Weighted Extrinsic Feedback in the Iterative Multiuser Decoding of Coded CDMA

We track the density of extrinsic information in iterative turbo decoders by actual density evolution, and also approximate it by symmetric Gaussian density functions. The approximate model is verified by experimental measurements. We view the evolution of these density functions through an iterative decoder as a nonlinear dynamical system with feedback. Iterative decoding of turbo codes and of serially concatenated codes is analyzed by examining whether a signal-to-noise ratio (SNR) for the extrinsic information keeps growing with iterations. We define a for the iterative decoder, such that the turbo decoder will converge to the correct codeword if the noise figure is bounded by a number below zero dB. By decomposing the code's noise figure into individual curves of output SNR versus input SNR corresponding to the individual constituent codes, we gain many new insights into the performance of the iterative decoder for different constituents. Many mysteries of turbo codes are explained based on this analysis. For example, we show why certain codes converge better with iterative decoding than more powerful codes which are only suitable for maximum likelihood decoding. The roles of systematic bits and of recursive convolutional codes as constituents of turbo codes are crystallized. The analysis is generalized to serial concatenations of mixtures of complementary outer and inner constituent codes. Design examples are given to optimize mixture codes to achieve low iterative decoding thresholds on the signal-to-noise ratio of the channel.

A hybrid Interference cancellation (HIC) technique employing both Parallel and Successive Interference Cancellation (PIC/SIC) is proposed for Overloaded Direct Sequence Code Division Multiple Access (OCDMA) system under imperfect synchronization. Two types of receivers are proposed to efficiently mitigate the multiuser interference (MUI). In type I receiver, the MUI within group-1 users are first cancelled using Parallel Interference Cancellation (PIC). Then the multistage Successive Interference Cancellation (m-SIC) is performed on the resultant output of PIC to efficiently cancel the interference of group-2 users on group-1 users. The type II receiver is designed by performing PIC immediately followed by SIC and this combined process is repeated till a major chunk of multi user interference is cancelled. The BER performance of type II receiver is proved to be better than other cancellation techniques such as PIC, SIC and the proposed type I receiver.

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.

The paper introduces an analytical tool to find the large-system performance of a multistage linear partial parallel interference cancellation (PPIC) receiver using the moments of the eigenvalues of the covariance matrix for a code division multiple access (CDMA) system over a frequency-flat fading channel. The figure of merit to evaluate the performance is the signal-to-interference-plus-noise ratio (SINR) that is calculated under a large-system condition. In this case, the number of active users and the processing gain tend towards infinity while their ratio is a fixed value. It is shown that the large-system performance is a function of the system load, the partial cancellation factor (PCF), the number of interference cancellation stages, the signal-to-noise ratio (SNR), and the received powers of the interfering users. Furthermore, for practical applications, the physical meaning of the large system is described by numerical simulations.

Chaotic sequences have good correlation properties and they can be used as address sequences in Spread Spectrum Communication. Chaotic functions are highly sensitive to initial condition and exhibit non-linear behavior. In Chaotic spread spectrum communication systems, different user may be assigned different sequences generated with different initial conditions. Set of binary sequences with good pair wise cross- correlation property finds applications in Direct Sequence- Code Division Multiple Access (DS-CDMA) systems. Security in Direct Sequence-Code Division Multiple Access systems is relatively limited. To enhance security in Direct Sequence- Code Division Multiple Access systems, chaotic sequences are generated. In this paper we analyze the generation of chaotic binary sequences and study of auto correlation and cross correlation properties.

We propose a layered space time (LST) transceiver in a multiuser code division multiple access (CDMA) mobile communication system. In particular, we examine a multiuser iterative receiver with joint detection, channel estimation and decoding in a frequency selective fading channel. The iterative parallel interference canceller (PIC) with statistics combining is used as the multiuser detection scheme and a least mean square (LMS) adaptive algorithm is applied to estimate the multiple input and multiple output (MIMO) frequency selective fading channel. Simulation results are presented to demonstrate the performance of this receiver. It is shown that the performance of the receiver with iterative channel estimation is close to that of the receiver with perfect channel state information (CSI).

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.

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.

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 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.

Semi-blind multiuser detection for the reverse link of a multi-cell code-division multiple-access (CDMA) system is considered. In such a system, although active users are present in the home cell and neighbouring cells, the base station receiver only knows the signal information of the users in its own cell. Three new adaptive semi-blind linear multiuser detectors are proposed, based on a parallel interference cancellation approach. Compared with some known semi-blind detectors, the proposed detectors share a rather simple implementation structure and involve reduced latency for processing, which are of particular importance in a time-varying CDMA system. Numerical results are presented to show that the detection performance of the proposed detectors is superior to that of some existing semi-blind multiuser detectors.

Since code division multiple access systems in multipath environments suffer from multiple access interference (MAI), multiuser detection schemes should be used in the receivers. Parallel interference cancellation (PIC) is a promising method to combat MAI due to its relatively low computational complexity and good performance. It is shown that the complexity of PIC is still high for realistic scenarios in terms of the symbol rate, the number of users, spreading gain, and multipath components. However, two novel methods are introduced to reduce significantly the complexity without sacrificing performance. The first approach, called reduced PIC, takes advantage of the composition of the interference to concentrate interference cancellation only on significant terms. The second approach, called differential PIC, exploits the multistage character of PIC to avoid unnecessary double calculations of certain terms in consecutive stages. It is shown that a combination of both approaches leads to a performance very close to the single-user bound whereas the complexity can be kept on the order of the conventional RAKE receiver.

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.