MMSE Equalizer Research Articles

Asymptotic Analysis of Reduced-Rank Chip-Level MMSE Equalizers in the Downlink of CDMA Systems

In this paper, we address the performance of downlink CDMA receivers that consist in a reduced rank Wiener chip-rate equalizer followed by a despreading operation. In particular, we tackle the question of whether a performance close to the optimum can be achieved for small values of the rank. To answer this question, it is standard to consider the output signal-to-interference plus noise ratio (SINR), and to study its convergence speed versus the rank of the receiver. Unfortunately, this task is difficult due to the fact that the SINR expressions depend on the spreading codes allocated to the various users in a rather complicated way. To be able to obtain positive results, we assume that the spreading factor and the number of users tend to infinity while their ratio remains finite. As in the third-generation UMTS system, the spreading codes we consider coincide with orthogonal Walsh-Hadamard codes scrambled by an independent identically distributed sequence. In this context, we show that the SINR of each reduced-rank receiver converges toward a deterministic limit which depends only of the rank of the receiver, and not of the spreading codes given to the various users. Using some previous results, we prove that the asymptotic SINRs converge exponentially to the SINR of the plain Wiener receiver when the rank of the receiver increases. We obtain the corresponding convergence rate, and exhibit the parameters that influence the convergence speed. We finally compare our asymptotic performance expressions with the results of numerical simulations. We observe a good agreement for spreading factors as low as 16

MMSE Equalizer for MIMO-ISI Channel With Shorten Guard Period

A finite-impulse-response minimum mean-square error equalizer (FIR-MMSEE) for block-based transmission over a multiple-input multiple-output (MIMO) intersymbol interference (ISI) channels is proposed in this correspondence. The proposed MMSE equalizer can avoid interblock interference (IBI) without requiring a guard period to be larger than or equal to the channel order. By reducing the number of redundant symbols in the transmitted signals, the spectral efficiency of the communication system with long channel dispersion can be greatly improved. We further investigated the parallel FIR-MMSEE equalization structure and proposed a group successive interference cancellation MMSE equalizer (GSIC-MMSEE). A group of elements in the output of the FIR-MMSEE will be used to successively cancel their own effect in the received signals after the signal detection in GSIC-MMSEE. Since the cancellation process eliminates some interference sources, the error performance of the subsequent equalization will be improved. Moreover, the interference cancellation is performed in a group-by-group basis. As a result, the computational complexity of the proposed equalizer can be controlled by adjusting the number of groups in the GSIC. Simulation results are presented to compare the performance of the proposed FIR-MMSEE and GSIC-MMSEE

Low Complexity Intercarrier Interference Equalization Technique in OFDM System

Intercarrier interference will cause the loss of subchannel orthogonality and increase the error floor in proportion to the Doppler frequency. In this paper, we firstly analyze the generation mechanism of intercarrier interference in OFDM. Then we propose an O(N log 2 N) complexity ICI equalizer for OFDM systems in the presence of double selective fading which is mainly bases on FFT operation. Simulation result shows that with only 6 iterations LCD-FFT can achieve better performance than the LS-equalizer. After 10 iterations LCD-FFT performs almost the same as MMSE equalizer.

Bit error rate evaluation of linear equalization in FWA systems

AbstractThe bit error rate performance of broadband wireless fixed access (FWA) systems over multipath fading channels is investigated in this paper. Linear MMSE equalization is examined theoretically for 16‐QAM and QPSK modulated FWA systems and shown to yield unsatisfactory performance. The theoretical analysis is validated by Monte‐Carlo simulations and proved to be reasonably accurate. It provides us an insight into the physical limitations imposed by the FWA channels and suggest solutions to improve the capacity and performance of future FWA systems. Copyright © 2006 John Wiley & Sons, Ltd.

Blind direct estimation of PSK symbols for zero padding OFDM in rapid channel variation cases

A totally blind ZP-OFDM receiver is derived in this letter. Unlike existing blind receivers that are based on the scheme that channels are estimated first and then equalizers (MMSE or ZF) are adopted to detect information symbols, our scheme is based on the symbol-block-by-symbol-block direct symbol estimation approach. The proposed receiver is also robust in rapid channel variation cases as long as channel responses remain almost constant during one symbol block. Simulation results show that the BER performance of our scheme and linear MMSE (and ZF) equalizers with perfect CSI are indistinguishable in SNR scenarios of interests, despite the fact that our scheme is totally blind.

Low-complexity search for optimal delay in linear FIR MMSE equalization

In this letter, an efficient algorithm to determine the optimal delay in linear finite impulse response equalizers based on the minimum mean-square-error criterion is proposed. The algorithm uses the Levinson-Durbin (L-D) recursion as a starting point to find the values of the mean-square error for equalizers with all nontrivial delays. Despite the exhaustive search approach, the complexity of the proposed algorithm is only doubled when compared to the calculation of the equalizer with one prescribed delay. Such increase in complexity may be fully justified in practice because it yields globally optimal equalizer's design.

Iterative mmse Receivers for Space–Time Trellis-Coded CDMA Systems in Multipath Fading Channels

We explore code-division multiple-access (CDMA) systems with multiple transmit and receive antennas combined with space-time trellis codes over a frequency-selective channel. The conventional noniterative multiuser minimum mean square error (NIMU-mmse) detector is generalized to accommodate multiple antennas and multiple paths and then extended to include the turbo principle in an iterative fashion, allowing interference regeneration and cancellation at the receiver. Iterative multiuser mmse (IMU-mmse) receivers employing chip- and symbol-level detectors are derived and their equivalence is demonstrated. Computer simulations show that the proposed iterative mmse equalizers completely remove the interference of the other users in a multiantenna environment; they provide a significant improvement over the NIMU-mmse detector and they effectively achieve the single-user performance, even in a fully loaded system. Two suboptimal iterative mmse detectors, which allow a computational complexity reduction of up to three orders of magnitude compared to the IMU-mmse and still outperform the NIMU-mmse detector, are introduced. The proposed iterative mmse equalizers are analyzed and supported by extensive computer simulations.

A recursive QR approach to adaptive equalization of time-varying MIMO channels

This paper presents a novel adaptive equalization algorithm for time-varying MIMO systems with ISI channel conditions. The algorithm avoids channel estimation before equalization and leads to a direct QR-based procedure for updating the equalizer coefficients to track the time- varying channel characteristics. Our approach does not require precise channel estimation and needs relatively few pilot symbols for satisfactory equalization. The theoretical foundations of the pro- posed algorithm are rooted in signal recovery results derived from the generalized Bezout identity and the finite alphabet property inherent in digital communication schemes. Concerning the con- vergence behavior of the algorithm, we address the following three issues: existence of fixed points, exclusiveness of fixed points, and robustness under noise disturbance and parameter selection. The equalizer demonstrates promising capability in achieving low symbol error rates for a very broad range of SNRs. Simulation results are presented confirming that this approach outperforms the more traditional recursive least squares (RLS) adaptive equalizer for this application and rivals the performance of MMSE equalizers requiring channel knowledge.

Performance evaluation of tap selection based MMSE equalization for UWB systems

Performance evaluation of tap selection based MMSE equalization for UWB systems

Time-domain and frequency-domain per-tone equalization for OFDM over doubly selective channels

In this paper, we propose new time- and frequency-domain per-tone equalization techniques for orthogonal frequency division multiplexing (OFDM) transmission over time- and frequency-selective channels. We present one mixed time- and frequency-domain equalizer (MTFEQ) and one frequency-domain per-tone equalizer. The MTFEQ consists of a one-tap time-varying (TV) time-domain equalizer (TEQ), which converts the doubly selective channel into a purely frequency-selective channel, followed by a one-tap frequency-domain equalizer (FEQ), which then equalizes the resulting frequency-selective channel in the frequency-domain. The frequency-domain per-tone equalizer (PTEQ) is then obtained by transferring the TEQ operation to the frequency-domain. While the one-tap TEQ of the MTFEQ optimizes the performance on all subcarriers in a joint fashion, the PTEQ optimizes the performance on each subcarrier separately. This results into a significant performance improvement of the PTEQ over the MTFEQ, at the cost of a slight increase in complexity. Through computer simulations we show that the MTFEQ suffers from an early and high error floor, while the PTEQ outperforms the MMSE equalizer for OFDM over purely frequency-selective channels, it can approach the performance of the block MMSE equalizer. An important feature of the proposed techniques is that no bandwidth expansion or redundancy insertion is required except for the cyclic prefix.

Multichannel Estimation and Equalization Algorithm for Asynchronous Uplink DS/CDMA

This paper addresses the problem of channel estimation and equalization in asynchronous uplink DS/CDMA. We propose a complete receiver structure which contains a multichannel estimator suitable for time-varying channels, a MMSE equalizer and a propagation delay estimator. The multichannel estimator is a stochastic gradient algorithm which does not require any knowledge about the propagation delays. It estimates a matrix which contains a linear combination of the impulse response of each channel and the propagation delays. This estimate is used to derive a MMSE equalizer. In the despreading stage knowledge on the integer part (chip period multiple) of the propagation delays is needed. Two reduced complexity schemes for propagation delay estimation are presented. The channel estimator convergence is studied in stationary conditions while the overall algorithm performance is demonstrated by using simulations in time-varying channel conditions.

Performance of Frequency-Time MMSE Equalizer for MC-CDMA over Multipath Fading Channel

A cyclic prefix of sufficient length is inserted (longer than the maximum delay spread of the channel) in Multi-Carrier CDMA (MC-CDMA) transmission over dispersive channels such that the channel, a...

An Improved Linear MMSE Detection Technique for Multi‐Carrier CDMA Systems: Comparison and Combination with Interference Cancellation Schemes

AbstractIn this paper, various detection techniques are studied in the synchronous case of a multiuser MC‐CDMA system operating in frequency selective Rayleigh channel. First of all, an improved linear detection approach named MMSE per user detection technique, which takes into account the despreading process, is presented. Simulation results over a Rayleigh fading channel show that this novel technique offers for non full load systems a significant gain compared to the MMSE equalization per camer method. Then, the performance of various parallel and successive interference cancellation techniques is compared to the performance of the linear MMSE per user scheme. When the power of the interferers is roughly equal to the power of the useful signal, the performance of the linear MMSE per user is really close to the performance of the non linear PIC and SIC schemes.

On linear H/sup ∞/ equalization of communication channels

As an alternative to existing techniques and algorithms, we investigate the merit of the H/sup /spl infin// approach to the linear equalization of communication channels. We first give the formulation of all causal H/sup /spl infin// equalizers using the results of and then look at the finite delay ease. We compare the risk-sensitive H/sup /spl infin// equalizer with the MMSE equalizer with respect to both the average and the worst-case BER performances and illustrate the improvement due to the use of the H/sup /spl infin// equalizer.

Performance of Shalvi and Weinstein's deconvolution criteria for channels with/without zeros on the unit circle

This correspondence shows that the Shalvi and Weinstein (1994) blind deconvolution criteria are applicable for finite SNR regardless of channels having zeros on the unit circle or not. The associated deconvolution filter is stable with a nonlinear relation to the nonblind MMSE equalizer and capable of performing perfect phase equalization for finite SNR.

Efficiently computed reduced-parameter input-aided MMSE equalizers for ML detection: a unified approach

A unified approach for computing the optimum settings of a length-N/sub f/ input-aided equalizer that minimizes the mean-square error between the equalized channel impulse response and a target impulse response of a given length N/sub b/ is presented. This approach offers more insight into the problem, easily accommodates correlation in the input and noise sequences, leads to significant computational savings, and allows us to analyze a variety of constraints on the target impulse response besides the standard unit-tap constraint. In particular, we show that imposing a unit-energy constraint results in a lower mean-square error at a comparable computational complexity. Furthermore, we show that, under the assumed constraint of finite-length filters, the relative delay between the equalizer and the target impulse response plays a crucial role in optimizing performance. We describe a new characterization of the optimum delay and show how to compute it. Finally, we derive reduced-parameter pole-zero models of the equalizer that achieve the high performance of a long all-zero equalizer at a much lower implementation cost.