An Analysis of the Distortion Effects of Nonlinear Amplifiers on CDMA Signals

This paper presents a direct method of analyzing the distortion effects of the power amplifier nonlinearity on the CDMA signal. In the analysis approach of the paper, the various user signals, which are composed of data-modulated Walsh codes, which are also modulated by various scrambling codes, are modeled as independent random sequences. These sequences are band limited, using square root raised cosine filters and modulated by a carrier frequency before being input to the nonlinear power amplifier. The paper derives a closed-form expression for the output signal power, the output signal power-to-distortion power spectral density ratio as a function of the input signal power back-off, the number of codes, and the amplifier transfer characteristics. The paper also shows that the amplifier possesses a maximum capacity defined as the maximum total bit rate R_T that can be present at the amplifier input for a specified output signal power back-off and the required output bit energy to distortion power spectral density ratio (E_b/N_0d). Alternatively at any given output back-off and user data rate, the (E_b/N_0d) ratio is a strong function of the number of codes M in the CDMA signal.