Direct Sequence Code Division Multiple Access Research Articles

ESTIMATION AND DESIGN OF MC-DS-CDMA FOR HYBRID CONCATENATED CODING IN HIGH SPEED VEHICULAR COMMUNICATION

The design of Multi Carrier Direct Sequence Code Division Multiple Access (MC-DS-CDMA) structure which generalizes serial and parallel concatenated code is investigated to this project. This model is ideal for designing various codes in the performance of both error floor and water floor region. We propose a concatenated code for transmitter block which is used for multi carrier direct sequence CDMA technique. Simulation results of MC-DS-CDMA uplink system using Cadence software shows the various parameters such as memory, Execution time and number of transient steps required for the Execution of MC-DS-CDMA uplink system was estimated and also power consumed was determined for each block in the transmitter. An improved concatenated code model is used for uplink mobile communication. Further system performance improvements can be obtained by concatenating inner code and outer code and the results of computer simulations demonstrate that the performance of the concatenated code was investigated.

Game-theoretic solutions through intelligent optimization for efficient resource management in wireless visual sensor networks

We propose a quality-driven cross-layer optimization scheme for wireless direct sequence code division multiple access (DS-CDMA) visual sensor networks (VSNs). The scheme takes into account the fact that different nodes image videos with varying amounts of motion and determines the source coding rate, channel coding rate, and power level for each node under constraints on the available bit rate and power. The objective is to maximize the quality of the video received by the centralized control unit (CCU) from each node. However, since increasing the power level of one node will lead to increased interference with the rest of the nodes, simultaneous maximization of the video qualities of all nodes is not possible. In fact, there are an infinite number of Pareto-optimal solutions. Thus, we propose the use of the Nash bargaining solution (NBS), which pinpoints one of the infinite Pareto-optimal solutions, based on the stipulation that the solution should satisfy four fairness axioms. The NBS results in a mixed-integer optimization problem, which is solved using the particle swarm optimization (PSO) algorithm. The presented experimental results demonstrate the advantages of the NBS compared with alternative optimization criteria.

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.

Performance of relay-aided downlink DS-CDMA system using transmitter preprocessing based on feedback information

In this contribution, we investigate the performance of relay-aided downlink (DL) direct sequence code division multiple access (DS-CDMA) system with multi-user transmitter preprocessing (MUTP) based on vector quantized channel impulse responses (VQ-CIRs). Specifically, the CIRs are estimated with the aid of training-sequence based estimation technique at each of the relays. These estimated CIRs are then VQ and the magnitudes and phases are fed back to the base station (BS) through feedback channels, that conflict noise and fading. At the BS, the CIRs recovered using a linear detector are then exploited to formulate the preprocessing matrix to mitigate the DL multi-user interference at the relays. Our study shows that the attainable bit-error-rate (BER) degrades when noise and fading contaminated VQ-CIRs are invoked to realize the preprocessing matrix. Nevertheless, the resultant BER performance of the MUTP based on VQ-CIRs acquired via ideal feedback remains close to that achieved with perfect CIRs assumption.

An information theory criteria based blind method for enumerating active users in DS-CDMA system

In this paper, a new and blind algorithm for active user enumeration in asynchronous direct sequence code division multiple access (DS-CDMA) in multipath channel scenario is proposed. The proposed method is based on information theory criteria. There are two main categories of information criteria which are widely used in active user enumeration, Akaike Information Criterion (AIC) and Minimum Description Length (MDL) information theory criteria. The main difference between these two criteria is their penalty functions. Due to this difference, MDL is a consistent enumerator which has better performance in higher signal-to-noise ratios (SNR) but AIC is preferred in lower SNRs. In sequel, we propose a SNR compliance method based on subspace and training genetic algorithm to have the performance of both of them. Moreover, our method uses only a single antenna, in difference to the previous methods which decrease hardware complexity. Simulation results show that the proposed method is capable of estimating the number of active users without any prior knowledge and the efficiency of the method.

Multiuser Detection for DS-CDMA Systems on Nakagami-0.5 Fading Channels with Impulsive Noise

Recent attention has been given to Nakagami-0.5 fading channel as a special case of Nakagami-m fading channel with m = 0.5 and as a worst-case (severe) fading scenario. This paper presents a study on the performance analysis of a robust multiuser detector for direct sequencecode division multiple access (DS-CDMA) systems including maximal ratio combining (MRC) receive diversity. A new M-estimator, which performs well in the heavy-tailed impulsive noise, proposed to robustify the detector is also studied and analyzed by deriving average probability of error over a single path Nakagami-0.5 fading channel. Numerical results show the efficacy of the proposed Mestimator, under severe fading conditions of Nakagami-m channel, in comparison with linear decorrelating detector, the Huber and the Hampel estimator based detectors.

Ternary Zero Correlation Zone Sequence Sets for Asynchronous DS-CDMA

In this paper we propose a new class of ternary Zero Correlation Zone (ZCZ) sequence sets based on binary ZCZ sequence sets construction. It is shown that the proposed ternary ZCZ sequence sets can reach the upper bound on the ZCZ sequences. The performance of the proposed sequences set in asynchronous Direct Sequence-Code Division Multiple Access (DS-CDMA) system is evaluated. In the simulation we used two types of channels: Additive White Gaussian Noise (AWGN) and frequency non-selective fading with AWGN noise. The proposed ternary ZCZ sequence sets show better results, in term of Bit Error Rate (BER), than Hayashi’s ternary ZCZ sequence sets.

A Six-Dimensional Hyperchaotic System Selection and Its Application in DS-CDMA System

In this paper, we analyze the hyperchaotic properties of cellular neural network (CNN) systems based on Lyapunov exponents. A four-step algorithm for selecting the hyperchaotic system is proposed. Firstly, calculate the maximum absolute value of time sequences, which is one of the properties of chaotic system, noted as boundedness. Secondly, check another property of chaotic system: sensitive dependence on the initial conditions. Several systems can be identified as non-chaotic systems through above two steps, which saves lots of time to calculate the Lyapunov exponents. Then, calculate the Lyapunov exponents. If the largest Lyapunov exponent is negative, the system is identified as non-chaotic system. Finally, determine the given system is chaotic or not by the strange attractors and the time sequences. The binary hyperchaotic spread spectrum sequences can be generated by a six- dimensional CNN system. Some of the binary hyperchaotic sequences can be used in a direct sequence code division multiple access (DS-CDMA) system after selecting through a special rule. Simulation results prove the effectiveness of the six-dimensional CNN hyperchaotic sequences, compared with the m-sequence and second-order Chebyshev polynomial function.

PN CODE ACQUISITION USING SMART ANTENNA AND ADAPTIVE THRESHOLDING CFAR BASED ON ORDERED DATA VARIABILITY FOR CDMA COMMUNICATIONS

Recently, a novel approach for PN code acquisition of direct sequence code division multiple- access (DS-CDMA) systems in Rayleigh fading multipath channel was proposed in (1). The authors considered a combination of adaptive thresholding constant false alarm rate (CFAR) and smart antennas to increase the system capacity and consequently enhance the detection performance. This paper considers still the problem of PN code acquisition for DS-CDMA communication systems over Rayleigh fading channels under the presence of multipath and multiple-access interference (MAI) signals. We propose and analyze an adaptive array acquisition system, which integrates an adaptive thresholding technique based on ordered data variability (ODV) index constant false alarm rate (ODV-CFAR) and digital beamforming where a low complexity least mean square (LMS) algorithm is used to calculate the optimal weighting coe-cients. This approach is expected to mitigate interferences caused by the presence of multiple access interference and multipath. Unlike other approaches based on a flxed censoring point when the number of interferences is assumed known, ODV-CFAR processing does not require prior knowledge about the number of interferences. The simulation results show a robust performance of the proposed system in varying mobile communication channels.

Achieving Pareto Optimal Power Tracking Control for Interference Limited Wireless Systems via Multi-Objective H_2/H_infty Optimization

To track a desired signal-to-interference-plus-noise-ratio (SINR) that allows for higher system throughput and better link quality in an interference limited wireless communication system, a feedback power controller is designed to obtain the optimal SINR tracking control. In this paper, we propose the multi-objective (MO) H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> optimal power control for interference limited wireless systems that provides quality solutions for both objectives. The considered multi-objective H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> power tracking control is complicated in nature and an equivalent formulation that minimizes the upper bounds of both objectives is proposed. Furthermore, the resulting constraints for the MO H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> power control problem is transformed as three linear matrix inequalities (LMIs), leading to a LMIs-constrained MO problem (MOP). By combining the LMI toolbox in MATLAB with an evolutionary searching algorithm, a set of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> solutions called Pareto optimal solutions can be obtained and a particular solution can be selected based on the tradeoff determined by the system designer. Finally, to illustrate our design procedure, we provide numerical simulations for a typical interference limited wireless system, i.e., a direct-sequence code division multiple access (DS-CDMA) cellular system. Simulation results demonstrate that the optimal tradeoff indeed can be achieved by the proposed MO H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> power control.

Performance of NLMS-based Parallel Interference Cancellation (PIC) For Up-Link CDMA Systems

Parallel interference cancellation (PIC) is a promising detection technique to suppress multiple access interference (MAI) for up-link direct-sequence CDMA (DS-CDMA) systems. Adaptive PICs are attractive due to its low implementation complexities and good performance. In this paper, we propose a general framework for analyzing the bit error rate (BER) and convergence performance of the normalized LMS (NLMS) based PIC. To further improve the convergence speed of the PIC system, a novel switch-mode noise-constrained NLMS (SNC-NLMS) algorithm for the adaptive multistage PIC (AMPIC) is also proposed. Simulation results show that the analytical results are reasonably accurate and the SNC-NLMS AMPIC outperforms the NLMS-based one, if the algorithm parameters are properly chosen.

Performance Enhancement of Downlink Multiuser DS-CDMA Detectors in Multipath Channels over AWGN Channels

In this paper, we have introduced the diversity scheme maximum ratio combining (MRC) as preprocessor to the adaptive multiuser-detector (MUD) in direct-sequence code-division multiple-access (DS-CDMA) downlink systems to enhance its performance in multipath channels over AWGN channels. The performance of DS-CDMA is limited by multiple-access interference (MAI). To reduce the effects of MAI, MUDs are used at the receiver side. MUDs work by taking into account the information like spreading codes of the users and system parameters, but these values are not constant and also not known beforehand. Thus MUDs were advanced to adaptive MUDs which take into account the changing parameters by employing adaptive technique for e.g. LMS or RLS algorithms. These adaptive MUDs require each transmitter to send a training sequence at startup which the receiver uses for initial adaptation. However, if there is any drastic change in interference environment, decision directed adaptation becomes unreliable, and data transmission must be temporarily suspended and yield to a fresh training sequence. Therefore MRC as preprocessor to the adaptive MUD of DS-CDMA system is used. As shown by simulation results, the BER performance achieved by the proposed detector outperforms the conventional MF detector with a good margin and is promising.

Optimal power allocation in DS-CDMA with adaptive SIC technique

This paper proposes an optimal power allocation in direct sequence-code division multiple access (DS-CDMA) system. The objective is to minimize total transmit power, while simultaneously meeting the certain sum channel capacity (data transmission rate) and outage probability constraints on Rayleigh fading channel. Then a weighted correlator with an adaptive successive interference cancelation (SIC) scheme is developed using neural network (NN) for an improvement in receiver performance. A closed mathematical form of joint probability of error (JPOE) is derived. This determines the number of active users' interfering effect that needs to be canceled in order to achieve a desired bit error rate (BER) value. Mathematical analysis shows that better receiver performance can be achieved through large change in weight up-gradation (w) for the strong users with a particular change in learning rate (η). Simulation results in terms of sum capacity as well as weak user's (users with poor channel gain) capacity, outage probability and BER performance duly support the effectiveness of the proposed scheme over the existing works.

다중 반송파 부호분할 다중접속 통신에서의 주파수 효율 향상을 위한 효율적인 전력 및 전송률 적응화 기법

We propose an efficient frequency-time domain resource allocation scheme in multi-carrier (MC) direct-sequence code-division multiple-access (DS/CDMA) communications. We consider, as a power allocation strategy in the frequency domain, transmitting each user`s DS waveforms over the user`s sub-band with the largest channel gain. We then consider rate adaptation in the time domain, where the data rate is adapted such that a desired transmission quality is maintained. We analyze the achievable average data rate of the proposed scheme with fixed average transmission power, and compare the performance to single carrier DS/CDMA systems with power and rate adaptations.

Spatial and temporal adaptive receiver for DS-CDMA systems

The objective of this paper is to propose a reception scheme to improve the performance of DS-CDMA (Direct Sequence Code Division Multiple Access) systems, compared to conventional direct channel receivers. The technique employs a spatial and temporal adaptive receiver composed of N spatial processors (associated with M antennas) and N Decision Feedback Equalizers (DFE), both of which are trained using a TMDP (Time Multiplexed Dedicated Pilot) sequence. The spatial and temporal receiver associated with TMDP offers considerable benefits when compared to the conventional Rake-Finger receiver. Performance results and details of the functioning of the proposed reception scheme are presented.

Blind Capon-like adaptive ST-BC MIMO-CDMA receiver based on constant modulus criterion

In this paper, we present a new Capon-like blind receiver based on linearly constrained constant modulus (LCCM) criterion for the multiple-input multiple-output (MIMO) antennas system along with space–time block code (ST-BC) using direct-sequence code division multiple access (CDMA) modulation technique. A time-varying channel model with generalized sidelobe canceller (GSC) associated with the recursive least squares (RLS) algorithm is implemented to reduce the complexity of receiver design. In our derived algorithm, the parameter of constant modulus, α, relating to the desired user power is updated adaptively via stochastic gradient algorithm to track userʼs amplitude variation. Also we prove theoretically that in the two-branch filter bank receiver design the weight vector of one branch can be updated simply using the other one, which has been obtained with our proposed CM-GSC-RLS algorithm, with simple pre-calculated transform. Hence computation complexity of the proposed adaptive blind receiver can be further reduced significantly. Via intense simulations it reveals that our proposed scheme has robust performance against the userʼs acquisition inaccuracies comparing with current available algorithms.

Effective Resource Management in Visual Sensor Networks With MPSK

The problem of resource management in a Direct Sequence Code Division Multiple Access (DS-CDMA) wireless Visual Sensor Network (VSN) with M-array Phase Shift Keying (MPSK) modulation in an Additive White Gaussian Network (AWGN) channel was considered in this paper. Achieving maximum video quality, in spite of the prevailing network resource constraints, is of utmost importance in VSN applications. Our optimization scheme is based on the Nash Bargaining Solution (NBS). The nodes in the network negotiate in order to determine their transmission parameters (transmission powers; source and channel coding rates for each node). The task is to optimize the transmission powers (which are continuous) and the source and channel coding rates (which are discrete) for all the network nodes, while taking advantage of the improved bandwidth spectral efficiency provided by the higher order constellation.

An Analysis of the DS-CDMA Cellular Uplink for Arbitrary and Constrained Topologies

A new analysis is presented for the direct-sequence code-division multiple access (DS-CDMA) cellular uplink. For a given network topology, closed-form expressions are found for the outage probability and rate of each uplink in the presence of path-dependent Nakagami fading and shadowing. The topology may be arbitrary or modeled by a random spatial distribution with a fixed number of base stations and mobiles placed over a finite area. The analysis is more detailed and accurate than existing ones and facilitates the resolution of network design issues including the influence of the minimum base-station separation, the role of the spreading factor, and the impact of various power-control and rate-control policies. It is shown that once power control is established, the rate can be allocated according to a fixed-rate or variable-rate policy with the objective of either meeting an outage constraint or maximizing throughput. An advantage of variable-rate power control is that it allows an outage constraint to be enforced on every uplink, which is impossible when a fixed rate is used throughout the network.

PSO-based multiuser detectors for high-order modulation DS/CDMA systems under spatial and multipath diversities

In this paper, the uplink direct-sequence code-division multiple access DS-CDMA multiuser detection MuD problem is studied from a heuristic perspective. The heuristic particle swarm optimisation PSO approach is due to the NP-complexity of the MuD optimisation problem. The challenge is to solve the associated hard complexity problem in a polynomial time. A complete parameter optimisation procedure for PSO applied to the MuD problem is carried out herein under different system configurations, such as high-order modulation and diversity exploitation. The PSO-MuD performance is analysed through Monte Carlo simulation MCS method. MCS results show that, after convergence, the performance achieved by the PSO-MuD is much better than that shown by the conventional detector, and somewhat closer to that given by the single-user bound. The flat Rayleigh channel is initially considered, but the results are further extended to the space-time wireless channels.

A maximum-likelihood bound approaching minimum bit error rate linear multiuser detector for frequency-selective DS-CDMA systems

In this letter, a minimum bit error rate (MBER) linear multiuser detector (MUD) is considered for direct-sequence code-division multiple-access (DS-CDMA) communication systems, distorted by time-varying and frequency-selective multipath fading channels. Based on the approach for finding filter coefficients of the proposed MBER MUD, an efficient Newton method with a barrier parameter is developed. The BER performance of the MBER MUD is compared to other conventional detectors. The study finds that the proposed MBER MUD has more than 2dB gain over the linear minimum mean-squared error (LMMSE) detector. Furthermore, in the high SNR region, the BER performance of the proposed MBER MUD approaches the performance of the maximum-likelihood (ML) detector.