LTE Physical Layer - 3G Network Solutions3gnets.in/files/documents/LTE-Protocol-Stack-Physical-layer.pdf · LTE Protocol Stack- 1 [email protected] LTE Development, Conformance

LTE Protocol Stack- 1 [email protected]

LTE Development, Conformance Test, Optimization

Certification Course Amateur Level (3PCA-L1)

3PCA-L1

LTE

Physical Layer LTE Protocol Stack

Author: Surya Patar Munda

LTE Physical Layer- 3PCA-L1 Certification email: [email protected] Page 2

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:

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-L1), you need this course. This

knowledge and level is required for the next level Professional Level (3PCP-L1) where you can

be trained for higher level with Hands on Projects and real implementation. Full Amateur level

courses are:

LTE Physical Layer - (3PCA-L1)

LTE L2 Layer - MAC, RLC, PDCP - (3PCA-L2)

LTE RRC (3PCA-RRC)

LTE NAS (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.299


Contents 1. Downlink Physical Layer ................................................................................................................. 7

1.1 OFDMA Principles ................................................................................................................... 7

1.1.1 OFDM - Orthogonal Multiplexing Principle ...................................................................... 7

1.1.2 Peak-to-Average Power Ratio and Sensitivity ................................................................ 9

1.1.3 Timing Offset and Cyclic Prefix Dimensioning ................................................................ 9

1.1.4 OFDMA Parameter Dimensioning .............................................................................. 10

1.1.5 Physical Layer Parameters for LTE .............................................................................. 10

1.1.6 Transmission Resource Structure ................................................................................. 11

1.2 Synchronization and Cell Search .......................................................................................... 15

1.2.1. Synchronization Sequences and Cell Search in LTE ................................................... 15

1.2.2. ZadoffChu Sequences ................................................................................................ 16

1.2.3. Primary Synchronization Signal (PSS) Sequences ...................................................... 17

1.2.4. PSS Generation ............................................................................................................ 17

1.2.5. Secondary Synchronization Signal (SSS) Sequences.................................................. 18

1.2.6. Cell Search Performance .............................................................................................. 19

1.2.7. Reference Signals and Channel Estimation ................................................................. 19

1.2.8. Design of Reference Signals in LTE ............................................................................. 19

1.2.9. Cell-Specific Reference Signals (CRS) ......................................................................... 20

1.2.10. UE-Specific Reference Signals(URS) ........................................................................... 21

1.2.11. RS-Aided Channel Modelling and Estimation ............................................................... 22

1.2.12. Frequency Domain Channel Estimation ....................................................................... 22

1.2.13. Time-Domain Channel Estimation ................................................................................ 22

1.2.14. Spatial Domain Channel Estimation(SD-MMSE) .......................................................... 23

1.3 Phy Data and Control Channels - DL .................................................................................... 25

1.3.1. Physical Broadcast Channel (PBCH) ............................................................................ 25

1.3.2. Physical Downlink Shared Channel (PDSCH) .............................................................. 26

1.3.3. Physical Multicast Channel (PMCH) ............................................................................. 27

1.3.4. Downlink Control Channels ........................................................................................... 27

1.3.5. Physical Control Format Indicator Channel (PCFICH).................................................. 28

1.3.6. Physical Downlink Control Channel (PDCCH) .............................................................. 29

1.3.7. PDCCH Candidate Selection ........................................................................................ 30

1.3.8. Formats for Downlink Control Information (DCI) ........................................................... 31

1.3.9. Physical Hybrid ARQ Indicator Channel (PHICH) ......................................................... 35

1.3.10. Resource Allocation ...................................................................................................... 36

1.3.11. DL Resource Allocation Rules ...................................................................................... 38

1.3.12. Resource Allocation Bitmap examples.......................................................................... 39

1.3.13. Uplink Grant .................................................................................................................. 40


1.3.14. PDCCH Transmission and Blind Decoding ................................................................... 41

1.3.15. Enhanced PDCCH (EPDCCH) ...................................................................................... 41

EPDCCH assignment procedure .................................................................................................. 41

Mapping to resource elements ...................................................................................................... 42

Resource mapping parameters for EPDCCH ............................................................................... 43

EPDCCH formats .......................................................................................................................... 43

1.3.16. Scheduling Process - Control Channel Viewpoint ........................................................ 45

1.4 Physical Layer Processing - DL ............................................................................................ 47

1.4.1. Link Adaptation and Feedback Computation ................................................................ 48

1.4.2. CQI Feedback in LTE .................................................................................................... 48

1.4.3. Channel Coding............................................................................................................. 49

1.4.4. Viterbi Algorithm (VA) (Example): ................................................................................. 49

1.4.5. LTE Contention-Free Interleaver ................................................................................... 51

1.4.6. Rate-Matching ............................................................................................................... 52

1.4.7. HARQ in LTE ................................................................................................................. 53

1.4.8. Coding for Control Channels in LTE ............................................................................. 54

1.4.9. General structure for downlink physical channels ......................................................... 54

1.4.10. Scrambling .................................................................................................................... 55

1.4.11. Modulation ..................................................................................................................... 55

1.4.12. Layer mapping ............................................................................................................... 55

1.4.13. Precoding ...................................................................................................................... 55

1.4.14. Mapping to resource elements ...................................................................................... 56

1.5 MIMO Techniques ................................................................................................................. 59

1.5.1. Introduction to MIMO ..................................................................................................... 59

1.5.2. Single-User (SU-) MIMO Techniques ........................................................................... 60

1.5.3. Multi-User Techniques .................................................................................................. 63

1.5.4. MIMO Schemes in LTE ................................................................................................. 64

1.5.5. Single-User Schemes ................................................................................................... 64

1.5.6. Beamforming Schemes ................................................................................................. 65

1.5.7. What is Spatial Multiplexing? ........................................................................................ 65

1.5.8. Precoding ...................................................................................................................... 66

1.5.9. Cyclic Delay Diversity (CDD) ........................................................................................ 67

1.5.10. Multi-User Schemes ...................................................................................................... 68

1.5.11. Physical-Layer MIMO Performance .............................................................................. 70

2. Uplink Physical Layer .................................................................................................................... 71

2.1. SC-FDMA Principles ............................................................................................................. 71

2.1.1. SC-FDMA Signal Generation (DFT-S-OFDM) .............................................................. 71

2.1.2. SC-FDMA Design parameters in LTE ........................................................................... 72


2.2. UL Physical Channel Structure ............................................................................................. 75

2.2.1 Uplink Shared Data Channel Structure ......................................................................... 75

2.2.2 Scheduling in LTE SC-FDMA Uplink............................................................................. 76

2.2.3 Uplink Control Channel Design ..................................................................................... 77

2.2.4 Physical Uplink Control Channel (PUCCH) .................................................................. 77

2.2.5 Multiplexing of UEs within a PUCCH Region ................................................................ 78

2.2.6 Control Signalling Information Carried on PUCCH ....................................................... 79

2.2.7 CQI Transmission on PUCCH (Format 2) ..................................................................... 79

2.2.8 Multiplexing CQI and ACK/NACK on PUCCH .............................................................. 79

2.2.9 CQI and ACK/NACK in Same RB (Mixed PUCCH RB) ................................................ 81

2.2.10 Scheduling Request (SR) on PUCCH (Format 1) ......................................................... 81

2.2.11 Control Signalling and UL-SCH multiplexing on PUSCH .............................................. 82

2.2.12 Multiple-Antenna Techniques ........................................................................................ 83

2.2.13 PUSCH UE Antenna Selection Indication ..................................................................... 83

2.2.14 Multi-User Virtual MIMO or SDMA ............................................................................... 83

2.3. Uplink Reference Signal ........................................................................................................ 85

2.3.1. UL RS Signal Sequence Generation ............................................................................ 85

2.3.2. Base RS Sequences and Sequence Grouping ............................................................. 85

2.3.3. Orthogonal RS via Cyclic Time-Shifts of a Base Sequence ......................................... 86

2.3.4. Sequence-Group Hopping and Planning ...................................................................... 87

2.3.5. Cyclic Shift Hopping ...................................................................................................... 88

2.3.6. Demodulation Reference Signals (DM RS) .................................................................. 88

2.3.7. Uplink Sounding Reference Signals (SRS) ................................................................... 89

2.4. Uplink Capacity and Coverage.............................................................................................. 91

2.4.1 Uplink Capacity - Factors Affecting Uplink Capacity ..................................................... 91

2.4.2 Uplink Power Control and Interference Management ................................................... 92

2.4.3 Uplink Control Channel Overhead ................................................................................ 92

2.4.4 Modulation and Number of HARQ Transmissions ........................................................ 92

2.4.5 Delay Constraints and VoIP .......................................................................................... 92

2.4.6 Number of eNodeB Receive Antennas ......................................................................... 92

2.4.7 Minimum Size of Resource Allocation........................................................................... 93

2.4.8 LTE Uplink Capacity Evaluation .................................................................................... 93

2.4.9 LTE Uplink Coverage and Link Budget ......................................................................... 93

2.5. Random Access .................................................................................................................... 95

2.6.1 Random Access Procedure .......................................................................................... 95

2.6.2 Contention-Based Random Access Procedure ............................................................ 95

2.6.3 Physical Random Access Channel Design ................................................................... 98

2.6.4 Preamble Sequence Theory and Design .................................................................... 102


2.6.5 PRACH Implementation .............................................................................................. 108

2.6.6 Time Division Duplex (TDD) PRACH .......................................................................... 109

2.6.7 Uplink Timing Control .................................................................................................. 110

2.6.8 Timing Advance Procedure ......................................................................................... 110

2.6.9 Power Control .............................................................................................................. 111

2.6. Miscellaneous...................................................................................................................... 116

2.6.1 Evaluation LTE Physical Layer Questions .................................................................. 116


1. Downlink Physical Layer

1.1 OFDMA Principles OFDMA (Orthogonal Frequency division Multiple Access) is a multicarrier scheme. Multicarrier schemes subdivide bandwidth into parallel subchannels, ideally each non-frequency-selective (spectrally-flat gain), overlapping but orthogonal. This avoids need of guard-bands, makeing OFDM highly spectrally efficient, as subchannels can be perfectly separated at the receiver. This makes receiver less complex, attractive for high-rate mobiles. Robustness has to be built up against time-variant channels by employing channel coding. LTE downlink combines OFDM with channel coding and Hybrid Automatic Repeat reQuest (HARQ). OFDM is ideal for broadcast/DL applications for low receiver complexity. OFDM has efficient implementation by means of the FFT. It uses Cyclic Prefix to avoid ISI, enabling block-wise processing. Orthogonal subcarriers avoid spectrum wastage in intersubcarrier guard-bands. Parameters flexibility allows balance the tolerance of Doppler and delay spread.

Key OFDMA points (a) Orthogonal subcarriers with very small inter-subcarrier guard-bands. (b) It makes use of a CP to avoid ISI, enabling block-wise processing. (c) Efficient implementation by means of the FFT. (d) Achieves high transmission rates of broadband transmission, with low receiver complexity. (e) Balanced tolerance of Doppler and delay spread depending on the deployment scenario. (f) It can be extended to a multiple-access scheme, OFDMA, in a straightforward manner. (g) Suited for broadcast or downlink applications because of low receiver complexity while

requiring a high transmitter complexity (expensive PA).

First OFDM patent filed at Bell Labs in 1966, initially only as analog. In 1971, Discrete Fourier Transform (DFT) was proposed. Later in 1980, application of the Winograd Fourier Transform (WFT) or Fast Fourier Transform (FFT) was employed. OFDM then became modulation of choice for ADSL and wireless systems. OFDM tended to focus broadcast systems such as - Digital Video Broadcasting (DVB) and Digital Audio Broadcasting (DAB), and WLANs. Main thing to control in OFDM was PAPR and thats why in low power WLAN it was good. First cellular mobile based on OFDM was proposed in 1985 by IEEE to LTE downlink. Other benefits of OFDM was to operate in different bandwidth according to spectrum availability.

1.1.1 OFDM - Orthogonal Multiplexing Principle Challenge is always in having a symbol period Ts < channel delay spread Td. This generates Intersymbol Interference (ISI), needing complex equalization procedure. Equalization complexity usually is in proportion to square of (channel impulse response length). Data symbols are first serial-to-parallel converted for modulation on M parallel subcarriers.This increases symbol duration by a factor of approx M, > channel delay spread.


Fig 2.1.1.1 OFDM Signal Processing This operation makes time-varying channel impulse response substantially constant during each modulated OFDM symbol. Resulting long symbol duration is virtually unaffected by ISI compared to the short symbol duration. A Serial to Parallel (S/P) converter collects serial data symbols into a data block Sk = [Sk [0] , Sk [1] , . . . , Sk [M 1]]T of dimension M, where the subscript k is the index of an OFDM symbol (spanning the M sub-carriers). The M parallel data streams are first independently modulated resulting in the complex vector Xk = [Xk [0] , Xk [1] , . . . , Xk [M 1]]T . In principle it is possible to use different modulations (e.g. QPSK or 16QAM) on each sub-carrier, the channel gain may differ between sub-carriers, and thus some sub-carriers can carry higher data-rates than others. The vector of data symbols Xk then passes through an Inverse FFT (IFFT) resulting in a set of N complex time-domain samples xk = [xk[0], . . . , xk[N 1]]T . In a practical OFDM system, the number of processed subcarriers is greater than the number of modulated sub-carriers (i.e. N M), with the unmodulated sub-carriers being padded with zeros.

Fig 2.1.1.2 OFDMA tramsmission and reception

A guard period is created at the beginning of each OFDM symbol, to eliminate the remaining impact of ISI. A Cyclic Prefix (CP) is added at the beginning of each symbol xk. The CP is generated by duplicating the last G samples of the IFFT output and appending them at the beginning of xk. This yields the time domain OFDM symbol [xk[N G], . . . , xk[N 1], xk[0], . . . , xk[N 1]]T . CP length G should be longer than the longest channel impulse response to be supported. The CP converts the linear (i.e. aperiodic) convolution of the channel into a circular (i.e. periodic) one which is suitable for DFT processing. The IFFT output is then Parallel-to-Serial (P/S) converted for transmission through frequency-selective channel. Here is an example of OFDM LTE signal. At the receiver, the reverse operations are performed to demodulate the OFDM signal, CP are removed and ISI-free block of samples is passed to the DFT. If number of subcarriers N is designed to be a power of 2, a highly efficient FFT implementation may be used to transform the signal back to the frequency domain. Among the N parallel streams output from the FFT, the modulated subset of M subcarriers are selected and further processed by the receiver. Let x(t) be the signal symbol transmitted at time instant t . The received signal in a multipath environment is then given by r(t) = x(t) * h(t) + z(t), where h(t) is the continuous-time impulse response of the channel, represents the convolution operation and z(t) is the additive noise. Assuming that x(t) is band-limited to [1/2Ts ,1/2Ts], the continuous-time signal x(t) can be sampled at sampling rate Ts such that the Nyquist criterion is satisfied. Due to multipath, several replicas of the transmitted signals arrive at the receiver at different delays. The received discrete-time OFDM symbol k including CP, under the assumption that the channel impulse response has a length smaller than or equal to G, Receiver has to process equalization to recover xk[n] signals. CP of OFDM changes the linear convolution into a circular one. The circular convolution is very efficiently transformed by an FFT into a multiplicative operation in frequency domain. Hence, the transmitted signal over a frequency-selective (multipath) channel is converted into a transmission over N parallel flat-fading channels in


the frequency domain: Rk[m] = Xk[m] H[m] + Zk[m]. As a result the equalization is much simpler than for single-carrier systems and consists of just one complex multiplication per subcarrier.

1.1.2 Peak-to-Average Power Ratio and Sensitivity Major drawbacks of OFDM is that it has a high Peak-to-Average Power Ratio (PAPR). The amplitude variations of OFDM signal can be very high, however PAs of RF transmitters are linear only within a limited dynamic range. Hence, OFDM signal is likely to suffer from non-linear distortion caused by clipping, giving out-of-band spurious emissions and in-band corruption of the signal. To avoid such distortion, PAs should have large power back-offs, leading to inefficient amplification. Let x[n] be the signal after IFFT. PAPR of an OFDM symbol is defined as the square of the peak

amplitude divided by the mean power, i.e. PAPR = Max,n{|x[n]|2} / E{|x[n]|

2}

It is observed that a high PAPR does not occur very often. However, when it does occur, degradation due to PA non-linearities may be expected. PAPR Reduction Techniques Many techniques are studied for reducing the PAPR, but not specified for downlink. An overview of possibilities is provided below.

1. Clipping and filtering . Signal may be clipped, but it causes spectral leakage into adjacent channels, resulting in reduced spectral efficiency, in-band noise, degrading BER. To avoid this problem, oversample the original signal by padding with zeros and processing it using a longer IFFT. Oversampled signal is clipped and then filtered to reduce the out-of-band radiation. This may be is used in LTE.

2. Selected mapping. Whichever phase vector gives Least PAPR, that is used. To recover phase information, separate control signalling is used to tell which phase vector was used. It is not used.

3. Coding techniques. Use code words with lowest PAPR. Complementary codes have good properties to combine both PAPR and forward error correction. It is not used.

Sensitivity to Carrier Frequency Offset and Time-Varying Channels OFDM orthogonality relies that transmitter and receiver operate with exactly same frequency reference, else perfect orthogonality of subcarriers is lost, causing subcarrier leakage (Inter-Carrier Interference (ICI). UE local oscillator frequency drifts are usually greater than in the eNodeB and are typically due to temperature and voltage variation and phase noise. This difference between the reference frequencies is referred as Carrier Frequency Offset (CFO). The CFO can be larger than subcarrier spacing - divided into integer part and fractional part. Frequency error fo = (T+e)df. Where, df is subcarrier spacing,, T is an integer and 0.5


where d is the timing offset in samples corresponding to a duration equal to To.

This phase shift can be recovered as part of the channel estimation operation, with cyclic prefix but not zero-padding.In the general case of a channel with delay spread, for a given CP length the maximum tolerated timing offset without degrading the OFDM reception is reduced by an amount equal to the length of the channel impulse response: To TCP Td. For greater timing errors, ISI and ICI occur. Timing synchronization becomes more critical in long-delay spread channels. Initial timing is achieved by the cell-search and synchronization procedures. Thereafter, for continuous tracking of timing-offset, either CP correlation or Reference Signals (RSs) is used. If an OFDM system, CP is sufficiently designed of lengthG samples such that Channel impulse Response L


further overhead, sub-carrier spacing is kept at 7.5kHz and an extended CP of approx 33 s is used.

Fig 2.1.5.1 -FDD Frame Structure

Fig 2.1.5.2 TDD Frame Structure

Note that, with normal CP, the CP for the first symbol in each 0.5 ms slot is slightly longer than the next six symbols, to accommodate an integer (7) number of symbols in each slot, with assumed FFT block-lengths of 2048. For 20 MHz, FFT order of 2048 is assumed for efficient implementation. However, in practice the implementer is free to use other Discrete Fourier Transform sizes. These parameterizations are designed to be compatible with a sampling frequency of 30.72 MHz, which is 8*3.84Mhz(UMTS sampling rate), for backward compatibility. Thus, the basic unit of time in LTE, is defined as Ts = 1/30.72 s. Lower sampling frequencies (and proportionally lower FFT orders) are always possible to reduce RF and baseband processing complexity for narrower BW: Example, for 5 MHz, FFT order and sampling frequency could be 512 and fs = 7.68 MHz respectively, while only 300 subcarriers are actually modulated with data. For simple implementation, direct current (d.c.) subcarrier is left unused, to avoid d.c. offset errors.

1.1.6 Transmission Resource Structure LTE downlink, consist of user-plane and control-plane data from higher protocol stack layers multiplexed with physical layer signalling. A DL resources possess dimensions of time(slot),


frequency(multiple of 180khz) and space(layer). Layer is defined by multiple antenna transmission and reception. The largest unit of time is the 10 ms radio frame, further subdivided into ten 1 ms subframes, each of which is split into two 0.5 ms slots. Each slot has seven OFDM symbols in normal CP (six if extended CP). In frequency domain, resources are grouped in units of 12 subcarriers (15*12kHz=180 kHz), such that one unit of 12 subcarriers for a duration of one slot is termed a Resource Block (RB).

Fig 2.1.6.1 FDD Downlink Frame sample

Smallest unit of resource is the Resource Element (RE) - one subcarrier for a duration of one OFDM symbol. A RB comprised of 84 REs in normal CP (72 RE in extended CP). Within certain RBs, some REs are reserved for synchronization signals (PSS/SSS), reference signals (RS), control signalling and critical broadcast system information (CFICH,PHICH,PDCCH). Remaining REs are used for data transmission(PDSCH), and are usually allocated in pairs (in time domain) of RBs.

Fig 2.1.6.2 TDD Downlink Frame sample

Two types of frame structure are defined:

1. Frame Structure Type 1(Frequency Division Duplex,FDD) assumes all subframes are available for DL, in paired radio spectrum, or standalone downlink carrier.

2. Frame Structure Type 2(Time Division Duplexing , TDD) in unpaired spectrum, basic structure of RBs and REs remains same, but only a subset of subframes are available for


downlink; remaining subframes are used for uplink or for special subframes which allow switching between DL & UL. In the centre of the special subframes a guard period is provided which allows UL timing to be advanced.

Signal Structure Physical layer translate data into reliable signal for transmission between eNodeB and UE. Each block of data is first protected against transmission errors, first with a Cyclic Redundancy Check (CRC), and then with coding; The initial scrambling stage is applied to all DL channels and helps interference rejection. Scrambling sequence uses order- 31 Gold code, which are not cyclic shifts of each other. Scrambling sequence generator is re-initialized every subframe (except PBCH), based on cell-id, subframe number (within a radio frame), UE identity and codeword id. Scrambling sequence generator is similar to pseudo-random sequence used for Reference Signals, only difference is the method of initialization. A fast-forward of 1600 places is applied at initialization to ensure low cross-correlation between sequences used in adjacent cells. Following scrambling, data bits from each channel are mapped to modulation symbols depending on modulation scheme, then mapped to layers, precoded, mapped to RE, and finally translated into a complex-valued OFDM signal by IFFT. To communicate with eNodeB cells, UE must first identify the DL from one of these cells and synchronize with it. This is achieved by means of special synchronization signals embedded into the OFDM structure by cell search and synchronization. Then UE estimates DL radio channel to perform demodulation of received DL signal, based on pilot signals (reference signals) inserted into DL signal. The channel designs are explained next.


1.2 Synchronization and Cell Search Cell Search executes synchronization for time and frequency parameters, necessary to demodulate DL and to transmit UL with correct timing and acquires some critical system parameters. Three major synchronization requirements:

1. symbol timing determines correct symbol start position and sets the FFT window position; 2. carrier frequency synchronization to reduce frequency errors by oscillator & Doppler shift; 3. sampling clock synchronization.

1.2.1. Synchronization Sequences and Cell Search in LTE Two relevant cell search procedures exist in LTE:

1. Initial synchronization,

UE detects a cell and decodes all information required to register. This is required, for example, when UE is switched on, or it has lost connection to the serving cell.

2. New cell identification,

When UE is already connected and is detecting a new neighbour cell. UE reports new cell measurements to Serving cell for handover. Procedure is repeated periodically until either Scell quality becomes satisfactory again, or UE moves to another cell.

Fig 2.2.1.1 FDD and TDD Synchronization Signalling

Synchronization procedure detects specially designed Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS). This enables time and frequency synchronization, provides the UE with physical cell identity (PCI) and CP, and informs UE whether cell uses FDD or TDD. Here is figure explaining relative location of PSS and SSS in frame structure of FDD and TDD respectively.

Fig 2.2.1.2 Cell Synchronization Process

In initial synchronization, UE proceeds to decode PBCH for critical system information (SI). For new cell identification, UE does not need to decode PBCH; it makes quality-level measurements (RSRP/RSRQ) and reports to the serving cell.


The sync signals are transmitted periodically, twice per 10 ms radio frame.

In FDD cell, PSS is always located in the last symbol of the 0th and 10

th slots of each frame,

thus enabling UE to acquire the slot boundary timing independently of the CP. SSS is located in the symbol immediately preceding PSS, for coherent detection of SSS relative to PSS.

In TDD cell, PSS is located in 2nd

symbol of the 2nd

and 12th slots, while the SSS is located 3

symbols earlier and falls in previous slot. Position of SSS changes depending on CP length of the cell. At this stage, CP length is unknown and SSS is blindly detected by checking for SSS at expected positions.

PSS in a given cell is same in every subframe, SSS may change & thus UE knows the position of the 10 ms radio frame boundary. PSS and SSS are transmitted in the central six Resource Blocks (RBs), irrespective of the system BW (6 to 110 RBs), without knowing BW. The PSS and SSS are each comprised of a sequence of length 62 symbols, mapped to the central 62 subcarriers around d.c. subcarrier which is left unused. Five REs at each extremity of each sync sequence are not used. Thus a UE can detect the PSS and SSS with size-64 FFT and a lower sampling rate if all 72 subcarriers were used. In case of MIMO at eNodeB, PSS and SSS are always transmitted from same antenna port in a subframe, while between different subframes they may be transmitted from different antenna ports for diversity. PSS and SSS sequence indicate one of 504 unique PCI, grouped into 168 groups of three identities. The three identities in a group are assigned to cells under same eNodeB. Three PSS sequences are used to indicate the cell identity within the group, and 168 SSS sequences are used to indicate the identity of the group. PSS uses ZadoffChu sequences

1.2.2. ZadoffChu Sequences ZadoffChu (ZC) sequences (Generalized Chirp-Like (GCL) sequences) are non-binary unit-amplitude sequences, which satisfy a Constant Amplitude Zero Autocorrelation (CAZAC) property. The ZC sequence of odd-length NZC is given by

aq(n) = exp [j2q (n(n + 1)/2 + ln)/ NZC ]

where q {1, . . . , NZC 1} is the ZC sequence root index, n = 0, 1, . . . , NZC 1, l N is any integer (In LTE l = 0). ZC sequences have the following important properties. Property 1. A ZC sequence has constant amplitude which limits PAPR and generates bounded and time-flat interference to other users, and its NZC-point DFT also has constant amplitude. Property 2. ZC sequences of any length have ideal cyclic autocorrelation (correlation with circularly shifted version of itself is a delta function). ZC periodic autocorrelation is exactly zero for 0 and it is non-zero for = 0, whereas PN periodic autocorrelation shows significant peaks, some above 0.1, at non-zero lags. CAZAC sequence allows multiple orthogonal sequences to be generated from the same ZC sequence. Indeed, if the periodic autocorrelation of a ZC sequence provides a single peak at the zero lag, the periodic correlation of the same sequence against its cyclic shifted replica provides a peak at lag NCS, where NCS is the number of samples of the cyclic shift. This creates a Zero-Correlation Zone (ZCZ) between the two sequences. As a result, as long as the ZCZ is dimensioned to cope with the largest possible expected time misalignment between them, the two sequences are orthogonal for all transmissions within this time misalignment. Property 3. The absolute value of the cyclic cross-correlation function between any two ZC sequences is constant and equal to 1/NZC, if |q1 q2| (where q1 and q2 are the sequence indices) is relatively prime with respect to NZC . Selecting NZC as a prime number results in NZC 1 Zaddoff-Chu sequences which have the optimal cyclic cross-correlation between any pair. Cyclic extension or truncation preserves both the constant amplitude property and the zero cyclic autocorrelation property for different cyclic shifts. The DFT of a ZC sequence xu(n) is a weighted cyclicly-shifted ZC sequence Xw(k) such that w = 1/u mod NZC. This means that a ZC sequence can be generated directly in the frequency domain without the need for a DFT operation.


1.2.3. Primary Synchronization Signal (PSS) Sequences There are 0 to 503 (total 504) PCI are available to be assigned. Every cell will have a PCI where

PCI=3* NID,1 + NID,2.

The NID,1 is the PCI group ID - , NID,1 can have values 0 to 167 and

NID,2 is the PCI local (may be sector) ID - NID,2 can have values 0 to 2.

How does the PCI optimization affect my Network? Well, this is the main parameter, by which the

PSS, SSS and reference signals will be generated. Even your scrambling code with which every DL

and UL signal will be scrambled, will depend on this. So, every generated signals uniqueness

depends on this parameter. Lets understand how some of the signals are generated based on PCI.

If PSS, SSS, RS and other generated signals are not unique, then my every operation will be affected

and it may reflect as latency in Synchronization detection, Interference and lower SINR values for

signals, which will end up in low CQI.

1.2.4. PSS Generation The PSS represented by d(u,n) is generated from a frequency-domain Zadoff-Chu sequence

according to

61,...,32,31

30,...,1,0)(

63

)2)(1(

63

)1(

ne

nend

nnuj

nunj

u

where the Zadoff-Chu root sequence index u is given by following table.

(2)IDN

Root index u

0 25

1 29

2 34

The mapping of PSS to resource elements depends on the frame structure, FDD or TDD. The

sequence d(u,n) is mapped to the resource elements according to

231

61,...,0 ,

RBsc

DLRB

,

NNnk

nnda lk


For FDD, the PSS is mapped to the last OFDM symbol in slots 0 and 10. For TDD, the PSS is apped

to the third OFDM symbol in subframes 1 and 6. Resource elements (k,l) in the OFDM symbols used

for transmission of the primary synchronization signal where

66,...63,62,1,...,4,5

231

RBsc

DLRB

n

NNnk

are reserved and not used for transmission of the primary synchronization signal. N=0,1,61 are used with above formulae.

PSS is constructed from a freq-domain ZC sequence of length 63, with middle element punctured to avoid transmitting on d.c. subcarrier. This set of roots for ZC sequences was chosen for its good periodic autocorrelation and cross-correlation properties. These sequences have a low-frequency offset sensitivity (maximum undesired autocorrelation peak /desired correlation peak) at a certain frequency offset, giving best robustness. Also the ZC sequences are robust against frequency drifts. Thus, PSS can be easily detected during the initial synchronization with a frequency offset up to 7.5 kHz. The selected root combination satisfies time-domain root-symmetry, sequences 29 and 34 are complex conjugates of each other and can be detected with a single correlator. UE must detect PSS without any prior knowledge of the channel, so noncoherent correlation is required for PSS timing detection.

1.2.5. Secondary Synchronization Signal (SSS) Sequences SSS maximum length M-sequences, can be created by cycling through every possible state of a shift register of length n, resulting in M-sequence of length 2n 1. Two length-31 BPSK secondary sync codes (SSC1(even (di)) and SSC2(odd d(i)) are interleaved to construct SSS sequence in frequency-domain. Two codes are two different cyclic shifts of a single length-31 M-sequence. Cyclic shift indices are derived from a function of PCI group. Two codes are alternated between the first and second SSS in each radio frame.

5 subframein )(

0 subframein )()12(

5 subframein )(

0 subframein )()2(

)(11

)(0

)(11

)(1

0)(

1

0)(

0

10

01

1

0

nzncns

nzncnsnd

ncns

ncnsnd

mm

mm

m

m

with

30,30

2)1(,2)1(

31mod131

31mod

(1)ID

(1)ID(1)

ID

01

0

NqqqN

qqqNm

mmm

mm

Thus UE determines the 10 ms radio frame timing from a single observation of a SSS. SSC2 is scrambled by a sequence that depends on the index of SSC1. Sequence is then scrambled by a code that depends on the PSS. Scrambling code is mapped to the PCI within the group corresponding to the target eNodeB. The resource mapping is done as per the following:


2 typestructure framefor 11 and 1 slotsin 1

1 typestructure framefor 10 and 0 slotsin 2

231

61,...,0 ,

DLsymb

DLsymb

RBsc

DLRB

,

N

Nl

NNnk

nnda lk

SSS sequences are spectrally flat. PSS, the SSS can be detected with a frequency offset up to 7.5 kHz. Channel is known based on the PSS sequence first and then SSS detection is done. However, in the case of synchronized neighbouring eNodeBs, coherent detector performance can be degraded. If an interfering eNodeB employs the same PSS, phase difference between them can have adverse impact on estimation of the channel coefficients. If BW of the channel is less than the six RB for SSS, impact may be bad, hence minimum 6 RB legth is chisen. M-sequence and WalshHadamard matrices are similar and index remapping is done. This reduces complexity of SSS

detector, as complexity = N log2 N with N=32, complexity= 32 log2 32 = 160.

1.2.6. Cell Search Performance A new cell detection delay for UE and report it to S-eNB should be less than acceptable threshold. eNodeB uses the reports to prepare intra- or inter-frequency handover. A multicell environment with three cells with different transmitted powers with synchronized and unsynchronized eNodeBs should be analysed. For the propagation channel, various multipath fading, at least two receive antenna, UE speeds, (5 km/h, 300 km/h) should be considered. Cell search performance is measured as 90-percentile(maximum time required to detect a target cell 90%of the time) identification delay. After detection of PSS-SSS, RSRP is measured. For initial synchronization case the time taken to decode PBCH is adapted, and not just of reporting of measurements on RS.For inter-frequency handover, performance can be derived from the intra-frequency performance timing.

Coherent Versus Non-Coherent Detection A coherent detector uses knowledge of the channel, while a non-coherent detector uses an optimization metric of average channel statistics. In PSS, non-coherent (No channel estimation available) detection is used, while for SSS, coherent (channel estimation) or non-coherent techniques can be used.

1.2.7. Reference Signals and Channel Estimation In any communication system signal x transmitted by A passes through a radio channel H (exhibit multipath fading, causing ISI) and suffers additive noise before being received by B. To remove ISI, equalization, detection algorithms and knowledge of Channel Impulse Response (CIR) is used. OFDMA is quite robust against ISI by CP which allows very good equalization at receiver. Coherent detection uses amplitude and phase information exchanged between eNodeBs and UEs. This comes at a price of overhead of channel estimation by exploiting known signals which do not carry any data, sacrificing spectral efficiency. Known reference signals are inserted into the transmitted signal structure. Reference signals(known) are multiplexed with data symbols (unknown at receiver) in either frequency, time or code domains. Time multiplexing, known preamble-based training transmission also is another technique. Orthogonal RS multiplexing is the most common technique. OFDM transmission is a two-dimensional lattice in time and frequency, which helps multiplexing of RSs mapped to specific REs according to a specific pattern. Since RS are sent only on particular OFDM REs (particular symbols and subcarriers), channel estimates for non-RS REs have to be computed via interpolation.

1.2.8. Design of Reference Signals in LTE In DL, three different types of RS are provided:


1. Cell-specific RSs ( common RSs). 2. UE-specific RSs, in the data for specific UEs. 3. MBSFN-specific RSs, for MBSFN.

1.2.9. Cell-Specific Reference Signals (CRS) In an OFDM-based system, an equidistant arrangement of RS in the lattice structure diamond shape achieves the Minimum Mean Squared Error (MMSE) estimate. In time domain, RS spacing in is governed by maximum Doppler spread (highest speed-540km/h(150m/s)) to be supported. Doppler shift is fd = (fc v/c) where fc is the carrier frequency, v is UE speed in m/s, and c= 3 * 10

8 m/s. Considering fc = 2 GHz (2*10

9Hz) and v = 500 km/h, fd

=(2*109*150/3*10

8) =1000 Hz. According to Nyquists sampling theorem, minimum sampling

frequency to reconstruct the channel is Tc = 1/(2fd)= 0.5 ms. This implies that two RS/slot are needed to estimate channel correctly.

Fig 2.2.8.1 Cell RS for 1, 2 and 4 antenna

In frequency domain, there is one RS every six subcarriers on each symbols including RS symbol, but staggered so that within each RB there is one RS every 3 subcarriers. This spacing is goverened by expected coherence BW of channel, governed by channel delay spread. The 90% and 50% coherence BW are given respectively by Bc,90% = 1/50d=20kHz and Bc,50% = 1/5d=200kHz where d is the r.m.s delay spread=1000ns. In LTE spacing between two RS in frequency, is 45 kHz (3 symbols), enough to resolve expected frequency domain variations of the channel. RS patterns are designed to work with MIMO antennas defined for multiple antenna ports at eNodeB. An antenna port may be either a single physical antenna, or a combination of multiple physical antenna elements. The transmitted RS in a given antenna port defines the antenna port from the point of view of the UE, and enables UE to derive channel estimate for that antenna port.


Fig 2.2.8.2 Antenna ports and Physical antennas

Up to eight cell-specific antenna ports may be used by eNodeB, requiring UE to derive up to eight separate channel estimates. For each antenna port, a different RS pattern is designed, to minimize intra-cell interference between multiple transmit antenna ports. Rp is used for RS Tx on antenna port p. Also, when a RE is used for RS on one antenna port, corresponding RE on other antenna ports is set to zero to limit interference. Mark that, density of RS for third and fourth antenna ports is half of the first two, to reduce overhead. In cells with a high prevalence of high-speed users, use of four antenna ports is unlikely, RSs with lower density can provide sufficient channel estimation accuracy.

Fig 2.2.8.3 Antenna port example of port 0 and port 5

All the RSs (cell-specific, UE-specific or MBSFN specific) are QPSK modulated to ensure low PAPR. The signal can be written as r(l,ns,m) = 1/2[1-2c(2m)] + j1/2[1-2c(2m+1)] where m is RS index, ns = slot number and l =symbol number within slot, c(i) is length-31 Gold sequence, with different initialization values depending on type of RSs. RS sequence carries unambiguously one of the 504 different cell identities, Ncell ID. For the cell-specific RSs, a cell-specific frequency shift (Ncell ID mod 6) is also applied. This shift avoids collisions between common RS from up to six adjacent cells. Transmission power of RS is boosted, up to max 6 dB relative to surrounding data symbols, designed to improve channel estimation. If adjacent cells also transmit high-power RS on same REs, interference will prevent the gain.

1.2.10. UE-Specific Reference Signals(URS) UE-specific RS may be used in addition to CRSs, embedded only in a specific UEs scheduled RBs, using a distinct antenna port. UE is expected to use them to derive the channel estimate for demodulating data in PDSCH RBs.


Fig 2.2.9 Port 5, UE specific Reference Signals

A typical usage of URSs is beam-forming of data transmissions to specific UEs. Rather than using physical antennas of other CRS antenna ports, eNodeB may use a correlated array of antenna elements to generate a narrow beam in the direction of a particular UE. Beam will experience different channel response requiring URSs to demodulate the beamformed data coherently. The URS structure is chosen not to collide with CRSs, and hence URSs does not affect CRSs. URSs have a similar pattern as CRSs allowing re-use similar channel estimation algorithms. Density is half of CRS, minimizing the overhead.

1.2.11. RS-Aided Channel Modelling and Estimation Channel estimation problem is related to physical propagation, number of transmit and receive antennas, BW, frequency, cell configuration and relative speed.

1. frequencies and BW determine the scattering. 2. Cell deployment governs multipath, delay spread and spatial correlation. 3. Relative speed sets time-variations.

Propagation conditions characterize the channel in three dimension (frequency, time and spatial) domains. Each MIMO multipath channel component can experience different scattering conditions across the three domains. LTE specifications do not mandate any specific channel estimation technique, and there is therefore complete freedom in implementation provided that the performance requirements are met and the complexity is affordable.

1.2.12. Frequency Domain Channel Estimation The natural approach to estimate the whole CTF is to interpolate its estimate between the reference symbol positions. As a second straightforward approach, the CTF estimate over all subcarriers can be obtained by IFFT interpolation. More elaborate linear estimators derived from both deterministic and statistical viewpoints are proposed -Least Squares (LS), Regularized LS, Minimum Mean-Squared Error (MMSE) and Mismatched MMSE. It is seen that IFFT and linear interpolation methods yield lowest performance. The regularized LS and the mismatched MMSE perform exactly equally. Optimal MMSE estimator outperforms any other estimator. MMSE-based channel estimation suffers the least band-edge degradation, while all the other methods presented are highly adversely affected.

1.2.13. Time-Domain Channel Estimation Time-Domain (TD) channel estimation requires sufficient memory for buffering soft values of data over several symbols while the channel estimation is carried out. However, correlation between consecutive symbols decreases as UE speed increases. TD correlation is inversely proportional to the UE speed sets a limit on the possibilities for TD filtering in high-mobility conditions.

Finite and Infinite Length MMSE-(TD-MMSE) The statistical TD filter which is optimal in terms of Mean Squared Error (MSE) can be approximated in the form of a finite impulse response filter. It can be observed that, unlike Frequency-Domain (FD) MMSE filtering, the size of the matrix to be inverted for a finite-length TD-MMSE estimator is independent of the channel length L but dependent on the chosen FIR order M. Similarly to the FD counterpart, the TD-MMSE estimator requires knowledge of the PDP, the UE speed and the noise variance.


Normalized Least-Mean-Square(NLMS) An adaptive estimation approach can be considered which does not require knowledge of second-order statistics of both channel and noise. A feasible solution is the Normalized Least-Mean-Square (NLMS) estimator. It can be observed that the TD-NLMS estimator requires much lower complexity compared to TD-MMSE as no matrix inversion is required, as well as not requiring any a priori statistical knowledge. Other adaptative approaches could also be considered such as Recursive Least Squares (RLS) and Kalman-based filtering.

1.2.14. Spatial Domain Channel Estimation(SD-MMSE) LTE UE is designed for MIMO. Consequently, whenever the channel is correlated in the spatial domain, the correlation can be exploited to provide a further means for enhancing the channel estimate. If it is desired to exploit spatial correlation, a natural approach is again offered by Spatial Domain (SD) MMSE filtering.


1.3 Phy Data and Control Channels - DL

1.3.1. Physical Broadcast Channel (PBCH) Basic system information (SI) carries configuration info via Broadcast Channel (BCH). The SI is divided into two categories:

1. Master Information Block (MIB), carries limited most frequently transmitted parameters essential for initial access to the cell, is carried on Physical Broadcast Channel (PBCH).

2. System Information Blocks (SIBs) multiplexed together with Physical Downlink Shared Channel(PDSCH).

PBCH requires to be: 1. Detectable without prior knowledge of system bandwidth; 2. Low system overhead; 3. Reliable reception right to the edge of the LTE cells; 4. Decodable with low latency and low impact on UE battery life.

Fig 2.3.1 PBCH transmission

Detectability without prior knowledge of BW is achieved by mapping PBCH only on central 72 subcarriers of OFDM signal, regardless of actual BW. UE will have first identified the centre-frequency synchronization signals. Low system overhead is achieved by deliberately keeping information on PBCH to a minimum (MIB is just 14 bits), and, since it is repeated every 40 ms, it is just 0.35 kbps.


Reliability of PBCH is achieved with time diversity, FEC coding and antenna diversity. Time diversity is exploited by spreading transmission of each MIB on PBCH over 40 ms ensuring reception despite loosing one transmission. The FEC coding for the PBCH uses a convolutional coder, as bits to be coded is small. Basic code rate is 1/3, after which a high degree of repetition of systematic bits and parity bits is used, such that each MIB is coded at a very low code-rate (1/48 over a 40 ms period) to give strong error protection. PBCH uses dual-antenna receive diversity enabling wider cell coverage with fewer cell sites. Transmit antenna diversity may be also employed at eNodeB to further improve coverage. The exact REs used by PBCH is independent of transmit antenna ports; REs used for RS are avoided by PBCH. Number of transmit antenna ports used by eNodeB must be determined blindly by the UE. Discovery of number of transmit antenna ports is helped by CRC on each MIB which is masked with a codeword representing the number of transmit antenna ports. Low latency and a low impact on UE battery life is also facilitated by low code rate with repetition. Full set of coded bits are divided into four subsets, each is self-decodable, which are sent in one of four different frames during the 40 ms. UE may decode the MIB correctly from the transmission in less than four radio frames, then UE does not need to receive other parts of PBCH in the remainder of 40 ms. On the other hand, if SIR is low, UE can receive further parts of MIB, soft-combining each part, until successful decoding is achieved.Timing of 40 ms interval is not indicated explicitly to UE; it is determined by scrambling and bit positions. UE can initially do four separate decodings of the PBCH and checking the CRC for each decoding to determine 40ms boundary. A simple approach is to perform decoding using soft combination of the PBCH over four radio frames, advancing 40 ms sliding window one radio frame at a time until the window aligns with 40 ms period of the PBCH and the decoding succeeds.

1.3.2. Physical Downlink Shared Channel (PDSCH) PDSCH is the main DL data channel. It is used for all user data, broadcast SI apart from PBCH, and paging messages. Data is transmitted on PDSCH in units of transport blocks(TB), each of which is a MAC-PDU. TB is passed from MAC to physical layer once per Transmission Time Interval (TTI), where a TTI is 1 ms, a subframe duration. General Use of the PDSCH one or, at most, two TB can be transmitted per UE per subframe, depending on transmission mode selected for the PDSCH for each UE:

Transmission Mode 1: Single eNodeB antenna port;

Transmission Mode 2: Transmit diversity;

Transmission Mode 3: Open-loop spatial multiplexing;

Transmission Mode 4: Closed-loop spatial multiplexing;

Transmission Mode 5: Multi-user Multiple-Input Multiple-Output (MU-MIMO);

Transmission Mode 6: Closed-loop rank-1 precoding;

Transmission Mode 7: Transmission using UE-Specific RS.

Transmission Mode 8: Transmission spatial multiplexing 2 layers

Transmission Mode 9: Transmission using 8 antenna, upto 8 layers

Transmission Mode 10: Transmission using 8 antenna, upto 8 layers Except Tx-mode 7, RS for demodulating PDSCH is given by CRS. Number of eNodeB antenna ports for PDSCH is same as PBCH. In Tx-mode 7, UE-specific RSs provide phase reference for the PDSCH. Tx-mode also affects DL control signalling, and CQI from UE. After coding and mapping to spatial layers, coded data bits are mapped to modulation symbols depending on radio channel conditions and data rate required. Modulation order may be between two bits per symbol (QPSK) and six bits per symbol (64QAM). The RE for PDSCH can be any which are not reserved for other purposes (i.e. RS, PSS, SSS, PBCH and control signalling(PCFICH,PHICH,PDCCH)). When UE is given a pair of RB of PDSCH, in a subframe, only the available RE within RB can carry PDSCH data. Allocation of RB to PDSCH for a


UE is signalled by dynamic control signalling at the start of the relevant subframe using Physical Downlink Control Channel (PDCCH). The mapping of data to RB is carried out in one of two ways:

1. localized mapping and 2. distributed mapping.

Localized resource mapping allocates all REs in a pair of RB to same UE. Distributed resource mapping separates two PRB in frequency giving frequency diversity for small amounts of data. Up to two pairs of RB may be transmitted to a UE in this way. Example of distributed Mapping: In Voice-over-IP (VoIP) service, certain frequency resources may be persistently-scheduled, on a periodic basis to a specific UE by RRC signalling rather than PDCCH. As data per UE for VoIP is small (one or two pairs of RB), degree of frequency diversity obtainable via localized scheduling is very limited. When dynamic channel-dependent PDCCH scheduling is not done, frequency diversity is achieved through distributed mapping. A frequency-hop occurs at slot boundary in the middle of subframe, block of UE data transmitted on one RB in first half of subframe and on a different RB in the second half. The potential number of VoIP users which can be accommodated in a cell increases by distributed mapping, compared to localised. Special Uses of the PDSCH Apart from normal user data transmission, PDSCH is used for Dynamic BCH, all SIBs, not carried on PBCH. The RBs for SIBs are indicated by PDCCH, same way as for other PDSCH data without any specific UE identity, but is, rather by fixed SI-RNTI(FFFF), known to all UEs.. Another special use is for Paging, as no separate physical channel provided. Normal PDCCH signalling is used to carry equivalent of a WCDMA paging indicator, with detailed paging information carried on PDSCH in a RB indicated by PDCCH, using single fixed identifier, P-RNTI (FFFE). Different UEs monitor different subframes for their paging messages with their Paging Frame and Paging Occasion calculations.

1.3.3. Physical Multicast Channel (PMCH) All UEs must be aware of possible existence of MBMS at physical layer. Basic structure of PMCH is very similar to PDSCH. PMCH is designed for single-frequency network operation, whereby multiple cells transmit the same modulated symbols with very tight time-synchronization, so that signals from different cells are received within CP, called MBSFN (MBMS Single Frequency Network) operation. The key differences between PDSCH & PMCH are as follows:

a. PDCCH and PHICH cannot occupy more than two OFDM symbols in MBSFN subframe. PDCCH is used only for UL resource grants and not for the PMCH, as scheduling of MBSFN data is carried out by higher-layer signalling.

b. RS symbols embedded in PMCH is different from PDSCH. c. The extended CP is always used. If non-MBSFN subframes use the normal CP,

normal CP is also used in OFDM symbols used for control signalling at the start of each MBSFN subframe.

Some spare time samples usage is unspecified between the end of the last control signalling symbol and the first PMCH symbol, PMCH remaining aligned with the end of the subframe; eNodeB may transmit an undefined signal or alternatively switch off its transmitter UE cannot assume anything about transmitted signal during these samples. A UE measuring a neighbouring cell does not need to know the allocation of MBSFN subframes, since UE knows that the first two OFDM symbols in all subframes use the same CP and RS pattern. The MBSFN subframes patterns in a cell is indicated in SI, which indicates if pattern of MBSFN subframes in neighbouring cells is same or different from current cell. If different pattern, then UE can only ascertain the pattern by reading SI of that cell.

1.3.4. Downlink Control Channels Requirements for Control Channel Design Control channels convey physical layer signals or messages which cannot be carried efficiently or quickly by higher layers. The UL resources on PUSCH is determined dynamically by UL scheduling in eNodeB, and therefore signalling must be provided to indicate to UEs which resources are granted permission to use, together with modulation and code rate. To facilitate efficient operation of HARQ


and ensure appropriate power levels, further physical layer signals are needed to convey ACK/NACK by eNodeB, and power control commands. Flexibility, Overhead and Complexity LTE allows operation in BW from six RB (1.08MHz) to 110 RB (19.8MHz). It is also designed to support very few users with high data rates, or very many users with low data rates. Both UL grants and DL allocations could be required for every UE in each subframe, and may be only one RB each, worst case. Control channel and HARQ is designed to minimize unnecessary overhead and power saving, scaleability and flexibility without undue decoding complexity. Coverage and Robustness If control channels reception fails, corresponding data transmission will also fail, and will impact throughput efficiency. Channel coding and frequency diversity can be used to make control channels robust. It is important to adapt transmission parameters of the control signalling for different UEs or groups of UEs, so that lower code rates and higher power levels are only applied for only relevant UEs as necessary (e.g. near cell border). System-Related Design Aspects LTE control channel is designed in a cell for a particular UE (or in some cases a group of UEs). To minimize latency, control channel transmission should be completed within one subframe, it must be possible to transmit multiple control channels within a subframe. Both common and dedicated control channel messages are supported. If the data arrives at the eNodeB on a regular basis, as VoIP, it can be controlled by persistent scheduling.

Control Channel Structure and Contents Downlink control channels (PDCCH) can be configured to occupy the first 1, 2 or 3 OFDM symbols in every subframe, over entire BW. There are two special cases: in subframes containing MBSFN, there may be 0, 1 or 2 symbols, while for narrow BW (BW< 10 RB), PDCCH may be 2, 3 or 4 to ensure sufficient coverage at cell border. This can adjust overheads of particular configuration, traffic scenario and channel conditions.

1.3.5. Physical Control Format Indicator Channel (PCFICH) PCFICH carries CFI indicating the number symbols used for control in each subframe. UE could deduced CFI blind decoding each possible number of symbols, but at a cost of significant processing load. For MBSFN carriers, there may not be any physical control channels, so PCFICH is not present.

Fig 2.3.5.1 PCFICH REG requirements

CFI values od 1,2,3 are used and 4 is reserved for future. To make it robust, each CFI is coded with 32 bit long codeword, mapped to 16 REs with QPSK modulation.


Fig 2.3.5.2 PCFICH Mapping on neighbour Cells

PCFICH is transmitted on same ports as PBCH, with transmit diversity if more than one antenna port is used. For frequency diversity, 16 REs are distributed across BW with a predefined pattern in the first symbol in each DL subframe, so that UEs can always locate. This is prerequisite to decode rest of the control signalling. A cell-specific (PCI based) frequency offset is applied to the positions of PCFICH REs. In addition, a cell-specific scrambling sequence (PCI based) is applied to the CFI codewords to uniquely be sent.

1.3.6. Physical Downlink Control Channel (PDCCH) PDCCH carries many Downlink Control Information (DCI) messages in each subframe, for resource assignments and other controls for a UE or group of UEs. Each PDCCH is transmitted using one or more Control Channel Elements (1CCE=9REG), each CCE corresponds to nine sets of four RE known as Resource Element Groups (1REG=4RE) . Four QPSK symbols are mapped to each REG. The RS RE are not included within REGs. REGs concept is used for PCFICH and PHICH as well.

Fig 2.3.6.1 REG Counting

Four PDCCH formats 0,1,2,3 are supported, based on 1,2,4 or 8 CCEs are used respectively for each

PDCCH.


Fig 2.3.6.2 Search Space, Aggregation levels & common and UE specific

CCEs are numbered and used consecutively. A PDCCH with a format consisting of n CCEs may only start with a CCE with a number equal to a multiple of n. Format is decided by eNB based on RF conditions. If a UE has good downlink RF (e.g. close to eNB), format 0 may be sufficient, but for a cell border UE, format 3 may be required for robustness with good power level of a PDCCH.

1.3.7. PDCCH Candidate Selection eNB should apply the following rules when allocating the PDCCH candidates. Let m = number of

PDCCH candidates to monitor in the given search space for all carriers and aggregation levels.

- SI-RNTI / P-RNTI / RA-RNTI, use Common Search Space. UL/DL C-RNTI/ SPS C-RNTI, and DL

Temp. C-RNTI, use UE-Specific Search Space. TPC-PUCCH-RNTI / TPC-PUSCH-RNTI and UL

Temp. C-RNTI is not considered for default CCE management.

- For SI-RNTI PDCCH candidate CCEs between 0 and (CS_Agr-1) is used and reserved in FDD and left

vacant if no SI-RNTI is scheduled. For TDD the default UL/DL configuration type 1, this PDCCH

candidate is reserved forS I-RNTI in sf 0 & 5 (and UL grant for C-RNTI/SPS-RNTI is not scheduled).

- CCEs between CS_Agr and (2*CS_Agr-1) can be used either for P-RNTI or RA-RNTI.

For FDD:

- For DL C-RNTI/SPS-RNTI/Temp C-RNTI the lowest m =m' from CCEs between 2*CS_Agr and

(Max_CCE-1) shall be used.

- For UL C-RNTI/SPS-RNTI the lowest m =m">m' from CCEs between 2*CS_Agr and (Max_CCE-1)

shall be used.

For TDD:

- For DL C-RNTI/SPS-RNTI/Temp C-RNTI the lowest m =m' which has a PDCCH available from

CCEs between 2*CS_Agr and (Max_CCE-1) shall be used.

- For UL C-RNTI/SPS-RNTI the lowest m =m">m' which from CCEs between 2*CS_Agr and

(Max_CCE-1) shall be used.

CCE resources utilized are well defined for default values of common search space aggregation level

=4, UE-specific search space aggregation L=2 resulting in 6 PDCCH candidates m=0..5. For different


bandwidth, Max_CCE =20(5 Mhz)/25(10 MHz)/37(15 MHz)/50(20 MHz) for FDD. These are in general

to be applied in MAC Transport block size.

Each TDD subframe (take sf config 1 as example) having different PHICH group number, and for

5/10/15/20 MHz bandwidth, each subframe has, therefore, different number of MAX_CCE. SF0 and

SF5 cannot be used for UL grant. SF1 and SF6 are not used for DL assignment. SF2, SF3, SF7 and

SF8 are not applicable to PDCCH CCE allocation since they are uplink subframes.

1.3.8. Formats for Downlink Control Information (DCI) The useful DCI content depends on the specific case of deployment. If no MIMO required, then MIMO parameters are not required. To minimize overhead, different message formats are designed, each with minimum payload required for a particular scenario. The number of bits required for resource assignment depends on BW. To avoid complexity, Formats 0 and 1A are designed to be always 42bits. Since for different BW different size is needed, to reduce complexity, the smaller format size is extended by adding padding bits to be the same size as the larger. As an example, DCI message formats is listed below. DCI Bits for bandwidth of 50 RBs and format Purpose four antennas at eNodeB) --------- -------------------------------------------------------- -------------------------------------------- 0 PUSCH grants 42 1 PDSCH assignments with a single codeword 47 1A PDSCH assignments using a compact format 42 1B PDSCH assignments for rank-1 transmission 46 1C PDSCH assignments using a very compact format 26 1D PDSCH assignments for multi-user MIMO 46 2 PDSCH assignments for closed-loop MIMO operation 62 2A PDSCH assignments for open-loop MIMO operation 58 3 Transmit Power Control (TPC) commands for multiple users 42

for PUCCH and PUSCH with 2-bit power adjustments 3A Transmit Power Control (TPC) commands for multiple users 42

for PUCCH and PUSCH with 1-bit power adjustments

Format 0. DCI Format 0 is used for resource grants for the PUSCH. DCI Formats D C I S T R Fields Size Description

DCIFormat - Format0

FreqHopping 1-bit PUSCH frequency hopping f lag

Allocation variable Resource block assignment/allocation

ModCoding 5-bits Modulation, coding scheme and redundancy version

NewData 1-bit New data indicator

TPC 2-bits PUSCH TPC command

CShiftDMRS 3-bits Cyclic shift for DM RS

CQIReq 1-bit CQI request

For TDD config 0, this f ield is the Uplink Index.

For TDD Config 1-6, this f ield is the Dow nlink Assignment Index.

Not present for FDD

Format0

TDDIndex 2-bits


Format 1. DCI Format 1 is used for resource assignments for single codeword PDSCH:

DCI Formats DCISTR Fields Size Description

Format1 DCIFormat - Format1

AllocationType 1-bit Resource allocation header: type 0, type 1

(only if downlink bandwidth is >10 PRBs)


ModCoding 5-bits Modulation and coding scheme

HARQNo 3-bits (FDD) HARQ process number

4-bits (TDD)


RV 2-bits Redundancy version

TPCPUCCH 2-bits PUCCH TPC command

TDDIndex 2-bits For TDD config 0, this field is not used. For TDD Config 1-6, this field is the Downlink Assignment Index. Not present for FDD

Format 1A. DCI Format 1A is used for compact resource assignments for single codeword PDSCH, and allocating a dedicated preamble signature to a UE for contention-free random access: DCI Formats

DCISTR Fields Size Description

Format1A

DCIFormat - Format1A

AllocationType 1-bit VRB assignment flag: 0 (localized), 1 (distributed)




4-bits (TDD)




TDDIndex 2-bits For TDD config 0, this field is not used.

For TDD Config 1-6, this field is the Downlink Assignment Index.

Format 1B. DCI Format 1B is used for compact resource assignments for PDSCH using closed loop precoding with rank-1 (transmission mode 6). Information is same as in Format 1A, but with addition of precoding vector indicator applied for the PDSCH.

Format1B DCIFormat - Format1B





4-bits (TDD)





TPMI 2-bits -(2 ants) PMI information

4-bits- (4 ants)

PMI 1-bit PMI confirmation



Not present for FDD.

Format 1C. DCI Format 1C is used for very compact PDSCH assignments. With 1C format, PDSCH is uses QPSK. This is used for paging, and some SI: DCI Formats


Format1C DCIFormat - Format1C



Format 1D. DCI Format 1D is used for compact signalling of resource assignments for PDSCH using multi-user MIMO (transmission mode 5). Information is similar as in Format 1B. Instead of one of precoding vector indicators bits, there is a single bit for power offset indicators, to show if transmitted power is shared between two UEs.


Format1D DCIFormat - Format1D





4-bits (TDD)




TPMI 2-bits (2 antenna) Precoding TPMI information

4-bits (4 antenna)

DlPowerOffset 1-bit Downlink power offset



Not present for FDD.

Format 2. DCI Format 2 is used for resource assignments for PDSCH for closed-loop MIMO (transmission mode 4):

DCI Formats








4-bits (TDD)

SwapFlag 1-bit Transport block to codeword swap flag

ModCoding1 5-bits Modulation and coding scheme for transport block 1


NewData1 1-bit New data indicator for transport block 1

RV1 2-bits Redundancy version for transport block 1




PrecodingInfo 3-bits - 2ants Precoding information

6-bits - 4 ants


For TDD Config 1-6, this field is the Downlink Assignment Index. Not present for FDD.

Format 2A. DCI Format 2A is used for resource assignments for PDSCH for open-loop MIMO (transmission mode 3). Info is the same as Format 2, except that if eNodeB has two antenna ports, there is no precoding information, and for four antenna ports two bits are used to indicate the transmission rank.


Format2A DCIFormat - Format2A

Allocation Type 1-bit Resource allocation header: type 0, type 1





4-bits (TDD)

SwapFlag 1-bit Transport block to codeword swap flag







Precoding Info 0-bits Precoding information

(2 antennas)

2-bits

(4 antennas)



Format 2B


Format2B

DCIFormat - Format2B






4-bits (TDD)


ScramblingId 1-bit Scrambling identity









Formats 3 and 3A. DCI Formats 3 and 3A are used for power control for PUCCH and PUSCH with 2-bit or 1-bit power adjustments respectively.



TPCCommands variable TPC commands for PUCCH and PUSCH


Format3A

DCIFormat - Format3A

TPCCommands variable TPC commands for PUCCH and PUSCH

CRC attachment. For UE to know whether it has received a PDCCH correctly, a 16-bit CRC is appended to each PDCCH. CRC is scrambled with UE identity for this to be identified for a particular UE. In UL MIMO, antenna may be indicated using Format 0 by antenna-specific mask to the CRC. This way, no extra bit needed. PDCCH construction. The PDCCH bits are encoded. The coded and rate-matched bits are then scrambled with a cell-specific scrambling sequence to distinguish from neighbouring cells. The scrambled bits are QPSK modulated and mapped to blocks of four REs (REGs). Interleaving is applied for frequency diversity, followed by RE mapping to symbols indicated by PCFICH, excluding PCFICH and PHICH. The PDCCHs are transmitted similar to PBCH, and diversity is applied if more antenna ports are used.

1.3.9. Physical Hybrid ARQ Indicator Channel (PHICH) PHICH carries the HARQ ACK(0)/NACK(1), indicating whether eNB has correctly received on PUSCH. Bit is repeated in each of three BPSK symbols for robustness. Multiple PHICHs are mapped to the same REs (of same PHICH group). Different PHICHs within group are separated through different complex orthogonal Walsh sequences of length four for normal CP (two for extended CP). Number of PHICHs in a group can be up to twice the sequence length. A cell-specific scrambling sequence is then applied. PHICH duration (symbols) in time domain, is configurable by SI to either one(normal) or three(extended) symbols. Each of the three instances of orthogonal code of a PHICH is mapped to a REG on one of the first three symbols of each subframe, such that each PHICH is partly transmitted on each available symbols. UEs to deduce to which remaining resource elements in the control region the PDCCHs are mapped.


Fig 2.3.8.1 PHICH Coding, duration and cyclic shift allocation

PHICH index is implicitly associated with the index of the lowest uplink RB used for PUSCH. The adjacent PUSCH RBs are associated with PHICHs in different PHICH groups, for load balancing.

Fig 2.3.8.2 PHICH bits to REG mapping

However, for MU-MIMO, this is not sufficient to enable multiple UEs to be allocated the same RBs for a PUSCH. In this case, different cyclic shifts of RS are configured for different UEs for the same PUSCH resources in time-frequency, and same cyclic shift index is then used to shift PHICH so that each UE will receive its ACK or NACK on a different PHICH.

1.3.10. Resource Allocation In each subframe, PDCCHs indicate resource allocations, normally localized, (Physical Resource Block (PRB) in first slot is paired with PRB in the second slot of the subframe). Explanation here is in terms of first slot only. The most flexible/simple approach is to send each UE a bitmap, each bit indicating a particular PRB. This is good for small BW, but for large BW (110 PRBs), bitmap would need 110 bits (

Documents

LTE Physical Layer - 3G Network Solutions3gnets.in/files/documents/LTE-Protocol-Stack-Physical-layer.pdf · LTE Protocol Stack- 1 [email protected] LTE Development, Conformance