DS-CDMA Communication Systems Research Articles

Design and Simulation of an Efficient Image Communication Scheme Via DS-CDMA

In this work, we try to design a DS-CDMA communication system model for the transmission of different types of images: binary, gray level and color. We adopted a transmission system of multiple users. During transmission of each image, we evaluated the noise and multiple access interference effects on the quality of DS-CDMA transmitted images which are inversely proportional to the length of the spreading sequence. Quality is assessed each time by the bit error rate (BER) estimation between received and original images. By increasing the signal to noise ratio to positive values, the BER tends to decrease, this results in a received image with fewer errors, thus without alteration which confirms Shannon equation. By using one user alone, the inter-user interference is canceled and the BER obtained is far lower than the BER obtained in presence of inter-user interferences. This can be resolved in hardware implementation using adequate filters.

Performance Evaluation of DS-CDMA System Using Chaotic Binary Sequences

In this paper generation of binary sequences derived from chaotic sequences defined over Z 4 is proposed. The six chaotic map equations considered in this paper are Logistic map, Tent Map, Cubic Map, Quadratic Map and Bernoulli Map. Using these chaotic map equations, sequences over Z 4 are generated which are converted to binary sequences using polynomial mapping. Segments of sequences of different lengths are tested for cross correlation and linear complexity properties. It is found that some segments of different length of these sequences have good cross correlation and linear complexity properties. The Bit Error Rate performance in DS-CDMA communication systems using these binary sequences is found to be better than Gold sequences and Kasami sequences.

Multiuser Detection for Uplink DS-CDMA Amplify-and-Forward Relaying Systems

We consider the uplink of a cooperative DS-CDMA communication system, where each user communicates with the base station through a direct link and with the help of relays. A multiuser detection approach for the joint estimation of spatial signatures, channel gains and transmitted symbols of all the users is proposed. Such an approach is derived from a trilinear tensor model for the received signal that combines the source-destination and relay-destination links. The proposed multiuser receiver provides an extension of the traditional trilinear DS-CDMA receiver to cooperative relaying systems, by including the users' relay-assisted links into the trilinear model. Moreover, in contrast to a previous work by the authors, where the relays of a cluster transmit over orthogonal channels, the proposed receiver operates in a more attractive scenario where all the relays simultaneously transmit towards the base station without requiring orthogonal codes. Simulation results illustrate the performance of the proposed receiver in comparison with alternative approaches.

Probability Distribution of Multiple-Access Interference in Chaotic Spreading Codes based on DS-CDMA Communication System

This paper is devoted to the evaluation of the probability distribution of the multiple access interference in asynchronous Direct Sequence Code Division Multiple Access (DS CDMA) communication system based on chaotic spreading sequences. Statistical properties of such interference in a system with K users causing the multiple-access interference are obtained by a computer simulation. Simulation is carried out for the spreading sequences produced by the logistic map; the length of sequences: N = 7, 31, 63 and the number of users correspondingly: K = (3, 8, 11); (11, 32, 65); (64, 129). Ill. 3, bibl. 8, tabl. 2 (in English; abstracts in English and Lithuanian). DOI: http://dx.doi.org/10.5755/j01.eee.123.7.2380

A two-stage Independent Component Analysis-based method for blind detection in CDMA systems

We propose an ICA-based method for blind detection of users in asynchronous DS-CDMA communications systems with multipaths channels with the only knowledge of the desired user@?s code. The method can handle both the uplink and the downlink situations, since it does not require the synchronism between users. We convert the received cyclostationary signal into an observations vector that follows the ICA model with instantaneous mixture. The selection of the estimated source is carried out by means of the desired user@?s code. Unlike previous works, we avoid to project the results after each iteration. Instead, we introduce a preprocessing based on a linear transformation of the data that enforces the extraction vector to lie in the desired user@?s subspace. The detection is done in two stages. The second stage is a fine tuning in which the constraint is removed from the data in order to obtain more accurate results. Computer simulations show that the proposed method compares favorably with other well-known methods, in terms of mean-square error (MSE) of the output, symbol error rate and robustness against the near-far problem.

Performance analysis of CDMA-based wireless communication systems using the simplified improved Gaussian approximation method

The limitations initially prevalent in CDMA-based systems have been gradually solved thereby making CDMA a very promising access mode for future communication systems. Following the move towards higher data transmission rates, however, a number of other problems have emerged that need to be tackled. As a result, current world-wide research on CDMA-based systems is geared towards high speed, highly mobile multimedia communication applications. In this paper, the performance of a multi-user DS-CDMA communication system operating in a Rayleigh fading environment is investigated. Performance expressions are derived and the variation of the performance with various system parameters is presented. Tanzania Journal of Science Vol. 30 (2) 2004: pp. 41-52

ICA based GSIC receiver for multirate DS-CDMA systems

In this paper, we propose an Independent Component Analysis (ICA) based Group-wise Successive Interference Cancellation (GSIC) receiver for multi rate DS-CDMA communication systems. By describing the multi rate DS-CDMA data model in terms of ICA model, we show that ICA can be employed for multiuser detection within a group of GSIC receiver. The advantage of ICA technique is that it is a blind approach based on independence of input signals. The multirate DS-CDMA system has been simulated using multiple processing gain and simulations have been carried out for both single path and multi path propagation conditions. It is shown that the proposed method exhibits good bit error rate performance.

Performance Analysis of Downlink Beamforming in FDD DS-CDMA Systems

In this letter, the effects of transmit beamforming on downlink performance in DS-CDMA communication systems are examined. We present a simple-to-use expression for the conditional instantaneous SINR after Rake combining. Assuming BPSK modulation, the performance of average bit error rate is evaluated. We compare the average BER performance obtained by different beamforming methods under frequency selective multipath fading channels.

A DS-CDMA Cellular System Using Band Division and Channel Segregation Distributed Channel Allocation

SUMMARY In DS-CDMA cellular communications systems, the single frequency reuse can be utilized. Since large other-cell interference is produced, the well known soft handover or site diversity must be used. If the single frequency reuse is not utilized to avoid the other-cell interference, we will face the frequency allocation problem, similar to FDMA systems. In this paper, a DS-CDMA cellular system using band division is proposed. The available wide frequency band is divided into several narrow frequency bands and the different frequency bands are allocated to adjacent cells so as to avoid the large other-cell interference. For the frequency allocation, the channel segregation distributed channel allocation (CS-DCA) algorithm is applied. The link capacity is evaluated by computer simulation.

Blind Multiuser Detection for DS-CDMA Using Independent Component Analysis Neural Network

In this paper, the use of Independent Component Analysis (ICA) neural networks for multiuser detection in multipath DS-CDMA communication systems operating in convolutive channels is examined, in both synchronous and asynchronous cases. To take advantage of known parameters, the ICA detector system was initialized by a detector that uses a subspace-based method of estimating channel noise followed by a multistage refinement based on the Hopfield neural network. In order to operate in a totally blind environment, the detector then makes use of an independent component analysis neural network. A number of different ICA learning algorithms were applied to the CDMA detection problem. We explored the use of nonlinear-Hebbian and natural gradient learning, which we believe to be a unique application to the multiuser detection problem. Nonlinear Hebbian learning was found to yield superior results in the most benign cases while natural gradient learning yielded superior results in the harshest environments. This detector gives superior performance over both the conventional single user detector and the LMMSE detector.

An AV-based RAKE-like receiver for DS-CDMA systems

Previously, several detectors based on auxiliary vector (AV) filters were presented to suppress multi-access interference (MAI) in direct-sequence/code-division-multiple-access (DS-CDMA) systems. Knowing the signature waveform of the desired user, these AV-based detectors are a class of promising candidates for MAI suppression due to their superior performance and low computational complexity. However, it is hard to obtain the desired user's signature waveform, which is the convolution of the user's spreading sequence and the channel impulse response, unless the channel vector is known or unless training signals are sent. In this letter, based on both of the merits of AV detectors and RAKE receivers, an AV-based RAKE-like receiver is proposed for DS-CDMA communication systems in multipath channels. Simulation results indicate that the performance of the new receiver is very close to that of the AV detectors, which know the exact signature waveform of the desired user.

Interference control and chip waveform design in multirate DS-CDMA communication systems

We study the interference effects in a multirate DS-code-division multiple-access (CDMA) system. Optimum chip waveform selection with arbitrary shapes is analyzed using a time domain approach. The problem is posed as an interference minimization problem under energy and time-bandwidth constraints and prolate spheroidal wave functions are used to arrive at a solution. Various factors affecting the interference are identified and the trade-off between competing factors is analyzed. The effect of the interchip interference on the optimum chip waveform design is also quantified under a practical bandwidth constraint. We study the benefits of employing two different chip waveforms for two classes of users. We compare the performance of systems employing two different chip waveforms with that of a single-chip waveform system such as IS-95. We show that when the power imbalance is large, it is advantageous to employ two different chip waveforms for different classes of users.

Diversity performance of DS-CDMA communication system on Nakagami fading with arbitrary parameters

This paper is concerned with the analysis of direct sequence-code division multiple access system operating over Nakagami fading channel having independent but arbitrary fading with unequal mean powers in the diversity branches. Performance is measured by the bit error rate and the outage probability under a generalized binary detection scheme for various multipath fading conditions. New analytical expressions are derived as a function of the number of branches, fading parameters, unequal mean signal power and the number of users. Analyses include both coherent and noncoherent binary detection schemes. The result indicates that the effect of arbitrary fading is not negligible.

Temporal diversity on DS-CDMA communication systems for blind array signal processing

Many array signal processing algorithms have been developed to reject interfering signals and, therefore, increase the quality of the communication link. Some of them are reference-based and some others are blind, i.e., without any side information. A specific type of deterministic blind beamforming algorithms, named self-reference, is treated in the paper. Self-reference beamforming is suitable for signals presenting a diversity or a redundancy structure. This type of signals exhibit the property of exact self-prediction that allows the definition of an error signal to be minimized. In the present paper, the time diversity present in the well-known DS-CDMA communication system is analyzed. Blind approaches are derived from the property of exact self-prediction arising from the implicit time redundancy encountered in the spreading codes, and from the explicit time redundancy that can also be induced by the transmitter. The proposed methods do not require, unlike most blind subspace-based techniques, any kind of spectral matrix decomposition, with the consequent reduction of complexity and computational burden. The presented methods were tested via simulations and compared to the classical time reference beamformer (TRB) in terms of SINR and BER, showing a performance slightly better than a TRB scheme with 10% of the bits as a training sequence.

A New Improved First-Order Approximate Decorrelating Detector for DS-CDMACommunication Systems

An improved first-order approximate decorrelating detector for direct-sequence code division multiple access (DS-CDMA) communication systems is proposed. This detector is based on the minimization of the multiple access interference (MAI) for each user. Through analysis and numerical results, the detector is shown to perform better than the original first-order approximate detector in various conditions at the expense of an additional complexity on the order of O(K2), where K is the number of active users in the system.

Blind RLS based space-time adaptive 2-D RAKE receivers for DS-CDMA communication systems

The goal of a blind two-dimensional (2-D) RAKE receiver for DS-CDMA is to cancel strong multiuser access interference (MUAI) while optimally combining the signal of interest's (SOI) multipath. The weight vector yielding the optimum SINR for bit decisions may be computed as the largest generalized eigenvector of a matrix pencil composed of the SOI plus MUAI and MUAI alone space-time correlation matrices. An alternative blind 2-D RAKE receiver is presented based on RLS-type space-time adaptive filtering that offers competitive performance. The applicability of the scheme to the IS-95 uplink is addressed.

Reduced-dimension blind space-time 2-D RAKE receivers for DS-CDMA communication systems

Blind space-time RAKE receivers for classical DS-CDMA that cancel strong multiuser access interference while optimally combining the desired user's multipath are presented. The delay spread is assumed to be fraction of a symbol interval as in the IS-95 CDMA standard. The post-correlation symbol interval is segmented into that which encompasses the RAKE fingers and that away from the RAKE fingers. The signal-plus-interference and interference alone space-time correlation matrices are estimated during the former and latter, respectively. Reduced-dimension space-time RAKE receivers are investigated for alleviation of the computational burden of computing the largest generalized eigenvector of the resulting matrix pencil. Theoretical analysis and supporting simulations reveal that if the space and time compressing transformations are designed judiciously, reduced-dimension two-dimensional (2-D) RAKE receivers offer faster convergence at the expense of only a slight loss in asymptotic signal-to-interference-plus-noise ratio (SINR) relative to the full-dimension space-time RAKE receiver. Applications to the IS-95 uplink are also presented along with supporting simulations.

Evolution of the power control techniques for DS-CDMA toward 3G wireless communication systems

In this article we present a survey of the power control techniques for modern wireless DS-CDMA communication systems. Power control is the single most important system requirement for DS-CDMA systems. Well-defined power control is essential for proper functioning of the DS-CDMA system. In the absence of power control the effect of near⁄far phenomena is dominant, and the capacity of the DS-CDMA mobile system is very low, even lower than that of mobile systems based on FDMA. However, when the power control in DS-CDMA systems exists, it allows users to share resources of the system equally between themselves. Also, with a proper power control it is possible to lower total transmitting power of the mobiles and prolong the battery life.

On adaptive minimum probability of error linear filter receivers for DS-CDMA channels

Receiver architectures in the form of a linear filter front-end followed by a hard-limiting decision maker are considered for DS-CDMA communication systems. Based on stochastic approximation concepts a recursive algorithm is developed for the adaptive optimization of the linear filter front-end in the minimum BER sense. The recursive form is decision driven and distribution free. For additive white Gaussian noise (AWGN) channels, theoretical analysis of the BER surface of linear filter receivers identifies the subset of the linear filter space where the optimal receiver lies and offers a formal proof of guaranteed global optimization with probability one for the two-user case. To the extent that the output of a linear DS-CDMA filter can be approximated by a Gaussian random variable, a minimum-mean-square-error optimized linear filter approximates the minimum BER solution. Numerical and simulation results indicate that for realistic AWGN DS-CDMA systems with reasonably low signature cross-correlations the linear minimum BER filter and the MMSE filter exhibit approximately the same performance. The linear minimum BER receiver is superior, however, when either the signature cross-correlation is high or the background noise is non-Gaussian.

Iterative rake receiver with map channel estimation for ds-cdma systems

We propose an iterative rake receiver structure using an optimum semi-blind channel estimation algorithm for ds-cdma mobile communication systems. This receiver performs an iterative estimation of the channel according to the maximum a posteriori criterion, using the expectation-maximization algorithm. This estimation process requires a convenient representation of the discrete multipath fading channel based on the Karhunen-Loeve orthogonal expansion theorem. The rake receiver uses pilot as well as unknown control and data symbols optimally for improving channel estimation quality. Moreover, it can take into account the coded structure of all unknown transmitted symbols when channel estimation quality is poor or unsatisfactory. The validity of the proposed method is highlighted by simulation results obtained for the FDD mode of the umts interface.