Wideband time-division and spread-spectrum multiple-access methods

The application of spread-spectrum techniques to digital mobile radio transmissions is addressed. A detailed review of the principles of cellular mobile radio systems, with particular emphasis on multiple-access methods and radio propagation is provided. When work on the thesis commenced the front-runner of the competing multiple-access (MA) systems for the pan-European digital cellular mobile radio system was the wideband time-division multiple-access (WB-TDMA) arrangement known as CD900. As a consequence a major objective of our research was to evaluate and hence to innovate ways of improving the performance of CD900. Although CD900 employed time-division multiple-access (TDMA), it also utilised spectrum spreading, converting 6-bit symbols into 31 or 32-chip code words. Our enhancements of CD900 included the introduction of correlation diversity (CD), adaptive CD (ACD), and the embedding of ACD into combinative (CD/ACD) and selective (SD/ACD) space diversity schemes of order Ms. These modifications yielded substantial reductions in symbol error rate (SER) for transmissions over urban mobile radio channels. Specifically, the basic CD900 had an asymptotic SER of the order of only 10-1. However, the introduction of CD/ACD operated with Ms= 5 significantly reduced the asymptotic SER, and for a SER of 10^-3 the channel SNR was only some 5 dB larger than that for a Gaussian channel. In addition, the SER performance of the CD900 was obtained for transmissions over Gaussian, flat Rayleigh fading, log-normal fading, and indoor wireless channels. Direct-sequence spread-spectrum multiple-access (DS/SSMA) communications was studied. For a Gaussian channel, the bit error rate (BER) performance was evaluated by the characteristic function, series expansion, Gauss-Quadrature Rule (GQR), Gaussian approximation and Monte-Carlo methods. The GQR was found to be the most computationally efficient and accurate, and was therefore employed when transmissions over a channel having a number of discrete, Rayleigh faded paths was investigated. We found asymptotic BER (P_R) of 2.2x10^-1 and 10^-1 when the number of multiple-access users K= 10 for the sequence length N= 31 and N= 127, respectively. When 5-th order switched (selective) diversity was used, the corresponding P_R decreased to 10^-1 and 10^-2; whereas the counterparts for MRC diversity were 3x10^-2 and 5x10^-4, respectively. The DS/SSMA over an urban mobile radio channel utilizing the ACD and space diversity schemes were investigated. Similar results to those obtained for the channel having a number of discrete, Rayleigh fading paths were achieved. Whereas the CD900 enhanced by our innovations provided a performance over wideband mobile radio channel which approaches that for a Gaussian channel, the DS/SSMA system performance degraded exponentially with the number of users, and was found to be significantly inferior to the CD900 in mobile radio applications. (DX84412)