Analytical Methods for CDMA Systems with Parallel Interference Cancellation: The Large Deviation Approach

For a simple CDMA system, we compute the bit-error probability (BEP)with soft-decisionparallel-interference-cancellation (SD-PIC). Instead of approximatingthe signal-to-noise ratio, we use a different measure to calculateperformance. This measure is the exponential rate of the BEP, i.e., thelimit of n−1 log(BEP) = −I, for the processinggain n→ ∞, where I depends only on the number of users. Weshow, using the rate as a measure, that SD-PIC improves the performance.The values of I follow as the solution of an optimization problem whichcan be calculated numerically. We use these results to derive theasymptotic behaviour of the rate for large k. We also derive results forthe second order asymptotics of the BEP. Inclusion of second orderasymptotics leads to excellent approximations.