Detection Code Division Multiple Access Research Articles

An Adaptive Bayesian Computation Model for CDMA Multiuser Detection using Evolutionary Computation Scheme

Demand of wireless communication is increasing drastically where CDMA (Code Division Multiple Access) is considered as most promising technique for real-time communication. However, due to extreme utilization of these technologies several challenges occur such as interference and packet drop resulting in poor communication. To address these issues, multi-user detection schemes have been adopted widely which are based on the filtering techniques and MMSE (Minimum Mean Square Error) based multiuser detection schemes. In this work we address these issues and proposed a novel approach of multi-user detection for asynchronous CDMA using combined optimization and evolutionary computation. Bayesian computation model is applied which helps to compute the Log Likelihood Ratio (LLR) using Monte Carlo simulation. Later genetic algorithm is incorporated to obtain the optimal solution for LLR probability resulting in reliable communication. An extensive simulation study is presented which shows significant improvement in the performance when compared with state-of-art multiuser detection schemes.

Second-Order Cone Programming Based Robust Blind Multiuser Detector for CDMA Systems

In this paper, we propose a robust blind multiuser detector for code-division multiple access (CDMA) systems against signature waveform mismatch (SWM) derived from the influences of time asynchronization or channel distortion. This blind detection method with SWM problem is formulated as blind source separation (BSS) model subject to the second-order cone (SOC) constraint. The resulting blind separation based on SOC programming problem is solved by approximate negentropy maximization using quasiNewton iterative methods. Theoretical analysis and simulation results show that the performance of the proposed blind detector is superior to those of the existing methods.

Multi-user Detection in DS-CDMA System Using Biogeography Based Optimization

To attain large information rates, code division multiple access (CDMA) is a well-built aspirant for the downlink of cell phone communications. However, while transmitting any signal over fading channel, the performance of CDMA system is highly affected. For that reason, Multiuser detection (MUD) and channel estimation play a significant function for overcoming the interference and characterizing the channel. In this paper, biogeography base optimization (BBO) algorithm introduced for multiuser detection CDMA system. The objective of BBO algorithm is used to minimize the error rate of the user transmitting signal. According to immigration rate and emigration rate, the optimal solution of the detection problem is decided. Therefore, the user detection complexity and the interference of user transmitting signal are resolved. Proposed multiuser detection algorithm is appropriate for run time user identification process; since the detection complication and the required time are concentrated. The proposed approach is implemented in MATLAB working platform and the performance measures are examined. The error rate of proposed algorithm is compared with harmony search algorithm, enhanced harmony search algorithm, and without multiuser detection procedure.

Performance Analysis of the Downlink CP-WCDMA System with Turbo Code in Frequency Domain

Modern wireless communications require an efficient spectrum usage and high channel capacity and high throughput. Turbo code, linear equalizers, multi-user detection and wideband code-division multiple access (WCDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate MAI, eliminate ISI and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Single-output downlink cyclic prefix CP-WCDMA systems. It employs frequency domain interference cancellation schemes to mitigate the interference caused by the multipath fading channel. Also, the proposed scheme is developed for the downlink Turbo code CP-WCDMA system to maximize the throughput of the proposed system.

Successive AF/DF Relaying in the Cooperative DS-CDMA Uplink: Capacity Analysis and Its System Architecture

A successive-relaying-aided network (SRAN) is designed for a multiuser spread-spectrum scenario conceived for noncoherent (NC) detection to convert the typical 50% half-duplex relaying induced throughput loss to a potential user-load reduction of the code-division multiple-access (CDMA) system, where the NC allows us to avoid the extra power consumption imposed by channel estimation. We commence by evaluating the NC discrete-input-continuous-output memoryless channel (DCMC) capacity of both the amplify-and-forward-based (AF) and decode-and-forward-based (DF) SRANs in the direct-sequence CDMA (DS-CDMA) uplink (UL). While NC detection has the added benefit of eliminating both the pilot overhead and the power-hungry channel estimation, it tends to form an error floor at high Doppler frequencies. We mitigate this problem using multiple-symbol detection, which increases the detection complexity upon extending the detection window. Finally, a relay-aided soft-input-soft-output multiple-symbol differential sphere detection (SISO-MSDSD) CDMA regime is proposed, which significantly reduces the system's complexity without sacrificing its performance.

Successive beamforming, multiuser detection and diversity reception for multicarrier DS-CDMA system with antenna array

Code-Division Multiple-Access (CDMA) systems are interference limited, and therefore efficient interference management is necessary to enhance the performance of a CDMA system. In this paper, a successive beamforming (spatial filtering), linear decorrelating MultiUser Detection (MUD, temporal filtering) and diversity reception structure for uplink multicarrier Direct Sequence CDMA (DS-CDMA) system with antenna array are proposed. By beamforming, the antenna array suppresses interference according to the distinct array signature. Subsequently, linear decorrelating MUD is applied to separate the signals of different users and eliminate Multiple Access Interference (MAI). Finally, the decorrelated signals at different subcarriers that belong to the same user are combined to achieve frequency diversity. Simulation results show that the proposed structure offers significant Bit Error Rate (BER) performance improvement by successively exploiting the space-time-frequency processing.

Improved detection in CDMA for biased sources

We consider the detection of biased information sources in the ubiquitous code-division multiple-access (CDMA) scheme. We propose a simple modification to both the popular single-user matched-filter detector and a recently introduced near-optimal message-passing-based multiuser detector. This modification allows for detecting modulated biased sources directly with no need for source coding. Analytical results and simulations with excellent agreement are provided, demonstrating substantial improvement in bit error rate in comparison with the unmodified detectors and the alternative of source compression. The robustness of error-performance improvement is shown under practical model settings, including bias estimation mismatch and finite-length spreading codes.

Dynamical replica theoretic analysis of CDMA detection dynamics

We investigate the detection dynamics of the Gibbs sampler for code-division multiple access (CDMA) multiuser detection. Our approach is based upon dynamical replica theory which allows an analytic approximation to the dynamics. We use this tool to investigate the basins of attraction when phase coexistence occurs and examine its efficacy via comparison with Monte Carlo simulations.

Efficient Bayesian inference for learning in the Ising linear perceptron and signal detection in CDMA

Efficient new Bayesian inference technique is employed for studying critical properties of the Ising linear perceptron and for signal detection in code division multiple access (CDMA). The approach is based on a recently introduced message passing technique for densely connected systems. Here we study both critical and non-critical regimes. Results obtained in the non-critical regime give rise to a highly efficient signal detection algorithm in the context of CDMA; while in the critical regime one observes a first-order transition line that ends in a continuous phase transition point. Finite size effects are also studied.

Generating functional analysis of CDMA detection dynamics

We investigate the detection dynamics of the parallel interference canceller (PIC) for code-division multiple-access (CDMA) multiuser detection, applied to a randomly spread, fully syncronous base-band uncoded CDMA channel model with additive white Gaussian noise (AWGN) under perfect power control in the large-system limit. It is known that the predictions of the density evolution (DE) can fairly explain the detection dynamics only in the case where the detection dynamics converge. At transients, though, the predictions of DE systematically deviate from computer simulation results. Furthermore, when the detection dynamics fail to convergence, the deviation of the predictions of DE from the results of numerical experiments becomes large. As an alternative, generating functional analysis (GFA) can take into account the effect of the Onsager reaction term exactly and does not need the Gaussian assumption of the local field. We present GFA to evaluate the detection dynamics of PIC for CDMA multiuser detection. The predictions of GFA exhibits good consistency with the computer simulation result for any condition, even if the dynamics fail to convergence.

Near-far resistance of optimum and suboptimum CDMA detectors under multipath

Near-far resistance is one of the most important performance measures in a multiuser communication system, especially in a code division multiple access (CDMA) system. The optimum multiuser detector can achieve the optimum near-far resistance. However, due to its high computational complexity, the optimum detector is impractical to implement. Nevertheless, computation of the near-far resistance of the optimum multiuser detector is still important since it is the least upper bound on the near-far resistance of any detector, as well as a benchmark for relative performance of any suboptimum detector. Lupas et al. and Zvonar et al. provided the computation of the optimum near-far resistance in the asynchronous AWGN and unipath channel case. In this paper, we extend the above results to the asynchronous multipath channel case. We derive the optimum near-far resistance under the assumption of a common identifiability condition in the blind multiuser detection/equalization literature. We also derive the ideal near-far resistance of three typical methods in blind multiuser detection/equalization (subspace, linear prediction, and constrained optimization) and compare them with that of the optimum detector. Two methods to enhance near-far resistance of suboptimum detectors are also presented. Furthermore, one possible cause that may affect the near-far resistance in a practical setting is analyzed. The computational complexity of the proposed methods is analyzed and compared. Simulations confirm the theoretical findings.

Robust blind multiuser detection against signature waveform mismatch based on second-order cone programming

Blind signal detection in multiuser code division multiple access (CDMA) system is particularly attractive when only the desired user signature is known to a given receiver. A problem common to several existing blind multiuser CDMA detectors is that the detection performance is very sensitive to the signature waveform mismatch (SWM) which may be caused by channel distortion. In this paper, we consider the design of a blind multiuser CDMA detector that is robust to the SWM. We present a convex formulation for this problem by using the second-order cone (SOC) programming. The resulting SOC problem can be solved efficiently using the recently developed interior point methods. Computer simulations indicate that the performance of our new robust blind multiuser detector is superior to those of many existing methods.

Regularized adaptive detectors for code-division multiple-access signals

This paper presents the development of regularized adaptive detectors for code-division multiple-access (CDMA) wireless-communication systems. Multiuser CDMA detection is formulated as an inverse problem in the presence of channel-induced interference and noise and the estimation of the detector parameters is accomplished using elements of regularization theory. The regularized solutions are derived by minimizing objective functions formed as a linear combination of a term representing the estimation error and a term representing a smoothness constraint. The smoothness constraint is determined by data-dependent procedures developed to reduce the effect of noise and the impact of interfering users on the performance of the resulting detector. The proposed adaptive detector is evaluated and compared with existing multiuser CDMA detectors in terms of their robustness to noise, resistance to the near-far problem, and ability to handle multipath signals.

New results for probability of error performance in MMSE multiuser detection for CDMA

The minimum-mean-square-error (MMSE) linear multiuser detector considers multiple access interference (MAI) and background Gaussian noise asymptotically Gaussian for a large number of users in asynchronous code division multiple access (CDMA) system. For this asymptotic condition, the MMSE detector outperforms the decorrelating detector only if the value of normalized cross-correlation (NCC) for any pair of code sequence is less than or equal to numerically derived upper bounded value. The available results in literature have been derived for two-user case. In this paper, we have presented a general formula to calculate upper bound on NCC for arbitrary number of users under near-far situation. We also propose Chernoff bound on error probability of MMSE multiuser detector. Its proof based on divergence theorem and study of leakage coefficients for more than two users imposes the stringent condition on signal-to-noise ratio of desired user.

Performance Evaluation of a Novel CDMA Detection Technique: The Two-State Approach

The use of code division multiple access (CDMA) makes third-generation wireless systems interference limited rather than noise limited. The research for new methods to reduce interference and increase efficiency led us to formulate a signaling method where fast impulsive silence states are mapped on zero-energy symbols. The theoretical formulation of the optimum receiver is reported and the asymptotic multiuser efficiency (AME) as well as an upper bound of the probability of error have been derived and applied to the conventional receiver and the decorrelating detector. Moreover, computer simulations have been performed to show the advantages of the proposed two-state scheme over the traditional single-state receiver in a multiuser CDMA system operating in a multipath fading channel.

Directly Estimated Adaptive Detectors for Code-Division Multiple-Access Signals

This paper presents adaptive multiuser detectors for code-division multiple-access (CDMA) signals in wireless communication systems. Directly estimated adaptive (DEA) detectors are developed by formulating CDMA detection as an inverse problem in the presence of channel-induced interference and noise. The detector parameters are computed by a fully sequential adaptive algorithm that requires no matrix inversion, and can be implemented online as the data arrive at the receiver. The proposed DEA detector is experimentally evaluated in terms of its robustness to noise, resistance to the near-far problem, and ability to handle multipath fading signals. This experimental study indicates that the proposed DEA detector requires shorter training sequences of bits to achieve the performance levels attained by existing adaptive implementations of the minimum mean-square error detector.

Linear CDMA Detection Algorithm Based on Statistical Neurodynamics and Belief Propagation and the Stability Conditions

Recently a modified algorithm of code-division multiple-access (CDMA) parallel interference canceler (PIC) has been proposed by Tanaka based on statistical neurodynamics. In this paper we apply the modified algorithm to the linear PIC (LPIC) and investigate its stability. We show that the stable (unstable) fixed points of the modified algorithm correspond to the stable (unstable) replica symmetry solutions with the Gaussian prior. We also show the modified algorithm is a special case of Kabashima’s belief-propagation algorithm with Gaussian prior.

Indirectly Estimated Adaptive Detectors for Code-Division Multiple-Access Signals

This paper presents the development of indirectly estimated adaptive (IDEA) detectors for code-division multiple-access (CDMA) signals in wireless communication systems. Multiuser CDMA detection is formulated as an inverse problem in the presence of noise, and the estimation of the detector parameters is accomplished using elements of regularization theory. The development of autonomous CDMA detectors that are capable of adjusting to the channel characteristics with the support of training sequences relies on data-dependent procedures designed to reduce the effect of noise and interfering users on the performance of the detector. The detector parameters can be computed by adaptive algorithms that require no matrix inversion. The proposed IDEA detector is evaluated and compared with existing multiuser CDMA detectors in terms of their robustness to noise, resistance to the near-far problem, and ability to handle multipath signals.

Adaptive robust detection for CDMA using a genetic algorithm

A novel adaptive robust multi-user detector for code-division multiple access (CDMA) using a real-coded genetic algorithm (GA) is proposed. The GA implicitly implements Huber's M-estimator and is robust against heavy-tailed impulsive noise. The novel feature of this GA-based detector lies in joint symbol detection and adaptive estimation of the cut-off parameter of the M-estimator's object function through a GA optimisation strategy. In particular, the GA-based detector carries out a multipoint search by manipulating and maintaining a population of candidate solutions for different values of the cut-off parameter to encourage information formation and exchange. Since the GA treats the cut-off parameter as one of its optimisation parameters, the need for a separate channel estimator is thus eliminated. Simulation results are provided to examine the evolutionary behaviour and the detection performance of the proposed GA-based detector. It is shown that the GA approach provides good adaptive impulsive noise rejection capability.

A PDA-Kalman approach to multiuser detection in asynchronous CDMA

The probabilistic data association (PDA) method for multiuser detection in synchronous code-division multiple-access (CDMA) communication channels is extended to asynchronous CDMA, where a Kalman filter or smoother is employed to track the correlated noise arising from the outputs of a decorrelator. The estimates from the tracker, coupled with an iterative PDA, result in impressively low bit error rates. Computer simulations show that the new scheme significantly outperforms the best decision feedback detector. The algorithm has O(K/sup 3/) complexity per time frame, where K is the number of users.