Improved Power Amplifier Efficiency via Walsh Code Assignment in Synchronous CDMA Systems

In this paper, we study the effect of the assigned Walsh function to each user in a synchronous CDMA system on the power spectral density (PSD) of the transmitted CDMA signal. We show that the PSD of the CDMA signal depends primarily on the PSD of the assigned Walsh functions. Also, based on the Walsh code properties, an algorithm for Walsh code assignment in CDMA systems is introduced. The algorithm achieves lower spectral regrowth in terms of adjacent channel power ratio (ACPR) at all values of input back off (IBO). Performance measurements show that using such an algorithm for 17 active channels, a reduction of about 6 dB and 5 dB in the ACPR is achieved at IBO = 1 dB and 2 dB, respectively. This reduction in the spectral regrowth implies that using this algorithm, the high power amplifier (HPA) can operate at high values of input power approaching the saturation level with minimal spectral regrowth, and in turn the HPA efficiency is improved.