An accurate approximation is obtained for the average probability of error in an asynchronous binary direct-sequence spreadspectrum multiple-access communications system operating over nonselective and frequency-selective Rician fading channels. The approximation is based on the integration of the characteristic function of the multiple-access interference which now consists of specular and scatter components. For nonselective fading, the amount of computation required to evaluate this approximation grows linearly with the product KN , where K is the number of simultaneous transmitters and N is the number of chips per bit. For frequency-selective fading, the computational effort grows linearly with the product KN2. The resulting probability of error is also compared with an approximation based on the signal-to-noise ratio. Numerical results are presented for specific chip waveforms and signature sequences.

Direct-Sequence Spread-Spectrum Multiple-Access Communications Over Nonselective and Frequency-Selective Rician Fading Channels

The effects of the cross correlation between user codes in an opticalcode-division multiple-access communication system are investigated. The system model is a multiaccess satellite repeater in which the uplink and downlink channels are direct-detection, optical-polarization modulation links. The error probability is derived in terms of the cross correlation between the intended and interfering user codes. It is shown that the system error rate can be minimized by using code sequences in which the normalized second moment of the cross correlation between codes is small. The signalto- noise ratio (SNR) on the uplink is shown to be proportional to 1/K while the SNR on the downlink is proportional to 1/K1/2, where K is the number of users which are simultaneously accessing the system.

Cross-Correlation Interference Effects in Multiaccess Optical Communications

: The error performance of direct-sequence spread spectrum multiple-access communications systems on nonselective fading channels is investigated. The channels considered are those for which the fading intensity is slow, nonselective, and exhibit a generalized Rayleigh distribution. The generalized Rayleig distribution encompasses the important special cases of a Rician (specular plus Rayleigh) fading, Nakagami m-distributed fading, and Rayleigh fading. Results are obtained for the special cases of a single cochannel interferer without additive noise and multiple independent cochannel interferers with additive white Gaussian noise. The results are developed as a series expansion in the moments of the cross-correlation between the spreading codes (single interferer) or a series expansion in the powers of the second moment of the code cross-correlation (multiple interferers). To develop simple estimates of the probability of error, empirical estimates of the low-order cross-correlation moments of the spreading codes (taken to be Gold codes) are developed from a generalized gamma density approximation based on the cross-correlation moments for both Gold codes and random codes. Numerical results for the probability of error are obtained with these estimates and compared with the results based on exact moments developed from the code cross-correlation probability density. (Author)

/pdf/error-performance-of-direct-sequence-spread-spectrum-systems-2h3tfd1w1j.pdf

Error Performance of Direct Sequence Spread Spectrum Systems on Nonselective Generalized rayleigh Fading Channel.

This thesis describes an investigation into the application and performance of direct sequence spread spectrum techniques for land mobile radio systems.
There is a brief description of the basic principles of operation of direct sequence systems.
The multiple user facility is analysed and values obtained for the maximum number of simultaneous system users in terms of system parameters. This clearly illustrates the need for power control. A possible method of providing power control is described. Comparison of user density is made against conventional narrowband modulation methods. There is some discussion of the effect of sequence cross-correlations on the number of system users. The system organisation is mentioned, showing possible application of a calling channel.
Consideration is given to the possibilities of bandsharing with narrowband modulation systems. Figures are derived for the resulting interference to existing systems which would be caused by such an arrangement.
A brief resume of the pertinent features of the land mobile radio channel is given. The effects of shadowing on the output quality and spectral efficiency of direct sequence systems is discussed. There is an analysis of the effects of shadowing on the user density in small cell schemes. An analysis shows the effects of multipath propagation on direct sequence performance by reference to a simple two path channel.
Details are given of a simple experimental direct sequence spread spectrum transmitter and receiver constructed. The measured results for the performance of the system against various forms of interference and channel degredation are compared with their theoretical values.
Finally ideas for future work are discussed.

Direct sequence spread spectrum techniques for land mobile radio applications

The effects of fading on the performance of a spread spectrum multiple-access communication system using direct sequence modulation are investigated. General series expansions are developed for the average probability of error and specific results are given for the irreducible error rate associated with propagation over independent Rician fading channels or correlated Rayleigh fading channels. Simple single-term approximations for the irreducible error rate, which are proportional to the second moment of the cross-correlation between the spreading codes employed by the users, are derived for these cases.

C. Gardner

Papers

Error Performance of Direct Sequence Spread Spectrum Systems on Non-Selective Fading Channels