Performance analysis of iterative detection for unequal power coded CDMA systems

The combination of forward error control coding (FEC) with code division multiple access (CDMA) using random spreading sequences is considered. Through tracking the input-output variance evolutions of the decoding components, which are the CDMA interference resolution function and the individual error control decoders, an analysis of the iterative joint detection was previously presented. It was shown this variance transfer (VT) analysis between component decoding devices gives very accurate description of the convergence properties of the iterative joint detector for a CDMA system with all users received at equal energy. In this paper, the VT analysis is extended to the more realistic cases where the different users have unequal power levels. It is shown that the effective VT functions modified w.r.t. the unequal power scenario can be used to analyze the iterative reception employing a wide variety of linear filters for interference resolution, in a manner very similar to that in the equal power case. Numerical simulations verify the results predicted by VT analysis.