Chapter 3
Incremental Redundancy for Coding
3.1 Introduction
This chapter focuses on the concept of reliability in the context of packet data transmission. Typically, data transmission is not strictly delay-sensitive but it requires a virtually error-free link. Error control methods such as automatic retransmission request (ARQ) can be used to provide such a level of reliability. These schemes exploit the availability of a reliable feedback link in order to provide an indication of correct receipt of a data packet. Error-detection bits are appended to the packet in order to detect whether the packet is correctly received. Polynomial codes such as the cyclic redundancy check (CRC) are examples of error-detecting codes.
In order to provide more robust schemes, particularly suited for wireless channels affected by propagation impairments such as fading, the ARQ can be combined with channel coding (forward error correction, FEC). The result is the so-called hybrid ARQ (HARQ) scheme. In brief, when fading varies slowly over the duration of a codeword, coding takes care of the channel noise while retransmissions take care of bad channel conditions (deep fades). Hence, HARQ performs better than ARQ in poor channel conditions but it can experience a significant loss in terms of spectral efficiency in good signal conditions.
The average number of transmissions of an ARQ process ...
