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

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.

Pseudo Convex Framework using Sparse Channel Estimation for Multipath Fading Channels in DS-CDMA Systems

A Direct-Sequence Code-Division Multiple-Access (DS-CDMA) with rake receiver for multiuser system sets up a new dimension in mobile communication system. We propose a pseudo convex framework using an optimum sparse channel estimation technique for DS-CDMA mobile communication systems. Further, the blind channel estimation problem will be examined for rake-based DS-CDMA communication framework with time variant multi-path fading channels This receiver accomplishes an effective assessment of channel according to a maximum convexity criterion, by means of the sparse technique. This estimation method requires a convenient representation of the discrete multipath fading channel based on the sparse theory. In this paper, we have defined a specialized interior-point method for solving large-scale ℓ1 - problem in multiuser detection. Our method can be generalized to handle a variety of extensions such as various channel conditions. A new solution to DS-CDMA based sparse channel estimation is presented in this paper that assures a global optimal solution. Also, it is proven that the said solution can be used as an apparent program that shall enable solutions utilizing interior point techniques involving polynomial time complexity. Through simulation the rationality of the techniques proposed in this paper has been highlighted by results obtained for various modulation schemes and channel parameters. The execution of a pseudo convex framework using sparse channel estimation technique with rake receiver in DS-CDMA framework for multipath fading channels is explored. The overall performance is evaluated in terms of bit error rate (BER) for a range of values of signal to noise ratio (SNR). This framework gives better performance under various modulation schemes using pseudo noise (PN) spreading code. Furthermore, performance of the proposed system is compared with different detectors.

Improved Whale Optimized MLP Neural Network-Based Learning Mechanism for Multiuser Detection in MIMO Communication System

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.

Performance of a novel evolutionary genetic-based multi-user detector for multi-carrier CDMA communication systems

In this paper, we investigate the performance of a modified evolutionary multi-user detector (MUD) in multi-carrier direct-sequence code-division multiple-access (MC-CDMA) communication systems over frequency-selective fading channels. The genetic algorithm (GA), which tries to reduce the complexity in optimal detection due to greedy exhausted searching, performs using the adequate sub-optimal multi-user detector for MC-CDMA systems. In this paper, we propose a forcing mutation scheme for the genetic algorithm (GA) to perform a forcing mutation evolutionary multi-user detector (FME-MUD) and to largely reduce computational complexity. Simulation results show that with six generations, the proposed FME-MUD scheme outperforms the conventional GA-MUD, especially at heavy system loads.

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.

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.

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.

페이딩 채널에서 직접 대역확산 부호분할 다중접속 통신을 위한 일반화된 혼합 전력/전송률 적응화 기법

We investigate a generalized combined power and rate adaptation scheme in direct-sequence (DS) code-division multiple-access (CDMA) communications over Nakagami fading channels. The transmission power allocated to user i is proportional to , where is the channel gain of user i and p is a real number, and the data rate (i.e., spreading gain) is jointly adapted so that a desired QoS is maintained. We analyze the average data rate of the proposed adaptation scheme subject to fixed average and peak transmission power constraints. Our results show that the proposed joint adaptation scheme provides a significant performance improvement over power-only and rate-only adaptation.

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.

Multiuser Detection Based on Adaptive LMS and Modified Genetic Algorithm in DS-CDMA Communication Systems

In this paper, we present an efficient evolutionary algorithm for the multi-user detection (MUD) problem in direct sequence-code division multiple access (DS-CDMA) communication systems. The optimum detector for MUD is the maximum likelihood (ML) detector, but its complexity is very high and involves an exhaustive search to reach the best fitness of transmitted and received data. Thus, there has been considerable interest in suboptimal multiuser detectors with less complexity and reasonable performance. The proposed algorithm is a combination of adaptive LMS Algorithm and modified genetic algorithm (GA). Indeed the LMS algorithm provides a good initial response for GA, and GA will be applied for this response to reach the best answer. The proposed GA reduces the dimension of the search space and provides a suitable framework for future extension to other optimization algorithms. Our algorithm is compared to ML detector, Matched Filter (MF) detector, conventional detector with GA; and Adaptive LMS detector which have been used for MUD in DS-CDMA. Simulation results show that the performance of this algorithm is close to the optimal detector with very low complexity, and it works better in comparison to other algorithms.

H∞Channel Estimation for DS-CDMA Systems: A Partial Difference Equation Approach

In the communications literature, a number of different algorithms have been proposed for channel estimation problems with the statistics of the channel noise and observation noise exactly known. In practical systems, however, the channel parameters are often estimated using training sequences which lead to the statistics of the channel noise difficult to obtain. Moreover, the received signals are corrupted not only by the ambient noises but also by multiple-access interferences, so the statistics of observation noises is also difficult to obtain. In this paper, we will investigate theH∞channel estimation problem for direct-sequence code-division multiple-access (DS-CDMA) communication systems with time-varying multipath fading channels. The channel estimator is designed by applying a partial difference equation approach together with the innovation analysis theory. This method can give a sufficient and necessary condition for the existence of anH∞channel estimator.

Contribution to Synchronization and Tracking Modelisation in a CDMA Receiver

We propose and analyze a noncoherent receiver with PN code tracking for direct sequence code division multiple access (DS-CDMA) communication systems. We employ the delay-lock loop (DLL) architectures for the tracking stage. The choice of DLL parameters is studied with special focus on DS-CDMA communication systems and orthogonality conditions. We described the modeling and simulation of the NCO using hardware description language VHDL. Details of the VHDL implementation are shown.

Distributed power control algorithms in the uplink of wireless code-division multiple-access systems

Power control is an important problem in today's wireless systems, which is related to battery utilisation in the mobile units. In the present study, this problem is addressed using a distributed approach. The uplink of a direct-sequence code-division multiple-access communication (DS-CDMA) system is studied, and through a proper selection of the tracking error function, the non-linear coupling among active users is transformed to individual linear loops, where equivalent power references incorporate the information of the remaining active users in the cell. It is concluded that the uplink channel variations do not destroy the stability of these feedback structures. However, the delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. A linear quadratic (LQ)-optimal power control strategy is derived and compared to a dead-beat approach. In this sense, the dead-beat algorithm shows sensitivity to the roundtrip delays estimation, but the LQ-optimal control can be adjusted to be robust to this estimation error. In fact, there is a severe compromise between robustness and performance. But through an appropriate selection of the LQ parameters, a stable closed-loop system can be always guaranteed independently on the uncertainty in the estimation of the roundtrip delay. Simulation results are presented to compare the proposed control algorithms to a standard single-step power correction approach.

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

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

PERFORMANCE COMPARISON OF NON-LINEAR INTERFERENCE CANCELLATION TECHNIQUES FOR MULTIUSER DETECTION IN DS-CDMA SYSTEM

Direct sequence code division multiple access (DS-CDMA) is a most popular mobile communication technology. In DS-CDMA communication, signals of all users overlap in time and frequencies and cause multiple access interference (MAI). The conventional DS-CDMA detectors follow the single user matched filter detection technique, in which each user is detected individually without regard for other users and ignores the existence of MAI. Its performance is not very satisfactory and is particularly limited by the near-far problem. Multi-user detection is the efficient technique to use the capabilities of code-division multiple access (CDMA), which is becoming the ubiquitous air-interface in future generation communication systems. The problem of multiple access interference (MAI) is vital for a CDMA system. The different types of multi-user detectors have been proposed to mitigate the MAI. The different sub optimal multi-user detector offers performance in between the single user matched filter and the optimal multiuser detectors with substantially reduced complexity. This paper presents the simulation results regarding performance comparison of some most popular non-linear multi-user detectors i.e., multistage detectors, decision- feedback detectors and subtractive interference cancellation.

A Blind Equalization Algorithm Based on Minimization of Normalized Variance for DS/CDMA Communications

This paper proposes a blind equalizer that pursues the maximization of a novel objective function. It consists of the ratio between the square of the mean signal power and the variance of the signal power. In this paper, important prepositions are first mathematically stated. The devised equalizer is asymptotically equivalent to the well-known constant-modulus algorithm (CMA) and the Kurtosis maximization algorithm (KMA) because they pursue the same ldquoRicianityrdquo criterion already employed in the ldquoRayleigh-nessrdquo test for spread-spectrum code acquisition. The performance of the addressed equalizers is then investigated by focusing on direct-sequence code-division multiple-access (DS/CDMA) communication environments in a downlink scenario. In particular, we have evidenced the effectiveness of the presented equalizer in the numerical examples in terms of robustness against the effect of interference, as well as in terms of convergence rate, in spite of a small amount of extra processing.

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

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

New Sequences with Low Correlation and Large Family Size

In direct-sequence code-division multiple-access (DS-CDMA) communication systems and direct-sequence ultra wideband (DS-UWB) radios, sequences with low correlation and large family size are important for reducing multiple access interference (MAI) and accepting more active users, respectively. In this paper, a new collection of families of sequences of length pn-1, which includes three constructions, is proposed. The maximum number of cyclically distinct families without GMW sequences in each construction is π(pn-1)/n · π(pm-1)/ m, where p is a prime number, n is an even number, and n = 2m, and these sequences can be binary or polyphase depending upon choice of the parameter p. In Construction I, there are pn distinct sequences within each family and the new sequences have at most d + 2 nontrivial periodic correlation {-pm-1, -1, pm-1,2pm-1,···,dpm-1}. In Construction II, the new sequences have large family size p2n and possibly take the nontrivial correlation values in {-pm-1, -1, pm-1, 2pm-1,···,(3d-4)pm-1}. In Construction III, the new sequences possess the largest family size p(d-1)n and have at most 2d correlation levels {-pm-1, -1, pm-1,2pm-1, ···,(2d-2)pm-1}. Three constructions are near-optimal with respect to the Welch bound because the values of their Welch-Ratios are moderate, WR ≐ d, WR ≐ 3d-4 and WR ≐ 2d-2, respectively. Each family in Constructions I, II and III contains a GMW sequence. In addition, Helleseth sequences and Niho sequences are special cases in Constructions I and III, and their restriction conditions to the integers m and n, pm ≠ 2 (mod 3) and n ≡ 0 (mod 4), respectively, are removed in our sequences. Our sequences in Construction III include the sequences with Niho type decimation 3 · 2m-2, too. Finally, some open questions are pointed out and an example that illustrates the performance of these sequences is given.

Performance of a Novel Adaptive Multistage Full Parallel Interference Canceller for CDMA Systems

In this paper, we investigate the statistical behavior of the adaptation partial cancellation weight (PCW) in partial parallel interference cancellation (PPIC) schemes based on least mean square (LMS) algorithms in which the adaptation PCW of the last chip in a symbol period reveals the information of the correctness of the bit decision for direct-sequence code-division multiple-access (DS-CDMA) communication systems. We obtained an adaptive decision threshold using a maximum a posteriori probability (MAP) detection criterion for the PCWs to judge the correctness of the bit decision. A bit-inversion procedure is proposed to invert the incorrect bit decisions. After bit inversion, full PIC is performed to cancel multiple-access interference (MAI). Simulation results show that the proposed three-stage LMS-based adaptive full PIC (AFPIC) multiuser detector reaches the performance of a single user bound over flat fading channels. Moreover, the AFPIC detector effectively cancels MAI over a two-path frequency-selective fading channel.

Low-Complexity Code Acquisition Method in DS/CDMA Communication Systems: Application of the Maximum Likelihood Method to Propagation Delay Estimation

Code acquisition performance in the Direct-Sequence Code-Division Multiple-Access (DS/CDMA) communication system is strongly related to the quality of the communication systems. The performance is assessed by (i) code acquisition time; (ii) precision; and (iii) complexity for implementation. This paper applies the method of maximum likelihood (ML) to estimation of propagation delay in DS/CDMA communications, and proposes a low-complexity method for code acquisition. First, a DS/CDMA system model and properties of outputs with a passive matched-filter receiver are reviewed, and a statistical problem in code acquisition is mentioned. Second, an error-controllable code acquisition method based on the maximum likelihood is discussed. Third, a low-complexity ML code acquisition method is proposed. It is shown that the code acquisition time with the low-complexity method is about 1.5 times longer than that with the original ML method, e. g. 13 data periods under 4.96dB.