Constant modulus algorithm (CMA) for CDMA communications systems

For high-speed code division multiple access (CDMA) systems, interchip interference that is caused by multipath propagation becomes severe and cannot be ignored. Together with the multiple access interference due to nonideal conditions in CDMA, they become major obstacles to overall system performance. In this paper, both batch and adaptive algorithms based on linear prediction are proposed for joint blind equalization and multiuser detection for asynchronous CDMA systems. It is shown that the new methods are insensitive to estimation error of propagation delay or chip timing. In addition, the adaptive algorithm is computationally efficient. Simulation results show that the proposed methods are also near-far resistant and compare favorably to many existing methods.

In synchronous code division multiple access (CDMA) systems, both multiple access interference (MAI) and intersymbol interference (ISI) arise. MAI is present due to the CDMA format while ISI is present due to channel multipath. One approach to overcome these problems is to detect the entire package of data for all users at once. This implies working with matrices with dimensions proportional to the product of the data package length and the number of active users. In this paper, we present an alternative approach. This alternative consists of a type of multistage detector that overcomes the MAI and ISI problems; in addition, it has complexity proportional to the number of CDMA users, is computationally efficient, and is suitable for pipeline implementation which allows fast data processing. Simulations are compared to a derived single user bound for the average bit error probability and the results prove the validity of the proposed detection method.

This paper carries comprehensive comparison of Interleave Division Multiple Access (IDMA) and Code Division Multiple Access (CDMA) systems in terms of performance (BER) and behavior to different number of users. Performance analysis of two systems whose transmitter and receiver structures are almost same are studied under Additive White Gaussian Noise (AWGN) and performance differences are illustrated by simulation results. Although Multiple Access Interference (MAI) and Intersymbol Interference (ISI) can affect the performance of the CDMA system, simulation results illustrates that IDMA system model achieves this type of problem using their interleavers. Results also show that large number of users can easily be adapted by the IDMA scheme much easier than CDMA scheme.

The capacity of code division multiple access (CDMA) systems is limited by interference and can therefore be increased by techniques that suppress interference. Adaptive techniques using multiple antennas are generally used for interference suppression in the receiver. Blind adaptive schemes are attractive as they do not require any training sequence. Most of the commonly used blind algorithms optimize some cost functions that are generally non-convex and nonlinear functions of tap weights, when implemented using linear filters. Therefore the use of nonlinear structure that can form nonconvex decision regions is more suitable for this purpose. In this paper a blind antenna beamformer using constant modulus algorithm (CMA) has been implemented, where the beamforming network has complex nonlinear functions. Simulation results indicate better performance than the linear filters in terms of signal to interference and noise ratio (SINR).

This paper studies detecting matrices in random dense and sparse Code Division Multiple Access (CDMA) systems. Detecting matrices were originally introduced in the coin weighing problem. Such matrices can be used in CDMA systems in over-loaded scheme where the number of users is greater than the number of chips. We drive some conditions in the large system limit for binary and bipolar random CDMA systems to ensure that any random matrix is a detecting matrix. Furthermore, we extend our results to sparse random ternary matrices that have been using in the sparse CDMA literature. Finally, a construction method for the sparse detecting matrices is introduced.

In this paper, we applied genetic algorithm (GA) scheme as a suboptimal detection of a bit-synchronous code division multiple access (CDMA) systems over a Gaussian channel. The GA scheme attempts to search for the user's transmitted bit sequence that optimizes the correlation matrix employed. Simulation results showed that the Bit Error Rate (BER) performance of our proposed multiuser detector is better than those of conventional receivers like Minimum Mean Square Error(MMSE), Matched Filter(MF) and Rake Receivers.

Multiple access interference (MAI) appears in Code Division Multiple Access (CDMA) systems when the communication channel is a multi-path channel and the spreading codes are not orthogonal. Orthogonality between spreading codes cannot be maintained at the CDMA receiver because the codes may be asynchronous due to channel delay and multi-path spread. The receiver cannot perfectly separate the different signals of the multiple access users, and the resultant MAI limits the capacity of CDMA systems. MAI is a function of the cross correlation property between used spreading codes. In this paper we focus on the cross correlation of the spreading codes, we propose a method to find spreading codes with minimum magnitude of cross correlation. Employing these codes will reduce the resultant MAI in the CDMA system; hence it will increase the system capacity. A great enhancement is shown by comparing found minimum cross correlation spreading codes (MCCSC) with Hadamard and Gold codes.

The use of multiple antennas at the transmitter or receiver provides a powerful means, of increasing the capacity of wireless systems. The goal of this paper is to illustrate ways in which analogies with multiuser code division multiple access (CDMA) systems may be used to provide design insights for space-time communication systems, for both single and multiple users. Three examples are considered: (a) noncoherent space-time codes for transmit antenna diversity against Rayleigh fading, constructed using an analogy with a CDMA downlink; (b) optimal multiuser transmit beamforming on a cellular downlink, where the algorithm for computing beamforming weights is obtained by transforming the problem into a CDMA uplink with MMSE interference suppression and power control; (c) design of high bandwidth efficiency systems exploiting both transmit and receive antenna arrays, using analogies with CDMA systems with random and slowly time-varying spreading waveforms.

In this treatise, the performance of Code division multiple access (CDMA) system is investigated using multi-user transmitter pre-processing (MUTP). The MUTP is promising signal processing technique which is carried out at the transmitter side to reduce the burden of detector and hence low complexity receiver is made feasible at mobile station (MS) in the context of downlink (DL) communication. Further, the unique signature sequence is considered to support more user population. In particular, the performance of MMSE-MUTP assisted CDMA system is investigated for realistic channel specifications based on Stanford University Interim (SUI) channel model. It has been demonstrated that the MMSE-MUTP assisted CDMA system provides achievable bit error rate with low complexity of receiver for DL transmission.

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.

The technique of joint blind channel estimation and multiple access interference (MAI) suppression for an asynchronous code-division multiple-access (CDMA) system is investigated in this research. To identify and track dispersive time-varying fading channels and to avoid the phase ambiguity that come with the second-order statistic approaches, a sliding-window scheme using the expectation maximization (EM) algorithm is proposed. The complexity of joint channel equalization and symbol detection for all users increases exponentially with system loading and the channel memory. The situation is exacerbated if strong inter-symbol interference (ISI) exists. To reduce the complexity and the number of samples required for channel estimation, a blind multiuser detector is developed. Together with multi-stage interference cancellation using soft outputs provided by this detector, our algorithm can track fading channels with no phase ambiguity even when channel gains attenuate close to zero.

In this paper, we explore some of the fundamentals of synchronous Code Division Multiple Access (CDMA) as applied to wireless and optical communication systems under very general settings (of any size) for the user symbols and the signature matrix entries. The channel is modeled by real/complex additive noise of arbitrary distribution. Two problems are addressed. The first problem concerns whether uniquely detectable overloaded matrices exist in the absence of additive noise under these general settings, and if so, whether there are any practical optimum detection algorithms. The second one is about the bounds for the sum channel capacity when user data and signature matrices employ any real or complex alphabets (finite or infinite). In response to the first problem, we have developed practical maximum likelihood detection algorithms for overloaded CDMA systems for a large class of alphabets. In response to the second problem, a general theorem has been developed in which the sum capacity lower bounds with respect to the number of users, spreading gain, and signal-to-noise ratio can be derived. To show the power and utility of the main theorem, a number of sum capacity bounds for special cases are evaluated. An important conclusion of this paper is that the lower and upper bounds of the sum capacity for small/medium-size CDMA systems depend on both the input and the signature symbols; this is contrary to the asymptotic results for large-scale systems reported in the literature (also confirmed in this paper) where the signature symbols and statistics disappear for signature matrices and input vectors with i.i.d. entries. Furthermore, upper and asymptotic bounds are derived and compared to other derivations.

The authors propose a novel use of zero correlation zone (ZCZ) codes to increase the performance of quasi- synchronous code division multiple access (CDMA) systems. In this new approach, each spread sequence associated to a active user is a concatenation of two or more ZCZ sequences. In the receiver, each of the component sequences is separately correlated in order to recover the original information. A comparison to the convencional receiver is fulfilled by analyzing the performance of both schemes in a multipath Rayleigh fading channel and similar system parameters.

A dispersive finite impulse response (FIR) channel model is often considered for indoor wireless channels. For the FIR channel model in this paper, we consider the joint estimation of channel parameters and data symbols in indoor code division multiple access (CDMA) systems. The least squares formulation is constructed for the joint estimation incorporating the decision feedback technique. This formulation is suitable for indoor CDMA systems that usually have a small processing gain. The formulation is further modified to provide a recursive approach to estimate the channel vector.

The W-CDMA system plays an important role in the 3G cellular systems because of its compatible networking architecture to the present popular Global System for Mobile Communications (GSM) systems as well as the salient features of a Code Division Multiple Access (CDMA) system, including multi-path fading tolerance, high system capacity, low power consumption, good performance and coverage. In CDMA system, a procedure used by a Mobile Station (MS) to search for the best cell site and achieve code, time, and frequency synchronization with it is referred as cell search. Fast cell search is essential to reduce switch on delay (initial cell search), increase stand by time (idle mode search) and maintain good link quality (active-mode search). This is particularly true for the Wideband Code Division Multiple Access-Frequency Division Duplex (W-CDMA-FDD) system since it employs nonsynchronous base stations instead of synchronous ones of other CDMA systems to extend coverage from outdoor to indoor. Cell search is critical for achieving code and time synchronization. The main process of achieving cell search is divided into three stages followed by code verification, tracking and carrier frequency adjustment: 1) slot synchronization, 2) frame synchronization and code group identification, 3) primary scrambling code identification. This paper addresses slot synchronization in downlink as cell search algorithm in W-CDMA. In the three step search, the receiver searches for the slot timing by correlating the received signal with the Primary Synchronization (PSCH) code using Matched Filter (MF). This paper propose a design for a hierarchical matched filter for Primary Synchronization Code (PSC). The aim is to reduce the hardware complexity using Field Programmable Gate Array (FPGA) logic resources required to realize the filter.