LTE - DL-SCH HARQ Modeling

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Introduction

The Downlink Shared Channel (DL-SCH) is described in TS36.212, Section 5.3.2. This example demonstrates how a transmitter retransmits a single codeword over a single layer, using a different Redundancy Version (RV) each time until the CRC of the received codeword indicates a successful transmission. The DL-SCH transmission is 16QAM modulated with a 1/2 target coding rate transmitted over single antenna port. The setup used in this example is based on Fixed Reference Channel R.3 defined in Table A.3.3.1-2 of TS36.101.

Hybrid Automatic Repeat reQuest (Hybrid-ARQ) is a combination of Forward Error Correction (FEC) and Automatic Repeat reQuest in an optimal manner. Hybrid-ARQ schemes are commonly used to facilitate reliable communication over noisy wireless channels. HARQ is able to compensate for link adaptation errors and provides a finer granularity of coding rate, resulting in better throughput performance than other FEC schemes.

There are three types of Hybrid ARQ: Type I, Type-II and Type-III.

Hybrid ARQ Type I

The simplest method: Hybrid ARQ Type I uses the CRC to detect whether or not an error in transmission has occurred. If a packet is found to be in error a retransmission request will be sent to the transmitter and the erroneous packet will be discarded. The transmitter will then retransmit the same packet until the packet is successfully decoded by the receiver or the maximum retransmission limit is reached.

Hybrid ARQ Type I can be extended to include packet combining, this is known as Hybrid ARQ Type I with Packet Combing or Chase Combining. After each failed retransmission the erroneous packets are stored in a buffer. The receiver then uses maximum ratio combining to combine each received channel bit with any previous transmissions of the same bit and the combined signal is fed to the decoder. Chase combining does not give any additional coding gain, it only increases the accumulated received signal to noise ratio  for each retransmission.

Hybrid ARQ Type II

In Hybrid ARQ Type II, also known as full Incremental Redundancy (IR), each retransmission is not necessarily identical to the original transmission. Instead, multiple sets of coded bits are generated and whenever a retransmission is required the retransmitted data represents a different set of coded bits than the previous transmission. The receiver combines the retransmission with previous transmission attempts of the same packet. As the retransmission contains additional parity bits, not included in the previous transmission attempts, the resulting code rate is generally lowered by subsequent retransmissions. Each transmission contains a different set of parity bits resulting in a higher coding gain when compared to chase combining.

Hybrid ARQ Type III

The final method Hybrid ARQ Type III, also known as partial IR, decreases the coding rate by sending additional redundancy bits in each retransmission. It does however ensure that retransmissions are able to self-decode. This means the retransmitted packet can be chase combined with previous packets to increase the diversity gain.

HARQ Process in LTE

LTE utilizes IR HARQ with a 1/3 turbo encoder used for FEC. The Transport Block (TB) CRC is used to detect errors. The receiver only receives different punctured versions of the same turbo-encoded data; each of these retransmissions are self decodable. Thus, it falls into category of a type III Hybrid ARQ.

In LTE retransmissions are sent with an initial coding rate of 1/2 or 3/4. The maximum number of simultaneous DL-HARQ processes (number of PDSCH transmissions catered for) is limited to 8 as specified in TS36.213, Section 7.

In LTE, the N-channel stop-and-wait protocol is used as the Hybrid ARQ protocol as it offers low buffering requirements and low Acknowledgment (ACK)/Negative Acknowledgment (NACK) feedback overhead.

Cell-wide Settings

Cell wide settings are specified in a structure enb.

enb.NDLRB = 50;                % No of Downlink RBs in total BW
enb.CyclicPrefix = ‘Normal‘;   % CP length
enb.PHICHDuration = ‘Normal‘;  % PHICH duration
enb.NCellID = 10;              % Cell ID
enb.CellRefP = 1;              % Single antenna ports
enb.DuplexMode = ‘FDD‘;        % FDD Duplex mode
enb.CFI = 2;                   % 2 PDCCH symbols
enb.Ng = ‘sixth‘;              % HICH groups
enb.NSubframe = 0;             % Subframe number 0

pdsch.NLayers = 1;                % No of layers to map the transport block
pdsch.TxScheme = ‘Port0‘;         % Transmission scheme
pdsch.Modulation = {‘16QAM‘};     % Modulation
pdsch.RV = 0;                     % Initialize Redundancy Version
pdsch.RNTI = 500;                 % Radio Network Temporary Identifier
pdsch.NTurboDecIts = 5;           % Number of turbo decoder iterations
pdsch.PRBSet = (0:enb.NDLRB-1).‘; % Define the PRBSet
pdsch.CSI = ‘On‘;                 % CSI scaling of soft bits

rvIndex = 0;                                  % Redundancy Version index
transportBlkSize = 12960;                     % Transport block size
[~,pdschIndicesInfo] = ltePDSCHIndices(enb,pdsch,pdsch.PRBSet);
codedTrBlkSize = pdschIndicesInfo.G;          % Available PDSCH bits
dlschTransportBlk = randi([0 1], transportBlkSize, 1); % DL-SCH data bits

% Possible redundancy versions (number of retransmissions)
redundancyVersions = 0:3;

LTE - DL-SCH HARQ Modeling

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原文地址：https://www.cnblogs.com/zzyzz/p/13246900.html