Multiple access interference properties of constant-envelope CDMA

CDMA systems transmitting multiple signals from a single source normally employ linear superposition of codes. The resulting signal has a high peak-to-average power ratio (PAPR), which is undesirable from a high power amplifier (HPA) viewpoint. Several techniques have been proposed to reduce PAPR and ease the HPA requirements. Among these, constant-envelope CDMA (CE-CDMA) is an alternative that eliminates envelope variation, achieving PAPR = 0 dB. It can use a saturating (Class C) amplifier that runs at lower peak power and may be more robust and inexpensive than the Class A HPA needed for linear superposition. The latter requires significant linear dynamic range and must operate at several dB output power backoff. Constant-envelope multiplex techniques are nonlinear and thus exhibit both a multiplexing efficiency loss-part of the price one pays for a substantial reduction in peak power-and multiple access interference (MAI) among the user signals. To date there has been no systematic, quantitative exploration of multiplexing efficiency and MAI in CE-CDMA. The principal results reported in this paper are the derivation of formulas for multiplexing loss and MAI in CE-CDMA and their use to discover, for the case of homogeneous users, multiplexing methods optimum with respect to minimizing the combined losses. A comparison of MAI for linearly multiplexed and CE-CDMA concludes the paper.