Noise Based PSK CDMA System for Secure Communications

Abstract

This paper presents the operation of a code division multiple access (CDMA) system that is based on application of random Gaussian spreading sequences. The system is presented in the form of a block schematic and the expressions for input–output signals are used to demonstrate the procedures of deriving bit error rate (BER) of the system and finding the capacity of the channel. The expressions for BER are analyzed for the cases when additive white Gaussian noise and fading are present in the channel. Due to the same statistical characteristics of the spreading sequences and the noise that represents the channel, the system is called Noise Based Phase Shift Keying (NBPSK) CDMA system. Because the transmitted signal is a sum of random signals the proposed system can be used to enhance the security in signal transmission that will depend on the goodness of random sequence generators that produce spreading sequences. Due to the Gaussian distribution of the CDMA signal at the input of channel, the capacity of the channel is confirmed to be equivalent to the maximal theoretically achievable value for the white Gaussian noise channel.