Asynchronous Direct-sequence Code-division Multiple-access Systems Research Articles

Improving performance of DS-CDMA systems using chaotic complex Bernoulli spreading codes

The most important goal of spreading spectrum communication system is to protect communication signals against interference and exploitation of information by unintended listeners. In fact, low probability of detection and low probability of intercept are two important parameters to increase the performance of the system. In Direct Sequence Code Division Multiple Access (DS-CDMA) systems, these properties are achieved by multiplying the data information in spreading sequences. Chaotic sequences, with their particular properties, have numerous applications in constructing spreading codes. Using one-dimensional Bernoulli chaotic sequence as spreading code is proposed in literature previously. The main feature of this sequence is its negative auto-correlation at lag of 1, which with proper design, leads to increase in efficiency of the communication system based on these codes. On the other hand, employing the complex chaotic sequences as spreading sequence also has been discussed in several papers. In this paper, use of two-dimensional Bernoulli chaotic sequences is proposed as spreading codes. The performance of a multi-user synchronous and asynchronous DS-CDMA system will be evaluated by applying these sequences under Additive White Gaussian Noise (AWGN) and fading channel. Simulation results indicate improvement of the performance in comparison with conventional spreading codes like Gold codes as well as similar complex chaotic spreading sequences. Similar to one-dimensional Bernoulli chaotic sequences, the proposed sequences also have negative auto-correlation. Besides, construction of complex sequences with lower average cross-correlation is possible with the proposed method.

Analysis of BPSK modulated asynchronous band-limited DS-CDMA systems with diversity receivers over Generalised-K fading channels

The authors studied bit-error rate (BER) performance of asynchronous band-limited direct-sequence code-division multiple-access (DS-CDMA) systems with various diversity-combining receivers over Generalised-K fading channels. The effects of band-limited pulse shapes, multitone jamming, multiple-access interference as well as both flat and frequency-selective fading are considered. The Generalised-K model is adopted in order to include the effects of shadowing and fading of a wireless channel. The authors consider binary phase-shift keying as the modulation technique. The analytical expressions are valid for any arbitrary value of Generalised-K distribution parameters. Two types of band-limited pulses, namely spectrum raised cosine and Beaulieu–Tan–Damen (BTD) pulses, are incorporated in the analysis. Numerical results show that the system with BTD pulse outperforms the one with SRC pulse for various diversity-combining receivers under various channel conditions. Furthermore, by incorporating a minimum mean-square error stage in the multipath diversity receiver, the BER performance can be further improved.

Joint Iterative Multiuser Detection and Channel Estimation for Differentially Coded Asynchronous CDMA Systems

In this paper, we propose a receiver structure that performs joint iterative multiuser detection and channel estimation for the uplink of differentially coded asynchronous direct-sequence code-division multiple-access systems in unknown multipath channels. The overall transmission scheme for each user is the serial concatenation of a recursive systematic convolutional (RSC) encoder and a differential encoder separated by an interleaver. The proposed receiver consists of two stages. The first stage performs channel estimation, soft interference cancellation, and soft-input soft-output multiuser filtering, followed by the second stage, which consists of a bank of serially concatenated single-user channel decoders and differential decoders. The single-user iterative decoder for each user consists of a powerful combination of an RSC decoder and a differential decoder, which work together in an iterative fashion. In terms of channel estimation, at the first iteration, the multipath channel is blindly estimated by exploiting the orthogonality property between the signal and noise subspaces, whereas, for the next iterations, the channel is estimated using the soft estimates of coded bits of each user provided by the single-user iterative decoders in conjunction with the output of the multiuser detector. By exchanging soft information between the different stages, the receiver performance is improved through iteration. Simulation results demonstrate that the performance of the proposed iterative multiuser detector with the integrated channel estimator approaches the single-user bound at high signal-to-noise ratio.

Interference Identification and Blind Multiuser Detection for Asynchronous CDMA Systems With Multipath Fading

We propose a blind user identification detection (UID) scheme based on linear prediction algorithm for asynchronous direct-sequence code-division multiple-access systems over multipath fading channels. Numerical results show that both the false alarm rate and bit error rate of the proposed algorithm are better than those of the existing subspace-based UID algorithms.

Minimum probability of error-based adaptive multiuser decision feedback detection

For synchronous direct-sequence code-division multiple access (DS-CDMA) systems, a new criterion based on minimum probability of error (MPE) is proposed for designing decision feedback detectors (DFDs) instead of the conventional minimum mean-squared error criterion. The MPE criterion-based DFD (MPEDFD) tries to minimize the average/conditional probability of error adaptively. Study of bit-error rate and convergence performance shows that the MPEDFDs work pretty well after a short training sequence. Moreover, the modified MPEDFD is applicable for the asynchronous DS-CDMA systems with multipath.

Precise analysis of bit-error probability for asynchronous multicode DS-CDMA systems

A precise bit-error probability (BEP) analysis method is derived for a multicode direct-sequence code-division multiple-access (DS-CDMA) system in an additive white Gaussian noise channel. The method is applicable to a multicode DS-CDMA system with an arbitrary number of multiple code sequences, and any selection of multiple code sequences. The precise method gives results that discriminate the effect of the selection of different multiple code sequences on the BEP, whereas Gaussian approximations (GAs) do not. Thus, the new method can be used to select the best multicode set for a given system, a task that cannot be achieved using GAs. A two-step analytical procedure enables deriving an explicit, compact form for the CF of the receiver decision statistic in a DS-CDMA system with an arbitrary number of multiple code sequences, and for any selection of multiple code sequences.

Accurate computation of bit error probabilities of band-limited asynchronous DS-CDMA systems using higher order moments

An existing method called the simplified improved Gaussian approximation (SIGA) was previously applied to compute the bit error probabilities (BEPs) of band-limited asynchronous direct sequence code division multiple access (DS-CDMA) systems employing general pulse shaping (IEEE Trans. Commun., vol. 50, p. 656, 2002). The SIGA method uses moments up to the second order and is more accurate than the standard Gaussian approximation (SGA) (IEEE Trans. Commun., vol. 50, p. 656, 2002). In this paper, a new method that uses moments up to the fourth order is proposed for computing the BEP. The method is derived from a five-point Chebyshev interpolation formula and is inherently more accurate than the SIGA. Like the SIGA, the new method requires the evaluation of only closed-form expressions and the error function. The new method achieves higher accuracy with a modest increase in computational complexity.

A Theoretical Evaluation of Parallel Interference Cancellation in M-Ary Orthogonal Modulated Asynchronous DS-CDMA System Over Multipath Rayleigh Fading Channels

In this paper, we tackle the problem of theoretical evaluation for the multistage parallel interference cancellation (PIC) scheme in a direct-sequence code division multiple access (DS-CDMA) system with orthogonal modulation and long scrambling codes. The studied system operates on the reverse link in a time varying multipath Rayleigh fading channel. By applying the Central Limit Theorem and some other approximations to multiple access interference (MAI) and intersymbol interference (ISI), as well as assuming identically distributed chips from a single interferer, the bit error rate (BER) performance of the PIC scheme at any stage can be recursively computed from the signal-to-noise ratio, number of users, the number of path per user, processing gain of the CDMA system, and the average received power of each path. For completeness, the BER expression is derived for chip synchronous and chip asynchronous systems over both equal and unequal power multipath channels. The proposed analysis is validated by the Monte Carlo simulations and proved to be reasonably accurate, and it gives insight into the performance and capacity one can expect from PIC-based receivers under different situations. For instance, the analytical results can be used to examine the convergence property, multipath diversity gains, and near-far resistance of the PIC scheme.

Analysis of beamformer configurations for DS-CDMA systems

Two different beamformer configurations for the base station receiver in direct-sequence code-division multiple access (DS-CDMA) systems, namely, a chip-based and a symbol-based configuration, are studied. In the chip-based configuration, different interfering components are rejected based on the spatial distribution of their power. In the symbol-based configuration, spatial diversity is exploited after despreading and different interfering components are rejected based on their interfering strength, which depends on both their power and code correlation with the signal of interest. For the symbol-based configuration, more effort is applied to rejecting the interfering components with higher interfering strength, and thus, a more selective and efficient system is achieved. Detailed performance analysis and simulations show that in the presence of multiple-access interference, the symbol-based configuration can lead to a significant improvement in the signal-to-interference-plus-noise ratio relative to that achieved with the chip-based configuration for both asynchronous and synchronous DS-CDMA systems.

Performance of iterative multiuser receivers for turbo-coded asynchronous DS-CDMA systems over flat-fading channels

The performance of iterative multiuser receivers in a turbo-coded asynchronous direct-sequence code-division multiple-access system over a fully interleaved Rayleigh flat-fading channel is presented. The full-complexity soft-in/soft-out (SISO) multiuser detector based on the logarithmic maximum a posteriori probability (log-MAP) algorithm has a computational complexity growing exponentially with the number of users. It is shown that the computational complexity can be significantly reduced by replacing the MAP multiuser detector in the receiver with suboptimal SISO detectors. A novel hybrid reduced-state algorithm that effectively combines the advantages of the T-algorithm and the channel memory reduction algorithm is proposed. The computational complexity of the proposed detector is determined by the reduced channel memory length and can be further reduced adaptively based on channel conditions. Simulation results show that a substantial reduction in complexity can be obtained at the expense of slight degradation in performance compared with the full-complexity MAP detector. The proposed algorithm is compared with other reduced-complexity schemes and the superiority of the proposed algorithm in terms of performance/complexity tradeoffs is demonstrated.

Effects of Array Weight Errors on Interference Cancellation Receiver in Uplink Synchronous and Asynchronous DS-CDMA Systems

This paper investigates the impacts of array weight errors (AWE) in an antenna array (AA) on a parallel interference cancellation (PIC) receiver in uplink synchronous and asynchronous direct sequence code division multiple access (DS-CDMA) systems. The performance degradation due to an AWE, which is approximated by a Gaussian distributed random variable, is estimated as a function of the variance of the AWE. Theoretical analysis, confirmed by simulation, demonstrates the tradeoffs encountered between system parameters such as the number of antennas and the variance of the AWE in terms of the achievable average bit error rate and the user capacity. Numerical results show that the performance of the PIC with the AA in the DS-CDMA uplink is sensitive to the AWE. However, either a larger number of antennas or uplink synchronous transmissions have the potential of reducing the overall sensitivity, and thus improving its performance.

Code-Timing Estimation for CDMA Systems With Bandlimited Chip Waveforms

In this paper, we present a novel code-timing estimator for uplink asynchronous direct-sequence code-division multiple-access systems utilizing bandlimited chip waveforms. The proposed estimator requires only the spreading code and training of the desired user. We start from a maximum likelihood (ML) approach that models the intersymbol interference and multiple-access interference as a colored Gaussian process with unknown covariance matrix in the frequency domain. The exact ML estimator is highly nonlinear and requires iterative searches over multi-dimensional parameter space that is impractical to implement. To deal with this difficulty, we invoke asymptotic (large-sample) approximations of the ML criterion and reparameterization techniques, which lead to an asymptotic ML estimator that yields code-timing and channel estimates via efficient noniterative quadratic optimizations. To benchmark the proposed estimator, we provide Crame/spl acute/r-Rao bound analysis for the code-timing estimation problem. Numerical simulation results are presented, which show that the proposed scheme is resistant to interference, fading, and modeling errors (e.g., sampling position errors), and compares favorably to several competing schemes in multipath fading channels.

Near-far resistance of linear receivers in chip-asynchronous CDMA systems with random signatures

The paper quantifies the robustness of linear receivers, such as minimum mean-squared error and decorrelating receivers against the near-far problem in chip asynchronous direct-sequence code-division multiple-access systems with random signatures, where both of those linear receivers achieve the optimum near-far resistance. Due to the asynchronism of the system, different symbols of a user see different interference structures which affect their performance differently. Thus, instead of concentrating on any particular symbol of a user, we analyze the average optimum near-far resistance of a randomly selected symbol of that user by providing simple tight upper and lower bounds on it.

Chip-locked space-time filtering for maximizing SINR in asynchronous DS-CDMA systems

This paper considers a chip-locked space-time (CLST) filtering technique for direct-sequence code-division multiple access (DS-CDMA) systems. CLST filtering exploits the knowledge of the multiple access interference (MAI) chip delays, as well as the fact that the MAI spectrum is colored in the time and space domains. Chip delays of interferers from the same cell as the desired user are available at the base station and can be used to improve performance of single-user receivers. CLST filtering reduces MAI through joint optimization of spatio-temporal filtering and exploitation of chip delays of locked interference, without the need of interferers spreading codes. A significant improvement in signal-to-interference plus noise ratio can be achieved through CLST filtering with respect to the case when the interference is unlocked. The capabilities of CLST filtering to suppress chip-delay-locked interference improves with increasing chip waveform excess bandwidth. Numerical results show that CLST filtering already provides significant performance gains with square-root raised cosine chip pulses of small excess bandwidth. Furthermore, it is also shown that CLST filtering for a long observation interval is suitable for DS-CDMA systems employing long sequence spreading.

DOA estimation in the asynchronous DS-CDMA system

It was previously shown that the number of array elements must exceed the number of sources for subspace-based directional-of-arrival (DOA) estimation. This is clearly not practical for a direct-sequence code-division multiple access (DS-CDMA) wireless system since the number of mobile users is very large. To overcome the restriction, a DOA's estimator employing the code-matched filters and parallel MUSIC algorithms is proposed. In the asynchronous DS-CDMA system, we show that the multiple access interference (MAI) throughout the code-matched filter tends toward the real Gaussian distribution with zero mean. In addition, based on the singular value decomposition (SVD) of a finite amount of data, we achieve a theoretical expression for the mean-squared error (MSE) of the DOA's estimation. With the advantage of code-matched filter inherent in the DS-CDMA system, we prove that the proposed technique can obtain an unbiased DOA estimation with low MSE, even in the environment where the number of mobile users far exceeds the number of array elements. Simulation results verify the theoretical derivation.

Performance analysis for multistage interference cancellers in asynchronous DS-CDMA systems

The authors propose an accurate analytical method for an asynchronous direct-sequence code-division multiple-access system using a multistage interference cancellation technique. In this analysis, the RAKE receiver is considered over uplink fading channels. Monte Carlo simulations show that the proposed analysis agrees more closely with simulation results than previous analyzes.

A simple and accurate method of probability of bit error analysis for asynchronous band-limited DS-CDMA systems

This paper considers probability of bit error (P/sub e/) analysis in asynchronous band-limited direct-sequence code-division multiple-access (DS-CDMA) systems. It presents a simple and accurate method of P/sub e/ analysis. The proposed method can serve as an attractive alternative to the only two techniques currently available for band-limited systems: the standard Gaussian approximation (SGA) and the characteristic function method. The former is prone to inaccuracy while the latter, large computational complexity. The method generalizes the simplified improved Gaussian approximation (SIGA) derived previously for rectangular pulses. This paper also outlines a generalization of another method referred to as the improved Gaussian approximation (IGA). Numerical examples demonstrate the far greater accuracy of the generalized SIGA with respect to the SGA. The examples consider the IS-95 and square-root raised cosine (Sqrt-RC) pulses as well as uniform and nonuniform received power conditions.

Chaos-based asynchronous DS-CDMA systems and enhanced rake receivers: measuring the improvements

Results of recent theoretical investigations highlighted that the use of chaos in direct-sequence code-division multiple access (DS-CDMA) systems may lead to nonnegligible improvements in communication quality for several scenarios. We here briefly review the main steps in this derivation and report the experimental verifications of the corresponding theoretical prediction. With this, we confirm that chaos-based spreading sequences outperform classical pseudo-random sequences in at least two important cases. Over nonselective channels, the ability of chaos-based spreading of minimizing multiple-access interference leads to a measured 60% improvement in P/sub err/ with respect to classical spreading. Over selective channels, the possibility of jointly optimizing chaos-based spreading and Rake receiver profiles leads to improvements of up to 38% in P/sub err/ with respect to systems with either conventional spreading or conventional Rake policies.

A comparison of long versus short spreading sequences in coded asynchronous DS-CDMA systems

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.

An asynchronous multirate decorrelator

This paper examines truncated window decorrelators for an asynchronous direct-sequence code-division multiple-access system supporting users transmitting at different bit rates. We decode a user by extending the observation window over a sufficient number of its bits. To characterize practical window sizes, simple upper and lower bounds for the asymptotic efficiency of both the truncated window and infinite window decorrelators are developed. Empirical results show that as the length of the observation window increases, the bounds converge rapidly to the asymptotic efficiency of the infinite window decorrelator. The complexity of the receiver depends strongly on the ratio of the maximum to minimum bit rates.