Direct-sequence Spread-spectrum Multiple-access Communication System 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.

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.

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.

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.

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.

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.

An asymptotic analysis of DS/SSMA communication systems with random polyphase signature sequences

Direct-sequence spread-spectrum multiple-access (DS/SSMA) communication systems with random m-phase sequences, for even m, are considered. By examining the asymptotic behavior of the normalized multiple-access interference (MAI), the authors find that a system with random m-phase sequences, for m>4, should have the same performance as one with random quadriphase sequences asymptotically. However, a system, with random m-phase sequences, for m/spl ges/4, may perform better than one with random binary sequences when the number of simultaneous users is relatively small. A new Gaussian approximation is proposed to estimate the probabilities of error in these systems. In two cases, the new approximation reduces to established results which have been shown to provide close estimates to the probabilities of error.

Performances of DS/SSMA communications with MPSK signaling and complex signature sequences

We study a direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system with M-ary phase-shift-keyed (MPSK) signaling. Users in the system employ complex signature sequences, which are modeled as deterministic sequences. We present upper and lower bounds for the probability of bit error (p.b.e.) of a reference transmission. First, the conditional p.b.e., given the multiple-user interference (MUI), is obtained. Then, the conditioning is removed by averaging over the discretized probability density function (p.d.f.) of the MUI. The result is an upper (or lower) bound when the discretized p.d.f. is a shifted-high (or shifted-low) version of a sectioned p.d.f. This process requires the two-dimensional p.d.f. of the MUI, which is derived. Arbitrarily tight bounds can be obtained at the cost of increased computation. Numerical results are presented for complex FZC and near optimal 4-phase sequences, as well as real binary Gold sequences, for BPSK, 4-PSK, and 8-PSK. Gray code bit mapping is assumed, although the analysis can be applied to other bit mappings. Results show that complex sequences can yield better performance than binary sequences in MPSK signaling. Using complex sequences, the number of available signature sequences is greatly enlarged. >

Generalised Barker-like PN sequences for quasisynchronous spread-spectrum multiple-access communication systems

The generalised Barker (1953)-like pseudonoise (PN) sequence is introduced. Three theorems are established for such a set of sequences. A computer search algorithm is derived based on these theorems to search for the generalised Barker-like PN sequences employing two design parameters m and l. The multiple-access-interference (MAI) performance of a quasisynchronous direct-sequence spread-spectrum multiple-access communication system (QS-DS/SSMA) employing the generalised Barker-like PN sequence is examined and compared with an asynchronous DS/SSMA communication system employing random binary sequence and Gold sequence. It was found that the MAI performance of the QS-DS/SSMA system is superior when both m and l are small satisfying the inequality 2N≥l2+3m (m-1)+1, where N is the period of sequences. It was also shown that the bit-error-rate performance of a synchronous DS/SSMA system employing the generalised Barker-like PN sequence is 13.65 dB superior to that of the Gold sequence.

A characterization of multiple-access interference in generalized quadriphase spread-spectrum communications

A proposition that is useful for studying multiple-access interference (MAI) in generalized quadriphase spread-spectrum systems is established. The proposition provides the conditional density functions and the conditional cumulative distribution functions of random variables that characterize the MAI. It is shown that these characterizing random variables converge almost surely to the true MAI. The proposition provides a method to study the effect of the chip waveform on the performance of the system and provides a means for the design and analysis of spectrally efficient systems with continuous-phase or M-ary phase-shift-keyed modulation. The proposition allows the evaluation of the average error probability with arbitrary accuracy without making a Gaussian approximation for the MAI and is also useful for studying systems that send data in packets, where the delays of the interferers are often fixed for the duration of a packet. For illustration, the proposition is applied to computing the average error probability of generalized quadriphase direct-sequence spread-spectrum multiple-access communication systems with various chip waveforms and offset parameters corresponding to commonly used modulation schemes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Joint signal detection and parameter estimation in multiuser communications

The problem of simultaneously detecting the information bits and estimating signal amplitudes and phases in a K-user asynchronous direct-sequence spread-spectrum multiple-access communication system is addressed. The joint maximum-likelihood (ML) estimator has a computational complexity that is exponential in the total number of bits transmitted and thus does not represent a practical solution to the problem. An estimator that combines a suboptimum tree-search algorithm with a recursive least-squares estimator of complex signal amplitude is considered. The complexity of this estimator is O(K/sup 2/) computations per decoded bit, and its performance is very close to that of the joint ML receiver. This receiver has the advantage that the transmitted signal powers and phases are extracted from the received signal in an adaptive fashion without using a test sequence.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Serial MSK spread-spectrum multiple-access communications

A serial minimum-shift-keyed (MSK) direct-sequence spread-spectrum multiple-access (DS/SSMA) communication system is described and analyzed to obtain the signal-to-noise ratio. The single-channel system generates the same transmitted signals as a corresponding parallel MSK/DS/SSMA system. Hence, the transmitted signals have a constant envelope and desirable spectral properties. However, hardware is required for only one channel, and the multiple-access capability of the system compares favorably with a standard binary DS/SSMA system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance of binary and quaternary direct-sequence spread-spectrum multiple-access systems with random signature sequences

The performance of synchronous and asynchronous, binary and quaternary (with and without offset) direct-sequence spread-spectrum multiple-access (DS/SSMA) communication systems using random signature sequences and arbitrary chip waveforms is investigated. The average probability of error at the output of the correlation receiver is evaluated using a characteristic-function approach for these systems. Numerical results are presented that illustrate performance comparisons between systems using random and deterministic signature sequences, synchronous and asynchronous systems, systems with rectangular or sinewave chip waveforms, and binary and quaternary systems with the same data rates and bandwidth. In all cases, the accuracy of the Gaussian approximation is also examined. >

Spread-spectrum multiple-access performance of orthogonal codes: impulsive noise

A direct-sequence spread-spectrum multiple-access (SSMA) communication system that assigned a set of M-orthogonal sequences to each user is analyzed. An accurate model is incorporated for the impulsive noise that characterizes the LF and MF bands, so that the SSMA receiver operates in a combination of multiple-access interference and impulsive (atmospheric) noise. The performance of a linear receiver operating in such an environment is analyzed, and probability-of-error curves are presented. The presence of impulsive noise motivates the derivation and analysis of a nonlinear receiver that use a variable-gain stage to suppress noise impulses. This receiver is effectively optimum when the signal amplitudes are below a certain bound and when the noise and interference samples are independent, or nearly so. However, the gain stage of this nearly optimum receiver depends on the noise model parameters including the various user delays. Consequently, a nonparametric receiver that incorporates a simple clipper is also analyzed. The asymptotic relative efficiency of both receivers is determined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Partial Correlation Effects in Direct-Sequence Spread-Spectrum Multiple-Access Communication Systems

Nearly all previous analytical results on the performance of direct-sequence spread-spectrum multiple-access (DS/SSMA) communication systems are restricted to systems in which the period <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</tex> of the signature sequence is equal to the number <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> of chips per data symbol. In many applications, however, it is necessary to employ signature sequences whose period <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</tex> is much larger than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> . Thus, successive <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> -chip segments of the signature sequence are used to phase-code the carrier during the transmission of successive data symbols. The performance of such systems depends on the partial correlation properties of the signature sequences (rather than the aperiodic correlation properties as in the case when <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N=p</tex> ). In this paper, we consider the performance of a DS/SSMA system for arbitrary values of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> . As special cases of our results, we delermine the effects of partial correlation in three classes of systems: those for which <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N =p</tex> , those for which <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</tex> are relatively prime, and those for which <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> is a divisor of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</tex> . We also provide two methods for the design of sequences for systems for which <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> is a divisor of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</tex> .

Conditions for synchronisation and corresponding user capacities for spread-spectrum multiple-access systems

User capacities for direct-sequence spread-spectrum multiple-access communication systems are obtained, for the situation where a receiver has to synchronise to its wanted signal in the presence of other users&apos; transmissions, under all relative phase conditions. The use of both conventional and rapid-acquisition codes is considered.