Title :
A comparison of long versus short spreading sequences in coded asynchronous DS-CDMA systems
Author :
Tang, Kai ; Siegel, Paul H. ; Milstein, Laurence B.
Author_Institution :
Dept. of Electr. & Comput. Eng., California Univ., San Diego, La Jolla, CA, USA
fDate :
8/1/2001 12:00:00 AM
Abstract :
The performance of turbo-coded asynchronous direct sequence code division multiple access (DS-CDMA) using long and short spreading sequences is compared by both analysis and simulation. For coded systems with a conventional matched filter (MF) receiver, three analytical methods with different complexity are compared: the standard Gaussian approximation, the improved Gaussian approximation (IGA), and the density function approach. It is shown that while the standard Gaussian approximation is fairly accurate for the long sequences, it is too optimistic for the short sequences. For the short-sequence systems, the IGA gives an accurate estimate for the performance with much less complexity than the density function approach. The analysis shows that for either the additive white Gaussian noise (AWGN) channel or the flat Rayleigh fading channel and a MF receiver, there is a degradation in the average performance of the turbo-coded short-sequence systems compared to the long-sequence systems due to the fact that the cross-correlations are not time-varying. However, the short-sequence systems are amenable to the use of an interference suppression technique designed to minimize the mean square error. Such a minimum mean square error (MMSE) receiver in the turbo-coded system is shown to outperform the long-sequence system with the MF receiver, especially when there is a near-far problem, as previously observed in a convolutionally-coded system. Finally, similar results are obtained by computer simulations for the turbo-coded CDMA systems on a frequency-selective Rayleigh fading channel
Keywords :
AWGN channels; Gaussian processes; Rayleigh channels; cellular radio; code division multiple access; correlation methods; interference suppression; least mean squares methods; matched filters; multiuser channels; radio receivers; radiofrequency interference; sequences; spread spectrum communication; turbo codes; AWGN channel; MMSE receiver; additive white Gaussian noise channel; average performance; cellular radio; complexity; computer simulations; convolutionally-coded system; cross-correlation; density function; direct sequence code division multiple access; flat Rayleigh fading channel; frequency-selective Rayleigh fading channel; improved Gaussian approximation; interference suppression; long spreading sequences; matched filter receiver; minimum mean square error; near-far problem; short spreading sequences; simulation; standard Gaussian approximation; turbo-coded asynchronous DS-CDMA systems; AWGN; Analytical models; Density functional theory; Direct-sequence code-division multiple access; Fading; Gaussian approximation; Mean square error methods; Multiaccess communication; Performance analysis; Turbo codes;
Journal_Title :
Selected Areas in Communications, IEEE Journal on