LTE ARQ and HARQ techniques for reliable data transmissions
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 1 technique requires receiver to correct the erroneous packet with information
contained in form of FEC bits. If receiver fails to decode the packet then a complete
re-transmission is requested. This technique has an advantage of a possibility of
packet correction without retransmission.
Drawback of HARQ Type I:
This technique results in increased packet with addition of FEC bits. This results
in limitation on channel bandwidth and data throughput.
LTE HARQ is of two types: Type I HARQ and Type II HARQ
Type I HARQ adds Forward Error Correction(FEC) bits to the data and ED codes. So
the packet looks like [Data bits + ED bits + FEC bits].
In LTE there is a signal quality threshold limit. When the signal quality is above
that limit, HARQ is used to correct Received Packets. When signal quality goes below
that threshold the L2 software automatically replaces HARQ with plain ARQ technique.
Type II HARQ is a complex but efficient technique to optimize Hybrid-ARQ strategy.
In this protocol the data, ED and FEC bits are grouped into Information Block (IB)
and FEC block (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. Complete HARQ Type II article next.
LTE ARQ technique - Cyclical Redundancy Checking (CRC) type Error Detecting (ED)
codes are added in Transmit packets. At receiver a sliding window technique is used
to identify the erroneous packet in data transmission.