only Master Information Block (MIB) whereas all the SIB are transmitted using BCCH mapped on DL-SCH.

BCCH mapped to BCH as well as DL-SCH.

Transport Block size of BCH is set to 24 bits.

On this 24 bits , 16 bits CRC is added and makes 40 bits.

On 40 bits,  1/3 convolution coding is applied and converted in 120 bits.

On 120 bits, rate matching is applied and converted into 1920 bits.

This 1920 bits are spread across 40ms ( 4 frames). It means 4 frames carrying the original 24 bits.

X : MIB on PBCH on slot 1 on subframe 0  
Y: MIB' on PBCH on slot 1 on Subframe 0

 <---(X)---> <---(X) ---> <---(X)---> <---(X)--->  <---(Y)---> <---(Y)---> <---(Y)---> <---(Y)--->
<-10ms->      <-10ms->      <-10ms->  <-10ms->    <-10ms->  <-10ms->   <-10ms->     <-10ms->

Primary and Secondary Synchronization Signals

Both primary and secondary synchronization signals are designed to detect all type of UEs.  The synchronization signals always occupy the 62 sub-carrier of the channel, which make the cell search procedure same regardless of channel bandwidth. Although 72 subcarriers (6 RB) are available, only 62 sub-carriers are used so that the UE can perform the cell search procedure.

The primary synchronization signal subcarriers are modulated using a frequency domain Zadoff-Chu Sequence. Each subcarrier has the same power level with its phase determined by the root index number in sequence generator as defined in 36.211 .

There are 504 unique physical-layer cell identities. The physical-layer cell identities are grouped into 168 unique physical-layer cell-identity groups, each group containing three unique identities. The grouping is such that each physical-layer cell identity is part of one and only one physical-layer cell-identity group. A physical-layer cell identity

N(cell, ID) = 3N(1, ID) + N(2, ID) 

is thus uniquely defined by a number N(1,ID) in the range of 0 to 167, representing the physical-layer cell-identity group, and a number N(2,ID) in the range of 0 to 2, representing the physical-layer identity within the physical-layer cell-identity group Three different cell id is used for the primary synchronization signal i.e N(2, ID). The root index corresponds to the cell identity N(2, ID).

The secondary signal is used to identify cell-identity groups. The number and position of subcarrier are same as for the primary synchronization signal: that is the central 62 sub carriers. The sequence generation function utilizes an interleaved concatenation of two length 31 binary sequences as defined in 36.211.

The secondary synchronization signal gives a cell-identity group number from 168 possible cell identities N (1, ID). The mapping of the sequence to resource elements depends on the frame structure.

For FDD,  frame structure type 1, the primary synchronization signal shall be mapped to the last OFDM symbol in slots 0 and 10. Let NRBDL = 50 (10MHz and NRBSC = 12) for Normal Cyclic prefix NDLsyml = 7 Resource elements  (k,l) in the OFDM symbols used for transmission of the primary synchronization signal. k will range from 269 to 330 subcarriers and l shall be equal to 6 (even slot) Resource elements  (k,l) in the OFDM symbols used for transmission of the secondary synchronization signal. k will range from 269 to 330 subcarriers and l shall be equal to 5 (even slot).


In LTE each scheduling grant is defined based on fixed size control channel elements (CCE). The four different CCE aggregation levels are defined for the transmission of a control channel.  Whatever LTE scheduler schedules for PDCCH, that should be mapped to the CCE size by applying different coding rates.

 For example if LTE scheduler schedules Format 1A DCI of length 28bits then Add CRC to 28bits   (28bits + 16 bits) = 44bits , Now this 44 bits can be mapped to different CCE as follows.

PDCCH format 0     (CCE size = 72 bits)    44bits are converted into 72 bits using coding rate 44/72.
PDCCH format 1     (CCE size = 144 bits)   44 bits are converted into 144 bits using coding rate 44/144.
PDCCH format 2    (CCE size = 288 bits)   44 bits are converted into 288 bits using coding rate 44/288.
PDCCH format 3    (CCE size = 576 bits)   44 bits are converted into 576 bits using coding rate 44/576.

 It is to be noted that the combination of PDCCH formats and CCE aggregation levels with code rates >3/4 (0.75) is not supported. All the PDCCHs transmitted in a subframe are multiplexed together, and then scrambled with a Cell Specific sequence prior to QPSK modulation.

Friday, May 17, 2013


UE shall need to check all possible combination of PDCCH locations, PDCCH formats, and DCI formats and act on those message with correct CRCs (taking into account that the CRC is scrambled with UE identity).  Carrying out such a 'blind decoding' of all the possible combinations would require the UE to make many PDCCH decoding attempts in every subframe.

For large system bandwidths, with a large number of  possible PDCCH locations, it would be significant burden, leading to excessive power consumption in the UE receiver. The alternative approach defined for LTE to reduce this burden. For each UE a limited set of CCE locations where a PDCCH may be placed.  The set of CCE locations in which the UE may find its PDCCH can be considered as search space. In LTE the search space is a different size for each PDCCH format. Moreover, separate dedicated and common search spaces are defined, where a dedicated search space is configured for each UE individually, while all UEs are informed of the extent of the common search space.

The physical downlink control channel carries scheduling assignments and other control information. A physical control channel is transmitted on an aggregation of one or several consecutive control channel elements (CCEs), where a control channel element corresponds to 9 resource element groups (REG). Each REG in turn has four Resource Elements (REs).  The number of resource-element groups not assigned to PCFICH or PHICH is N(Reg) .

The CCEs available in the system are numbered from 0 and  N(cce) - 1, where N(cce) = floor(N(Reg) 9) . PDCCH is modulated with QPSK modulation.

PDCCH format Number of CCEs Number of REG No of Res 1 REG = 4 REs Number of PDCCH bits 1 symbols contains 2 bits
0 1 9 36 72
1 2 18 72 144
2 4 36 144 288
3 8 72 288 576

Why LTE has 15Khz sub carrier spacing

One argument for adopting a 15 kHz subcarrier spacing for LTE was that it may simplify the implementation of WCDMA/HSPA/LTE multi-mode terminals. Assuming a power-of-two FFT size and a subcarrier spacing f = 15 kHz, the sampling rate fs = f · NFFT will be a multiple or sub-multiple of the WCDMA/HSPA chip rate fcr = 3.84 MHz. Multi-mode WCDMA/HSPA/LTE terminals can then straightforwardly be implemented with a single clock circuitry.In addition to the 15 kHz subcarrier spacing, a reduced subcarrier spacing flow = 7.5 kHz is also defined for LTE.


Why first and sixth subframe are always assigned to DL in LTE TDD

The first and sixth subframe of each frame (subframe 0 and subframe 5) are always assigned to downlink transmission while remaining subframes can be flexibly assigned to be used for either downlink and uplink. The reason for the predefined assignment of the first and sixth subframe for downlink transmission is that these subframes include the LTE Synchronization signals. The synchronization signals are transmitted on the downlink of each cell and are intended to be used for initial cell search as well as for neighbor-cell search.

 From Book :


3G Americas has published a educational white paper on MIMO and SMART ANTENNAS.