Effect of tracking errors on the coding-spreading tradeoff in CDMA systems

The issue of allocating a fixed DS-CDMA bandwidth expansion between coding and spreading is considered in the presence of code-tracking errors. In the absence of tracking errors, it has been shown that it is desirable to decrease the rate of a convolutional coder. The resulting coding gain may, however, result in increased tracking errors due to the lowering of required signal-to-interference level. Thus, tracking errors push the bandwidth tradeoff between coding and spreading towards more spreading. It is shown how the tradeoff in allocating the bandwidth expansion between coding and spreading is sensitive to the standard deviation of the code-tracking errors. The tradeoff between the effects of tracking errors and tracking loop time-constant is quantified.