An adaptive asynchronous CDMA multiuser detector forfrequency-selective Rayleigh fading channels

An adaptive asynchronous code-division multiple-access (CDMA) multiuser detector is proposed that uses a recently derived extended Kalman filter based algorithm (see Lim, T.J. and Rasmussen, L.K., IEEE Trans. Commun., vol.45, p.213-20, 1997) to perform joint data detection and parameter tracking in frequency-selective Rayleigh fading channels. A receiver structure based on this adaptive multiuser detector is presented and its performance in terms of parameter tracking and bit error rate (BER) is investigated. The receiver is a form of an adaptive RAKE that exploits multipaths to achieve performance gain     

异步CDMA系统中的CM多用户检测器

本文将恒模（CM）算法应用于异步CDMA系统的多用户信号检测。该CM检测器只要求了解目标用户的信息，运算较简单，且由于采用了自适应方法，检测器无需估计信号的幅度。分析及仿真结果均表明，CM检测器的误码性能和抗-近效应能力均优于传统检测器，且对于用户数目的变化有很好的适应性。     

Adaptive multiuser CDMA detector for asynchronous AWGNchannels-steady state and transient analysis

A two-stage adaptive multiuser detector in an additive white Gaussian noise code-division multiple-access channel is proposed and analyzed. Its first stage is an asynchronous one-shot decorrelator which in terms of computational complexity only requires inversion of K symmetric K×K matrices for all K users. In addition, the K inversions can be done in parallel, and the computed results for one user can be reused by all other users as well, resulting in a latency of only one bit, same as its synchronous counterpart. The decorrelated tentative decisions are utilized to estimate and subtract multiple-access interference in the second stage. Another novel feature of the detector is the adaptive manner in which the multiple-access interference estimates are formed, which renders prior estimation of the received signal amplitudes and the use of training sequences unnecessary. Adaptation algorithms considered include steepest descent (as well as its stochastic version), and a recursive least squares-type algorithm that offers a faster transient response and better error performance. Sufficient conditions for the receiver to achieve convergence are derived. The detector is near-far resistant, and is shown to provide substantial steady-state error performance improvement over the conventional and decorrelating detector, particularly in the presence of strong interfering signals     

Iterative multiuser detection with Gauss-Seidel soft detector asfirst stage for asynchronous coded CDMA

A new iterative detection and decoding structure for asynchronous convolutionally coded and turbo coded CDMA is proposed. The new scheme is based on the combination of Gauss-Seidel soft detection and parallel interference cancellation. The complexity of the new scheme is linear to the number of users. It is shown that for a heavily loaded system, near-optimal performance can be obtained     

Coupling cancellation based multiuser detector for asynchronousDS/CDMA

A three-shot multiuser detector is proposed for the reverse link of asynchronous DS/CDMA systems. By taking advantage of the preceding (already reconstructed) bit and the matched filter output for the following two bits, the coupling between temporally adjacent bits (TABs, which always exists for asynchronous systems) is cancelled in a three step fashion. The obtained detector has several useful advantages over existing schemes     

An extrinsic Kalman filter for iterative multiuser decoding

One powerful approach for multiuser decoding is to iterate between a linear multiuser filter (which ignores coding constraints) and individual decoders (which ignore multiple-access interference). Subject to clearly formulated statistical assumptions and the history of input signals provided by the outer decoders over all previous iterations, an extrinsic Kalman filter is suggested. This approach is motivated by the recent observation that decoder outputs are loosely correlated during initial iterations. Numerical results show that iterative decoding using this filter provides better performance in terms of the supportable load and convergence speed as compared to previously suggested linear-filter-based iterative decoders.     

Genetic-algorithm-assisted multiuser detection in asynchronous CDMA communications

In an asynchronous direct-sequence code-division multiple-access, a specific bit of the reference user is interfered by two asynchronously arriving surrounding bits of all the other users supported by the system. Hence, for optimum multiuser detection, the entire input bit sequence influencing the current bit decisions must be considered, which results in a high detection delay as well as a high receiver complexity. Suboptimal multiuser-detection methods have been proposed based on a truncated observation window, in which the so-called "edge" bits are tentatively estimated by some other means. Using a similar approach, a multiuser detector is developed in this contribution that invokes genetic algorithms (GAs) in order to estimate both the desired bits as well as the edge bits within the truncated observation window. Using computer simulations, we showed that by employing GAs for improving the estimation reliability of the edge bits, our proposed multiuser detector is capable of achieving a near-optimum bit-error-rate performance, while imposing a lower complexity than the optimum multiuser detector.     

Blind multiuser detector based on orthogonal projection [for CDMA]

A new stable blind multiuser detector based on signal subspace orthogonal projection is presented. By carrying out a pseudoeigendecomposition (PED) analysis, a technique proposed for the first time, the signal of the interested user can be approximated by the orthogonal projection of a received multiuser signal onto the related subspace of the interested user. Simulations show that compared with the previous subspace-based MMSE blind multiuser detector, the new detector offers better performance without leading to an increase in the computational complexity     

The Kalman filter as the optimal linear minimum mean-squared errormultiuser CDMA detector

It is shown that a first-order linear state-space model applies to the asynchronous code-division multiple-access (CDMA) channel, and thus the Kalman filter produces symbol estimates with the minimum mean-squared error (MMSE) among all linear filters, in long- or short-code systems for a given detection delay. This result may be used as a benchmark against which to compare the performance of other linear detectors in asynchronous channels. It also reveals that a time-varying recursive filter with a fixed and finite complexity implements the fixed-lag linear MMSE (LMMSE) detector, which hitherto has been assumed to require a processing window (and hence complexity) that grows with time     

A low-complexity SISO multiuser detector for iterative decoding of asynchronous CDMA systems with convolutional codes

The optimal decoding scheme for asynchronous code-division multiple-access (CDMA) systems that employ convolutional codes results in a prohibitive computational complexity. To reduce the computational complexity, an iterative receiver structure was proposed for decoding multiuser data in a convolutional coded CDMA system. At each iteration, extrinsic information is exchanged between a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders. A direct implementation of the optimal SISO multiuser detector, however, has exponential computational complexity in terms of the number of users which is still prohibitive for channels with a medium to large number of users. This paper presents a low-complexity SISO multiuser detector using the decision-feedback scheme, of which tentative hard decisions are made and fed back to the SISO multiuser from the previous decoding output. In the proposed scheme, the log-likelihood ratios (LLR) as well as the tentative hard decisions of code bits are fed back from the SISO decoders. The hard decisions are used to constrain the trellis of the SISO multiuser detector and the LLRs are used to provide a priori information on the code bits. The detector provides good performance/complexity tradeoffs. The computational complexity of the detector can be set to be as low as linear in the number of users. Simulations show that the performance of the low-complexity SISO multiuser detector approaches that of the single-user system for moderate to high signal-to-noise ratios even for a large number of users.     

Iterative reduced-state multiuser detection for asynchronous coded CDMA

The conventional maximum a posteriori receiver for coded code-division multiple-access (CDMA) systems has exponential computational complexity in terms of the number of users and the memory of the channel code. In this letter, we propose a low-complexity soft-input soft-output (SISO) multiuser detector based on the reduced-state a posteriori probability algorithm. Per-survivor processing and soft interference cancellation are used to remove the residual past and future interference in the branch metric computation. The complexity of the proposed receiver is related to the reduced memory of the CDMA channel and can be adjusted according to the complexity/performance tradeoff. Simulation results show that for asynchronous convolutionally coded systems, the proposed receiver can achieve the near-single-user performance for moderate to high signal-to-noise ratios.     

Blind multiuser channel estimation in asynchronous CDMA systems

In asynchronous code division multiple access (A-CDMA) systems transmitting over multipath channels, both intersymbol interference (ISI) as a result of interchip interference (ICI) and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users' signature waveform. Recently, Liu and Xu (see Proc. 29th Asilomar Conf. Signals, Systems and Computers, 1996) introduced a blind estimation algorithm for synchronous CDMA (S-CDMA) systems to estimate the multiuser channels. However, this algorithm cannot be directly applied to an asynchronous CDMA (A-CDMA) system. In this paper, we study a similar blind estimation scheme that provides estimates of the multiuser channels by exploiting the structure information of the data output and the users' delays. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of channels and, hence, the signature waveforms. By utilizing antenna arrays, we extended our approach to overloaded systems, where the number of users may exceed the spreading factor     

Iterative multiuser detection for asynchronous CDMA withconcatenated convolutional coding

We propose iterative multiuser detectors for asynchronous code-division multiple-access with parallel-concatenated convolutional codes (turbo codes) and with serially concatenated convolutional codes (SCCC). At each iteration we update and exchange the extrinsic information from the multiuser detector and channel decoders and regenerate soft information between constituent convolutional decoders. Simulation results show that with the proposed structure, near-single-user performance can be achieved. We also propose two reduced-complexity techniques, i.e., the reduced-state iterative multiuser detector based on the T-MAP algorithm and the iterative interference canceler based on a noise-whitening filter. Simulation results show a small performance degradation for these two techniques, particularly for the T-MAP receiver     

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.     

The Cholesky-iterative detector: a new multiuser detector forsynchronous CDMA systems

The decorrelating decision-feedback (DDF) multiuser detector based on Cholesky factorization has been shown to alleviate the performance degradation of the users in the detection process, especially for relatively low cross-correlation values between user signals. A new detection concept for multiple users described in this paper employs two triangular matrices (upper and lower) and soft output information to improve the data estimates. Simulation results show that significant performance gains can be achieved over the DDF. Also, the users tend to have their bit error probabilities clustered. Thus, the performance of a given user is less dependent on its position in the detection process than for the DDF detector     

Blind adaptive multiuser detection for asynchronous dual-rateDS/CDMA systems

In this paper, the authors consider an asynchronous direct-sequence code division multiple access (DS/CDMA) system wherein users are allowed to transmit their symbols at one out of two available data rates. Three possible access schemes are considered, namely, the variable spreading length (VSL), the variable chip rate (VCR), and the variable chip rate with frequency shift (VCRFS) formats. Their performance is compared for the case that a linear one-shot multiuser receiver is employed. It is also shown that detection of the users transmitting at the higher rate requires a periodically time-varying processing of the observables. Moreover, the problem of blind adaptive receiver implementation is studied, and a cyclic blind recursive-least-squares (RLS) algorithm is provided which is capable of converging to the periodically time-varying high-rate users detection structure. Numerical results show that the proposed receivers are near-far resistant, and that the VCRFS access technique achieves the best performance. Finally as to the adaptive blind receiver implementation, computer simulations have revealed that the cyclic RLS algorithm for blind adaptive high-rate users demodulation outperforms the conventional RLS algorithm in most cases of primary importance     

Blind asynchronous multiuser CDMA receivers for ISI channels usingcode-aided CMA

A code-aided constant modulus algorithm (CMA) based approach is presented for blind detection of asynchronous short-code DS-CDMA (direct sequence code division multiple access) signals in intersymbol interference (ISI)/multipath channels. Only the spreading code of the desired user is assumed to be known; its transmission delay may be unknown. A linear equalizer is designed by minimizing the Godard/CMA cost function of the equalizer output with respect to the equalizer coefficients subject to the fact that the equalizer lies in a subspace associated with the desired user's code sequence. Constrained CMA leads to the extraction of the desired user's signal whereas unconstrained minimization leads to the extraction of any one of the active users. The results are further improved by using unconstrained CMA initialized by the results of the code-aided CMA. Identifiability properties of the approach are analyzed. Illustrative simulation examples are provided     

Memory search multiuser detector in DS/CDMA systems

The optimal multiuser detector (MUD) in DS/CDMA systems requires the solution of a NP-hard combinatorial optimisation problem (COP). A near optimal MUD based on solving the COP with memory search method is proposed. With almost the same computational complexity as the parallel interference cancellation (PIC) MUD, it produces a better performance gain than PIC-MUD     

Suboptimal multiuser detector for frequency-selective Rayleighfading synchronous CDMA channels

Proposes a suboptimal low-complexity multiuser receiver for synchronous CDMA frequency-selective Rayleigh fading channels. In contrast to the conventional RAKE receiver, which suffers from near-far effects due to channel fading, the proposed multiuser receiver is shown to alleviate the near-far problem while preserving multipath diversity gain. This is demonstrated by comparing the symbol error probability and asymptotic multiuser efficiency of the proposed multiuser detector and RAKE receiver     

A low complexity multiuser detector for synchronous CDMA system

In a code-division multiple-access (CDMA) system, multiuser detection (MUD) can exploit the information of signals from other interfering users to increase the system capacity. However, the optimum MUD for CDMA systems requires the solution of an NP-hard combinatorial optimization problem. It is well known that the computational complexity of the optimum multiuser detector is exponential with the number of active users in the system. In this paper, we apply a hybrid algorithm to develop a suboptimal MUD strategy. The result of symmetric successive overrelaxation (SSOR) preconditioned conjugate-gradient method is first used to initialize the reduced-complexity recursive (RCR) multiuser detector. Then, RCR algorithm is applied to detect the received data bit by optimizing an objective function in relation with the linear system of decorrelating detector. Simulation results for the synchronous case show that the performance of our proposed SSOR-RCR multiuser detector is promising and outperform the decorrelator and linear minimum mean squared error multiuser detector with lower computational complexity.