Multiuser Detection For DS-CDMA Systems Using Honeybees Mating Optimization Algorithm

This paper introduces an adaptive ( /spl mu/ + /spl lambda/ ) evolutionary algorithm (EA) for multiuser detection (MUD) in direct sequence code division multiple access (DS-CDMA) systems. The EA based multiuser detector takes the output of parallel interference cancellation (PIC) detector as the initial population and adopts maximum likelihood decision rule to detect the user bit sequences. During an EA running, the offspring population size is adaptively adjusted according to the current success probability. The major advantage of the adaptive EA based multiuser detection scheme is that it obtains rather good bit error rate (BER) performance with lower computational complexity. Monte Carlo simulation results show the EA based multiuser detector can always converge to the optimal solution with a small number of generations. Numerical analysis shows its computational time is polynomial complexity and less than other heuristic search algorithms.

In this paper, we present two layered multiuser detectors (MUDs) for MIMO frequency-time-domain (FT-domain) multi-carrier (MC) direct sequence code division multiple access (DS-CDMA) systems with an antenna array at the base station. We assume that multiple users are active and individually utilize multiple transmit antennas in the MC DS-CDMA system. The users are organized into groups, and each user is assigned a unique Time-domain (T-domain) signature code. Moreover, users in the same group share a unique F-domain signature code. As a result, they can exploit the T-domain and F-domain signature codes to spread their multiple symbols in parallel, and then transmit the FT-domain spread signals from the corresponding multiple antennas over the fading channels to the base station. However, because of the non-ideal channel effect and/or the use of non-orthogonal signature codes, the FT-domain spread MC DS-CDMA system is affected by multiple access interference (MAI) in the same way as CDMA-like systems. To mitigate the effects of MAI and improve the system’s performance, we propose two layered MUDs that exploit the layered soft decision errors. Specifically, in a trade-off between the performance and the computational complexity, only the soft decision errors of one user/one group are used in the proposed layered MUDs. The results of simulations and a complexity analysis demonstrate that the proposed layered MUDs outperform existing approaches and the computational complexity is modest.

This paper describes the effect of power imbalance on multiuser detection of Direct Sequence Code Division Multiple Access (DS-CDMA) systems. Unlike the conventional single-user detection technique which considers interference between users or multiple access interference (MAI) as noise, in multiuser detection scheemes of DS-CDMA systems MAI is eliminated successively. However in the present of near-far problems and fading channels, where recived power among different users are of diffetrent levels, the performance of multiuser detection methods degrade due to unreliable detections of users with low power levels. In this paper multiuser detection is described in a simple model and the performance for various number of users is evaluated using computer simulation. The effect of power imbalance among different users is presented to show the performance degradation.

The nonlinear Multiuser Detection (MUD) in Direct Sequence Code Division Multiple Access (DS/CDMA) system can be viewed as a two-class classification task. A new classification method called Probabilistic Tangent Subspace (PTS) is introduced to be used as an MUD. Due to the mobility of communicator, wireless communication channels are in fact time variant. The uncertainties of the time-varying channel’s coefficients cause the uncertainties of the Multiuser Interference (MUI). On the other hand, the probabilistic tangent subspace method is designed to encode the pattern variations. Therefore, we are motivated to adopt this method to develop a classifier as a multiuser detector for time-varying channels. Simulation results show that this MUD performs better than that based on Support Vector Machine (SVM) for Rayleigh fading channel in DS/CDMA system.KeywordsSupport Vector MachineMinimum Mean Square ErrorCode Division Multiple AccessRayleigh Fading ChannelMultiuser DetectionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

We propose a multistage joint adaptive array (AA) and spatial interference cancellation (IC) structure for a direct sequence code division multiple access (DS/CDMA) system. The structure utilizes both space and time processing to improve the performance of the base station receiver under the influence of multiple access interference (MAI) and in the absence of power control. The AA comprises a bank of temporal matched filters followed by a beamformer. IC is then used to remove the detected strong user from the received array input. This stage can be repeated for recover and remove more users. Computer simulation shows that, in the absence of power control, the proposed algorithm efficiently detects the weak user information at MAI levels of 30 dB where the conventional space-time structure fails.

A new RLS adaptive algorithm with variable forgetting factor for direct sequence code division multiple access (DS-CDMA) communication system is presented. Intersymbol interference (ISI) arises due to multipath propagation, which destroys the code orthogonality, gives rise to multiple access interference(MAI). Therefore, the objective of this paper is to minimize the effect of ISI with a long impulse response environment based on the proposed receiver. We consider an adaptive Laguerre lattice equalizer which applies an optimal method based on the variable step-size least mean square (VSLMS) algorithm for adaptable forgetting factor RLS algorithm to improve bit error rate (BER) performance of the receiver.

We present a novel multiuser detection (MUD) technique, based on ant colony optimization (ACO), for synchronous direct sequence code division multiple access (DS-CDMA) systems. ACO algorithms are based on the cooperative foraging strategy of real ants. While an optimal MUD design using an exhaustive search method is prohibitively complex, we show that the ACO based MUD converges to the optimal BER performance in relatively few iterations, providing 9.5% saving in computational complexity.

Innovations of modern digital radio technology has enabled many large mobile and distributed information systems, such as inventory tracking systems, to provide readily accessible voice and data services to end users despite mobility of data servers. These systems often contain components that are equipped with radio frequency identification (RFID) tags and interrogators for wireless connectivity. However, there are usually large number of these tagged items in these systems that are highly mobile. The wireless connections among them are also prone to failure. In this paper, we evaluate and compare the performance of two RF tag data access protocols in a single channel per cell implementation: slotted ALOHA Time Division Multiple Access (slotted ALOHA/TDMA) and Direct Sequence Code Division Multiple Access (DS/CDMA).We use a discrete event simulation model to study the impact of these protocols on data service under a diversity of highly mobile operating conditions. Mobile conditions are captured separately by various levels of burst error, uplink and downlink disconnections, and tag population. The results show that performance of DS/CDMA dominates in all mobile conditions. In the most general cases, DS/CDMA outperforms slotted ALOHA/TDMA by 1.5 times faster message time. DS/CDMA is particularly superior in the case of pure uplink disconnections. For burst error and pure downlink disconnections, DS/CDMA renders as much as 2.0 times and 4.5 times faster message time respectively.

A DS/CDMA (direct-sequence code division multiple access) system using BPSK (binary phase shift keying) modulation is simulated to determine the bit error probabilities for various numbers of users with a processing gain close to that of a cellular network. The results are compared to results obtained using the Gaussian approximation equations. The results obtained are to the feasibility of a DS/CDMA cellular network. It is concluded that DS/CDMA cellular networks appear to offer higher subscriber capacities than existing analog or TDMA (time division multiple access) formats. In addition, DS/CDMA offers additional advantages such as inherent privacy through the use of spreading codes and the ability to combat problems such as multipath fading and narrowband noise. >

Nowadays, Multicarrier Direct sequence code division multiple access (MC DS-CDMA) systems are used in mobile communication. Performance of these systems are limited by multiple access interference (MAI) created by spread-spectrum users in the channel as well as background channel noise. This paper proposes an incremental gradient descent (IGD) multi-user detection (MUD) for MC DS-CDMA system that can achieve near-optimum performance while the number of users is linear in its implementation complexity. The IGD algorithm make an effort to perform optimum MUD by updating one user's bit decision each iteration in the best way. This algorithm accelerates the gradient algorithm convergence by averaging. When a minimum mean square error (MMSE) MUD is employed to initialize the proposed algorithm, in all cases tested the gradient search converges to a solution with optimum performance. Further, the iterative tests denote that the proposed IGD algorithm provides significant performance for cases where other suboptimum algorithms perform poorly. Simulation compares the proposed IGD algorithm with the conventional detectors.

In most direct sequence code division multiple access (DS-CDMA) systems, the modulation scheme used is a simple one. In this paper, employing a combined modulation scheme in DS-CDMA is proposed. By combining two or more modulation schemes, the resultant symbol energy can be increased, and thus a better symbol error rate performance can be achieved. For the combined modulation scheme, the symbol error rate is determined by the less powerful one of the basic modulation schemes. So a proper selection of modulation combination is essential for power efficiency. In a DS-CDMA system, unlike a narrow-band system, the bandwidth efficiency is closely related to its power efficiency. Thus a combined modulation concept may result in a bandwidth efficient DS-CDMA system. In this paper, we present two DS-CDMA schemes by combining MFSK and coherent MPSK and differential BPSK. These two schemes are respectively denoted as (1) MFSK-MPSK/DS-CDMA, and (2) MFSK-DPSK/DS-CDMA. The proposed schemes have better performance than the conventional CDMA schemes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Direct-sequence code-division multiple-access (DS-CDMA) is a popular multiple-access technology for wireless communications. However, its performance is limited by multiple-access interference and multipath distortion. Multiuser detection and space-time processing are two signal processing techniques employed to improve the performance of DS-CDMA. Two minimum probability of error-based space-time multiuser detection algorithms are proposed in this paper. The first algorithm, minimum joint probability of error (MJPOE), aims to minimize the joint probability of error for all users. The second algorithm, minimum conditional probability of error (MCPOE), minimizes the probability of error of each user conditioned on the transmitted bit vector, for each user individually. In both the algorithms, the optimal filter weights are computed adaptively using a gradient descent approach. The MJPOE algorithm is blind and offers a bit-error-rate (BER) performance better than the nonadaptive minimum mean squared error (MMSE) algorithm, at the cost of higher computational complexity. An approach for reducing the computational overheads of MJPOE using Gram-Schmidt orthogonalization is suggested. The BER performance of the MCPOE algorithm is slightly inferior to MMSE, however, it has a computational complexity linear in the number of users. Both blind and training-based implementations for MCPOE are proposed. Both MJPOE and MCPOE have a convergence rate much faster than earlier known adaptive implementations of the MMSE detector, viz. least mean square and recursive least squares. Simulation results are presented for synchronous single path channels as well as asynchronous multipath channels, with multiple antennas employed at the receiver.

The multiuser detection (MUD) performance analysis in a synchronous direct sequence code division multiple access (DS-CDMA) system using decorrelating and decision feedback (DFB) receivers, including single path Nakagami fading channel is presented. The lower bounds of the receivers bit error probability (BER) are derived. The analytical and numerical results show that the DFB receiver performs better in the presence of the high Nakagami fading channels.

The probability of packet success for asynchronous direct sequence code division multiple access (DS-CDMA) over the multipath fading channel is investigated under different fading conditions and for different cases. We studied the bit-to-bit dependence caused by the multipath fading and by the multiple access interference (MAI) originating from the chip and phase offsets between the desired and interfering signals. We investigated the effect of using the Gaussian versus the improved Gaussian process on the computation of probability of packet success as well as the effect of the bit-to-bit dependence within a packet. The improved Gaussian approximation provides more accurate values for the probability of bit error for any number of simultaneous users, while it also allows us to incorporate the effects of bit-to-bit dependence into the calculations of the probability of packet success.

Spread spectrum signals are widely used for secure communications, as well as for multiple access interference (MAI).They have many interesting properties, including low probability of interception.Indeed, direct sequence spread spectrum systems (DSSSS) use a periodical pseudo-random sequence to modulate the baseband signal before transmission.Interception of DSSS signals is a difficult task due to the hiding of the PN sequence, especially if the accompanied noise is non-Gaussian and the channel is frequency selective.In this paper, a receiver is proposed to intercept asynchronous direct sequence code division multiple access (DS-CDMA) signals received from different users and estimate the PN sequences of each one of them.The signals are assumed to be transmitted over frequency selective fading channel and contaminated with Class-A impulsive noise.The proposed interception receiver is based on Independent Component Analysis (ICA), the pseudo random sequences is estimated from the eigenvector subspace in case of single path.In multipath situations, the estimation is the overlapped form of the pseudo random sequences, a triple correlation function (TCF) is introduced to eliminate the influence of multipath.The performance of proposed receiver is evaluated in terms of the number of correct estimated codes for m-sequence over typical urban channel in presence of impulsive noise.The results show that the interception receiver performs is 100% code estimation at -8dB signal to noise ratio.