Noncoherent MMSE interference suppression for DS-CDMA

A novel robust noncoherent receiver for minimum mean-squared error (MMSE) interference suppression for direct-sequence code-division multiple access (DS-CDMA) is proposed. The receiver consists of a linear MMSE filter and a decision-feedback differential detector (DF-DD). The performance of the proposed scheme is investigated analytically and by computer simulations. It is shown that the loss compared to coherent MMSE interference suppression is limited and can be made arbitrarily small by increasing the observation window used for calculation of the reference symbol of the DF-DD. Hence, the regarded noncoherent receiver is near-far resistant. For adjustment of the MMSE filter coefficients three noncoherent adaptive algorithms are proposed. In contrast to coherent adaptive algorithms, these noncoherent algorithms have the important advantage that they also converge if the channel phase is time-variant     

Linear space-time multiuser detection for multipath CDMA channels

We consider the problem of detecting synchronous code division multiple access (CDMA) signals in multipath channels that result in multiple access interference (MAI). It is well known that such challenging conditions may create severe near-far situations in which the standard techniques of combined power control and temporal single-user RAKE receivers provide poor performance. To address the shortcomings of the RAKE receiver, multiple antenna receivers combining space-time processing with multiuser detection have been proposed in the literature. Specifically, a space-time detector based on minimizing the mean-squared output between the data stream and the linear combiner output has shown great potential in achieving good near-far performance with much less complexity than the optimum space-time multiuser detector. Moreover, this space-time minimum mean-squared error (ST-MMSE) multiuser detector has the additional advantage of being well suited for adaptive implementation. We propose novel trained and blind adaptive algorithms based on stochastic gradient techniques, which are shown to approximate the ST-MMSE solution without requiring knowledge of the channel. We show that these linear space-time detectors can potentially provide significant capacity enhancements (up to one order of magnitude) over the conventional temporal single-user RAKE receiver     

Blind adaptive multiuser detection

The decorrelating detector and the linear minimum mean-square error (MMSE) detector are known to be effective strategies to counter the presence of multiuser interference in code-division multiple-access channels; in particular, those multiuser detectors provide optimum near-far resistance. When training data sequences are available, the MMSE multiuser detector can be implemented adaptively without knowledge of signature waveforms or received amplitudes. This paper introduces an adaptive multiuser detector which converges (for any initialization) to the MMSE detector without requiring training sequences. This blind multiuser detector requires no more knowledge than does the conventional single-user receiver: the desired user's signature waveform and its timing. The proposed blind multiuser detector is made robust with respect to imprecise knowledge of the received signature waveform of the user of interest     

Properties of the single-bit single-user MMSE receiver for DS-CDMAsystems

The single-bit single-user minimum mean-squared error (MMSE) receiver for direct-sequence code-division multiple access systems is studied in great detail. It is shown that the MMSE receiver is near-far resistant and asymptotically optimum as the interference or noise disappear (i.e., in interference or noise dominated scenarios). It is experimentally shown that the popular Gaussian approximation of the error probability is reasonable for the MMSE receiver over a broad range of operating conditions-i.e., signal-to-noise ratios and interference situations-even though this may not be the case for the standard matched-filter receiver. The concept of dimension reduction is introduced to reduce the complexity of the adaptive part of the receiver and the optimum dimension reduction transform is derived. Unfortunately, the optimum transform is dependent on unknown side-information and is, therefore, of limited practical use. However, several practical schemes are proposed (one which is asymptotically optimum). The schemes are shown to outperform previously known methods     

Training analysis of adaptive interference suppression fordirect-sequence code-division multiple-access systems

This paper studies the transient behavior of an adaptive near-far resistant receiver for direct-sequence (DS) code-division multiple-access (CDMA) known as the minimum mean-squared error (MMSE) receiver. This receiver structure is known to be near-far resistant and yet does not require the large amounts of side information that are typically required for other near-far resistant receivers. In fact, this receiver only requires code timing on the one desired signal. The MMSE receiver uses an adaptive filter which is operated in a manner similar to adaptive equalizers. Initially there is a training period where the filter locks onto the signal that is sending a known training sequence. After training, the system can then switch to a decision-directed mode and send actual data. This work examines the length of the training period needed as a function of the number of interfering users and the severity of the near-far problem. A standard least mean-square (LMS) algorithm is used to adapt the filter and so the trade-off between convergence and excess mean-squared error is studied. It is found that in almost all cases a step size near 1.0/(total input power) gives the best speed of convergence with a reasonable excess mean-squared error. Also, it is shown that the MMSE receiver can tolerate a 30-40 dB near-far problem without excessively long convergence time     

Matched filters with interference suppression capabilities forDS-CDMA

This paper considers the design of near-far resistant matched filters (MFs) for direct-sequence code-division multiaccess (DS-CDMA) systems. We show how additional information on the individual powers and chip delays of locked users, that is available at the base station, can be exploited for the design of these MFs. The proposed receiver can specifically reduce the effects of the locked interferers at the expense of a partial increase in complexity over the conventional MF. Furthermore, this detection technique is a link between the linear minimum mean squared error (MMSE) centralized multiuser detector and the decentralized noise-whitening MF. Numerical results for the rectangular chip pulse illustrate the signal-to-noise ratio (SNR), near-far resistance, and bit-error rate (BER) improvement that the proposed MF yields over the conventional MF and noise-whitening MF     

MMSE interference suppression for direct-sequence spread-spectrumCDMA

We consider interference suppression for direct-sequence spread-spectrum code-division multiple-access (CDMA) systems using the minimum mean squared error (MMSE) performance criterion. The conventional matched filter receiver suffers from the near-far problem, and requires strict power control (typically involving feedback from receiver to transmitter) for acceptable performance. Multiuser detection schemes previously proposed mitigate the near-far problem, but are complex and require explicit knowledge or estimates of the interference parameters. In this paper, we present and analyze several new MMSE interference suppression schemes, which have the advantage of being near-far resistant (to varying degrees, depending on their complexity), and can be implemented adaptively when interference parameters are unknown and/or time-varying, Numerical results are provided that show that these schemes offer significant performance gains relative to the matched filter receiver. We conclude that MMSE detectors can alleviate the need for stringent power control. In CDMA systems, and may be a practical alternative to the matched filter receiver     

Linear Differentially Coherent Multiuser Detection for Multipath Channels

By combining multipath processing, differential signal detection, and multiuser detection techniques, we develop a class of near-far resistant linear detectors for differentially coherent multipath signals. We derive and establish performance relationships among the following detectors: an optimally near-far resistant detector, a suboptimum detector which does not require knowledge of the signal coordinates, and a minimum mean square error (MMSE) detector which achieves near-optimum asymptotic efficiency. We present an adaptive multiuser detector which converges to the MMSE detector without training sequences and which requires less information than the conventional single user rake receiver.     

Iterative Group-Blind Multiuser Detection and Decoding With Signal Rank Estimation for Coded CDMA Systems

In this paper, a turbo receiver structure is proposed for the uplink of coded code-division multiple-access (CDMA) systems in the presence of unknown users. The proposed receiver consists of two stages following each other. The first stage performs soft interference cancellation and group-blind linear minimum mean square error (MMSE) filtering, and the second stage performs channel decoding. The proposed group-blind linear MMSE filter suppresses the residual multiple-access interference (MAI) from known users based on the spreading sequences and the channel characteristics of these users while suppressing the interference from other unknown users using a subspace-based blind method. The proposed receiver is suitable for suppressing intercell interference in heavily loaded CDMA systems. Since the knowledge of the number of unknown users is crucial for the proposed receiver structure, a novel estimator is also proposed to estimate the number of unknown users in the system by exploiting the statistical properties of the received signal. Simulation results demonstrate that the proposed estimator can provide the number of unknown users with high accuracy; in addition, the proposed group-blind receiver integrated with the new estimator can significantly outperform the conventional turbo multiuser detector in the presence of unknown users.      

Near-far resistance of multiuser detectors in asynchronous channels

Consideration is given to an asynchronous code-division multiple-access environment in which receiver has knowledge of the signature waveforms of all the users. Under the assumption of white Gaussian background noise, the authors compare detectors by their worst case bit error rate in a near-far environment with low background noise, where the received energies of the users are unknown to the receiver and are not necessarily similar. Conventional single-user detection in a multiuser channel is not near-far resistant, and the substantially higher performance of the optimum multiuser detector requires exponential complexity in the number of users. The authors explore suboptimal demodulation schemes which exhibit a low order of complexity while not exhibiting the impairment of the conventional single-user detector. It is shown that there exists a linear detector whose bit-error-rate is independent of the energy of the interfering users. It is also shown that the near-far resistance of optimum multiuser detection can be achieved by a linear detector. The optimum linear detector for worst-case energies is found, along with existence conditions, which are always satisfied in the models of practical interest     

Demodulation and code acquisition using decorrelator detectors forQS-CDMA

A linear detector for a quasisynchronous code-division multiple-access (QS-CDMA) cellular system is presented, which is designed according to a minimum mean-square error (MMSE) criterion. By using a time-averaged version of the interfering signal covariance matrix, it is shown that multiuser interference can be rejected without the need to estimate signal time-of-arrival. Furthermore, unlike previous MMSE receiver designs, estimation of the received signal covariance matrix is not required. An asymptotic form of the MMSE detector, corresponding to a decorrelator implemented using a projection operator, is also obtained. Bit-error rate (BER) results are presented which demonstrate the superiority of the MMSE receiver over the conventional matched-filter detector under strong near-far conditions. An analysis of the expected acquisition time T¯_ACQ is given for the decorrelator detector using a serial search scheme. The results obtained show that T¯_ACQ is far less for a code acquisition method using the decorrelator as opposed to a conventional matched filter     

A low complexity reduced-rank MMSE receiver for DS/CDMA communications

The minimum mean squared error (MMSE) receiver is a linear filter which can achieve optimal near-far resistance in direct-sequence code-division multiple-access communications. However, one of the main problems of this receiver is the required number of filter taps, which is typically large. This is especially true in systems with a large processing gain in which case the receiver's computation burden becomes very high. As a result, methods for reducing the complexity of the MMSE receiver have been of great interest in recent years. We propose an efficient partitioned MMSE receiver based on a classification algorithm. It is shown that the computational complexity (in terms of the filter taps) of the proposed receiver can be reduced significantly while good performance is maintained. Based on the special structure of our proposed receiver, we also propose a release-merge adaptive partition algorithm which can update the partition and the receiver's coefficients simultaneously. In particular, it is demonstrated that the proposed receiver can perform much better than previously proposed reduced-rank MMSE receivers, such as the partial despreading MMSE receiver and the cyclically shifted filter bank receiver, with even a smaller number of taps.     

A Study of the Technique of Linear Multiuser

The technique of linear multiuser detection in DS-CDMA systems is studied in this paper. The purpose is to find a receiver structure with good performance and moderate complexity, so that the receiver can efficiently suppress multiple-access interference(MAI) and multipath interference and has good near-far resistant ability, which may improve the system's capability while reducing the requirement for power control. The main work of the dissertation can be summarized as follows: the performance of MMSE multiuser detector in synchronous/asynchronous DS-CDMA systems over different channels is analyzed in chapter 2 of the dissertation. Using matrix method, we analyze the relation between performance measurement and spreading code correlation matrix, Signal-Interference-Ratio(SIR) and near-far factor, and prove that the performance of MMSE detector is better than that of the decorrelating detector. For fading channel, we analyze the performance of MMSE detector in DS-SS system firstly. Results show that the detector can efficiently suppress multipath interference. Extending to synchronous/asynchronous DS-CDMA systems over fading channels, we propose a simple linear detector structure that accomplishes despreading, detection and combining. Thus, the receiver is easy for implementation. Based on the proposed notion of combined spreading codes, we prove that the synchronous/asynchronous CDMA system is equivalent to the synchronous CDMA system over AWGN channel with double users. Therefore, the MMSE detector can efficiently suppress MAI and multipath interference in steady state, and has good near-far resistant ability. In chapter 3, we study the adaptive algorithm based on MMSE criterion. Firstly, the approach to the blind algorithm based on subspace is analyzed. We improve the algorithm in the part of channel estimation, which decreases the computational complexity while guaranteeing the performance. Meanwhile, we point out that CMOE-RLS algorithm is essentially an algorithm based on subspace approach. Also, it is shown from simulation that PASTd subspace tracking algorithm is not applicable for multiuser detection. Secondly, we propose an adaptive algorithm based on pilot channel, called PCA/PCRA. The algorithm does not require channel estimation, and has a rapid convergence rate. The steady state performance can be achieved by increasing the transmitting power in pilot channel. Computational complexity is only O(N2). Therefore, PCA/PCRA is suitable for the engineering application. The cost is that a pilot channel is needed for each user in the system. In chapter 4, constant algorithms for multiuser detection are studied. Firstly, we analyze the capture performance of CMA, and point out there exist many local stationary points. Initializations to guarantee CMA converges to the desired point are discussed. Results show that the convergence of CMA is decided by constant, step-size, spreading code correlation matrix and near-far factor. Secondly, we propose the constrained constant algorithm (C-CMA) for multiuser detection. It is shown that when the constant is greater than the triple power of the desired user, C-CMA globally converges to the desired point. Simulations illustrate that C-CMA has a rapid convergence rate and the steady state performance is good. However, great step-size can also reult in dispersion for the algorithm. Since C-CMA is a variable step-size CMOE-LMS algorithm, we propose a variable step-size constraint algorithm (VSCA). VSCA has the advantages of both CMOE-LMS and C-CMA such as robust, rapid convergence rate and good steady state performance. Thus, VSCA is suitable for engineering application. But the improper selection of step-size coefficients may degrade performance seriously. The computational complexity of the above constant algorithms is only O(N). In Section 5, the cyclostationarity of spreading signals is analyzed in the first part. We prove that spreading signals are ergodic cyclostationary signals with a cyclic period that is equal to the period of spreading code. Methods for processing cyclostationary signals are then given. It is shown that this method can mitigate the interference from a stationary noise for channel estimation. But the computational complexity for cyclostationary correlation is high, which prevents its application in implementation. In the second part, we discuss the application of oversampling technique in spreading communication systems. Although the oversampling can improve the performance of the linear multiuser detector, the improvement is trivial. On the contrary, oversampling increases the computational complexity of the weight vector greatly, which prevents its application in implementation. Additionally, we prove that FSE plus despreading or despreading pus FSE is equivalent to the linear detector with different lengths of delay line. However, the two kinds of structure have lower computational complexity.     

Multistage detection in asynchronous code-division multiple-accesscommunications

A multiuser detection strategy for coherent demodulation in an asynchronous code-division multiple-access system is proposed and analyzed. The resulting detectors process the sufficient statistics by means of a multistage algorithm based on a scheme for annihilating successive multiple-access interference. An efficient real-time implementation of the multistage algorithm with a fixed decoding delay is obtained and shown to require a computational complexity per symbol which is linear in the number of users K. Hence, the multistage detector contrasts with the optimum demodulator, which is based on a dynamic programming algorithm, has a variable decoding delay, and has a software complexity per symbol that is exponential in K. An exact expression is obtained and used to compute the probability of error is obtained for the two-stage detector, showing that the two-stage receiver is particularly well suited for near-far situations, approaching performance of single-user communications as the interfering signals become stronger. The near-far problem is therefore alleviated. Significant performance gains over the conventional receiver are obtained even for relatively high-bandwidth-efficiency situations     

Iterative multiuser detection for coded CDMA signals in AWGN andfading channels

A new iterative receiver for joint detection and decoding of code division multiple access (CDMA) signals is presented. The new scheme is based on a combination of the minimum mean square error (MMSE) criterion and the turbo processing principle by Hagenauer (see Proc. Int. Symp. Turbo Codes and Related Topics, Brest, France, p.1-9, 1997). The complexity of the new scheme is of polynomial order in the number of users. The new scheme is applicable to two situations: (a) when the receiver is capable of decoding the signals from all users and (b) when the receiver is only capable of decoding the signals from a subset of users. In the first scenario, we establish that the proposed receiver achieves superior performance to the iterative soft interference cancellation technique under certain conditions. On the other hand, in the second scenario, we argue that the proposed receiver outperforms both the iterative soft interference canceler and the iterative maximum a posteriori (MAP) receiver because of its superior near-far resistance. For operation over fading channels, the estimation of the complex fading parameters for all users becomes an important ingredient in any multiuser detector. In our scheme, the soft information provided by the decoders is used to enhance this estimation process. Two iterative soft-input channel estimation algorithms are presented: the first is based on the MMSE criterion, and the second is a lower-complexity approximation of the first. The proposed multiuser detection algorithm(s) are suitable for both terrestrial and satellite applications of CDMA     

On the average near-far resistance for MMSE detection of directsequence CDMA signals with random spreading

The performance of a near-far-resistant, finite-complexity, minimum mean squared error (MMSE) linear detector for demodulating direct sequence (DS) code-division multiple access (CDMA) signals is studied, assuming that the users are assigned random signature sequences. We obtain tight upper and lower bounds on the expected near-far resistance of the MMSE detector, averaged over signature sequences and delays, as a function of the processing gain and the number of users. Since the MMSE detector is optimally near-far-resistant, these bounds apply to any multiuser detector that uses the same observation interval and sampling rate. The lower bound on near-far resistance implies that, even without power control, linear multiuser detection provides near-far-resistant performance for a number of users that grows linearly with the processing gain     

A Low Complexity Near Optimal Signal Detection for Large Scale MIMO SC-FDMA Uplink System

This article proposes an improved Newton algorithm as a low complexity signal detection scheme for linear receiver in large scale multiple- input multiple- output (LS-MIMO) single carrier frequency division multiple access (SC-FDMA) uplink system, where a large number of antennas are set up at the base station and active users are with a single antenna system. Data detection for uplink SC-FDMA system is one of the specific challenges due to the significant rise in the dimension of antennas and number of subcarriers. Especially for symbol detection process, LS-MIMO SC-FDMA system with linear detector requires to perform a large matrix inverse computation. Even though linear detectors such as zero forcing (ZF) and minimum mean square error (MMSE) can achieve near-optimal detection performance, they still introduce high computational complexity and obliviously involve in the computation of matrix inversion. Therefore, a design of complexity reduction algorithm based near-optimal detector for LS-MIMO SC-FDMA system attains research interest. The improved Newton algorithm is employed to obtain linear detection solution which iteratively performs matrix free-inversion operation. The new algorithm performs matrix–matrix multiplication into matrix–vector multiplication, which substantially reduces receiver detection complexity. The efficacy of the proposed method is investigated at 16-QAM. Both ZF and MMSE criteria are proposed and compared through simulations. Simulation results illustrate that the proposed scheme outperforms the conventional detection schemes and exhibits near-optimal performance with a small number of iterations. Further, bit-error-rate performance is closer to classical linear detector with affordable computational complexity.

     

Identification of active users in synchronous CDMA multiuserdetection

The level of multiple access interference (MAI) in code division multiple access (CDMA) communication systems is a time-varying parameter related to the number of active users. Almost all existing multiuser detection schemes were designed based on a priori information of the active users. In many situations, however, the multiuser receiver does not know the number of active users, and the receiver designed for the detection of all users may lead to poor performance. To develop a more efficient detection scheme in practical applications, we propose a two-stage detection structure consisting of preprocessing (identification) and postprocessing (detection). In the preprocessing, we apply the subspace concept and a method based on the multiple signal classification (MUSIC) algorithm to identify the active users while requiring only a priori knowledge of all of the users' signature sequences. The proposed preprocessor is shown to be asymptotically near-far resistant, and to have the ability to identify the active users in a simple and reliable way. While in the detection process, as we efficiently use the active users' information in every observation interval, the performance is clearly improved compared to the conventional structure without identification. Moreover, the effect of imperfect identification on the decorrelating detector is also extensively analyzed. Though the decorrelating detector's inherent near-far resistant characteristic is impaired by imperfect identification, the proposed structure still outperforms the conventional structure in the general near-far environment     

A new symbol-alphabet-aware multi-user detection scheme

In this letter, a new multi-user detection scheme is proposed. It employs a novel nonlinear minimum mean square error (MMSE) estimator that exploits the knowledge of symbol alphabet to estimate the interfering signals. After cancelling the estimated interference, a conventional linear multi-user detector such as the MMSE detector or, simply, the matched-filter receiver (MFR) is then used to detect the desired user signal. Simulation results demonstrate that the proposed detector significantly outperforms the conventional linear MMSE detector.     

Multistage MMSE PIC Space–Time Receiver With Non-Mutually Exclusive Grouping

The arrival of new data services for wireless mobile communications requires an efficient use of the available bandwidth. Interference-limited cellular systems based on code-division multiple access (CDMA) can benefit from multiuser detection (MUD) and beamforming with antenna array to reduce multiple-access interference. Group-based techniques have been proposed to reduce the complexity of space-time MUD and have been shown to provide a performance-complexity tradeoff between matched filtering and full MUD. In this paper, the intergroup interference, which is a limiting factor in group-based systems, is reduced using multistage parallel interference cancellation after group-based minimum mean square error (MMSE) linear filtering. In addition, the extra resources that are available at the receiver are exploited by sharing users among groups. The proposed receiver is shown to converge, as the number of stages increases, to the full space-time MMSE linear MUD filter. The results show that the new approach provides bit error rate (BER) performance close to the full MUD receiver at a fraction of the complexity.