Optimum modulation and demodulation techniques and channel coding schemes for spread spectrum multiple access systems

Abstract

Spread-spectrum (SS) communication systems have been extensively investigated and considered for practical applications: typical examples of the applications of SS communication systems are those in military communication and in commercial mobile communication. There exist primarily two SS techniques. One is the direct sequence (DS) SS, which is fundamentally an amplitude modulation technique, and the other is the frequency hopping (FH) SS, which is a frequency modulation technique. An SS communication system differs markedly from the time division (TD) and frequency division (FD) multiple access (MA) communication systems in that each transmitted signal in the SS communication system uses the entire frequency band of the system and the entire time of the communication. First, we analyze the performance of direct-sequence spread spectrum multiple access (DS/SSMA) systems, which use trellis coding with asymmetric phase shift keying (PSK) modulation. By designing the signal constellation to be asymmetric, we obtain a performance gain for the DS/SSMA system over the systems using the traditional symmetric signal constellation. Bit error probability analysis of the proposed system is carried out for some cases. Examples are given to show the performance gain due to the asymmetry of the signal constellation. Secondly, we investigate the inherent noise reduction characteristic of a modulated orthogonal sequence. Using the amplitudes of the received sequences, we first estimate the smallest amplitude noise. Then the noise is reduced by the suggested system. Next, a new generation and an information construction methods for a modulated orthogonal sequence are suggested: with the new method, we can generate code sequences by only integer sums and modular techniques. The autocorrelation and cross-correlation characteristics of the sequence are investigated via a new procedure. A modified sequence also having the orthogonality and satisfying the mathematical lower boun...