Distributed and centralized multiuser detection with antenna arrays

In this letter, we have analyzed and compared two different multiuser detection receiver structures that are appropriate for code-division multiple-access systems with multiple antennas. We compare the distributed decorrelator (where signal decorrelation is performed at each receiving antenna) and the centralized decorrelator (where decorrelation is performed once collectively). We show that they provide the same performance in additive Gaussian noise channels. The distributed decorrelator provides better performance in flat fading channels.     

Joint decorrelating multiuser detection and channel estimation inasynchronous CDMA mobile communications channels

The asymptotic efficiencies of two decorrelators, path-by-path and channel-matched decorrelators, are analyzed in fading multipath propagation environments, and based upon the analytical results, a new joint multiuser detection and channel estimation scheme is proposed for asynchronous code division multiple access (CDMA) mobile communications channels. In the path-by-path decorrelator, each of the received signals corresponding to one of the multiple propagation paths is regarded as an independent interference source. On the contrary, in the channel-matched decorrelator, each composite signal transmitted from an identical user is regarded as a response of the multipath channel to the corresponding user's spreading sequence. The asymptotic efficiency of the path-by-path decorrelator is shown to drop rapidly as the number of simultaneous users increases. It is shown that the asymptotic efficiency can be made independent of the number of the propagation paths by the channel-matched decorrelator at the expense of requiring knowledge about the fading complex envelopes of all the propagation paths. The proposed joint multiuser detection and channel estimation scheme uses both path-by-path and channel-matched decorrelators. The path-by-path decorrelator is used for providing the channel estimator with the (noisy) channel information path-by-path, and decisions are made on the output of the channel-matched decorrelator. The decision results are fed back to the channel estimator, and used as the reference signals. The received complex envelope of each of the propagation paths is estimated in the channel estimator. Results of a series of exhaustive computer simulations are presented in order to demonstrate the overall performance of the proposed scheme, both in non-fading and fading multipath propagation environments     

A simulation comparison of multiuser receivers for cellular CDMA

Multiuser detection has gained significant notoriety as a potential advanced enabling technology for the next generation of CDMA systems. Due to the limitations of the conventional correlation receiver, the capacity of a single cell using CDMA is limited by self-interference and is subject to the near-far problem. To overcome these drawbacks, several advanced receiver structures have been proposed. Unlike the conventional receiver which treats multiple access interference (MAI) as if it were AWGN, multiuser receivers treat MAI as additional information to aid in detection. Although each of the multiuser types have been the subject of much literature, there is little published work comparing all structures on the basis of common assumptions. We present a comparison of five of the most discussed receiver structures: the decorrelator, the minimum mean square error (MMSE) receiver, the multistage parallel interference cancellation receiver, the successive interference cancellation receiver, and the decorrelating decision feedback receiver. Comparisons are based on both theoretical analysis and simulation results, examining bit error rate (BER) performance in AWGN, Rayleigh fading, and near/far channels. Additionally, receiver structures are compared on the basis of computational complexity as well as robustness to code phase misalignment. Finally, we present simulation results for noncoherent architectures of the aforementioned receivers     

Analysis of a partial decorrelator in a multicell DS-CDMA system

For a multicell code-division multiple-access (CDMA) system, we propose a partial decorrelator that decodes a user by suppressing only the in-cell interferers. As a result, each user suffers only from other-cell interference and enhanced receiver noise. By analysis, we show that in random CDMA systems, the partial decorrelator outperforms the conventional receiver, within the operating regime of the conventional receiver. In simulation, we observe that when users have equal received powers at their respective receivers, a multicell system with partial decorrelator receivers yields roughly 1.5 times the capacity of the conventional system.     

Vector Channel Estimation and Multiuser Detection for Multicarrier DS CDMA in Time and Frequency Selective Fading Channels

In a multicarrier direct-sequence code-division multiple access (MC DS CDMA) system, different fading channels for different users and/or different carriers are correlated in general; thus a vector channel model is more appropriate than disjoint scalar channel models. For multiuser MC DS CDMA systems, we propose (1) a generalized vector autoregressive model which accounts for correlation between different user/carrier fading channels, (2) the use of a two-phased algorithm to obtain the proposed model’s parameters, and (3) a receiver structure that consists of a generalized decorrelator followed by maximal-ratio combining (MRC) of uncorrelated carrier channel outputs of each user. The estimated fading coefficients provide the necessary quantities to MRC. The computer simulation results show that the proposed scheme has performance close to the case in which the channel is perfectly known, and outperforms separate scalar channel estimation case.     

Spectral efficiency of multicarrier CDMA

We analyze the spectral efficiency (sum-rate per subcarrier) of randomly spread synchronous multicarrier code-division multiple access (MC-CDMA) subject to frequency-selective fading in the asymptotic regime of number of users and bandwidth going to infinity with a constant ratio. Both uplink and downlink are considered, either conditioned on the subcarrier fading coefficients (for nonergodic channels) or unconditioned thereon (for ergodic channels). The following receivers are analyzed: a) jointly optimum receiver, b) linear minimum mean-square error (MMSE) receiver, c) decorrelator, and d) single-user matched filter.     

Analysis of an Approximate Decorrelating Detector

In this paper an approximate decorrelating detector is analyzed on the basis of a first order approximation to the inverse crosscorrelation matrix of signature waveforms. The approximation is fairly accurate for systems with low crosscorrelations and is exact in the two-user synchronous case. We present an exact as well as approximate analysis of the bit-error-rate performance of this detector on a channel that is subject to flat fading, and also specifically for the case of random signature waveforms. The detector outperforms the conventional matched filter receiver in terms of BER. The approximate decorrelator (while not being near-far resistant like the decorrelating detector) is fairly robust to imperfections in power control. Power trade-off regions are identified which characterize the significant advantage that the approximate decorrelator provides over the matched filter receiver. The reduced complexity of the approximate decorrelator and performance gains over the conventional matched filter makes it a viable alternative for implementation in practical CDMA systems, in particular in those where the signature waveforms span many symbol intervals.     

A Low-Complexity Multiuser Detector for Asynchronous CDMA QPSK Adaptive Antenna Array Systems

This paper proposes a low complexity joint space-time multiuser detection algorithm for asynchronous DS/CDMA antenna array systems. The proposed multiuser detector is composed of an adaptive antenna array, used as a linear beamformer, and a sliding window decorrelator. A QPSK modulation scheme is used in order to increase bandwidth efficiency. Numerical results are given in terms of Bit Error Rate (BER) under the assumption of a frequency-selective Rayleigh slow fading channel. In particular, the proposed receiver is shown to be near-far resistant, even in worst fading cases, and to exploit completely array introduction while maintaining acceptable computational complexity. The proposed architecture avoids linear filter realization of the decorrelator, which is impractical in the case of a large number of users, and operates with relatively short data frames instead of the complete information sequence. Finally, this receiver is very flexible to changes in timing configuration.     

The impact of frequency-flat fading on the spectral efficiency ofCDMA

The capacity of the randomly spread synchronous code-division multiple-access (CDMA) channel subject to frequency-flat fading is studied in the wide-band limit of large number of users. We find the spectral efficiency as a function of the number of users per chip, the distribution of the flat fading, and the signal-to-noise ratio (SNR), for the optimum receiver as well as linear receivers (single-user matched filter, decorrelator, and minimum mean-square error (MMSE)). The potential improvements due to both decentralized transmitter power control and multi-antenna receivers are also analyzed     

Linear multiuser receivers in random environments

We study the signal-to-interference (SIR) performance of linear multiuser receivers in random environments, where signals from the users arrive in “random directions.” Such a random environment may arise in a DS-CDMA system with random signature sequences, or in a system with antenna diversity where the randomness is due to channel fading. Assuming that such random directions can be tracked by the receiver, the resulting SIR performance is a function of the directions and therefore also random. We study the asymptotic distribution of this random performance in the regime where both the number of users K and the number of degrees of freedom N in the system are large, but keeping their ratio fixed. Our results show that for both the decorrelator and the minimum mean-square error (MMSE) receiver, the variance of the SIR distribution decreases like 1/N, and the SIR distribution is asymptotically Gaussian. We compute closed-form expressions for the asymptotic means and variances for both receivers. Simulation results are presented to verify the accuracy of the asymptotic results for finite-sized systems     

A NOVEL ONE—SHOT DECORRELATOR IN CDMA SYSTEMS

A novel one-shot decorrelator for asynchronous CDMA systems is developed.Com-pared with existing one-shot decorrelator, it can reduce complexity and has better performance while eliminating all MAI.This decorrelator is shown to be near-far resistant in both AWGN and fading channel.     

An asynchronous multirate decorrelator

This paper examines truncated window decorrelators for an asynchronous direct-sequence code-division multiple-access system supporting users transmitting at different bit rates. We decode a user by extending the observation window over a sufficient number of its bits. To characterize practical window sizes, simple upper and lower bounds for the asymptotic efficiency of both the truncated window and infinite window decorrelators are developed. Empirical results show that as the length of the observation window increases, the bounds converge rapidly to the asymptotic efficiency of the infinite window decorrelator. The complexity of the receiver depends strongly on the ratio of the maximum to minimum bit rates     

ITERATIVE "ONE-SHOT" DECORRELATION IN DS-CDMA SYSTEMS

In this paper, an iterative "one-shot" decorrelator for asynchronous code Division Multiple Access(CDMA) systems is proposed. This decorrelator feedbacks the decorrelated symbols of interfering users before decision to mitigate their interferences. As a result, this decorrelator is a linear detector and does not need any amplitude information of users as in decision-feedback detectors. Compared with existing "one-shot" decorrelator, it has reduced complexity and provided better performance while eliminating all Multiple Access Interference(MAI). The performance of this decorrelator is analyzed thoeritically, and analytical result is given. Numerical results show near-far resistant capability of this decorrelator in both AWGN and fading channel.     

Karhunen-Loeve expansion of the WSSUS channel output and itsapplication to efficient simulation

This paper derives a Karhunen-Loeve (K-L) expansion of the time-varying output of a multipath Rayleigh fading wide-sense-stationary uncorrelated-scattering (WSSUS) channel. It is shown that under the same mean-squared error condition, the number of terms required by the truncated K-L expansion is less than that of the series expansion obtained by using the discrete-path approximation of the channel so that simulation using the K-L expansion is more efficient. This computational advantage becomes more significant as higher simulation accuracy is required. The derived K-L expansion is applied to develop an efficient simulation technique for digital transmission over a multipath Rayleigh fading WSSUS channel using an optimum receiver. We show that the proposed technique requires shorter computation time than two other known simulation techniques     

DPSK Receiver with Implicit Diversity for the Linearly Frequency-Selective Rayleigh Fading Channel

A simple DPSK receiver for the linearly frequency-selective fading channel is presented. The channel is modelled as an f-power series truncated to the first two terms [1], [2]. Two specifically designed time invariant filters are used to remove the channel-induced ISI and separate the two implicit diversity branches. The received samples from each filter are then differentially decoded and linearly combined. Analytical and simulation results for binary DPSK demonstrate that this receiver shows significant improvement over the conventional receiver, which uses matched filtering and a product demodulator.     

ITERATIVE “ONE－SHOT” DECORRELATION IN DS－CDMA SYSTEMS

In this paper, an iterative “one-shot“ decorrelator for asynchronous code Division Multiple Access(CDMA) systems is proposed. This decorrelator feedbacks the decorrelated symbols of interfering users before decision to mitigate their interferences. As a result, this decorrelator is a linear detector and does not need any amplitude information of users as in decisionfeedback detectors. Compared with existing “one-shot“ decorrelator, it has reduced complexityand provided better performance while eliminating all Multiple Access Interference(MAI). The performance of this decorrelator is analyzed thoeritically, and analytical result is given. Numerical results show near-far resistant capability of this decorrelator in both AWGN and fading channel.     

Detection of CPM signals in fast Rayleigh flat-fading usingadaptive channel estimation

Differential detection techniques, which are commonly used in fast fading environments, are characterized by an irreducible error rate that increases with fading rate. The main source of this error floor is the phase error introduced by the multiplicative fading process. The paper describes a detection technique for continuous phase modulation (CPM) that employs decision feedback carrier recovery and adaptive channel estimation. This receiver was evaluated by software simulation and the results show a substantial reduction of the error floor relative to that of differential detection. Furthermore, in additive white Gaussian noise (AWGN) channels, the adaptive nature of the receiver allows it to perform close to ideal coherent detection of differentially encoded phase shift keying (DE-CPSK)     

Performance Comparison of Multiuser Detectors with Channel Estimation for Flat Rayleigh Fading CDMA Channels

Since the conventional detector performs poorly in bandwidth-efficient Code Division Multiple Access (CDMA) systems, several multiuser detectors were proposed recently. In this paper, we compare performance of the decorrelator, the two-stage detector (2S), and the decision-feedback detector (DF) for the Rayleigh flat fading synchronous CDMA channel. First, assuming perfect channel estimation, we show that the 2S and the DF have much lower bit error rate (BER) than the decorrelator, and prove that the ideal DF has unity Asymptotic Multiuser Efficiency (AME). Furthermore, we evaluate the performance of these detectors in the presence of channel mismatch. We model the Rayleigh flat fading channel as the second order Auto Regressive (AR) process, and use the Kalman filter as the channel estimator. The lower bounds on the BER of the 2S and the DF are derived. The analytical results and the simulations show that the estimation error accumulated due to cancellation of other users limits the performance of the 2S and the DF. Therefore, in the presence of channel mismatch, the decorrelator offers comparable or even better performance than more complex decision-feedback and two-stage detectors.     

SER performance of linear multiuser detectors for DS-CDMA downlink with transmitter nonlinear distortions

The aim of this paper is to examine the effects of the nonlinear distortions introduced by the high-power amplifier in the downlink of direct-sequence code-division multiple-access systems. By modeling the nonlinear distortion effects as a signal power loss and a superimposed colored Gaussian noise, we derive the symbol-error rate (SER) of the minimum mean-squared error (mmse) detector in additive white Gaussian noise channels. Successively, we make use of a semianalytical approach to obtain the SER performance of linear multiuser detectors, such as the RAKE, decorrelator, and mmse receiver, in frequency-selective fading channels. Simulation results confirm the effectiveness of the theoretical approach.     

Precoded random spreading multiple access system in AWGN channels

In this paper, we compare the asymptotic performance of the decorrelator approaches to multiuser detection in code-division multiple-access (CDMA). Precoding is the decorrelation which is performed at a common transmitter such as the base station or satellite that allows single-user detection at the handset receiver. Receiver-based decorrelator decorrelates multiple user signals at the receiver side. We analyze the performance of the aforementioned decorrelation approaches with time-varying random spreading sequences in additive white Gaussian noise channels. The results demonstrate that the performance of precoding and that of receiver-based decorrelator are asymptotically equivalent.