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Systems based on spread spectrum technique are inherently interference resistant. However, asynchronous transmission and multipath propagation causes manifestation of multiuser interference. Multiuser interference in a multiple access system limits the capacity of the system. Hence, it becomes imperative to mitigate the interference. This work presents the design and mathematical analysis of Hybrid Interference Cancellation (HIC) detector for direct-sequence code division multiple access (DS-CDMA). The detector is analysed for single path as well as multipath propagation over a Rayleigh fading channel. Simulation results are compared with the results of Successive Interference Canceller (SIC). Furthermore, the SIC and HIC detectors are tested for convergence. An HIC detector is proposed for MC-CDMA system. The interference canceller discussed in this paper dispenses with the RAKE receiver for diversity combining and data estimation, which averts the likelihood of errors because of incorrect data estimates.

Systems based on spread spectrum technique are inherently interference resistant. However, asynchronous transmission and multipath propagation causes manifestation of multiuser interference. Multiuser interference in a multiple access system limits the capacity of the system. Hence, it becomes imperative to mitigate the interference. This work presents the design and mathematical analysis of Hybrid Interference Cancellation (HIC) detector for direct-sequence code division multiple access (DS-CDMA). The detector is analysed for single path as well as multipath propagation over a Rayleigh fading channel. Simulation results are compared with the results of Successive Interference Canceller (SIC). Furthermore, the SIC and HIC detectors are tested for convergence. An HIC detector is proposed for MC-CDMA system. The interference canceller discussed in this paper dispenses with the RAKE receiver for diversity combining and data estimation, which averts the likelihood of errors because of incorrect data estimates. [ABSTRACT FROM AUTHOR]

Direct-Sequence Code Division Multiple Access (DS-CDMA) is a digital method to spread spectrum modulation for digital signal transmission. We propose to detect signal in DS-CDMA communication using the learning mechanism. Initially, the user signals are spread using the respective pseudo-noise (PN) code where the input signal is multiplied with the code which is then modulated using the quadrature phase shift keying (QPSK) modulator. The modulated signal is then transmitted in a 3G/4G channel considering all types of fading. The transmitted signal is received by the antenna array which is performed by demodulation. We propose to adaptively assign the weights by employing Improved Whale Optimized Multi-Layer Perceptron Neural Network (IWMLP-NN)-based learning mechanism. To design IWMLP-NN, Improved Whale Optimization Algorithm is combined with multilayer perceptron neural network. This is used instead of the normal Multiple Signal Classification (MUSIC) and least mean squares (LMS)/root-mean-square (RMS) algorithms used in beam-forming networks. After assigning weight through IWMLP-NN-based learning mechanism, we de-spread to get the original user data. We have compared our proposed technique with the normal techniques with the help of plots of Bit Error Rate (BER) versus Signal-to-Noise Ratio (SNR). We use both the AWGN channel and fading channel for analysis. Experimental results prove that our proposed method achieves better BER performance results even with deep fading. [ABSTRACT FROM AUTHOR]

This paper analyzes the method of power allocation for wireless communication networks, limited by multiple access interference under fading Rayleigh channels. The method takes into account the statistical variation of the desired and interfering signals power and optimally and dynamically allocates the power resources in the system. This allocation is made being taken into account the restrictions imposed by the outage probability for each receiver/transmitter pair. Based on the Perron-Frobenius eigenvalue and geometric programming (GP) theories, some interesting results were found for the optimization problem analyzed. A GP is a special type of optimization problem that can be transformed in a non-linear convex optimization problem simply by the change of variables, and then to be globally solved. Also, an iterative method for fast convergence was shown to find the optimally power allocation seeking to minimize the outage probability in the communication.

In this paper, a blind adaptive detector is proposed for blind separation of user signals and blind estimation of spreading sequences in DS-CDMA systems. The blind separation scheme exploits a charrelation matrix for simple computation and effective extraction of information from observation signal samples. The system model of DS-CDMA signals is modeled as a blind separation framework. The unknown user information and spreading sequence of DS-CDMA systems can be estimated only from the sampled observation signals. Theoretical analysis and simulation results show that the improved performance of the proposed algorithm in comparison with the existing conventional algorithms used in DS-CDMA systems. Especially, the proposed scheme is suitable for when the number of observation samples is less and the signal to noise ratio (SNR) is low.

In this paper, we propose a new hyperchaos-based spreading codes generator (HSCG) for multi-users direct sequence spread spectrum (DSSS) communication systems. For experimental validation, we implement the proposed generator in Xilinx FPGA Virtex-5 technology. Contrary to the traditional spreading codes, the proposed HSCG offers interesting statistical properties such as good autocorrelation, very small cross-correlation, no repetition of the generated spreading codes and high degree of security. In addition, the developed hardware architecture used for implementing the HSCG permits to generate simultaneously an infinite number of multi-length spreading codes with high hardware performances in terms of logic area consumption, throughput and latency. Real-time results are given for 32-, 64- and 128-bit chaotic spreading codes. All of these properties are validated and compared with that of classical codes (Gold sequences, Walsh–Hadamard) through bit error rate (BER) performance evaluation in a DS-CDMA system in AWGN and time-invariant multipath channels. With its interesting performances, the proposed HSCG is very useful for the next generations of mobile communication systems. [ABSTRACT FROM AUTHOR]

Generalized RAKE (G-RAKE) reception reduces the total amount of interference and provides enhanced diversity by comprising extra fingers to collect information about interference and further using channel and impairment correlation estimates for fingers allocation. However, the hardware complexity and the excessive computational requirements of GRAKE receivers may restrict their application in real systems; thus, suboptimal solutions are commonly used. In this paper, we propose and evaluate three maximum likelihood G-RAKE structures for colored noise with suboptimal finger placement. In all implementations, the fingers are optimally distributed within a time window that spans from several chip periods before the first arriving multipath to several chip periods after the latest one. The first receiver has its fingers at integer multiples of the chip period while in the rest two structures the search window is segmented in halves and tenths of the chip duration. This work also extends earlier studies by thoroughly investigating the impact of fractionally spaced finger placement on system performance. Our analysis shows that a suboptimal finger allocation reduces hardware complexity with negligible performance loss. The impact of channel delay spread and processing gain on system performance is also investigated and gives interesting results.

Over the past decade, wireless sensor network research primarily relied on highly-integrated commercial off-the-shelf radio chips. The rigid silicon implementation of the radio stack restricted access to the lower layers; thus, research focused mainly on the medium access control (MAC) layer and above. SRAM field-programmable gate array (FPGA)-based software-defined radios (SDR), on the other hand, provide a flexible architecture to experiment with any and all layers of the radio stack, but usually require desktop computers and draw high currents that prohibit mobile or longer-term outdoor deployments. To address these issues, we have developed a modular flash FPGA-based wireless research platform, called Marmote SDR, that has computational resources comparable to those of SRAM FPGA-based radio platforms, but at a reduced power consumption, with duty cycling support. We discuss the design decisions underlying Marmote SDR and evaluate its power consumption. Furthermore, we present and evaluate an asynchronous and multiple access communication protocol specifically designed for data-gathering wireless sensor networks.

Chaos based communications have drawn increasing attention over the past years. Chaotic signals are derived from non-linear dynamic systems. They are aperiodic, broadband and deterministic signals that appear random in the time domain. Because of these properties, chaotic signals have been proposed to generate spreading sequences for wide-band secure communication recently. Like conventional DS-CDMA systems, chaos-based CDMA systems suffer from multi-user interference (MUI) due to other users transmitting in the cell. In this paper, we propose a novel method based on radial basis function (RBF) for both blind and non-blind multiuser detection in chaos-based DS-CDMA systems. We also propose a new method for optimizing generation of binary chaotic sequences using Genetic Algorithm. Simulation results show that our proposed nonlinear receiver with optimized chaotic sequences outperforms in comparison to other conventional detectors such as a single-user detector, decorrelating detector and minimum mean square error detector, particularly for under-loaded CDMA condition, which the number of active users is less than processing gain.

In Body Sensor Networks (BSNs), two types of events should be addressed: periodic and emergency events. Traffic rate is usually low during periodic observation, and becomes very high upon emergency. One of the main and challenging requirements of BSNs is to design Medium Access Control (MAC) protocols that guarantee immediate and reliable transmission of data in emergency situations, while maintaining high energy efficiency in non-emergency conditions. In this paper, we propose a new emergency aware hybrid DTDMA/DS-CDMA protocol that can accommodate BSN traffic variations by addressing emergency and periodic traffic requirements. It takes advantage of the high delay efficiency of DS-CDMA in traffic burst, and the high energy efficiency of DTDMA in periodic traffic. The proposed scheme is evaluated in terms of delay, packet drop percentage, and energy consumption. Different OPNET simulations are performed for various number of nodes carrying emergency data, and for various payload sizes. The protocol performance is compared to other existing hybrid protocols. Results show that the proposed scheme outperforms the others in terms of delay and packet drop percentage for different number of nodes carrying emergency data, as well as for different payload sizes. It also offers the highest energy efficiency during periodic observation, while adjusting the energy consumption during emergency by assigning spreading codes only to nodes holding emergency data.

This work makes a review of the main Adaptives Multi-user Detectors (MuD-Adpt) for Direct Sequence - Code Division Multiple Access (DS-CDMA) systems. The MuD-Adpt based on Minimum Mean Square Error (MMSE) and Decorrelator (MuD-Dec) are focused. Multi-user detectors show great resistance to the near-far effect and combat effectively the Multiple Access Interference (MAI). Comparative numeric results characterize the substantial performance improvement of those detectors in relation to the matched filter conventional receiver (Conv).

Spatial transmit diversity schemes are now well integrated into third-generation cellular mobile communication system specifications. When DS-CDMA-based technology is deployed in typical macro- and microcell environments, multipath diversity and spatial diversity may be exploited simultaneously by a 2D RAKE receiver. The work presented in this paper focuses on taking advantage of spatial transmit diversity in synchronising the 2D RAKE structure. We investigate the use of coherent and noncoherent techniques for tracking the timing parameters of each multipath component. It is shown that both noncoherent and coherent techniques benefit from transmit diversity. Additionally the performance gap between these two techniques increases with the number of antennas.

In this paper, a generalized spectral subtraction approach for reducing additive impulsive noise, narrowband signals, white Gaussian noise and DS-CDMA interferences in MIMO channel DS-CDMA wireless communication systems is investigated. The interference noise reduction or suppression is essential problem in wireless mobile communication systems to improve the quality of communication. The spectrum subtraction scheme is applied to the interference noise reduction problems for noisy MIMO channel systems. The interferences in space and time domain signals can effectively be suppressed by selecting threshold values, and the computational load with the FFT is not large. Further, the fading effects of channel are compensated by spectral modification with the spectral subtraction process. In the simulations, the effectiveness of the proposed methods for the MIMO channel DS-CDMA is shown to compare with the conventional MIMO channel DS-CDMA.

In this paper the continuous dynamic model of Verhulst is used. It had been initially elaborated to describe the population growth of biological species with food restriction and physical space, in order to synthesize a new distributed power control algorithm (DPCA), applicable in direct sequence code division multiple access (DS-CDMA) systems. The discretization of the corresponding differential equation is accomplished via numeric integration Euler method (NIE). The properties of the proposed recursive algorithm, as Euclidian distance from optimum vector after convergence, convergence speed, normalized mean squared error (NSE), average power consumption per user and implementation complexity, are investigated through simulations. The simulation results are confronted with the other DPCAs: the classic algorithm derived by Foschini and Miljanic and the sigmoidal by Uykan and Koivo. With estimate errors, the proposed DPCA showed smaller discrepancy from the optimum power vector allocation after convergence and better convergence. Additionally, the Gerchgorin Circles theory (GC) is applied for the feasibility of the power allocation problem.

Çoklu erişim girişimi, bir doğrudan dizili/kod bölmeli çoklu erişim (DD/KBÇE) sisteminin başarımını sınırlayan temel etkidir ve yayma dizilerinin (kodlarının) bu etki üzerindeki rolü oldukça önemlidir. Bu çalışmada, adım kırmık ağırlıklandırma dalgaformlarıyla ağırlıklandırılmış toparlama dizileri kullanan bir DD/KBÇE sistemi için verilen bir kod seti içerisinde daha düşük bit hata oranları oluşturan dayanak yayma kodlarının belirlenmesine hizmet eden bir kriter önerilmektedir. Nümerik sonuçlar, önerilen kritere göre belirlenen yayma kodlarının, dayanak olarak kullanılmaya en uygun kodlar olduğunu göstermektedir.

This study analyses the performance of directsequence 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. [ABSTRACT FROM AUTHOR]

A compact (20 x 20 mm² size), coplanar waveguide (CPW) fed, octagonal slotted, dual-band antenna is presented in this paper. The proposed antenna has a simple structure consisting of an octagonal slot, L-shape stub, and a two stepped rectangular patch. The introduction of the L-shaped metallic stubs in the ground plane generates a lower resonance frequency at 2.42 GHz. The proposed antenna is fabricated on a low cost FR4 substrate having thickness of 1.6 mm. The measured impedance bandwidth of the proposed antenna is from 2.40 to 2.46 GHz and 3.2 to 6.2 GHz. It is also shown that the proposed antenna has stable radiation patterns of almost dumb-bell shape in the E-plane and omni-directional shape in the H-plane. The effects of various design parameters on the impedance bandwidth are also studied and presented in detail. The proposed antenna can be used for 3.5/5.5 GHz worldwide interoperability for microwave access (WiMAX), 2.4/5.2/5.8 GHz wireless local area network (WLAN) and 3.1-6.0 GHz Direct-Sequence Code Division Multiple Access (DS-CDMA) / Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) ultra wideband (UWB) applications. [ABSTRACT FROM AUTHOR]

Future wireless systems aim to provide higher transmission data rates, improved spectral efficiency and greater capacity. In this paper a spectral efficient two dimensional (2-D) parallel code division multiple access (CDMA) system is proposed for generating and transmitting (2-D CDMA) symbols through 2-D Inter-Symbol Interference (ISI) channel to increase the transmission speed. The 3D-Hadamard matrix is used to generate the 2-D spreading codes required to spread the two-dimensional data for each user row wise and column wise. The quadrature amplitude modulation (QAM) is used as a data mapping technique due to the increased spectral efficiency offered. The new structure simulated using MATLAB and a comparison of performance for serial one-dimensional (1-D) CDMA and parallel (2-D) CDMA is made under Additive White Gaussian Noise (AWGN), flat fading and multi-path selective fading channels conditions. It is found that 2-D CDMA has better speed and performance than serial 1-D CDMA.

Generalized RAKE (G-RAKE) reception reduces the total amount of interference and provides enhanced diversity by comprising extra fingers to collect information about interference and further using channel and impairment correlation estimates for fingers allocation. However, the hardware complexity and the excessive computational requirements of GRAKE receivers may restrict their application in real systems; thus, suboptimal solutions are commonly used. In this paper, we propose and evaluate three maximum likelihood G-RAKE structures for colored noise with suboptimal finger placement. In all implementations, the fingers are optimally distributed within a time window that spans from several chip periods before the first arriving multipath to several chip periods after the latest one. The first receiver has its fingers at integer multiples of the chip period while in the rest two structures the search window is segmented in halves and tenths of the chip duration. This work also extends earlier studies by thoroughly investigating the impact of fractionally spaced finger placement on system performance. Our analysis shows that a suboptimal finger allocation reduces hardware complexity with negligible performance loss. The impact of channel delay spread and processing gain on system performance is also investigated and gives interesting results. [ABSTRACT FROM AUTHOR]

To improve the BER performance of the DS-CDMA system over a frequency-selective fading channel, the frequency domain equalization (FDE) can be applied, in which one-tap equalization is carried out on each subcarrier component obtained by fast Fourier transform (FFT). In this paper, the DS-CDMA system with joint FDE and space antenna diversity combining is analyzed. Then, a channel interleaving method, called chip interleaving is used to improve the performance of the DS-CDMA system.

This paper investigates quality-driven cross-layer optimization for resource allocation in direct sequence code division multiple access wireless visual sensor networks. We consider a single-hop network topology, where each sensor transmits directly to a centralized control unit (CCU) that manages the available network resources. Our aim is to enable the CCU to jointly allocate the transmission power and source-channel coding rates for each node, under four different quality-driven criteria that take into consideration the varying motion characteristics of each recorded video. For this purpose, we studied two approaches with a different tradeoff of quality and complexity. The first one allocates the resources individually for each sensor, whereas the second clusters them according to the recorded level of motion. In order to address the dynamic nature of the recorded scenery and re-allocate the resources whenever it is dictated by the changes in the amount of motion in the scenery, we propose a mechanism based on the particle swarm optimization algorithm, combined with two restarting schemes that either exploit the previously determined resource allocation or conduct a rough estimation of it. Experimental simulations demonstrate the efficiency of the proposed approaches. [ABSTRACT FROM PUBLISHER]

This paper describes an ultra-wideband (UWB) antenna that uses a ring resonator concept. The proposed antenna can operate in the entire UWB, and the IEEE 802.11a frequency band can be rejected by inserting a notch stub into the ring resonator. The experiment results indicate that the measured impedance bandwidth of the proposed antenna is 17.5 GHz (2.5 GHz to at least 20 GHz). The proposed UWB antenna has omnidirectional radiation patterns with a gain variation of 3 dBi (1 dBi to 4 dBi). [ABSTRACT FROM AUTHOR]

Over the past decade, wireless sensor network research primarily relied on highly-integrated commercial off-the-shelf radio chips. The rigid silicon implementation of the radio stack restricted access to the lower layers; thus, research focused mainly on the medium access control (MAC) layer and above. SRAM field-programmable gate array (FPGA)-based software-defined radios (SDR), on the other hand, provide a flexible architecture to experiment with any and all layers of the radio stack, but usually require desktop computers and draw high currents that prohibit mobile or longer-term outdoor deployments. To address these issues, we have developed a modular flash FPGA-based wireless research platform, called Marmote SDR, that has computational resources comparable to those of SRAM FPGA-based radio platforms, but at a reduced power consumption, with duty cycling support. We discuss the design decisions underlying Marmote SDR and evaluate its power consumption. Furthermore, we present and evaluate an asynchronous and multiple access communication protocol specifically designed for data-gathering wireless sensor networks. [ABSTRACT FROM AUTHOR]

The optimal receiver for detecting direct sequence code division multiple access (DS-CDMA) signals suffers from computational complexity that increases exponentially with the number of users. Several suboptimal multiuser detectors (MUDs) have been proposed to overcome this problem. Due to the nonlinear nature of the decision boundary of the optimal receiver, it is known that nonlinear receivers outperform linear receivers. Radial basis function (RBF) MUD is a nonlinear suboptimal receiver that can perfectly approximate this decision boundary and it needs no training since it is fully determined when the spreading codes of all users and the channel impulse response (CIR) are known. However, the RBF MUD suffers from structural complexity since the number of hidden nodes (center functions) in its structure increases exponentially with the number of users. In this study, we propose a new method to minimize the number of center functions of the RBF MUD using a genetic algorithm (GA) and the least mean squares (LMS) algorithm. With simulations performed in AWGN and multipath channels it is shown that the proposed method immensely reduces the complexity of the RBF MUD with a negligible performance degradation. [ABSTRACT FROM AUTHOR]

Este trabalho faz uma revisão dos principais detectores multiusuários adaptativos ((MuD-Adpt) subótimos para sistemas DS-CDMA (Direct Sequence - Code Division Multiple Access). São enfocados os detectores adaptativos baseados no erro quadrático médio mínimo (MMSE - Minimum Mean Square Error) e os de descorrelação (MuD-Dec). Detectores multiusuários apresentam ótima resistência ao efeito perto-longe (near-far effect) e combatem efetivamente a interferência de múltiplo acesso (MAI). Resultados numéricos comparativos caracterizam o aumento substancial de desempenho desses detectores adaptativos em relação ao receptor convencional com filtro casado (Conv).

Neste artigo, utiliza-se o modelo dinâmico contínuo de Verhulst, elaborado inicialmente para descrever o crescimento populacional de espécies biológicas com restrição de alimento e espaço físico, na síntese de um novo algoritmo de controle de potência distribuído (DPCA, Distributed Power Control Algorithm),aplicável em sistemas de múltiplo acesso por divisão de código (DS-CDMA, Direct Sequence-Code Division Multiple Access). Realiza-se a discretização da equação diferencial correspondente por meio do método de Euler de integração numérica (NIE, Numerical Integration Euler Method). São investigadas via simulações computacionais as propriedades do algoritmo recursivo assim obtido, como distância Euclidiana do vetor ótimo em regime, velocidade de convergência, erro quadrático médio normalizado (NSE, Mean Normalized Squared Error) na condição de regime, consumo médio de potência por usuário e complexidade de implementação. Os resultados são confrontados com resultados de simulações análogas para o algoritmo clássico de Foschini e Miljanic e o sigmoidal de Uykan e Koivo. Na presença de erros de estimação, o DPCA proposto apresentou menor discrepância em regime em relação ao vetor ótimo e maior velocidade de convergência. Adicionalmente, aplica-se a teoria dos Círculos de Gerschgorin (GC) na determinação de condições suficientes para que o problema de alocação de potência seja factível.

Abstract: In this work a simple and efficient methodology for tuning the input parameters applied to the ant colony optimization multiuser detection (ACO-MuD) in direct sequence code division multiple access (DS-CDMA) is proposed. The motivation in using a heuristic approach is due to the nature of the NP complexity posed by the wireless multiuser detection optimization problem. The challenge is to obtain suitable data detection performance in solving the associated hard complexity problem in a polynomial time. Previous results indicated that the application of heuristic search algorithms in several wireless optimization problems have been achieved excellent performance-complexity tradeoffs. Regarding different system operation and channels scenarios, a complete input parameters optimization procedure for the ACO-MuD is provided herein, which represents the major contribution of this work. The performance of the ACO-MuD is analyzed via Monte-Carlo simulations. Simulation results show that, after convergence, the performance reached by the ACO-MuD is much better than the conventional detector, and somewhat close to the single user bound (SuB). Flat Rayleigh channels is initially considered, but the input parameter optimization methodology is straightforward applied to selective fading channels scenarios, as well as to joint time-spatial wireless channels diversities. [Copyright &y& Elsevier]

Abstract: Wavelet theory has emerged as a new mathematical tool and the important area of application of wavelets in communication: multiple accesses. Among the multiple access applications one of the most notable work is wavelet packet-based multiple access communication. The two new multiple access systems are Scale-Time-Code Division Multiple Access (STCDMA) and Scale-Code Division Multiple Access (SCDMA). In a STCDMA system, the channel is partitioned into different scales, and each scale into different time slots. This system uses Direct-Sequence (DS) Code-Division Multiple Access (CDMA) in each time slot to identify multiple users. These systems are analysed over a synchronous Additive White Gaussian Noise (AWGN) by using a conventional detector and a multiuser detector. Kasami sequences are optimal since the maximum cross correlation value achieves the Welch Lower Bound. The main purpose of using Kasami sequences is that, it decreases the multiple access interference. This paper deals with STCDMA over synchronous AWGN, Rayleigh channel and for different modulation schemes by using a multiuser detector. The main aim of multiuser detector is to mitigate the multiple access interference (MAI) and to improve the performance of STCDMA. [Copyright &y& Elsevier]

Computation of the Cholesky decomposition of the autocorrelation matrix is essential in developing zero-forcing, minimum mean square error (MMSE), and decision feedback detectors for a long code-division multiple-access (CDMA) system. However, computing the Cholesky decomposition of the time-varying correlation matrix requires many computer operations. This is because the computation of the filter weights is segment specific (varies from one symbol interval to next). In this paper, a novel and fast algorithm is developed to compute the Cholesky decomposition of the autocorrelation matrix for a long-code CDMA system. The smart update algorithms, to compute the solution (filter weights) of one segment from the solution of the previous segment, reduce the computational complexity, particularly for large-sized autocorrelation matrices. We exploit the fact that the channel matrix of a long-code CDMA system is block banded with a shift structure. Although the channel matrix is not Toeplitz for a long-code CDMA system, the shift structure of the matrix, along with its banded structure, allows the development of fast smart update recursive algorithms for data estimation. The relevance of this algorithm is discussed for channels with multipath delays extending over many symbol intervals. Simulations show that the proposed algorithm performs the same as competing multiuser detector algorithms for long-code CDMA at much reduced computational complexity. [ABSTRACT FROM PUBLISHER]

In this paper, we propose an efficient successive interference cancellation (SIC) method in direct sequence code division multiple access (DS-CDMA) system using neural network (NN). Neural network is used here to estimate the amplitudes of different users signals under frequency selective Rayleigh fading channel. The correlation values between the received signal and the signature /spreading waveforms for different users are given as the inputs to a NN and the output acts as an estimation of corresponding user's signal amplitude. A closed mathematical form of joint probability of error (JPOE) is developed to determine the number of active users needs to be canceled to achieve a desired bit error rate (BER) value. Simulation results strongly support the mathematical results. Mathematical analysis shows that better performance results can be achieved through large change in weight up-gradation (w) for the strong users with a particular change in learning rate (η). Performance of the SIC system has been studied for initial wrong ordering of a couple of pairs of interfering users based on the correlation values. Simulation results show that BER performance is better when users are ordered based on signal to interference ratio (SIR) values rather than correlation values. [ABSTRACT FROM AUTHOR]

A convolutionally coded M-ary orthogonal direct sequence code division multiple access (DS-CDMA) system in time-varying frequency-selective Rayleigh fading channels is considered in this work. We propose several novel soft demodulation algorithms based on interference cancellation and suppression techniques that can be coupled with soft decoding to improve the system performance in an iterative manner. The performance of the proposed demodulation algorithms is evaluated numerically and proved to achieve substantial bit error rate (BER) performance gain compared with the conventional detection schemes. [ABSTRACT FROM AUTHOR]

An iterative joint channel estimation, symbol detection, phase recovery and interference cancellation structure is proposed for asynchronous code-division multiple-access systems over frequency-selective fading channels. Based on the expectation maximization (EM) algorithm, a recursive channel estimator is developed for blind channel tracking, using a novel stochastic signal processing technique. To perform symbol detection given the phase ambiguities of the resultant EM channel estimates, a noncoherent scheme is developed to compute the a posteriori probabilities (APPs) of data symbols. Moreover, by incorporating the APPs into the proposed recursive channel estimator, phase ambiguity due to the EM channel estimation can be resolved, which enables soft multiple access interference cancellation for multiuser detection. Based on these new signal processing schemes, an iterative structure is proposed for joint channel estimation and multiuser detection over fast fading channels. [ABSTRACT FROM AUTHOR]

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In this paper, we proposed a new multicarrier, direct sequence code division multiple access, the so-called premulticoded MC-DS-CDMA system. In this scheme, the basic MC-DS-CDMA system is augmented with a precoder and a multicode encoder at the transmitter. The multicode encoder is an inverse Walsh-Hadamard transform (IWHT) unit, while the precoder is a recently suggested "constant amplitude" encoder that guarantees that the IWHT output stream has binary level. While IWHT, multicarrier (MC) transmission, and direct sequence spreading provide us with frequency diversity, the constant amplitude precoder ensures that the transmitted waveform is spread BPSK signal, minimizing peak to average-power ratio (PAPR) problem. The multicode and (MC) combination also addresses the need for flexible high data rate services in present day wireless communications. In the receiver, after matched filtering or correlation demodulation for each subcarrier and WHT operation, a detection strategy based on minimum-mean-square-error (MMSE) criterion is used. We have derived an upper bound for the bit error rate (BER) performance in Nakagami fading channel. Numerical results are compared with those obtained from a single carrier DS-CDMA system (SC-DS-CDMA) and a MC-DS-CDMA scheme without the precoding and IWHT units. We show that our proposed MC.DS-CDMA scheme has a much better BER performance compared with both of these schemes. [ABSTRACT FROM AUTHOR]

Power control is an important factor for direct-sequence code division multiple access (DS-CDMA) cellular radio systems to achieve higher communication link quality and better system capacity. In order to track the desired signal-to-interference-plus-noise ratio (SINR) under round-trip delay, multiple access interference (MAI), channel fading, and noise, a time delay-based state-space model is developed for representing the tracking error dynamics and a state feedback controller is introduced for SINR tracking control. Then the power tracking problem can be regarded as a control problem. In this paper, a robust H∞ power tracking control design is proposed to achieve a robust optimal SINR tracking from the minimization of the worst-case effect point of view. This robust optimal power tracking design problem can be transformed to solving the eigen-value problem (EVP) under some linear matrix inequality (LMI) constraints. The LMI Matlab toolbox can be used to efficiently solve the EVP via convex optimization to achieve a robust optimal SINR tracking design. Under the proposed distributed framework, the information of channel gain is not needed. [ABSTRACT FROM AUTHOR]

An adaptive multi-user detector was developed for direct-sequence code division multiple access (DS-CDMA) systems corrupted by non-Gaussian channel noise, which can be quite detrimental to the performance of the multi-user detectors based on classical white Gaussian assumption. This receiver simultaneously combats multipleaccess interference (MAI) and non-Gaussian impulsive noise. The channel parameters are estimated and transmitted signals are jointly detected by a simple recursive algorithm derived from the EM/SAGE algorithm. Analytical and simulation results show that the proposed technique is robust with wider applicability than conventional multi-user detectors in terms of near-far resistance and bit-error ratio (BER) when either MAI or non-Gaussian impulsive noise is dominant. [ABSTRACT FROM AUTHOR]

An antenna array based base station receiver for multicarrier Direct Sequence Code Division Multiple Access (DS-CDMA) system is proposed. The main advantage of the receiver is that the spatial diversity is achieved by combining signals of array elements. Based on the detailed analysis of multiuser interference and noise characteristics, the performance of the proposed receiver is analyzed. Theoretical analysis shows significant performance improvement in terms of system capacity due to the use of antenna arrays compared with the conventional single antenna multicarrier DS-CDMA approach. Simulation results confirm the theoretical analysis. [ABSTRACT FROM AUTHOR]

In this paper, we present a systematic study of the subspace-based blind channel estimation method. We first formulate a general signal model of multiple simultaneous signals transmitted through vector channels, which can be applied to a multitude of modern digital communication systems. Based on this model, we then propose a generalized subspace-based channel estimator by minimizing a novel cost function, which incorporates the set of kernel matrices of the signals sharing the target channel via a weighted sum of projection errors. We investigate the asymptotic performance of the proposed estimator, i.e., bias, covariance, mean square error (MSE), and Cramér-Rao bound, for large numbers of independent observations. We show that the performance of the estimator can be optimized by increasing the number of kernel matrices and by using a special set of weights in the cost function. Finally, we consider the application of the proposed estimator to a downlink code division multiple access (CDMA) system operating in a frequency-selective fading channel with negligible intersymbol interference (ISI). The results of the computer simulations fully support our analytical developments. [ABSTRACT FROM AUTHOR]

Detection structures for a direct sequence (US) code division multiple access (CDMA) multiuser uplink using codes with a period of several symbols, called long codes, are investigated. In a first part, we propose a multirate description of the uplink. Based on this formalism, we revisit the linear and decision feedback minimum mean square error (MMSE) detectors. In order to identify the best attainable performance, we focus on the infinite size versions. In a second part, reduced complexity solutions are put forward and discussed. Finally, the performance of the proposed detectors is investigated and compared with that of the classical rake receiver and the single-bit horizon detectors. The relevance of the different solutions is discussed in the light of the channel length. [ABSTRACT FROM AUTHOR]

In this paper, three novel stochastic gradient algorithms for adjustment of the widely linear (WL) minimum mean-squared error (MMSE) filter for multiple access interference (MAI) suppression for direct-sequence code-division multiple access (DS-CDMA) are introduced and analyzed. In particular, we derive a data-aided WL least-mean-square (LMS) algorithm, a blind WL minimum-output-energy (MOE) algorithm, and a WL blind LMS (BLMS) algorithm. We give analytical expressions for the steady-state signal-to-interference-plus-noise ratios (SINRs) of the proposed WL algorithms, and we also investigate their speed of convergence. Wherever possible, comparisons with the corresponding linear adaptive algorithms are made. Both analytical considerations and simulations show, in good agreement, the superiority of the novel WL adaptive algorithms. Nevertheless, all proposed WL algorithms require a slightly lower computational complexity than their linear counterparts. [ABSTRACT FROM AUTHOR]

Most blind source separation algorithms assume the channel noise to be Gaussian. This paper considers the problem of noncooperative, blind detection of synchronous direct-sequence code-division multiple-access communications (no knowledge of the spreading sequences or training data) in non-Gaussian channels. Three iterative algorithms with different performance and complexity tradeoffs are proposed. Simulation results show that they significantly outperform Gaussian-optimal blind source separation algorithms in non-Gaussian channels. The Cramér-Rao lower bound for this problem is computed, and the performance of the proposed algorithms is shown to approach this bound for moderate signal-to-noise ratios. [ABSTRACT FROM AUTHOR]

A novel one-shot decorrelator for asynchronous CDMA systems is developed. Compared with existing one-shot decorrelator, it can reduce complexity and has better performance while eliminating all MAI. This decorrelator is shown to be near-far resistant in both AWGN and fading channel. [ABSTRACT FROM AUTHOR]

The robustness of the subspace method for blind signature waveform estimation with respect to channel order is analyzed in asynchronous DS-CDMA systems theoretically. Theoretical analysis and simulation results show that the overestimating of the channel order will lead to the degradation of the quality of the estimated signature waveform. So we should adopt the channel order as small as possible. [ABSTRACT FROM AUTHOR]

This paper describes a model of a system with wideband CDMA that is a proposal of user access for future UMTS. This model has been implemented using OPNET tool. The model enables network architecture, radio interface, mobile station motion, call generation, signalling, etc. The examples of simulation results of call and handover blocking probability are also presented in this paper.