[ Introduction | Research Plan | Reference ]
Hybrid ARQ is not new. To
reduce the number of transmissions, we add redundancy in each transmission.
In more detail, Hybrid ARQ has been proposed for years and there have been many
analyses about the complexity and performance. Two types of Hybrid ARQ methods
have been proposed . One scheme is Chase Combining (CC) and the other is
Incremental Redundancy (IR). CC is simple HARQ and requires small
receiver buffer. IR requires larger receiver buffer than CC but can achieve
better performance than CC. Incremental Redundancy Hybrid ARQ is called as
type-II or type-III (when each packet is self-decodable).
1. Chase Combining
This scheme is to send a number of repeats of coded data and decoders combine
multiple coded packets before decoding. This scheme achieves gain with small
buffer size in a receiver. The buffer size becomes the number of coded symbols
of one coded packet. (This is not Type I)
2. Incremental Redundancy
This scheme is to transmit additional redundant information in each
retransmission and receiver decode on each retransmission. This incremental
redundancy scheme is called type-II Hybrid ARQ. If each retransmission packet is
self-decidable this scheme is called type-III Hybrid ARQ. IR requires larger
size of buffer in a receiver than Chase Combining. The buffer size becomes the
number of coded bits of total transmitted coded packets.
Simulation results on CC and IR have been shown and it has been reported that in
some cases Incremental Redundancy can achieve better performance than Chase
Combining.
Type-I Hybrid FEC/ARQ: In this scheme, the receiver discards the packet upon erroneous reception (i.e when errors remain after FEC decoding) at a particular code rate and asks for an entirely new retransmission. Retransmissions take place at either the same or lower code rate until the packet is correctly received or until a pre-set number of retransmissions have taken place. Although this method does not require a large buffer at the receiver, it is a very inefficient method of implementing ARQ.
However, if we apply it in a multi-hop scenario, things become different, each node in the several-hop path get a transmission chance both to the sink and next hop. How to arrange this is a whole new topic.