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.

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.

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.

LTE - 4G Wireless Technology

Digital fundamentals.

Interview Questions.

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].

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.

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.


Verilog Tutorial.

LTE Tutorial.

Memory Tutorial.

Hope you liked! this page. Don't forgot to access relevant previous and next sections with links below.