Coding and Interleaving for Fading Channels
For good performance convolutional codes, block codes and coded modulation are designed in AWGN channel.
Fading Channels :
- In fading channels errors associated with the demodulator tend to occur in bursts, corresponding to the times when the channel is in a deep fade
- Most codes designed for AWGN channels cannot correct for the long bursts of errors exhibited in fading channels.
- Codes design for AWGN channels can exhibit worse performance in fading than an uncoded system.
- To improve performance of coding in fading channels, coding is typically combined with interleaving to mitigate the effect of error bursts
- The basic of coding and interleaving is to spread error bursts due to deep fades over many codewords such that each received codeword only exhibits at most a few simultaneous symbol errors, which can be corrected for
- The spreading out of burst errors is accomplished by an interleaver and the error correction is accomplished by the code
- The size of the interleaver must be large enough so that fading is independent across a received codeword.
- Slowly fading channels require large interleavers, which in turn can lead to large delay
- Coding and interleaving is a form of diversity, and performance of coding and interleaving is often characterized by the diversity order associated with the resulting probability of error
- This diversity order is typically a function of the minimum Hamming distance of the code.
- Thus, designs for coding and interleaving on fading channels must focus on maximizing the diversity order of the code, rather than on metrics like Euclidean distance which are used as a performance criterion in AWGN channels.