Power Division in Spread Spectrum Systems with Limiting

Abstract

This paper finds power division through a bandpass nonlinearity when the input is two spread spectrum signals plus noise. This model applies to code division multiple access (CDMA) satellites or to multiple transmission of CDMA through a common Earth terminal transmitter. The solution requires finding a single attenuation constant for each input signal including the input noise. This approach provides the first exact power division solutions for spectrally overlapping, possibly coherently generated, spread spectrum signals plus noise through a bandpass limiter. Two examples illustrate the approach. Simultaneous transmission of coherently generated QPSK spread signals through a hard limiter without noise may cause whichever is the smaller of the two to suffer a sharp 6 dB suppression. This suppression is highly phase dependent and can be alleviated by proper phasing of the signals prior to hard limiting. In the second example, input/output SNR relations are found and compared with those given by Jones for two unmodulated sine waves and noise. A significant reduction in output SNR exists for spread signals compared to the two sine wave case examined by Jones.