Asynchronous Spread Spectrum Multiple Access Research Articles

Performance Analysis of SSMA Communication Systems With Spreading Sequences of Markov Chains: Large Deviations Principle Versus the Central Limit Theorem

In this paper, we consider asynchronous spread spectrum multiple access (SSMA) communication systems with spreading sequences of Markov chains. We show that, given phase shifts and time delays, the empirical mean of the multiple access interference (MAI) for such systems satisfies the conditional large deviations principle (LDP) with the convex, good rate function. By refinement of the large deviations analysis, we also obtain exact asymptotics of bit-error probabilities in such systems. Comparing theoretical expressions of bit-error probabilities with experimental results, we confirm that for too small numbers of users compared to the lengths of spreading sequences, the central limit asymptotics become invalid, but the large deviations asymptotics turn out to be relevant.

Phase-Shift-Free $M$-Phase Spreading Sequences of Markov Chains

We consider the optimum spreading sequences of Markov chains in terms of the mean value of the limit of variance of the normalized multiple-access interference for asynchronous spread-spectrum multiple access communication systems. We first define a novel class of spreading sequences, namely the phase-shift-free M(ges 3)-phase spreading sequences. Then we theoretically and experimentally show that the optimum phase-shift-free M-phase spreading sequences of Markov chains are superior to the optimum binary spreading sequences of Markov chains in terms of the bit-error probabilities based on the central limit theorem for periods N > 5.

Performance of UWB SSMA using orthogonal PPM-TH over dense multipath channels

In this work we study ultra wideband (UWB) communications over dense multipath channels using orthogonal pulse position modulation (PPM) for data modulation and time-hopping (TH) for code modulation. We consider the effects of the multiple access interference (MUI) in asynchronous spread spectrum multiple access (SSMA) based on random TH codes. We use a realistic multipath channel to analyze the effects of the transmission rate in the number of users for different bit error rate (BER) values.

Design of Optimum M-Phase Spreading Sequences of Markov Chains

We design M(≥3)-phase spreading sequences of Markov chains optimal in terms of bit error probabilities in asynchronous SSMA (spread spectrum multiple access) communication systems. To this end, we obtain the distributions of the normalized MAI (multiple access interference) for such systems and find a necessary and sufficient condition that the distributions become independent of the phase shifts.

The Central Limit Theorem for the Normalized Sums of the MAI for SSMA Communication Systems Using Spreading Sequences of Markov Chains

We extend the sliding block code in symbolic dynamics to transform J(≥2) sequences of Markov chains with time delays. Under the assumption that the chains are irreducible and aperiodic, we prove the central limit theorem (CLT) for the normalized sums of extended sliding block codes from J sequences of Markov chains. We apply the theorem to the system analysis of asynchronous spread spectrum multiple access (SSMA) communication systems using spreading sequences of Markov chains. We find that the standard Gaussian approximation (SGA) for estimations of bit error probabilities in such systems is the 0-th order approximation of the evaluation based on the CLT. We also provide a simple theoretical evaluation of bit error probabilities in such systems, which agrees properly with the experimental results even for the systems with small number of users and low length of spreading sequences.

Multiuser detectors with decision feedback for asynchronous spread-spectrum multiple access in multipath fading channels

This paper extends earlier work of the authors to the multipath fading channel. For a normalized Doppler spread of f/sub D/T=0.005 we show that a reduced state sequence estimation reduced-complexity receiver with K/sub s/=2 users per state and a processing gain of N/sub s/=127 performs only 0.5 dB worse than the single-user receiver for a bit-error rate of 10/sup -4/.

Modulatable orthogonal sequences and their application to SSMA systems

A set of N-1 orthogonal sequences of period N/sup 2/ is proposed, where N is a natural number. Each orthogonal sequence proposed can be modulated by N complex numbers of absolute value 1, so the modulated sequence is also orthogonal. When N is an odd prime number, the absolute value of the cross-correlation function between any two of the N-1 orthogonal sequences is constant and satisfies the mathematical lower bound. This property of the cross-correlation function is not changed when each of the two orthogonal sequences is modulated by N complex numbers of absolute value 1. Two spread-spectrum multiple-access (SSMA) systems using these sequences are proposed. One system is an asynchronous SSMA system, using the proposed sequences unmodulated. The cochannel interference peak between any two channels in this system realizes the mathematical lower bound for an asynchronous SSMA system using a set of orthogonal sequences. The other system is a synchronous SSMA system without cochannel interference which uses the modulated form of the proposed sequences. >

Cancellation techniques of co‐channel interference in asynchronous spread spectrum multiple access systems

AbstractA spread spectrum multiple access (SSMA) communication scheme employing asynchronous code‐multiplexing technique has been used in both mobile and satellite communication systems. One shortcoming of the SSMA system is the co‐channel interference due to the correlations among the pseudo noise (PN) sequences being used by multiple channels. Such co‐channel interference in the reception limits the number of channels that can share one common frequency band. Furthermore, there is a distant‐dependent problem of different received‐powers from different transmitters.This paper describes a technique for cancelling the co‐channel interference in asynchronous SSMA systems. This technique does not treat the co‐channel interference as thermal noise. It considers the co‐channel interference as components that can be recreated at each receiver. If the correlations among the PN sequences of all the active channels become known, the co‐channel interference can be cancelled using the linear algebraic approach. As a result, a larger number of channels can share the same frequency band more efficiently. Further, if initial synchronization can be implemented then the distant‐dependent problem can be alleviated.

Error Probability of Asynchronous Spread Spectrum Multiple Access Communication Systems

Several approaches for the evaluation of upper and lower bounds on error probability of asynchronous spread spectrum multiple access communication systems are presented. These bounds are obtained by utilizing an isomorphism theorem in the theory of moment spaces. From this theorem, we generate closed, compact, and convex bodies, where one of the coordinates represents error probability, while the other coordinate represents a generalized moment of the multiple access interference random variable. Derivations for the second moment, fourth moment, single exponential moment, and multiple exponential moment are given in terms of the partial cross correlations of the codes used in the system. Error bounds based on the use of these moments are obtained. By using a sufficient number of terms in the multiple exponential moment, upper and lower error bounds can be made arbitrarily tight. In that case, the error probability equals the multiple exponential moment of the multiple access interference random variable. An example using partial cross correlations based on codes generated from Gold's method is presented.