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