Robust blind PIC with adaptive despreader and improved interference estimator

An effective design of multistage parallel interference cancellation (PIC) receiver using blind adaptive (BA) despreader and pre‐respreader interference estimator for uplink CDMA is proposed and analysed. A novel algorithm is designed, which exploits constant modulus (CM) property of the users' transmitted signals and inherent channel condition to perform adaptive despreading based on minimum error variance criteria. This is carried out by BA weighting of each chip signal for accurate tracking of the desired user's signal power and hence for more improved data detection at the output of each stage of PIC. Furthermore, the despreader weights are used within the adaptive pre‐respreader interference estimation and cancellation to obtain online scaling factors during every symbol period, without any knowledge of users' channels or the use of training sequences. It is found that this way of estimation is optimal in minimum mean squared error sense, and hence, significant reduction in interference and noise variance is observed in detection and estimation of the desired users' signals compared with conventional PIC. Bit error probability of the proposed PIC is obtained using Gaussian Approximation method. Extensive simulation results are shown, which demonstrate impressive performance advantage in fading environments, high system loading, and severe near far conditions. Copyright © 2008 John Wiley & Sons, Ltd.     

Iterative vector channel estimation/MAP/PIC for CDMA systems in time-selective correlated multipath fading channels

Recently, Gao et al. proposed the expectation-maximization (EM) vector channel estimation for single-user direct-sequence code-division multiple-access (DS-CDMA) systems to estimate the correlated fading channel coefficients of the paths of a user. They also used one D-lag decision-feedback maximum a posteriori (MAP) detector to detect the information bits. A straightforward extension (noniterative) to multiuser synchronous CDMA cases, i.e., adding the parallel interference cancellation (PIC) multiuser detector into their scheme, results in poor performance. Therefore, we propose an iterative scheme which combines vector channel estimation, decision-feedback MAP detection, and PIC. The partial PIC is applied in the sequence hypothesis stage of MAP in each iteration before the final MAP decision. The proposed scheme has twice the decision delay of the previous scheme for single-user cases. The performance of the proposed system in the time-selective, correlated multipath fading channel environment is investigated. Simulation results show that the proposed scheme significantly outperforms the noniterative scheme. Simulation results also show that the proposed scheme performs better than its scalar channel-estimation version.     

Iterative multiuser detection with integrated channel estimation for coded DS-CDMA

A practically interesting approach for iterative channel estimation, multiuser detection, and single-user decoding based on maximum a posteriori symbol-by-symbol estimation for direct sequence/code-division multiple-access (DS-CDMA) is proposed. The receiver relies on the output of a bank of matched filters for each user and each path, and combines interference cancellation with iterated soft-decision feedback to improve channel estimation accuracy and data symbol reliability in course of a few iterations. We show that in this way, near single-user channel phase and amplitude estimation accuracy is achieved for frequency-selective fading channels, even in highly loaded systems, and illustrate that reliable data symbol estimation can be performed.     

A fuzzy PIC multiuser detection/vector channel estimation scheme in Rayleigh fading channels

We propose a fuzzy parallel interference cancellation (PIC) multiuser detection/vector channel estimation (VCE) scheme in Rayleigh fading channels. The VCE is based on a first‐order auto‐regressive model and an expectation‐maximization algorithm. The signal‐to‐interference ratio and signal‐to‐noise ratio are estimated from the vector channel model's parameters, and we adapt the weight of each interference cancellation path via fuzzy inference mechanism. The proposed fuzzy PIC and VCE cooperate in a way such that some of the fuzzy PIC input parameters come from the channel predictor and the fuzzy PIC makes the channel predictor be more accurate at the next PIC stage. Computing weights via fuzzy adaptive method adds insignificant complexity because it involves only table lookup. The simulation results show that the proposed fuzzy PIC/VCE scheme performs better than the improved PIC/vector channel estimation scheme with optimal parameter in fast‐varying Rayleigh fading channels. Copyright © 2007 John Wiley & Sons, Ltd.     

Blind adaptive multiuser detection in multipath CDMA channels basedon subspace tracking

A blind adaptive technique for signal demodulation in multipath code-division multiple-access (CDMA) communication channels is proposed. This technique is based on signal subspace estimation. The receiver employs a bank of linear filters (decorrelating filters or linear MMSE filters) at the front end to mitigate the multiple-access interference and the multipath interference. A channel estimator is used to estimate the channel state for diversity combining. It is shown that through the use of signal subspace estimation, both the decorrelating filterbank and the linear MMSE filterbank can be obtained blindly, i.e., they can be estimated from the received signal with the prior knowledge of only the signature waveform of the desired user. Two forms of the subspace-based linear filterbanks are developed and their equivalence in terms of the interference suppression capability is established. These subspace-based blind adaptive interference suppression techniques require, at each symbol epoch, the eigenvalues and the eigenvectors of an appropriate signal subspace, which ran be obtained using computationally efficient sequential adaptive eigendecomposition (subspace tracking) algorithms. Moreover, a blind adaptive method for estimating the channel state is developed, which also produces the postcombining decision statistic as a byproduct     

Performance and optimization of multistage partial parallel interference cancellation for wideband CDMA systems in multipath fading channels

In this paper, a closed-form bit error rate (BER) of hard-decision multistage partial parallel interference cancellation (PPIC) having perfect and imperfect channel estimation for complex spreading code-division multiple access (CDMA) over multipath Rayleigh channels was derived. The effects of receiving BER, power ratio of pilot to data channels, multiple-access interference (MAI) and multipath interference, additive white Gaussian noise, and other factors affecting channel estimation error are analyzed. The expression determining the optimal interference cancellation weight at path level for PPIC is first derived. Then, the oscillatory behavior of user-level-optimized PIC and the convergence of perfect channel estimation PIC and path-level-optimized PIC are validated. Results indicate that all factors determining the optimal weight certainly affect the BER performance of PPIC and that the lower BER limit of multistage PPIC depends on the adjustable factors, which include power ratio, observation length, and signal-to-noise ratio, and an unchangeable factor, i.e., number of users. In addition, the variance of MAI for complex spreading sequence is obtained, and the analytical expression can also be employed to predict the performance of multicode CDMA.     

Adaptive joint multiuser detection and channel estimation in multipath fading CDMA channels

The problem of joint multiuser detection and channel estimation in frequency-selective Rayleigh fading CDMA channels is considered. First the optimal multiuser detector for such channels is derived, which is seen to have a computational complexity exponential in the product of the number of users and the length of the transmitted data sequence. Two suboptimal detectors are then developed and analyzed, both of which employ decorrelating filters at the front-ends to eliminate the multiple-access interference and the multipath interference. The symbol-by-symbol detector uses a Kalman filter and decision feedback to track the fading channel for diversity combining. The per-survivor sequence detector is in the form of the Viterbi algorithm with the trellis updates being computed by a bank of Kalman filters in the per-survivor fashion. Both suboptimal detectors require the knowledge of all waveforms of all users in the channel and the channel fading model parameters. Adaptive versions of these suboptimal detectors that require only the knowledge of the waveform of the user of interest are then developed. The adaptive receivers employ recursive-least-squares (RLS) minimum-mean-square-error (MMSE) filters at the front-end to mitigate the interference, and use a bank of linear predictors to track the fading channels. It is shown that the front-end RLS-MMSE filters can be implemented using systolic arrays to exploit massively parallel signal processing computation, and to achieve energy efficiency. Finally, the performance of the suboptimal detectors and their adaptive versions are assessed by simulations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Multiuser receivers for CDMA systems in Rayleigh fading channels

Multiuser demodulation in relatively fast fading channels is analyzed. The optimal maximum likelihood sequence detection (MLSD) receiver is derived and a general suboptimal receiver to approximate the MLSD is proposed. The performance of the parallel interference cancellation (PIC) and decorrelating receivers is compared. The PIC receiver is demonstrated to achieve better performance in known channels than the decorrelating receiver, but it is observed to be more sensitive to the channel coefficient estimation errors than the decorrelator. At high channel loads the PIC receiver suffers from bit error rate (BER) saturation, whereas the decorrelating receiver does not. The performance of data-aided (DA) and decision-directed (DD) multiuser channel estimation is also compared. DA channel estimation is shown to be more robust than DD channel estimation, which may suffer from BER saturation caused by hangups at high signal-to-noise ratios (SNRs). The impact of channel estimation filter impulse response on the BER is studied by comparing optimal and suboptimal channel estimation filters. The implementation complexity of the decorrelating and PIC receivers is compared in terms of required floating point operations and clock cycles in a practical communication scenario. It is observed that the PIC receiver is only moderately more complex to implement than the conventional matched filter bank receiver, whereas the decorrelating receiver is significantly more complex     

An iterative maximum SINR receiver for multicarrier CDMA systems over a multipath fading channel with frequency offset

A robust iterative multicarrier code-division multiple-access (MC-CDMA) receiver with adaptive multiple-access interference (MAI) suppression is proposed for a pilot symbols assisted system over a multipath fading channel with frequency offset. The design of the receiver involves a two-stage procedure. First, an adaptive filter based on the generalized sidelobe canceller (GSC) technique is constructed at each finger to perform despreading and suppression of MAI. Second, pilot symbols assisted frequency offset estimation, channel estimation and a RAKE combining give the estimate of signal symbols. In order to enhance the convergence behavior of the GSC adaptive filters, a decisions-aided scheme is proposed, in which the signal waveform is first reconstructed and then subtracted from the input data of the adaptive filters. With signal subtraction, the proposed MC-CDMA receiver can achieve nearly the performance of the ideal maximum signal-to-interference-plus noise ratio receiver assuming perfect channel and frequency offset information. Finally, a low-complexity partially adaptive (PA) realization of the GSC adaptive filters is presented as an alternative to the conventional multiuser detectors. The new PA receiver is shown to be robust to multiuser channel estimation errors and offer nearly the same performance of the fully adaptive receiver.     

Performance comparisons of channel estimation techniques in multipath fading CDMA

The problem of pilot-symbol-aided estimation of multipath fading channels in up-link code-division multiple-access (CDMA) systems is considered. The transmitted symbol streams of each user are divided into time-slots; and each time-slot contains a number of pilot-symbols followed by information data symbols. Channel estimation is based on interpolation of the channel values corresponding to the pilot symbols in adjacent time-slots. Existing channel estimation techniques, including the weighted multislot average method and the wavelet expansion method, are studied. Two new channel estimation methods, namely, the robust channel interpolator, and the polynomial channel interpolator, are developed and are compared with these techniques. It is seen that the two new channel estimation methods significantly outperform the existing methods in multipath fading CDMA systems, for a wide range of Doppler values, and under various receiver schemes (with single or multiple receive antennas), such as the RAKE receiver, the interference cancellation receiver, and a receiver which performs iterative channel estimation and interference cancellation.     

基于信道跟踪的异步多用户OFDM系统接收机

本文针对空分多址接入OFDM系统上行链路提出了一种基于信道跟踪的异步多用户OFDM系统接收机,在每帧数据的开头设置导频符号得到初始信道估计,然后针对每一个数据符号在逐子载波的MMSE检测器、干扰抵消和信道估计之间迭代,得到最终的符号输出.仿真和分析结果表明这种接收机在保留了逐子载波处理简单结构的同时可以有效地跟踪信道.同时,用户在一定的移动速度下,异步时延越小,用户间干扰越弱,系统性能越好;帧长越短,信道跟踪能力越强.     

Nonlinear techniques for the joint estimation of cochannel signals

Cochannel interference occurs when two or more signals overlap in frequency and are present concurrently. Unlike in spread-spectrum multiple-access systems where the different users necessarily share the same channel, cochannel interference is a severe hindrance to frequency- and time-division multiple-access communications, and is typically minimized by interference rejection/suppression techniques. Rather than using interference suppression, we are interested in the joint estimation of the information-bearing narrow-band cochannel signals. Novel joint estimators are proposed that employ a single-input demodulator with oversampling to compensate for timing uncertainties. Assuming finite impulse-response channel characteristics, maximum likelihood (ML) and maximum a posteriori (MAP) criteria are used to derive cochannel detectors of varying complexities and degrees of performance. In particular, a (suboptimal) two-stage joint MAP symbol detector (JMAPSD) is introduced that has a lower complexity than the single-stage estimators while accruing only a marginal loss in error-rate performance at high signal-to-interference ratios. Assuming only reliable estimates of the primary and secondary signal powers, a blind adaptive JMAPSD algorithm for a priori unknown channels is also derived. The performance of these nonlinear joint estimation algorithms is studied through example computer simulations for two cochannel sources     

Partially blind adaptive MMSE interference rejection inasynchronous DS/CDMA networks over frequency-selective fading channels

In this work, the problem of joint suppression of multiple-access and narrow-band interference (NBI) for an asynchronous direct-sequence code-division multiple-access (CDMA) system operating on a frequency-selective fading channel is addressed. The receiver structure we consider can be deemed as a two-stage one: the first stage consists of a bank of minimum mean-square-error (MMSE) filters, each keyed to a given replica of the useful signal, and aimed at suppressing the overall interference; the second stage, assuming knowledge of the fading channel coefficients realizations, combines the MMSE filters outputs according to a maximal-ratio combining rule. Due to the presence of the NBI, the resulting structure is in general time-varying, and becomes periodically time-varying if the NBI bit-rate has a rational ratio to that of the CDMA system. Moreover, enlarging the observation window beyond the signaling interval and oversampling the signal space may yield a noticeable performance improvement. For the relevant case that the said ratio is rational, a new cyclic blind recursive least squares (RLS)-based algorithm is introduced, capable of tracking the periodically time-varying receiver structure, and allowing adaptive interference cancellation with a moderate complexity increase. We also come up with a closed-form expression for the conditional bit-error rate (BER), which is useful both to evaluate semi-analytical methods to assess the unconditional BER and to derive bounds on the system near-far resistance. The results indicate that the receiver achieves very satisfactory performance in comparison to previously known structures. Computer simulations also demonstrate that the cyclic blind RLS algorithm exhibits quite fast convergence dynamics     

Optimal two-stage decoupled partial PIC receivers for multiuser detection

Parallel interference cancellation (PIC) is considered a simple yet effective multiuser detector for direct-sequence code-division multiple-access (DS-CDMA) systems. However, system performance may deteriorate due to unreliable interference cancellation in the early stages. Thus, a partial PIC detector in which partial cancellation factors (PCFs) are introduced to control the interference cancellation level has been developed as a remedy. Although PCFs are crucial, complete solutions for their optimal values are not available. We consider a two-stage decoupled partial PIC receiver. Using the minimum bit error rate (BER) criterion, we derive a complete set of optimal PCFs. This includes optimal PCFs for periodic and aperiodic spreading codes in additive white Gaussian channels and multipath channels. Simulation results show that our theoretical optimal PCFs agree closely with empirical ones. Our two-stage partial PIC using derived optimal PCFs outperforms not only a two-stage, but also a three-stage full PIC.     

Performance of multiuser detection with adaptive channel estimation

An adaptive multipath decorrelating multiuser receiver is considered for application in Rayleigh fading multipath channels with significant Doppler spread. Coherent diversity combining is performed using adaptively obtained channel estimates in a manner that minimizes the impact of estimation errors on data detection. The bit-error rate of the receiver is evaluated analytically, showing dependence on the fading rate of the channel and tracking capabilities of adaptive least mean square and recursive least square algorithms, in addition to the order of multipath diversity and the number of active code-division multiple-access users     

Multiuser detection of DS-CDMA signals using partial parallel interference cancellation in satellite communications

Multiuser detection (MUD) using parallel interference cancellation (PIC) technique provides a good complexity, latency, and performance compromise. This technique is suitable for satellite systems using either code-division multiple-access (CDMA) or a combination of time-division multiple-access (TDMA) and CDMA. We offer a new scheme that is a combination of soft and hard PIC detectors whose performance is superior to that of the other famous suboptimal detectors. In soft partial parallel interference cancellation (PPIC), in the first few stages, when the performance is still poor, the accurate knowledge of power and phase cannot be of much use. However, in the following stages, accurate power and phase estimation can improve the performance. This coincides with the time when the decisions are reliable enough to be used for parameter estimation. In our scheme, after a few stages of soft interference cancellation (IC), estimation of the parameters will start. Having these estimates, in the subsequent stages hard IC is performed. The complexity of this scheme grows linearly with the number of users. Moreover, this scheme is much faster than other receivers such as successive interference cancellation (SIC). PIC detectors are usually studied in equal-power case, i.e., a perfect power control scheme is assumed. In this paper, PIC detector in a near-far condition where user signals arrive at the receiver with different power levels is also investigated.     

Adaptive parallel interference cancellation for CDMA systems—A weight selection and filtering scheme

Parallel interference cancellation (PIC) is a well-known multiuser detection algorithm in direct-sequence code-division multiple-access (DS-CDMA) systems. It is typically implemented with a multi-stage architecture. One problem associated with the PIC is that unreliable interference cancellation may occur in the early stages and the system performance may be degraded. Thus, the partial PIC detector was developed to control the cancellation level by use of interference cancellation factors. Partial PIC can be implemented with an adaptive form, in which optimal weights are derived using the least mean square (LMS) algorithm. In this paper, we propose an algorithm improving the conventional adaptive partial PIC. The main idea is to reduce the number of active weights in the LMS algorithm, and to perform weight post-filtering such that the resultant excess mean square error can be reduced. We also analyze the performance of the proposed algorithm and derive the bit error rate of the second stage output. Simulation results verify that the proposed algorithm outperforms the conventional partial PIC, and derived analytical results are accurate.     

Channel estimation for OFDM transmission in multipath fadingchannels based on parametric channel modeling

We present an improved channel estimation algorithm for orthogonal frequency-division multiplexing mobile communication systems using pilot subcarriers. This algorithm is based on a parametric channel model where the channel frequency response is estimated using an L-path channel model. In the algorithm, we employ the ESPRIT (estimation of signal parameters by rotational invariance techniques) method to do the initial multipath time delays acquisition and propose an interpath interference cancellation delay locked loop to track the channel multipath time delays. With the multipath time delays information, a minimum mean square error estimator is derived to estimate the channel frequency response. It is demonstrated that the use of the parametric channel model can effectively reduce the signal subspace dimension of the channel correlation matrix for the sparse multipath fading channels and, consequently, improve the channel estimation performance     

Multistage interference cancellation with diversity reception forasynchronous QPSK DS/CDMA systems over multipath fading channels

This paper introduces a multistage interference cancellation (MIC) technique with diversity reception for quadrature phase shift keying (QPSK) asynchronous direct-sequence code division multiple access (DS/CDMA) systems over frequency-selective multipath Rayleigh fading channels. Unlike the previous MIC, which tries to remove the lump sum of the multiple-access interference (MAI) and self-interference (SI), this introduced MIC attempts to cancel only the MAI and part of the SI due to the intersymbol interference, while treating the remaining SI created by the current symbol as useful information for symbol decision. In this technique, the RAKE combining is used to collect signal replicas over multiple fading paths. Upper and lower bounds on the bit error probability are derived using a Gaussian approximation and the characteristic function method. Furthermore, effects of channel estimation error on the performance are studied. Analytical and simulation results show that the introduced MIC can provide a performance extremely close to that in an ideal single-user environment and outperforms the previous MIC even in the presence of channel estimation error     

Adaptive DFE Multiuser Receiver for CDMA Systems using Two-Step LMS-Type Algorithm: An Equalization Approach

This paper presents an adaptive decision feedback equalizer (DFE) based multiuser receiver for code division multiple access (CDMA) systems over smoothly time-varying multipath fading channels using the two-step LMS-type algorithm. The frequency-selective fading channel is modeled as a tapped-delay-line filter with smoothly time-varying Rayleigh-distributed tap coefficients. The receiver uses an adaptive minimum mean square error (MMSE) multiuser channel estimator based on the reduced Kalman least mean square (RK-LMS) algorithm to predict these tap coefficients (Kohli and Mehra, Wireless Personal Communication 46:507–521, 2008). We propose the design of adaptive MMSE feedforward and feedback filters by using the estimated channel response. Unlike the previously available Kalman filtering algorithm based approach (Chen and Chen, IEEE Transactions on Signal Processing 49:1523–1532, 2001), the incorporation of RK-LMS algorithm reduces the computational complexity of multiuser receiver. The computer simulation results are presented to show the substantial improvement in its bit error rate performance over the conventional LMS algorithm based receiver. It can be inferred that the proposed multiuser receiver proves to be robust against the nonstationarity introduced due to channel variations, and it is also beneficial for the multiuser interference cancellation and data detection in CDMA systems.