LTE HARQ Type two (or II) (or 2) technique.

HARQ is Hybrid Automatic Repeat Request. Its an enhancement over Layer 2 ARQ protocol. These techniques are used to transfer data packets reliably over radio link. Lets discuss details for 4G LTE standard.

Type II HARQ is a complex but efficient technique to optimize Hybrid-ARQ strategy. In this protocol the data and ED bits are grouped into Information Block (IB). So we have IB and FEC bits (FECB).

While computing the first packet for transmission a major part of IB is selected and a smaller part of FECB is added. So in good channel conditions the packet is self sufficient to be decoded and no retransmission is required and overhead due to FECB is avoided. Also during erroneous packet detection, a small set of remaining IB and large part of FECB is re-transmitted. This retransmitted packet is known and Redundancy Version or RV. So this technique is more efficient than Type I but needs complex algorithms in hardware or software to implement it. LTE uses four RVs that are piped through the RF link until the packet is received correctly.

LTE - 4G Wireless Technology

Digital fundamentals.

Interview Questions.

Type II HARQ packet format: [Information + Redundancy(1) + Redundancy(2 only after 1 fails) + Redundancy(3 only after 1 and 2 fails) + Redundancy(4 only after 1,2 and 3 fails)]

LTE Type II HARQ employs Turbo codes which are highly efficient Forward Error Correction technique at a given code rate. The code rate used in 3GPP LTE turbo encoder is 1/3.

HARQ in LTE frequency-division duplex (FDD)

Allocation for HARQ are eight 1-ms sub-frames. LTE uses multiple HARQ parallel processes offset in time from each other. Where each process transmits only one block of data. The subsequent transmissions are based on ACK/NACK from previously transmitted packets.

HARQ in LTE time-division duplex (TDD) supports a software programmable number of HARQ processes.

For FDD, Downlink follows asynchronous timing , whereas each uplink HARQ process is assigned to a specific subframe (synchronous HARQ).


Verilog Tutorial.

LTE Tutorial.

Memory Tutorial.

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