Spread-spectrum Multiple-access Communication Research Articles

Quaternary DS-SSMA Communications Employing Generalized Detector over Rayleigh Fading Channels

This paper deals with an employment of the complex modulators and generalized receivers constructed based on the generalized approach to signal processing (GASP) in noise in quaternary direct-sequence spread-spectrum multiple-access (DS-SSMA) communication systems with complex signature sequences in flat Rayleigh fading. Owing to availability of large sets of complex spreading sequences possessing perfect correlation characteristics, the interest to use the complex spreading sequences in DS-SSMA communication systems has increased. The complex spreading sequences investigated in the present paper are based on the recently introduced orthogonal unified complex Hadamard transform (UCHN) sequences. In the present paper, the complex processing in modulators and generalized receivers is also employed with the purpose to take advantage of the correlation properties of complex signature sequences. We derive the average bit error rate (BER) for quarternary synchronous communications system based on the generalized approach to signal processing in noise first, and then the BER for quaternary asynchronous ones is evaluated using the characteristic function approach. We presented some results based on the Gaussian approximation method for asynchronous communication systems constructed on the basis of the generalized approach to signal processing in noise Computer modeling results demonstrate that the communication systems using UCHT spreading sequences perform generally better than the GOLD sequences and the 4-phase family A sequences. Comparative analysis between the asynchronous communication systems constructed on the basis of the generalized approach to signal processing in noise demonstrates superiority over the asynchronous communication systems employing the correlation receiver.

Concatenated Codes with Two Inner Coding Schemes in Frequency-Hopping Spread Spectrum Multiple-Access Channels

In this paper, we analyze the performance of a concatenated code with two different inner decoding schemes. One is the error- detecting inner coding, and the other is the error-detecting-and-correcting inner coding scheme. We compare the performance of the two decoding schemes for finite and infinite block length cases when the concatenated code is applied to slow frequency-hopping spread-spectrum multiple access (FH-SSMA) communication systems. Our results show that the error-detecting-and-correcting inner decoding scheme outperforms the error-detecting inner decoding scheme for finite block lengths, while there exists a performance trade-off for infinite block length case.

Performance Comparison of Two Inner Coding Structures in Concatenated Codes for Frequency-Hopping Spread Spectrum Multiple-Access Communications

Performance Comparison of Two Inner Coding Structures in Concatenated Codes for Frequency-Hopping Spread Spectrum Multiple-Access Communications

ANALYSIS OF THE SEQUENCES WITH OPTIMAL CROSS-CORRELATION PROPERTY

The design of large family size with the optimal cross-correlation property is important in spread spectrum and code division multiple access communication systems. In this paper we present the sequences with the decimation $d

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.

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.

Average bit-error-rate performance of band-limited DS/SSMA communications

Direct-sequence spread-spectrum multiple-access (DS/SSMA) communications, strictly band-limited transmitter chip waveforms with excess bandwidth in the interval between zero and one, pseudo-random spreading sequences, an additive white Gaussian noise channel, and matched filter receivers are considered. First, a new expression for the average bit error rate (BER) is derived for systems with quaternary phase-shift keying (QPSK) spreading, the conventional matched filter receiver, a coherent detector for binary phase-shift keying (BPSK) data symbols, and chip waveforms that result in no interchip interference. The expression consists of a well known BER expression based on the standard Gaussian approximation to multiple-access interference and a few correction terms. It enables accurate BER evaluations without any numerical integration for various choices of system parameters of interest. The accuracy of the expression is guaranteed as long as the conditional Gaussian approximation to the cross-correlation coefficients between the desired user's spreading sequence and the interfering users' spreading sequences is valid. The expression well reflects the effect of filtering on the system performance. Extensions of the expression are discussed for systems with QPSK spreading and different detection schemes, systems with BPSK spreading, and systems with different transmit and receive filters. Monte Carlo simulation results are also provided to verify the accuracy.

Correlation properties of binary spatiotemporal chaotic sequences and their application to multiple access communication.

This paper studies the correlation property and the spectral density of the binary spatiotemporal chaotic sequences that are generated by the one-way coupled map lattice. The results show that this kind of chaotic sequences possesses excellent randomlike property and could be used directly in the spread spectrum multiple access (SSMA) communications. The multiple access interference in the additional white Gaussian noise background is then analyzed, and the corresponding formulas are presented. The simulation and computation results indicate that the communication system adopting such spreading sequences possesses as good performance as the one employing Gold sequences. But the former has larger capacity and higher complexity. Therefore, the binary spatiotemporal chaotic sequences are good candidates for the spreading sequences in SSMA communications.

Blind adaptive multiuser detection for DS/SSMA communications with generalized random spreading

A new spreading scheme and an accompanying blind adaptive receiver structure are proposed for direct-sequence spread-spectrum multiple-access communications in a slowly-varying, frequency-selective fading channel. Each user's spreading sequence is given by the Kronecker product of a long-period pseudonoise (PN) sequence, which is accurately modeled by a random sequence, and a short-length deterministic signature code. This spreading scheme bridges the gap between pure PN spreading and pure short-code spreading schemes. It is shown that with this spreading scheme, the channel response to the desired signal component is easily estimated without relying on the spectral decomposition of the signal correlation matrix. With the estimate of the channel response, the receiver suppresses interference based on the maximum signal-to-interference ratio criterion. The blind adaptive receiver requires only coarse timing information and a priori knowledge of the desired user's PN sequence for adaptation. Numerical results show that the adaptive receiver significantly suppresses interference by successfully estimating the channel response and the interference statistics with a low computational complexity. An extension to spatio-temporal processing using an array antenna is also discussed.

Performance of a spatio-temporal matched filter receiver for DS/SSMA communications

A spatio-temporal matched filter receiver for direct-sequence spread-spectrum multiple-access (DS/SSMA) communications with aperiodic random quadriphase spreading sequences is derived, and the system performance is analyzed. It is shown with the method of characteristic functions that the cross-correlation coefficients between the desired user's and the interfering users' spreading sequences tend, in distribution, to independently and identically distributed circularly symmetric complex Gaussian random variables as the processing gain goes to infinity. Based on this Gaussian approximation, the structure of the spatio-temporal matched filter receiver is derived and a bit error rate formula is obtained. Using Monte-Carlo simulations, as well as analytical methods, it is shown that the spatio-temporal matched filter receiver achieves a significant performance improvement over the conventional, temporal, and spatial matched filter receivers by effectively suppressing the multiple access interference.

A spread-spectrum communication system protecting information data from interception

This correspondence tries to construct a direct-sequence spread-spectrum multiple-access communication system, so that the difficulty of wiretapping from insiders' and outsiders' attacks increases and the worst case error probability decreases, as much as possible. Bent sequences with optimal periodic correlation properties are assigned to users, as spreading sequences corresponding to secret keys.

Overlapping M-ary frequency shift keying spread-spectrum multiple-access systems using random signal sequences

A multiple-access spread-spectrum communication system using binary frequency shift keying (BFSK) or M-ary frequency shift keying (MFSK) and noncoherent demodulation is considered. In contrast to previous work typically assuming that the frequency shift keying (FSK) tones are nonoverlapping after direct-sequence (DS) spreading, here we consider a spread-spectrum multiple-access (SSMA) system under the assumption that the DS spread signals of different FSK tones are only orthogonal over the information symbol duration. Consequently, the frequency band of a spread FSK tone may be fully or partially overlapping with the other spread signals. An estimate of the variance of the multiple-access interference is obtained by assuming that the phase angles and time delays of the received signals are mutually independent random variables, provided that random signature sequences are employed for spreading. On the basis of the above assumptions, the bit error rate (BER) of our DS spread-spectrum multiple-access (DS-SSMA) and that of our hybrid DS slow frequency-hopping spread-spectrum multiple-access (DS-SFH SSMA) systems using FSK modulation is analyzed, when the channel impairments are constituted by a combination of additive white Gaussian noise (AWGN) and multiple-access interference. From our analysis and the numerical results, we concluded that, for a given system bandwidth and for a certain value of M, the system's BER performance can be optimized by controlling the amount of overlapping and that the systems with optimized overlapping outperformed the systems using no overlapping.

An optimum control strategy of channel input traffic in FH/SSMA systems with central controllers

An optimum channel input-traffic control (OCIC) scheme is proposed and investigated for slotted frequency-hopped spread spectrum multiple access communication systems. The conditional throughput is obtained, the state transition probability is derived, the steady-state throughput is obtained, and the mean packet delay is calculated. It is shown that the performance of the OCIC scheme is better than that of other schemes. It is also shown that the smaller the number of frequency slots is, the more the OCIC scheme outperforms the other schemes.

TCM/SSMA communication systems with cascaded sequences and PAM/QAM signal sets

An expression in matrix form for the multiple-access interference (MAI) in an asynchronous direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system with cascaded sequences (CVs), arbitrary chip waveforms, and trellis-coded modulation (TCM) with a pulse amplitude modulation (PAM) or quadrature amplitude modulation (QAM) signal set is obtained. TCM provides significant coding gain while the CVs decrease the correlation between the MAI of adjacent data intervals. The expression is used to calculate arbitrarily accurate probability density functions (PDFs) of the MAI in the TCM system and to derive an accurate approximation of the MAI variance. It also helps illustrate some properties of the MAI by separating contributing parameters into different matrices. We derive an approximation of the upper union bound on the bit-error probability and investigate its applicability. The results show that CV schemes can greatly reduce the pairwise error probabilities (PEPs) until the length of the CV becomes greater than that of the error weight sequence (EWS) under consideration.

An asymptotic analysis of DS/SSMA communication systems with general linear modulation and error control coding

We investigate direct-sequence spread-spectrum multiple-access (DS/SSMA) communication systems with general linear modulation and error control coding. By the central limit theorem for martingale difference arrays, it is proved that under general conditions, the overall multiple-access interference (MAI) across different symbol intervals tends to a jointly complex Gaussian random vector as both the number of chips per symbol interval and the number of interferers tend to infinity. The result allows the calculation of the limiting exact error probabilities, and justifies the use of the popular standard Gaussian approximation. The asymptotic properties of the MAI from each individual interferer are also examined. The results lead to another approximation, the conditional Gaussian approximation, which provides better estimates to the error probabilities, especially in near-far situations.

A linear receiver for direct-sequence spread-spectrum multiple-access systems with antenna arrays and blind adaptation

A linear receiver for direct-sequence spread-spectrum multiple-access communication systems under the aperiodic random sequence model is considered. The receiver consists of the conventional matched filter followed by a tapped delay line with the provision of incorporating the use of antenna arrays. It has the ability of suppressing multiple-access interference (MAI) and narrowband interference in some weighted proportions, as well as combining multipath components without explicit estimation of any channel conditions. Under some specific simplified channel models, the receiver reduces to the minimum variance distortionless response beamformer, the RAKE receiver, a notch filter, or an MAI suppressor. The interference rejection capability is made possible through a suitable choice of weights in the tapped delay line. The optimal weights can be obtained by straightforward but computationally complex eigenanalysis. In order to reduce the computational complexity, a simple blind adaptive algorithm is also developed.

Bounds on the pairwise error probability of coded DS/SSMA communication systems in Rayleigh fading channels

Arbitrarily tight upper and lower bounds on the pairwise error probability (PEP) of a trellis-coded or convolutional-coded direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system over a Rayleigh fading channel are derived. A new set of probability density functions (PDFs) and cumulative distribution functions (CDFs) of the multiple-access interference (MAI) statistic is derived, and a modified bounding technique is proposed to obtain the bounds. The upper bounds and lower bounds together specify the accuracy of the resulting estimation of the PEP, and give an indication of the system error performance. Several suboptimum decoding schemes are proposed and their performances are compared to that of the optimum decoding scheme by the average pairwise error probability (APEP) values. The approach can be used to accurately study the multiple-access capability of the coded DS/SSMA system without numerical integrations.

Design of demodulator for the chaotic modulation communication system

Chaotic modulation has recently been proposed for spread spectrum (SS) and code division multiple access (CDMA) communication. It embeds the signal of transmission in the bifurcating parameter of a chaotic dynamical system, and uses the wide-band output of the chaotic system as the transmitted signal. Here we consider the demodulation of this communication scheme using an adaptive filter. Not only can an adaptive filter reduce the effect of channel noise, but it also estimates the bifurcating parameter (i.e., the signal of transmission) sequentially which is required in a communication system. Three kinds of adaptive filters: the least mean square (LMS) algorithm, the recursive least square (RLS) technique and the Kalman filter, are considered here. It is found that the demodulators based on these adaptive filters outperform the standard inversion approach used in the literature.