Design of Joint Source/Channel Codes for Rayleigh Fading Channels

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9314221 Vaishampayan We address the issue of combined source and channel coding for fading channels. It is well-known that channel coding, together with interleaving, can be used to achieve an improvement n diversity order. However, these codes have traditionally offered uniform error protection for all bits transmitted. In reality, for many sources, some bits from the quantizer are more important than others. We show that unequal error protection schemes can offer greatly improved performance on the fading channel given a fixed diversity level in the system. We also provide a method for designing unequal error protection schemes in order to minimize a distortion criterion, subject to an overall system diversity level. Additionally, we note that the interleaving delay required in interleaved systems is very large compared to the diversity orders that the code provides. In speech coding applications the entire communications system must be designed to meet a delay budget. The interleaving delay thus restricts the delay that can be allocated to the source coder/decoder. We show that the construction of a channel code with a constraint on the interleaving delay can be treated as a problem of designing a code for a restricted number of independently slow fading channels in which interleaving is not allowed on any of the slow fading channels. The diversity order that the code can achieve is restricted to the number of independent channels available. However, the rate of the code is then restricted to the reciprocal of the diversity order. In order to get around this constraint, we use the fact that for continuous alphabet sources, not every transition probability for the effective channel as seen by the source coder/decoder need exhibit the maximum order of diversity. This allows the rate to the code to be increased. We will examine structured techniques for constructing combined source and channel codes for the Rayleigh fading channel. ***