Direct Sequence Code Division Multiple Access Signals Research Articles

Spatial assessment of array antenna based joint SDMA-TDMA architecture for DS-CDMA signals

Fifth Generation (5G) technology is looking forward to coagulation of Multiple Access (MA) techniques for integrating individual benefits. Equidistant Direct-Sequence Code Division Multiple Access (DS-CDMA) signals with no relative separation by Propagation Delay suffer from severity of Multiple Access Interference (MAI). Joint implementation of Space Division Multiple Access (SDMA) and Time Division Multiple Access (TDMA) is proposed here in overcoming the limitations for equidistant sources. Spatial assessment toward effectiveness of DS-CDMA signals with appropriate beamforming in SDMA-TDMA architecture is explored here. Quadriphase Shift Keying (QPSK) modulated equidistant DS-CDMA sources having non-overlapping time deployment are transmitted through Additive White Gaussian Noise (AWGN) channel in occurrence of Near-Far Effect. Direction of Arrival (DOA) estimate and spatial behaviour of DS-CDMA sources are exemplified utilizing MUSIC (Multiple Signal Classification), Root-MUSIC and gold-MUSIC algorithms for Uniform Linear Array (ULA) antenna.

Blind spreading sequence estimation algorithm for long‐code DS‐CDMA signals in asynchronous multi‐user systems

Despreading signal at receiver side requires prior knowledge of the spreading sequences in direct-sequence code division multiple access (DS-CDMA) system. However, the knowledge of spreading sequences is always unknown to the receiver in non-cooperative communication. Therefore the receiver has to estimate the spreading sequences in a blind manner. This study presents a novel algorithm to estimate the spreading sequences for asynchronous long-code (LC) DS-CDMA signals. By investigating the projection characteristics of the signal subspace, the authors treat the spreading sequence estimation problem as a discrete optimisation search problem that can be solved efficiently by modern optimisation search methods. Moreover, they design a successive subspace deflation scheme to alleviate interference from other users. Numerical experiments demonstrate that the proposed algorithm provides good estimation of the spreading sequences for LC-DS-CDMA signals. Compared with the existing algorithms, the proposed algorithm exhibits much better performance in large system load scenario with low signal-to-noise ratio.

Estimation of PN Code of DS-CDMA Signals over Frequency selective Fading Channel in presence of Impulsive noise

Spread spectrum signals are widely used for secure communications, as well as for multiple access interference (MAI). They have many interesting properties, including low probability of interception. Indeed, direct sequence spread spectrum systems (DSSSS) use a periodical pseudo-random sequence to modulate the baseband signal before transmission. Interception of DSSS signals is a difficult task due to the hiding of the PN sequence, especially if the accompanied noise is non-Gaussian and the channel is frequency selective. In this paper, a receiver is proposed to intercept asynchronous direct sequence code division multiple access (DS-CDMA) signals received from different users and estimate the PN sequences of each one of them. The signals are assumed to be transmitted over frequency selective fading channel and contaminated with Class-A impulsive noise. The proposed interception receiver is based on Independent Component Analysis (ICA), the pseudo random sequences is estimated from the eigenvector subspace in case of single path. In multipath situations, the estimation is the overlapped form of the pseudo random sequences, a triple correlation function (TCF) is introduced to eliminate the influence of multipath. The performance of proposed receiver is evaluated in terms of the number of correct estimated codes for m-sequence over typical urban channel in presence of impulsive noise. The results show that the interception receiver performs is 100% code estimation at -8dB signal to noise ratio.

Performance Analysis of Independent Component Analysis Algorithms for Multi-user Detection of DS-CDMA

conventional detection process of direct sequence code division multiple access (DS-CDMA) is limited by multiple access interference (MAI) due to loss of orthogonality between spreading waveforms in multipath propagation channel environment.. In this paper RADICAL Independent Component Analysis (ICA) algorithm is proposed for detection of DS-CDMA signals and compared with FastICA, JADE ICA algorithm. Independent component analysis (ICA) is statistical technique based on higher order statistics, represent set random variables as linear transformation of statistically independent components and these conditions are satisfied in multi-user CDMA environment. Conventional methods mitigate sources of interference by taking into account all available information and at times seeking additional information channel characteristics, direction of arrival, etc. Combining an ICA element to conventional signal detection reduces multiple access interference (MAI) and enables a robust, computationally efficient structure. The proposed algorithm takes advantage of differential entropy estimation and converges quickly. Bit error rate simulations of these algorithms have been given and compared for different number of users, SNR. The simulation results show that RADICAL ICA algorithm performs best on detecting the source signals from the mixed CDMA signals.

Blind Despreading of Short-Code DS-CDMA Signals with Bayesian Techniques

This paper introduces an algorithm for joint spreading codes and information sequences estimation based on reversible jump Markov chain Monte Carlo (RJ-MCMC) for direct-sequence code division multiple access (DS-CDMA) signals with low signal-to-noise ratio (SNR) in non-cooperative systems, by analyzing signal model. The proposed algorithm establishes a joint posterior distribution model of signal parameters and user number, and obtains the samples of distribution to be estimated through iterative sampling. The algorithm is able to construct a reversible Markov chain sampler that jumps between parameter subspaces of different dimensionality, so that the posterior distribution of parameters to be estimated is obtained. Simulation results indicate that the proposed algorithm can be applied to low SNR with equal or unequal power and to different user number. Moreover, the estimation perfor-mance of this algorithm is a significant improvement of the existing method.

Blind multiuser spreading sequences estimation algorithm for the direct-sequence code division multiple access signals

By combining the multiple signal classification (MUSIC)-based algorithm, proposed by Haghighat and Soleymani, and the segmentation-based idea, a blind multiuser spreading sequences estimation algorithm for the direct-sequence code division multiple access (DS-CDMA) signals is presented in this study. The main idea of the proposed algorithm is to divide the spreading sequences into short-time segments so as to recover them in a sequential procedure. First, the received signal samples are divided into K collections of temporal windows. Each collection is corresponding to a group of short-time segments of the spreading sequences. Secondly, by performing a sub-segmentation-based estimation scheme on the first collection of temporal windows, the first short-time segments of all the users' spreading sequences are extracted. Finally, with the help of the segments acquired in advance, the remaining segments of the spreading sequences can be recovered chip-by-chip through K−1 subspace projection loops. This algorithm can be applied not only to the synchronous short-code (SC) DS-CDMA signals, but also to the long-code (LC) DS-CDMA signals in non-cooperative contexts, even with minus SNR. Simulations are presented to illustrate the performance of the proposed method. In addition, for the synchronous SC-DS-CDMA signals, the computational cost of the proposed algorithm is much lower than that of the MUSIC-based algorithm. Furthermore, a derivation for blind synchronisation of the synchronous SC-DS-CDMA signals is also presented.

QPSK DS-CDMA System over Rayleigh Channel with a Randomly-Varying Frequency Narrow-Band Interference: Frequency Tracking Analysis

This paper analyses frequency tracking characteristics of a complex-coefficient adaptive infinite-impulse response (IIR) notch filter used for suppression of narrow-band interference (NBI) with a randomly-varying frequency in a quadriphase shift keying (QPSK) modulated direct-sequence code-division multiple-access (DS-CDMA) communication system. The QPSK DS-CDMA signals are transmitted over a frequency non-selective Rayleigh fading channel. The analysis is based on a first-order real-coefficient difference equation with respect to steady-state instantaneous frequency tracking error from which a closed-form expression that relates frequency tracking mean square error (MSE) with number of DS-CDMA active users and NBI power is obtained. Closed-form expressions for optimum notch bandwidth coefficient and step size constant that minimize the frequency tracking MSE are also derived. Computer simulations are included to substantiate the accuracy of the analyses.

Iterative Detection of Multicode DS-CDMA Signals With Strong Nonlinear Distortion Effects

Whenever a direct-sequence code-division multiple-access (DS-CDMA) signal is the sum of several components associated with different spreading codes [e.g., the DS-CDMA signal to be transmitted by the base station (BS) in the downlink or any multicode DS-CDMA signal], it has high envelope fluctuations and a high peak-to-mean envelope power ratio (PMEPR), setting strong linearity requirements for the power amplifiers. For this reason, it is desirable to reduce the envelope fluctuations of the transmitted signals. The use of clipping techniques combined with frequency-domain filtering was shown to be an effective way of reducing the envelope fluctuations (and, inherently, the PMEPR) of DS-CDMA signals, while maintaining the spectral occupation of the corresponding conventional DS-CDMA signals. To avoid PMEPR regrowth effects, the clipping and filtering operations can be repeated several times. However, the performance degradation due to nonlinear distortion effects on the transmitted signals can be relatively high, particularly when a very low PMEPR is intended (e.g., when a low clipping level and several iterations are adopted). This can particularly be serious if different powers are assigned to different spreading codes. To avoid significant performance degradation in these situations, we consider an improved receiver where there is an iterative estimation and cancellation of nonlinear distortion effects. Our performance results show that the proposed receiver allows significant performance improvements after just a few iterations, even when we have strong nonlinear distortion effects.

Subspace based direction of arrival estimation of DS-CDMA signals using orthogonal projection

In this paper a new method for direction of arrival estimation of direct sequence code division multiple access (DS-CDMA) signals in multipath fading environments is proposed. The method is based on suppressing the effect of coherent signals from different paths by orthogonal projection of the antenna's spatial-temporal output vector onto the undesired subspace. The method assumes the knowledge of the spreading code sequence and delays of the paths of the desired user and then applies the MUSIC algorithm with new transformed vectors. This method is applicable to all array geometries and does not need any spatial smoothing that is usually applied for coherent signals. The estimates of the proposed method are consistent and its performance is investigated through simulations.

Subspace-Based Schemes for NBI and MAI Suppression in DS-CDMA Communications

This paper deals with narrowband interference (NBI) and multiple access interference (MAI) suppression based on subspace-based approaches for direct-sequence code-division multiple-access (DS-CDMA) systems. First, the signal subspace-based and the interference subspace-based techniques are developed to estimate the noisy DS-CDMA signal and the interference signal, respectively. In conjunction with a modified multiple constrained minimum variance multiuser detector, we develop two efficient schemes to deal with the NBI and MAI that do not need any prior knowledge about the characteristics of the interference and specific training sequence. Next, an adaptive implementation based on a signal subspace tracking algorithm and a recursive least mean square algorithm is also developed. Several computer simulation examples in terms of bit error rate are provided for illustrating the effectiveness of the proposed schemes.

Blind Deconvolution of DS-CDMA Signals by Means of Decomposition in Rank-(1, L, L) Terms

In this paper, we present a powerful technique for the blind extraction of direct-sequence code-division multiple access (DS-CDMA) signals from convolutive mixtures received by an antenna array. The technique is based on a generalization of the canonical or parallel factor decomposition (CANDECOMP/PARAFAC) in multilinear algebra. We present a bound on the number of users under which blind separation and deconvolution is guaranteed. The solution is computed by means of an alternating least squares (ALS) algorithm. The excellent performance is illustrated by means of a number of simulations. We include an explicit expression of the Cramér–Rao bound (CRB) of the transmitted symbols.

Recursive Code-Timing Estimation for DS-CDMA Signals in a Time-Varying Environment Through a Stochastic Optimization Approach

A modified simulated-annealing method is proposed for recursively minimizing the nonlinear cost function in the subspace-based code-timing estimation for direct-sequence code-division multiple-access signals over a multipath fading channel. The new algorithm has low complexity, because of the recursive updating of the final code-timing estimates together with the noise-subspace tracking

Space-Time Cyclic Delay Transmit Diversity for a Multi-Code DS-CDMA Signal with Frequency-Domain Equalization

Frequency-domain equalization (FDE) can take advantage of the frequency-selectivity of the channel to improve the transmission performance in a frequency selective fading channel. To further improve the transmission performance, the transmit diversity technique can be used. Cyclic delay transmit diversity (CDTD) can strengthen the frequency-selectivity while space-time transmit diversity (STTD) can achieve the antenna diversity gain. In this paper, we propose a 4-antenna space-time cyclic delay transmit diversity (STCDTD), which is a combination of 2-antenna STTD and 2-antenna CDTD schemes, for orthogonal multi-code direct sequence code division multiple access (DS-CDMA) using FDE. We evaluate the BER performance and the throughput performance by computer simulation and compare them with the original CDTD and STTD schemes.

Performance analysis of asynchronous direct sequence code division multiple access for general chip waveforms over multi-path Rayleigh fading channels

The probability of error (Pe) performance of asynchronous direct sequence code division multiple access (DS-CDMA) signals over multi-path Rayleigh fading channels for general (i.e. time-limited and band-limited) chip waveforms is investigated. The numerical values of the Pe are calculated for DS-CDMA signals using conventional time-limited chip waveforms and a band-limited square root raised cosine (SRRC) chip waveform. Analytical calculations are verified by conducting simulations. In terms of the Pe, the SRRC-based system results in a worse performance value than those of the time-limited chip waveform-based systems. However, due to its compact bandwidth, the SRRC-based system provides the highest capacity defined as the number of users per Hertz for a same Pe performance level.

Multiuser Receiver for DS-CDMA Signals in Multipath Channels: An Enhanced Multisurface Method

This paper deals with the problem of multiuser detection in direct-sequence code-division multiple-access (DS-CDMA) systems in multipath environments. The existing multiuser detectors can be divided into two categories: (1) low-complexity poor-performance linear detectors and (2) high-complexity good-performance nonlinear detectors. In particular, in channels where the orthogonality of the code sequences is destroyed by multipath, detectors with linear complexity perform much worse than the nonlinear detectors. In this paper, we propose an enhanced multisurface method (EMSM) for multiuser detection in multipath channels. EMSM is an intermediate piecewise linear detection scheme with a run-time complexity linear in the number of users. Its bit error rate performance is compared with existing linear detectors, a nonlinear radial basis function detector trained by the new support vector learning algorithm, and Verdu's optimal detector. Simulations in multipath channels, for both synchronous and asynchronous cases, indicate that it always outperforms all other linear detectors, performing nearly as well as nonlinear detectors.

Decision Feedback Chip-Level Maximum Likelihood Detection for DS-CDMA in a Frequency-Selective Fading Channel

In direct sequence code division multiple access (DS-CDMA), variable rate transmission can be realized by simply changing the spreading factor SF for the given chip rate. In a frequency-selective fading channel, the transmission performance can be improved by using rake combining. However, when a very low SF is used for achieving a high transmission rate, error floor is produced due to insufficient suppression of inter-chip interference (ICI). In this paper, decision feedback chip-level maximum likelihood detection (DF-CMLD) is proposed that can suppress the ICI. An upper-bound for the conditional bit error rate (BER) is theoretically derived for the given spreading sequence and path gains. The theoretical average BER performance is numerically evaluated by Monte-Carlo numerical computation using the derived conditional BER. The numerical computation results are confirmed by computer simulation of DS-CDMA signal transmission with DF-CMLD.

Joint frequency-domain differential detection and equalization for DS-CDMA signal transmissions in a frequency-selective fading channel

It has been revealed that direct sequence code-division multiple access (DS-CDMA) can achieve a good bit-error rate (BER) performance, comparable to multicarrier CDMA (MC-CDMA), by using coherent frequency-domain equalization (FDE) instead of coherent Rake combining. However, coherent FDE requires accurate channel estimation. Pilot-assisted channel estimation is a practical solution, but its accuracy is sensitive to the Doppler spread. In this paper, a frequency-domain differential encoding and detection scheme is proposed for a DS-CDMA mobile radio. Joint frequency-domain differential detection and equalization (FDDDE) based on minimum mean-square error (MMSE) criterion is presented, where a simple decision feedback filter is used to provide a reliable reference signal for MMSE-FDDDE. Also presented is an approximate BER analysis. It is confirmed by both approximate BER analysis and computer simulation that MMSE-FDDDE provides good BER performance close to the coherent MMSE-FDE and shows high robustness against the Doppler spread; it outperforms coherent MMSE-FDE for large Doppler spreads. The proposed MMSE-FDDDE can also be applied to MC-CDMA. A performance comparison between uncoded DS- and MC-CDMA shows that DS-CDMA with MMSE-FDDDE achieves better BER performance than MC-CDMA with MMSE-FDDDE for small spreading factors.

Performance of optimum wavelet waveform for DS-CDMA chip waveform over QS-AWGN channel: Research Articles

In this paper, we search for a better chip waveform based on orthogonal wavelets for direct sequence-code division multiple access (DS-CDMA) signals to improve the probability of error (Pe) performance with minimal signal bandwidth variations. First, we derive the Pe expression over a quasi-synchronous additive white Gaussian noise channel for DS-CDMA signals, which use various pulse shaping waveforms including orthogonal wavelets as chip waveforms. It is observed that this expression depends on the chip waveform. Then, we design an optimum wavelet by using the relationship between wavelets and filter coefficients to reduce the probability of error. The DS-CDMA system using the optimum wavelet waveform results in a lower probability of error than those using the conventional chip waveforms such as raised cosine, half-sine and rectangular waveforms. Especially, the Pe of the optimum wavelet-based scheme becomes significantly better than those of the conventional chip waveforms-based schemes under the heavy loading that is the case for commercial wireless systems. When the systems work with full load (i.e. the number of users equals the processing gain), the optimum wavelet-based system results in 0.5, 2.1 and 4 dB better SNR values than those of the raised cosine, half-sine and rectangular-based systems, respectively, for a Pe value of 10-3. Copyright © 2005 John Wiley & Sons, Ltd.

Performance of optimum wavelet waveform for DS-CDMA chip waveform over QS-AWGN channel

In this paper, we search for a better chip waveform based on orthogonal wavelets for direct sequence-code division multiple access (DS-CDMA) signals to improve the probability of error (Pe) performance with minimal signal bandwidth variations. First, we derive the Pe expression over a quasi-synchronous additive white Gaussian noise channel for DS-CDMA signals, which use various pulse shaping waveforms including orthogonal wavelets as chip waveforms. It is observed that this expression depends on the chip waveform. Then, we design an optimum wavelet by using the relationship between wavelets and filter coefficients to reduce the probability of error. The DS-CDMA system using the optimum wavelet waveform results in a lower probability of error than those using the conventional chip waveforms such as raised cosine, half-sine and rectangular waveforms. Especially, the Pe of the optimum wavelet-based scheme becomes significantly better than those of the conventional chip waveforms-based schemes under the heavy loading that is the case for commercial wireless systems. When the systems work with full load (i.e. the number of users equals the processing gain), the optimum wavelet-based system results in 0.5, 2.1 and 4 dB better SNR values than those of the raised cosine, half-sine and rectangular-based systems, respectively, for a Pe value of 10−3. Copyright © 2005 John Wiley & Sons, Ltd.

Super-Exponential Methods for Blind Detection of Asynchronous CDMA Signals Over Multipath Channels

In this letter, code-constrained super-exponential algorithms (CSEA) are presented for blind detection of asynchronous short-code direct-sequence code-division multiple-access signals over multipath channels. Constrained SEA leads to the extraction of the desired user whereas unconstrained SEA leads to the extraction of any one of the actives users. The results are further improved by following the constrained SEA by unconstrained SEA. Convergence of the constrained SEA is proved and simulation examples are provided to illustrate the proposed approaches.