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

Blind beamforming-based strong interference suppression in underwater acoustic direct-sequence code-division multiple-access systems.

In an underwater acoustic direct-sequence code-division multiple-access (DS-CDMA) system, the performance of the conventional receiver is severely limited by the strong multiple-access interferences, which is due to the near-far effect. In this paper, a time-frequency-time with eigendecomposition-based blind beamformer (TFT-EBB) method is proposed for a distant user to suppress strong interference from a nearby user in DS-CDMA underwater acoustic communications. When the interference level is moderately higher than that of the desired signal, the TFT-EBB method can estimate the composite steering vector of all multipath components of the weak desired user signal via the eigendecomposition of the array covariance matrix at each frequency bin. Then the proposed energy detection-based RAKE receiver is cascaded to achieve multipath diversity, in which it requires less prior information compared with the conventional RAKE receiver. The proposed method is evaluated by using both simulated and field experimental data, which verifies its effectiveness.

Channel Estimation for DS-CDMA Rake Receiver using Sparse Recovery Approach

In the literature of direct-sequence code-division multiple-access (DS-CDMA), for issues related to channel estimation, some exclusive architectures have been proposed, which are characterized by known channel noise statistics and noise observation. But in reality, the channel parameters are frequently assessed utilizing training sequences that lead to difficulty in obtaining the channel noise statistics. Channel estimation quality has been proved to play an important role in the performance of rake receiver. This paper addresses the issues of optimizing DS–CDMA rake receiver channel estimation equipped with an Iterative least square sparse recovery (IL2SR) channel estimator. Moreover, the ambient noises corrupt the signal received and multiple-access interference further aggravates it. Because of this observation noises become hard to acquire. Hence this paper proposes as an iterative least square structure for channel estimation algorithm in rake receiver employed in DS-CDMA communication systems. Further, examination of blind channel estimation problem for rake-based DS-CDMA communication framework having multi-path fading channels with time variation is also attempted. The validity of the proposed techniques has been verified through results obtained from simulation for different channel parameters and spreading codes. Further exploration has been carried out with execution of the IL2SR with Rake receiver in DS-CDMA framework for multi-path fading channels. It is found that better performance is obtained with this framework under various channels with different spreading codes. The proposed system is compared with Kalman based techniques and it was found that DS-CDMA framework under additive white gaussian noise (AWGN)channel with IL2SR receiver reveals better outcomes in terms of bit error rate (BER). Also, there has been improvement in video quality while using the proposed IL2SR receiver with increase in the values of ratio of signal to noise ratio.

Non orthogonal multiple access for DS-CDMA systems

In direct sequence code division multiple access (DS-CDMA), the symbols of a single user are transmitted over a given spreading code. In this paper, we propose to increase the throughput of CDMA sy...

Convergence and BER Approximation of HIC Detector for DS-CDMA System in Rayleigh Fading Multipath Environment

In this paper we present mathematic analysis and implementation of hybrid interference canceller (HIC) for direct sequence-code division multiple access (DS-CDMA) system in multipath environment over Rayleigh channel. The HIC detector is examined for convergence. The bit error rate (BER) for the system is obtained using Simplified Improved Gaussian Approximation (SIGA). Using this expression, BER is plotted for the Rayleigh channel having exponentially decaying power delay profile (PDP) as well as flat PDP. The theoretical results are compared with the simulated system results.

Identical Code Cyclic Shift Multiple Access—A Bridge Between CDMA and NOMA

A traditional direct sequence CDMA (DS-CDMA) system works based on spreading codes, which should have good auto-correlation and cross-correlation properties to eliminate multiple access interference (MAI) and multipath interference (MI). However, to find a large number of good spreading codes is extremely difficult, and thus the capacity of a DS-CDMA system is limited strictly by the number of available spreading codes. This work proposes a downlink DS-CDMA design to enable multiple access using only one spreading code, namely identical code cyclic shift (ICCS) code, where in-phase and quadrature channels are used to transmit data and pilot, respectively. In particular, the ICCS code is used at a transmitter to perform DS modulation for user data, which is then sent to the in-phase channel. At the same time, the pilots are sent over the quadrature channel in a time-division multiplex (TDM) mode to construct a correlation matrix of ICCS code. At a receiver, MAI is removed by so-designed pilots plus a matched filter-bank. Simulation results show that the proposed ICCS multiple access (ICCSMA) can achieve a better performance than a traditional DS-CDMA system in a high signal to noise ratio region. ICCSMA serves as a bridge to turn CDMA to NOMA to enable massive connectivity in 5 G and beyond communications.

Extending Distributed-Based Transversal Filter Method to Spectral Amplitude Encoded CDMA

In this paper, the analog code modulation characteristics of distributed-based transversal filters (DTFs) suitable for use in spectrally encoded CDMA systems are presented. The DTF is verified as an appropriate method to use in high-speed CDMA systems as opposed to previously proposed methods, which are intended for Direct Sequence (DS) CDMA systems. The large degree of freedom of DTF design permits controlling the filter pulse response to generate well specified temporal phase-coded signals. A decoder structure that performs bipolar detection of user subbands giving rise to a Spectral-Amplitude Encoded CDMA system is considered. Practical implementations require truncating the spreading signals by a time window of duration equal to the span time of the tapped delay line. Filter functions are chosen to demodulate the matched channel and achieve improved user interference rejection avoiding the need for transversal filters featuring a large number of taps. As a proof-of-concept of the electronic SAE scheme, practical circuit designs are developed at low speeds (3-dB point at 1 GHz) demonstrating the viability of the proposal.

Computationally Efficient Chaotic Spreading Sequence Selection for Asynchronous DS-CDMA

Abstract The choice of the spreading sequence for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems plays a crucial role for the mitigation of multiple-access interference. Considering the rich dynamics of chaotic sequences, their use for spreading allows overcoming the limitations of the classical spreading sequences. However, to ensure low cross-correlation between the sequences, careful selection must be performed. This paper presents a novel exhaustive search algorithm, which allows finding sets of chaotic spreading sequences of required length with a particularly low mutual cross-correlation. The efficiency of the search is verified by simulations, which show a significant advantage compared to non-selected chaotic sequences. Moreover, the impact of sequence length on the efficiency of the selection is studied.

A comprehensive performance analysis of relay-aided CDMA communications over dissimilar fading channels

In this paper, bit error probability (BEP), outage probability (OP) and channel capacity (CC) of direct-sequence code-division multiple access systems with amplify-and-forward relaying are presented for different fading scenarios. In the first scenario, the source-destination link is assumed to experience Rayleigh fading while it is subject to Nakagami-m fading in the second scenario. The source-relay and relay-destination channels are considered to have Nakagami-m fading conditions in two scenarios. First, analytical expressions for the end-to-end probability density function (PDF) are derived by using the convolution integral. Then, BEP, OP and CC are obtained based on these PDFs in terms of infinite series. Truncation error analyses are presented for different parameter values in order to show that truncation error arising from the infinite series is negligible. Simple and easy-to-compute asymptotic expressions are also introduced for BEP and OP in order to simplify the performance analysis in high signal-to-noise ratio region. Simulation results are provided to show the accuracy of the proposed approximate and asymptotic expressions.

Outage performance of cooperative DS‐CDMA systems with best path selection over α − η − μ fading channels

Outage probability (OP) of amplify-and-forward direct-sequence code-division multiple-access (DS-CDMA) systems is analysed with best path selection over α − η − μ fading channels. First, cumulative distribution function (CDF) of α − η − μ fading is derived and then, lower bound of the OP is obtained for the considered system. An asymptotic CDF expression is also proposed in order to get more insights on system behaviour. The OP performance is presented for different relay and fading parameter configurations. Analytical derivations are validated by the close agreement with simulations.

Average symbol error rate and outage probability of DS-CDMA systems with AF relaying over asymmetric fading channels

In this paper, approximate outage probability (OP) and average symbol error rate (SER) of cooperative direct-sequence code-division multiple access (DS-CDMA) systems with amplify-and-forward (AF) relaying are evaluated over asymmetric fading channels. Practically, the channels between source (S), relay (R) and destination (D) nodes in a DS-CDMA system can be subject to different fading due to the nature of wireless medium. Motivated by this reality, the S–D path is assumed to be Nakagami-m distributed which is a general fading model for different types of channel conditions while S–R and R–D paths are considered to experience Rician fading in order to include line of sight conditions. First, closed-form solutions are obtained for both the cumulative distribution function and the probability density function of S–R–D paths. Then, a closed-form total moment generating function (MGF) of the considered system is derived. The lower bound of the OP is calculated with the help of the inverse Laplace transform of the derived MGF expression while average SER is analyzed by evaluating the integral over the derived MGF. Finally, an asymptotic MGF expression is proposed for high signal-to-noise ratio (SNR) analysis of AF DS-CDMA systems. The simulation results are provided to verify the correctness of the analytical derivations. It is shown that the approximate OP and average SER results are in well agreement with the simulations and the asymptotic results are tight from medium to high SNR regime.

Distributed Resource Allocation Assisted by Intercell Interference Mitigation in Downlink Multicell MC DS-CDMA Systems

This paper investigates the allocation of resources, including subcarriers and spreading codes, as well as intercell interference (ICI) mitigation for multicell downlink multicarrier direct-sequence code division multiple-access systems, which aim to maximize the system's spectral efficiency (SE). The analytical benchmark scheme for resource allocation and ICI mitigation is derived by solving or closely solving a series of mixed integer non-convex optimization problems. Based on the optimization objectives the same as the benchmark scheme, we propose a novel distributed resource allocation assisted by ICI mitigation scheme referred to as resource allocation assisted by ICI mitigation (RAIM), which requires very low implementation complexity and demands little backhaul resource. Our RAIM algorithm is a fully distributed algorithm, which consists of the subcarrier allocation (SA) algorithm named RAIM-SA, spreading code allocation (CA) algorithm called RAIM-CA and the ICI mitigation algorithm termed RAIM-IM. The advantages of the RAIM are that its CA only requires limited binary ICI information of intracell channels, and it is able to make mitigation decisions without any knowledge of ICI information. Our simulation results show that the proposed RAIM scheme, with very low complexity required, achieves significantly better SE performance than other existing schemes, and its performance is very close to that obtained by the benchmark scheme.

Buffer-aided distributed space-time coding schemes and algorithms for cooperative DS-CDMA systems

In this work, we propose buffer-aided distributed space-time coding (DSTC) schemes and relay selection algorithms for cooperative direct-sequence code-division multiple access (DS-CDMA) systems. We first devise a relay pair selection algorithm that can exploit all possible relay pairs and then select the optimum set of relays among both the source-relay phase and the relay-destination phase according to the signal-to-interference-plus-noise ratio (SINR) criterion. Multiple relays equipped with dynamic buffers are then introduced in the network, which allows the relays to store data received from the sources and wait until the most appropriate time for transmission. A greedy relay pair selection algorithm is then developed to reduce the high cost brought by the exhaustive search that is required when a large number of relays are involved in the transmission. The proposed techniques effectively improve the quality of the transmission with an acceptable delay as the buffer size is adjustable. An analysis of the computational complexity of the proposed algorithms and the delay and a study of the greedy algorithm are then carried out. Simulation results show that the proposed dynamic buffer-aided DSTC schemes and algorithms outperform prior art.

Performance of Two-Hop DS-CDMA Systems Using Amplify-and-Forward Protocol over Different Fading Channels

This study analyses the performance of direct-sequence code division multiple access (DS-CDMA) based on two-hop amplify-and-forward protocol over Weibull symmetric fading channels as well as Rayleigh/Rician, Rician/Rayleigh asymmetric fading phenomenas. We investigate the bit-error rate (BER) of the considered system using multiple relays by considering the effect of Weibull fading parameter and Rician K factor on the system performance. Our simulation results demonstrate the positive impacts of the value of fading parameter, Rician K factor and increasing number of relay nodes on BER performance. It is also confirmed that the Rician K factor is more effective on the system performance over Rician/Rayleigh fading channels in comparison with Rayleigh/Rician fading environment. DOI: http://dx.doi.org/10.5755/j01.eee.22.1.14116

Joint interference cancellation and relay selection algorithms based on greedy techniques for cooperative DS-CDMA systems

In this work, we propose interference cancellation techniques and a multi-relay selection algorithm based on greedy methods for the uplink of cooperative direct-sequence code-division multiple access (DS-CDMA) systems. We first devise low-cost list-based successive interference cancellation (GL-SIC) and parallel interference cancellation (GL-PIC) algorithms with RAKE receivers as the front-end that can approach the maximum likelihood detector performance and be used at both the relays and the destination of cooperative systems. Unlike prior art, the proposed GL-SIC and GL-PIC algorithms exploit the Euclidean distance between users of interest and the potential nearest constellation point with a chosen threshold in order to build an effective list of detection candidates. A low complexity multi-relay selection algorithm based on greedy techniques that can approach the performance of an exhaustive search is also proposed. A cross-layer design strategy that brings together the proposed multi-user detection algorithms, and the greedy relay selection is then developed along with an analysis of the proposed techniques. Simulations show an excellent bit error rate performance of the proposed detection and relay selection algorithms as compared to existing techniques.

Hyperchaotic spread spectrum sequences selection and its application in DS-CDMA system

In this paper we propose a new algorithm for hyperchaotic spread spectrum sequences selection. Firstly, binary hyperchaotic spread spectrum sequences are generated by a 6-order cellular neural network (CNN). Secondly, we analyze the properties of the sequences according to the P-values and correlation function. Finally, a new algorithm proposed for hyperchaotic spread spectrum sequences selection is based on the properties of hyperchaotic sequences. In order to prove the effectiveness of the proposed selection algorithm, some of the selected hyperchaotic sequences are applied in direct sequence code division multiple access (DS-CDMA) system. Simulation results show that the selected hyperchaotic sequences have better performance than m-sequences and have similar performance with Gold sequences. Compared with m-sequences or Gold sequences, hyperchaotic spread spectrum sequences are better suitable for secure communication systems.

Uplink Performance Evaluation of Packet Combining ARQ for MPR Prefix-Assisted DS-CDMA

Prefix-assisted direct-sequence code-division multiple access (DS-CDMA) is a viable transmission technique for high-data-rate wireless broadband systems, capable of coping with highly dispersive channels at the uplink. Packet combining (PC) ARQ with an iterative block decision feedback equalization (IB-DFE) technique can be used to cope with errors. However, extracting the IB-DFE's packet error rate (PER) for MAC-layer simulations can be time consuming. Most works that model the DS-CDMA behavior with PC ARQ use simplified asymptotic PER models in the study of the system performance, which are not precise for low signal-to-noise ratios and for a low number of mobile terminals (MTs). In this paper, the MAC performance is modeled using a discrete-time Markov chain that relies on a PER model. As an example, this paper describes a PER analytical model for an uncoded prefix-assisted DS-CDMA system with PC ARQ that considers the following inputs: the number of MTs accessing the channel, the MTs' number of transmissions, and the channel realizations for a given bit energy over noise ratio. The results show that the model portrays with better accuracy the simulations' results, compared with an alternative asymptotic model.

Adaptive RAKE receiver in chaos based pilot-added DS-CDMA system

This paper presents the theoretical analysis of adaptive multiuser RAKE receiver scheme in frequency selective fading channel for direct-sequence code division multiple access (DS-CDMA) system. Least mean square (LMS) algorithm is used to estimate the channel coefficients. Chaotic sequences are used as spreading sequence and corresponding bit error rate (BER) in closed form is derived for imperfect channel estimation conditions. Performances of chaotic sequences are compared with pseudorandom noise (PN) sequences. Under perfect synchronization assumption, various simulation results are shown to investigate the performance of the proposed system.

Widely‐linear minimum‐mean‐squared error multiple‐candidate successive interference cancellation for multiple access interference and jamming suppression in direct‐sequence code‐division multiple‐access systems

In this paper, the authors propose a widely-linear (WL) receiver structure for multiple access interference (MAI) and jamming signal (JS) suppression in direct-sequence code-division multiple-access systems. A vector space projection (VSP) scheme is also considered to cancel the JS before detecting the desired signals. They develop a novel multiple-candidate successive interference cancellation (MC-SIC) scheme which processes two consecutive user symbols at one time to process the unreliable estimates and a number of selected points serve as the feedback candidates for interference cancellation, which is effective for alleviating the effect of error propagation in the successive interference cancellation (SIC) algorithm. WL signal processing is then used to enhance the performance of the receiver in non-circular modulation scheme. By bringing together the techniques mentioned above, a novel interference suppression scheme is proposed which combines the WL MC-SIC minimum-mean-squared error (MMSE) algorithm with the VSP scheme to suppress MAI and JS simultaneously. Simulations for binary phase shift keying modulation scenarios show that the proposed structure achieves a better MAI suppression performance compared with previously reported SIC MMSE receivers at lower complexity and a superior JS suppression performance.

Robust Blind Multiuser Detection Algorithm Using Fourth-Order Cumulant Matrices

A new blind detection algorithm, based on fourth-order cumulant matrices, is presented and applied to the multiuser symbol estimation problem in direct sequence code division multiple access (DS-CDMA) systems. The blind detection is to estimate multiple symbol sequences in the downlink of a DS-CDMA communication system using only the received wireless data and without any knowledge of the user spreading codes. The proposed algorithm takes advantage of higher cumulant matrix properties to reduce the computational load and enhance the performance. Bit error rate simulations of this algorithm are performed for different numbers of users, signal-to-noise ratios, and numbers of symbols per user in comparison with the FastICA and RobustICA algorithms. The results show that the proposed algorithm outperforms both ICA-based detectors in estimating the symbol signals from the received mixed signals. Moreover, the proposed blind detector is computationally fast and exhibits high convergence speed in extracting user symbols.

BER performance analysis of Subtractive interference cancellation (SIC) Using Rayleigh channel in DS-CDMA Receiver system

In this paper, we present and analyze the performance of a parallel interference cancellation (PIC) scheme for multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) systems. In order to mitigate the multi-path interference (MPI) in the DS CDMA system. At each cancellation stage in the proposed PIC scheme, on each subcarrier, a weighted sum of the soft outputs of the other users in the current stage is cancelled from the soft output of the desired user to form the input to the next stage. At the last stage, the interference cancelled outputs from all the subcarriers are maximal ratio combined (MRC) to form the decision statistic. Parallel interference elimination is first proposed in this paper the multi-path interference are evaluated by tentative decision and known user information. Then the performance over Rayleigh fading channel are analyzed and compared to Matched filter, Decorelator, successive interference cancellation (SIC) and conventional parallel interference cancellation (PIC). It is shown that PIC performance can be improved greatly by using this method with simple structure and easy implementation.