Performance of direct-sequence spread-spectrum multiple-access systems in mobile radio

Abstract

The performance of coherent directsequence spread-spectrum multiple-access (DS/SSMA) communications over different mobile radio channels is investigated. Analytical techniques and numerical methods were used for a channel having a number of resolvable, discrete Rayleigh fading paths, while simulations were employed for transmissions over urban mobile radio channels. We found that even a flat Rayleigh fading channel caused a severe degradation in system performance compared with the additive white Gaussian noise channel, and that the system was unusable even for a few users when a wideband urban mobile radio channel was used. By transmitting a sounding sequence of chip length Ns = 511 to allow the receiver to estimate the channel impulse response, spreading the symbol over N = 127 chips, and on deploying fifth-order combinative space diversity equipped with adaptive correlation diversity of Kβ = 1.0 (CD/ACD), we were able to obtain a residual probability of bit error PR of less than 10−3 for 10 users, in spite of the urban mobile radio channel having 19 resolvable fading paths. By increasing N to 511 and deploying fifth-order maximal-ratio-combining (MRC) space diversity, a PR of 4 × 10−5 for 20 users was achieved when the radio channel consisted of 10 resolvable independent Rayleigh fading paths.