LTE Protocol Stack Physical Layer

8/21/2019 LTE Protocol Stack Physical Layer

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LTE Protocol Stack- 1 [email protected]

LTE Development, Conformance Test, Optimization

Certification CourseAmateur Level (3PCA-L1)

3PCA-L1

LTE

Physical LayerLTE Protocol Stack

Author: Surya Patar Munda


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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. Pleasedo 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 ofunderstanding 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 thenext 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. Hehas 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


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Contents1. 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 OFDMAParameter 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


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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


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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


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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


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1. Downlink Physical Layer

1.1 OFDMA PrinciplesOFDMA (Orthogonal Frequency division Multiple Access) is a multicarrier scheme. Multicarrierschemes subdivide bandwidth into parallel subchannels, ideally each non-frequency-selective(spectrally-flat gain), overlapping but orthogonal. This avoids need of guard-bands, makeing OFDMhighly spectrally efficient, as subchannels can be perfectly separated at the receiver. This makesreceiver 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 codingand Hybrid Automatic Repeat reQuest (HARQ). OFDM is ideal for broadcast/DL applications for lowreceiver complexity. OFDM has efficient implementation by means of the FFT. It uses Cyclic Prefix toavoid ISI, enabling block-wise processing. Orthogonal subcarriers avoid spectrum wastage inintersubcarrier guard-bands. Parameters flexibility allows balance the tolerance of Doppler and delayspread.

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 FourierTransform (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 DigitalAudio Broadcasting (DAB), and WLANs. Main thing to control in OFDM was PAPR and thats why inlow power WLAN it was good. First cellular mobile based on OFDM was proposed in 1985 by IEEEto LTE downlink. Other benefits of OFDM was to operate in different bandwidth according to spectrumavailability.

1.1.1 OFDM - Orthogonal Multiplexing PrincipleChallenge is always in having a symbol period Ts < channel delay spread Td. This generatesIntersymbol Interference (ISI), needing complex equalization procedure. Equalization complexityusually 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 afactor of approx M, > channel delay spread.


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Fig 2.1.1.1OFDM Signal ProcessingThis operation makes time-varying channel impulse response substantially constant during eachmodulated OFDM symbol. Resulting long symbol duration is virtually unaffected by ISI compared tothe short symbol duration. A Serial to Parallel (S/P) converter collects serial data symbols into a datablockSk = [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 vectorXk = [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, thechannel gain may differ between sub-carriers, and thus some sub-carriers can carry higher data-ratesthan others. The vector of data symbols Xk then passes through an Inverse FFT (IFFT) resulting in aset of N complex time-domain samplesxk = [xk[0], . . . , xk[N 1]]T .In a practical OFDM system, the number of processed subcarriers is greater than the number ofmodulated sub-carriers (i.e. N M), with the unmodulated sub-carriers being padded with zeros.

Fig 2.1.1.2OFDMA tramsmission and reception

A guard period is created at the beginning of each OFDM symbol, to eliminate the remaining impactof ISI. A Cyclic Prefix (CP) is added at the beginning of each symbol xk. The CP is generated byduplicating the last G samples of the IFFT output and appending them at the beginning of xk. Thisyields the time domain OFDM symbol [xk[N G], . . . , xk[N 1], xk[0], . . . , xk[N 1]]T . CP lengthG 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 forDFT processing. The IFFT output is then Parallel-to-Serial (P/S) converted for transmission throughfrequency-selective channel. Here is an example of OFDM LTE signal.

At the receiver, the reverse operations are performed to demodulate the OFDM signal, CP areremoved and ISI-free block of samples is passed to the DFT. If number of subcarriers N is designedto be a power of 2, a highly efficient FFT implementation may be used to transform the signal back tothe frequency domain. Among the N parallel streams output from the FFT, the modulated subset of Msubcarriers 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 multipathenvironment is then given by r(t) = x(t) * h(t) + z(t), where h(t) is the continuous-time impulseresponse of the channel, represents the convolution operation and z(t) is the additive noise. Assumingthat x(t) is band-limited to [1/2Ts ,1/2Ts], the continuous-time signal x(t) can be sampled at samplingrate Ts such that the Nyquist criterion is satisfied. Due to multipath, several replicas of the transmittedsignals arrive at the receiver at different delays. The received discrete-time OFDM symbol k includingCP, 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 linearconvolution into a circular one. The circular convolution is very efficiently transformed by an FFT intoa 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


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the frequency domain: Rk[m] =Xk[m] H[m] + Zk[m]. As a result the equalization is much simpler thanfor 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 amplitudevariations of OFDM signal can be very high, however PAs of RF transmitters are linear only within alimited dynamic range. Hence, OFDM signal is likely to suffer from non-linear distortion caused byclipping, giving out-of-band spurious emissions and in-band corruption of the signal. To avoid suchdistortion, 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, degradationdue to PA non-linearities may be expected.

PAPR Reduction TechniquesMany 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 inreduced spectral efficiency, in-band noise, degrading BER. To avoid this problem,oversample the original signal by padding with zeros and processing it using a longerIFFT. 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 tocombine both PAPR and forward error correction. It is not used.

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


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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 butnot zero-padding.In the general case of a channel with delay spread, for a given CP length themaximum tolerated timing offset without degrading the OFDM reception is reduced by an amountequal 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 timingis achieved by the cell-search and synchronization procedures. Thereafter, for continuous tracking oftiming-offset, either CP correlation or Reference Signals (RSs) is used.

If an OFDM system, CP is sufficiently designed of lengthG samples such that Channel impulseResponse L


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

Fig 2.1.5.1 -FDD Frame Structure

Fig 2.1.5.2TDD Frame Structure

Note that, with normal CP, the CP for the first symbol in each 0.5 ms slot is slightly longer than thenext six symbols, to accommodate an integer (7) number of symbols in each slot, with assumed FFTblock-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 inLTE, 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, whileonly 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 layersmultiplexed with physical layer signalling. A DL resources possess dimensions of time(slot),


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frequency(multiple of 180khz) and space(layer). Layer is defined by multiple antenna transmissionand reception.The largest unit of time is the 10 ms radio frame, further subdivided into ten 1 ms subframes, each ofwhich is split into two 0.5 ms slots. Each slot has seven OFDM symbols in normal CP (six if extendedCP). 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 2.1.6.1FDD Downlink Frame sample

Smallest unit of resource is the Resource Element (RE) - one subcarrier for a duration of one OFDMsymbol. A RB comprised of 84 REs in normal CP (72 RE in extended CP). Within certain RBs, someREs are reserved for synchronization signals (PSS/SSS), reference signals (RS), control signallingand 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 2.1.6.2TDD 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


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

Signal StructurePhysical layer translate data into reliable signal for transmission between eNodeB and UE. Eachblock 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 helpsinterference rejection. Scrambling sequence uses order- 31 Gold code, which are not cyclic shifts ofeach 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 toensure low cross-correlation between sequences used in adjacent cells.Following scrambling, data bits from each channel are mapped to modulation symbols depending onmodulation scheme, then mapped to layers, precoded, mapped to RE, and finally translated into acomplex-valued OFDM signal by IFFT.To communicate with eNodeB cells, UE must first identify the DL from one of these cells andsynchronize with it. This is achieved by means of special synchronization signals embedded into theOFDM structure by cell search and synchronization. Then UE estimates DL radio channel to performdemodulation of received DL signal, based on pilot signals ( reference signals) inserted into DL signal.The channel designs are explained next.


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1.2 Synchronization and Cell SearchCell Search executes synchronization for time and frequency parameters, necessary to demodulateDL 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 errorsby oscillator & Doppler shift;3. sampling clock synchronization.

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

1. Initial synchronization,

UE detects a cell and decodes all information required to register. This is required, forexample, 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 newcell measurements to Serving cell for handover. Procedure is repeated periodicallyuntil either Scell quality becomes satisfactory again, or UE moves to another cell.

Fig 2.2.1.1FDD and TDD Synchronization Signalling

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

Fig 2.2.1.2Cell Synchronization Process

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


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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

thslots of each frame,

thus enabling UE to acquire the slot boundary timing independently of the CP. SSS is locatedin 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 12thslots, 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 andSSS 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 the10 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 eachcomprised 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. Thusa UE can detect the PSS and SSS with size-64 FFT and a lower sampling rate if all 72 subcarrierswere used.In case of MIMO at eNodeB, PSS and SSS are always transmitted from same antenna port in asubframe, while between different subframes they may be transmitted from different antenna ports fordiversity.

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 areused to indicate the cell identity within the group, and 168 SSS sequences are used to indicate theidentity of the group.PSS uses ZadoffChusequences

1.2.2. ZadoffChu SequencesZadoffChu (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 NZCis 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, . . . , N ZC 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 andtime-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 circularlyshifted version of itself is a delta function). ZC periodic autocorrelation is exactly zero for 0 and itis 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 ZCsequence. Indeed, if the periodic autocorrelation of a ZC sequence provides a single peak at the zerolag, the periodic correlation of the same sequence against its cyclic shifted replica provides a peak atlag 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 thelargest possible expected time misalignment between them, the two sequences are orthogonal for alltransmissions within this time misalignment.Property 3. The absolute value of the cyclic cross-correlation function between any two ZCsequences is constant and equal to 1/NZC, if |q1 q2| (where q1 and q2 are the sequence indices) isrelatively prime with respect to NZC. Selecting NZCas a prime number results in NZC 1 Zaddoff-Chusequences which have the optimal cyclic cross-correlation between any pair. Cyclic extension ortruncation preserves both the constant amplitude property and the zero cyclic autocorrelation propertyfor different cyclic shifts.The DFT of a ZC sequence xu(n) is a weighted cyclicly-shifted ZC sequence Xw(k) such that w = 1/umod NZC. This means that a ZC sequence can be generated directly in the frequency domain withoutthe need for a DFT operation.


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1.2.3. Primary Synchronization Signal (PSS) SequencesThere 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,1is the PCI group ID - , NID,1 can have values 0 to 167 and

NID,2is the PCI local (may be sector) ID - NID,2can 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 GenerationThe 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


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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 synchronizationsignal. 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 toavoid 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 undesiredautocorrelation 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 duringthe initial synchronization with a frequency offset up to 7.5 kHz.

The selected root combination satisfies time-domain root-symmetry, sequences 29 and 34 arecomplex conjugates of each other and can be detected with a single correlator. UE must detect PSSwithout any prior knowledge of the channel, so noncoherent correlation is required for PSS timingdetection.

1.2.5. Secondary Synchronization Signal (SSS) SequencesSSS maximum length M-sequences, can be created by cycling through every possible state of a shiftregister 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 toconstruct SSS sequence in frequency-domain. Two codes are two different cyclic shifts of a singlelength-31 M-sequence. Cyclic shift indices are derived from a function of PCI group. Two codes arealternated between the first and second SSS in each radio frame.

5subframein)(

0subframein)()12(

5subframein)(

0subframein)()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 isscrambled by a sequence that depends on the index of SSC1. Sequence is then scrambled by a codethat 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:


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2typestructureframefor11and1slotsin1

1typestructureframefor10and0slotsin2

231

61,...,0,

DL

symb

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 7.5kHz. 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 bedegraded. If an interfering eNodeB employs the same PSS, phase difference between them can haveadverse impact on estimation of the channel coefficients. If BW of the channel is less than the six RBfor 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 Nwith N=32, complexity= 32 log232 = 160.

1.2.6. Cell Search PerformanceA 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 andunsynchronized 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 cell90%of the time) identification delay. After detection of PSS-SSS, RSRP is measured. For initialsynchronization case the time taken to decode PBCH is adapted, and not just of reporting ofmeasurements on RS.For inter-frequency handover, performance can be derived from the intra-frequency performance timing.

Coherent Versus Non-Coherent DetectionA coherent detector uses knowledge of the channel, while a non-coherent detector uses anoptimization metric of average channel statistics. In PSS, non-coherent (No channel estimationavailable) detection is used, while for SSS, coherent (channel estimation) or non-coherent techniquescan be used.

1.2.7. Reference Signals and Channel EstimationIn any communication system signal x transmitted by A passes through a radio channel H (exhibitmultipath 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 notcarry any data, sacrificing spectral efficiency. Known reference signals are inserted into thetransmitted signal structure. Reference signals(known) are multiplexed with data symbols (unknownat receiver) in either frequency, time or code domains. Time multiplexing, known preamble-basedtraining transmission also is another technique.Orthogonal RS multiplexing is the most common technique. OFDM transmission is a two-dimensionallattice in time and frequency, which helps multiplexing of RSs mapped to specific REs according to aspecific 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 LTEIn DL, three different types of RS are provided:


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

to estimate channel correctly.

Fig 2.2.8.1Cell RS for 1, 2 and 4 antenna

In frequency domain, there is one RS every six subcarriers on each symbols including RS symbol, butstaggered so that within each RB there is one RS every 3 subcarriers. This spacing is goverened byexpected 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 whered is the r.m.s delay spread=1000ns. In LTE spacing between two RS in frequency, is 45 kHz (3symbols), enough to resolve expected frequency domain variations of the channel.

RS patterns are designed to work with MIMO antennas defined for multiple antenna ports ateNodeB. An antenna port may be either a single physical antenna, or a combination of multiplephysical antenna elements. The transmitted RS in a given antenna port defines the antenna port fromthe point of view of the UE, and enables UE to derive channel estimate for that antenna port.


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Fig 2.2.8.2Antenna ports and Physical antennas

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

Fig 2.2.8.3Antenna 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 differentinitialization values depending on type of RSs. RS sequence carries unambiguously one of the 504different 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, designedto 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 addit ion 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 fordemodulating data in PDSCH RBs.


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Fig 2.2.9Port 5, UE specific Reference SignalsA typical usage of URSs is beam-forming of data transmissions to specific UEs. Rather than usingphysical antennas of other CRS antenna ports, eNodeB may use a correlated array of antennaelements to generate a narrow beam in the direction of a particular UE. Beam will experience differentchannel 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. URSshave 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 EstimationChannel estimation problem is related to physical propagation, number of transmit and receiveantennas, 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 conditionsacross the three domains. LTE specifications do not mandate any specific channel estimation

technique, and there is therefore complete freedom in implementation provided that the performancerequirements are met and the complexity is affordable.

1.2.12. Frequency Domain Channel EstimationThe natural approach to estimate the whole CTF is to interpolate its estimate between the referencesymbol positions. As a second straightforward approach, the CTF estimate over all subcarriers can beobtained by IFFT interpolation.More elaborate linear estimators derived from both deterministic and statistical viewpoints areproposed -Least Squares (LS), Regularized LS, Minimum Mean-Squared Error (MMSE) andMismatched 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 estimatoroutperforms 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 EstimationTime-Domain (TD) channel estimation requires sufficient memory for buffering soft values of data overseveral symbols while the channel estimation is carried out. However, correlation betweenconsecutive symbols decreases as UE speed increases. TD correlation is inversely proportional to theUE 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 approximatedin 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 FDcounterpart, the TD-MMSE estimator requires knowledge of the PDP, the UE speed and the noisevariance.


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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 complexitycompared to TD-MMSE as no matrix inversion is required, as well as not requiring any a prioristatistical knowledge.

Other adaptative approaches could also be considered such as Recursive Least Squares (RLS) andKalman-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 spatialdomain, the correlation can be exploited to provide a further means for enhancing the channelestimate. If it is desired to exploit spatial correlation, a natural approach is again offered by SpatialDomain (SD) MMSE filtering.


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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 2.3.1PBCH transmission

Detectability without prior knowledge of BW is achieved by mapping PBCH only on central 72subcarriers of OFDM signal, regardless of actual BW. UE will have first identified the centre-frequencysynchronization signals.

Low system overheadis achieved by deliberately keeping information on PBCH to a minimum (MIBis just 14 bits), and, since it is repeated every 40 ms, it is just 0.35 kbps.


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Reliabilityof PBCH is achieved with time diversity, FEC coding and antenna diversity. Time diversityis exploited by spreading transmission of each MIB on PBCH over 40 ms ensuring reception despiteloosing one transmission. The FEC coding for the PBCH uses a convolutional coder, as bits to becoded is small. Basic code rate is 1/3, after which a high degree of repetition of systematic bits andparity 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. Transmitantenna 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 avoidedby 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 acodeword representing the number of transmit antenna ports.

Low latencyand 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 offour different frames during the 40 ms. UE may decode the MIB correctly from the transmission in lessthan four radio frames, then UE does not need to receive other parts of PBCH in the remainder of 40ms. 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 isdetermined by scrambling and bit positions. UE can initially do four separate decodings of the PBCHand 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 periodof 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, andpaging messages. Data is transmitted on PDSCH in units of transport blocks(TB), each of which is aMAC-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 PDSCHone or, at most, two TB can be transmitted per UE per subframe, depending on transmission modeselected 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 portsfor PDSCH is same as PBCH. In Tx-mode 7, UE-specific RSs provide phase reference for thePDSCH. 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 symbolsdepending on radio channel conditions and data rate required. Modulation order may be between twobits 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, PBCHand control signalling(PCFICH,PHICH,PDCCH)). When UE is given a pair of RB of PDSCH, in asubframe, only the available RE within RB can carry PDSCH data. Allocation of RB to PDSCH for a


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

1. localized mapping and2. 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 maybe persistently-scheduled, on a periodic basis to a specific UE by RRC signalling rather thanPDCCH. As data per UE for VoIP is small (one or two pairs of RB), degree of frequency diversityobtainable via localized scheduling is very limited. When dynamic channel-dependent PDCCHscheduling is not done, frequency diversity is achieved through distributed mapping. A frequency-hopoccurs at slot boundary in the middle of subframe, block of UE data transmitted on one RB in first halfof 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 distributedmapping, compared to localised.

Special Uses of the PDSCH

Apart from normal user data transmission, PDSCH is used for Dynamic BCH, all SIBs, not carried onPBCH. The RBs for SIBs are indicated by PDCCH, same way as for other PDSCH data without anyspecific 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 PDCCHsignalling is used to carry equivalent of a WCDMA paging indicator, with detailed paging informationcarried 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 andPaging 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 isvery 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 fromdifferent 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 MBSFNsubframe. PDCCH is used only for UL resource grants and not for the PMCH, asscheduling 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 ofeach MBSFN subframe.

Some spare time samples usage is unspecified between the end of the last control signalling symboland 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 anythingabout 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 MBSFNsubframes in neighbouring cells is same or different from current cell. If different pattern, then UE canonly ascertain the pattern by reading SI of that cell.

1.3.4. Downlink Control ChannelsRequirements for Control Channel DesignControl channels convey physical layer signals or messages which cannot be carried efficiently orquickly 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 grantedpermission to use, together with modulation and code rate. To facilitate efficient operation of HARQ


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and ensure appropriate power levels, further physical layer signals are needed to convey ACK/NACKby eNodeB, and power control commands.

Flexibility, Overhead and ComplexityLTE allows operation in BW from six RB (1.08MHz) to 110 RB (19.8MHz). It is also designed tosupport 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 impactthroughput efficiency. Channel coding and frequency diversity can be used to make control channelsrobust. It is important to adapt transmission parameters of the control signalling for different UEs orgroups of UEs, so that lower code rates and higher power levels are only applied for only relevantUEs as necessary (e.g. near cell border).

System-Related Design AspectsLTE control channel is designed in a cell for a particular UE (or in some cases a group of UEs). Tominimize latency, control channel transmission should be completed within one subframe, it must bepossible to transmit multiple control channels within a subframe. Both common and dedicated controlchannel messages are supported. If the data arrives at the eNodeB on a regular basis, as VoIP, it canbe controlled by persistent scheduling.

Control Channel Structure and ContentsDownlink control channels (PDCCH) can be configured to occupy the first 1, 2 or 3 OFDM symbols inevery subframe, over entire BW. There are two special cases: in subframes containing MBSFN, theremay be 0, 1 or 2 symbols, while for narrow BW (BW< 10 RB), PDCCH may be 2, 3 or 4 to ensuresufficient coverage at cell border. This can adjust overheads of particular configuration, trafficscenario 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 coulddeduced CFI blind decoding each possible number of symbols, but at a cost of significant processingload. For MBSFN carriers, there may not be any physical control channels, so PCFICH is not present.

Fig 2.3.5.1PCFICH REG requirementsCFI values od 1,2,3 are used and 4 is reserved for future. To make it robust, each CFI is coded with32 bit long codeword, mapped to 16 REs with QPSK modulation.


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Fig 2.3.5.2PCFICH Mapping on neighbour Cells

PCFICH is transmitted on same ports as PBCH, with transmit diversity if more than one antenna portis used. For frequency diversity, 16 REs are distributed across BW with a predefined pattern in thefirst symbol in each DL subframe, so that UEs can always locate. This is prerequisite to decode restof the control signalling.

A cell-specific (PCI based) frequency offset is applied to the positions of PCFICH REs. In addition, acell-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 ormore Control Channel Elements (1CCE=9REG), each CCE corresponds to nine sets of four REknown 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 2.3.6.1REG CountingFour PDCCH formats 0,1,2,3 are supported, based on 1,2,4 or 8 CCEs are used respectively for each

PDCCH.


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

CCEs are numbered and used consecutively. A PDCCH with a format consisting of n CCEs may onlystart with a CCE with a number equal to a multiple of n. Format is decided by eNB based on RFconditions.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 SelectioneNB should apply the following rules when allocating the PDCCH candidates. Let m =number ofPDCCH 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


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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 MIMOparameters are not required. To minimize overhead, different message formats are designed, eachwith minimum payload required for a particular scenario.The number of bits required for resource assignment depends on BW. To avoid complexity, Formats0 and 1A are designed to be always 42bits. Since for different BW different size is needed, to reducecomplexity, the smaller format size is extended by adding padding bits to be the same size as thelarger. 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 421 PDSCH assignments with a single codeword 471A PDSCH assignments using a compact format 421B PDSCH assignments for rank-1 transmission 461C PDSCH assignments using a very compact format 261D PDSCH assignments for multi-user MIMO 462 PDSCH assignments for closed-loop MIMO operation 622A PDSCH assignments for open-loop MIMO operation 583 Transmit Power Control (TPC) commands for multiple users 42

for PUCCH and PUSCH with 2-bit power adjustments3A 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 Form ats 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 field 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


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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 DownlinkAssignment 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:

DCIFormats

DCISTR Fields Size Description

Format1A

DCIFormat - Format1A

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




4-bits (TDD)





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

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

Format1B DCIFormat - Format1B





4-bits (TDD)





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TPMI 2-bits -(2 ants) PMI information

4-bits- (4 ants)

PMI 1-bit PMI confirmation


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

Not present for FDD.

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

DCIFormats


Format1C DCIFormat - Format1CAllocation variable Resource block assignment/allocation


Format 1D. DCI Format 1D is used for compact signalling of resource assignments for PDSCH usingmulti-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 transmittedpower 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):

DCIFormats








4-bits (TDD)

SwapFlag 1-bit Transport block to codeword swap flagModCoding1 5-bits Modulation and coding scheme for transport block 1


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NewData1 1-bit New data indicator for transport block 1

RV1 2-bits Redundancy version for transport block 1

ModCoding2 5-bits Modulation and coding scheme for transport block 2



PrecodingInfo 3-bits - 2ants Precoding information

6-bits - 4 antsTDDIndex 2-bits For TDD config 0, this field is not used.

For TDD Config 1-6, this field is the DownlinkAssignment 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 thetransmission 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)


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

Assignment Index.

Format 2BDCI Formats DCISTR Fields Size Description

Format2B

DCIFormat - Format2B






4-bits (TDD)


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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 isappended to each PDCCH. CRC is scrambled with UE identity for this to be identified for a particularUE. 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 thenscrambled with a cell-specific scrambling sequence to distinguish from neighbouring cells. Thescrambled bits are QPSK modulated and mapped to blocks of four REs (REGs). Interleaving isapplied for frequency diversity, followed by RE mapping to symbols indicated by PCFICH, excludingPCFICH and PHICH. The PDCCHs are transmitted similar to PBCH, and diversity is applied if moreantenna 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 onPUSCH. 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 throughdifferent 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 scramblingsequence is then applied. PHICH duration (symbols) in time domain, is configurable by SI to eitherone(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 firstthree symbols of each subframe, such that each PHICH is partly transmitted on each availablesymbols. UEs to deduce to which remaining resource elements in the control region the PDCCHs aremapped.


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Fig 2.3.8.1PHICH Coding, duration and cyclic shift allocation

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

Fig 2.3.8.2PHICH bits to REG mapping

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

1.3.10. Resource AllocationIn 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 interms 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 (too large). Thismay make PDCCH larger than the data itself. One possible solution could have been to send acombined message to all UEs, but that would need high power to ensure to reach each UE reliably.Some methods are given below.


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Fig 2.3.9.1Resource Allocation Type 0

Resource allocation Type 0. A bitmap indicates Resource Block Groups (RBG

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