Adaptive maximum SINR RAKE filtering for DS-CDMA multipath fadingchannels

The conventional signature-matched RAKE processor for multipath direct-sequence code division multiple access channels is viewed as a regular linear tap-weight filter of length equal to the sum of the system processing gain and the user channel memory. In this paper, performance improvements are sought in the context of adaptive filtering under maximum signal-to-interference-plus-noise-ratio criteria. The minimum-variance-distortionless-response RAKE (RAKE-MVDR) filter and the lower complexity scalar optimized auxiliary-vector RAKE (RAKE-AUX) filter are developed. Bit error rate (BER) comparisons with the conventional RAKE signature-matched filter are carried out for training sets of reasonably small size, perfectly known, and mismatched/estimated channel coefficients, and extreme near-far system configurations     

Real-time adaptive multipath compensation

Multipath propagation interference in urban mobile communications can be minimised by using spread spectrum modulation and a RAKE filter. The Letter describes the simulation and practical operation of an adaptive or programmable RAKE filter, employing maximal ratio combining, which can handle time varying multipath channels in either the base station or mobile receiver.<>     

Bit error probability of a matched filter in a Rayleigh fadingmultipath channel in the presence of interpath and intersymbolinterference

The exact bit error probability of a matched filter receiver in a known discrete Rayleigh fading multipath channel is determined for a binary antipodal system. The matched filter receiver or the RAKE is one form of a diversity receiver. Traditionally, both the interpath interference (IPI) and the intersymbol interference (ISI) caused by the non-ideal autocorrelation function of the bit waveform are neglected in the analysis of the RAKE. Previously, the exact matched filter bounds; which include IPI, were derived. This bound gives the performance of the matched filter if only one symbol is transmitted. In our analysis, a symbol sequence is transmitted instead of only one symbol. Thus, our analysis includes both IPI and ISI as well as the correlation between the paths, The analysis permits direct comparisons between different kinds of bit waveforms     

RAKE接收机中功率测量滤波器的设计

本文分析了多径衰落信道的相关特性，计算了利用RAKE接收机实时测量信号功卓的误差。结果表明，采用RAKE接收机进行测量可以显著提高测量的准确性．同时，研究了各种因素对测量滤波器性能的影响。     

A RAKE receiver design for WCDMA FDD uplink with an RLS-based adaptive beamforming scheme

In this paper, we present a RAKE receiver design with adaptive antenna arrays for the wide-band code-division multiple-access (WCDMA) frequency-division duplexing (FDD) uplink. The RLS-based adaptive beamforming scheme is proposed and can be built with the existing one-dimensional RAKE receiver. We adaptively calculate the beamforming weight vector for each multipath of the desired user, and use maximum ratio combining (MRC) to combine each multipath signal in the demodulation process. Two matched filters based on the spreading waveforms are designed in our scheme for WCDMA FDD uplink application. The proposed scheme has the ability of suppressing strong multiuser access interference and the other types of interferers through spatial ing. The tracking capability of the proposed algorithm is demonstrated in the simulation results.     

A linear MMSE receiver for multipath asynchronous random-CDMA with chip pulse shaping

This paper studies the design and implementation of a linear minimum mean-square error (LMMSE) receiver in asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems that employ long-code pseudonoise (PN) sequences and operate in multipath environments. The receiver is shown to be capable of multiple-access interference (MAI) suppression and multipath diversity combining without the knowledge of other users' signature sequences. It maximizes output signal-to-noise ratio (SNR) with the aid of a new chip filter which exploits the cyclostationarity of the received signal and combines all paths of the desired user that fall within its supported time span. The performance of the LMMSE receiver is compared with that of the coherent selective RAKE receiver. The achieved gain is on the order of 0.6-1.8 dB in dense multipath environments of current narrow-band settings and nonuniform power distribution scenarios of next-generation CDMA systems. An example of adaptive implementation of the LMMSE receiver is presented and accompanied by complexity analysis, training curves, and quantitative performance comparisons illustrating the convergence rate and steady-state performance of the adaptive algorithms.     

A Blind RAKE Receiver With Robust Multiuser Interference Cancelation for DS/CDMA Communications

In this paper, a blind RAKE receiver with robust multiuser access interference cancellation is presented for frequency-selective Rayleigh fading channels. In contrast to a conventional receiver, here, only knowledge of the spreading code and rough timing of the desired user is required. By investigating the code space of the multipath signals and the data vector space, a RAKE filtering vector is developed to extract the desired data from all the paths of the desired user. Our proposed technique not only exploits the characteristics of multipath propagation but also the characteristics of timing offsets that may occur in the receiver, to facilitate the application of a blind linear filter-optimization technique for robust interference suppression. Based on the RAKE filtering vector, interference rejection is implemented by using the auxiliary-vector (AV) technique. Our approach, however, effectively overcomes the sensitivity of the original AV method to multipath propagation and timing offsets. To mitigate the signal cancellation at relatively high signal-to-interference and noise ratios (SINR) resulting from the estimation errors of the RAKE filtering vector, robust strategies are introduced in addition to the linear filter optimization. Simulation results are presented to show the effectiveness of the proposed techniques.     

A research of UWB rake receiver based on novel RLS adaptive algorithm

A modified RAKE receiver based on novel Recursive Least Squares （RLS） adaptive algorithm is proposed. The receiver uses L-fingered correlators, which are composed of RLS adaptive filters, to enhance the performance of multipath receiving. It can also track the amplitude of the received signal to form a real-time amplitude estimation which is correlated with the power of excess delay bin. The simulation results based on the IEEE UltraWide Band （UWB） channel models （CMI to CM4） show that the novel RLS algorithm can alter the attenuation estimation with the finger＇s power delay profile, and RAKE receiver with few fingers can be employed to get high performance.     

Bidirectional iterative ISI canceller for high-rate DSSS/CCK communications

This paper proposes a new RAKE receiver incorporated with a bidirectional iterative intersymbol interference (ISI) canceller in order to reinforce multipath robustness of high-rate direct-sequence spread-spectrum complementary code keying (DSSS/CCK) systems. The proposed RAKE receiver first combines multipath signal components through a channel matched filter (CMF) and removes postcursor-ISI by employing a codeword decision feedback equalizer (DFE). Then, a CCK codeword detector tentatively determines the current CCK codeword symbol and reuses it to subtract precursor-ISI from the previous symbol. Therefore, the ultimate symbol decision is made using the delayed signal with both postcursor-ISI and precursor-ISI cancelled. The detection performance can be more improved through an iterative refinement processing between the postcursor and the precursor components. Simulation results exhibit a significantly improved error rate performance of the proposed receiver compared with that of the legacy RAKE receiver employing only a postcursor DFE. The additional cost for realization of the proposed receiver is one symbol decision delay and reuse complexity of the DFE and the codeword detector.     

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.     

TDD-CDMA系统的一种新的多径发送分集方案

本文提出了ＴＤＤ－ＣＤＭＡ系统的一种新的多径分集方案。它在发送端采用一个ＦＩＲ滤波器,面在接收端采用ＲＡＫＥ接收机实现多径合并。分析结果表明对ＦＩＲ滤波器的系统进行优化的问题椒于求解矩阵的牲玫特征向量的问题,而最大的特征值所对应的特征向量就是最佳ＦＩＲ滤波器的抽头系数。本文还分析了多天线的情况,此时相当于２维时空信号处理。数值分析表明采用上述方案的系统的输出信噪比（ＳＮＲ）较之通道的ＲＡＫＥ系统或     

Effects of Nakagami-Fading Parameters and Power Control Error on Performance of DS-CDMA Cellular Systems with Adaptive Beamforming

It is well known that power control error (PCE) is a critical issue in CDMA cellular systems. In this paper, the bit error rate (BER) of a direct sequence-code division multiple access (DS-CDMA) receiver with imperfect power control, adaptive beamforming, and voice activity is derived in frequency-selective Nakagami fading channels. We discuss the effects of PCE, Nakagami-m fading parameter, and channel’s multipath intensity profile as average signal strength and rate of average power decay and their effects on the BER performance of DS-CDMA cellular systems. In this paper, the RAKE receiver consists of three stages. In the first stage, with conjugate gradient adaptive beamforming algorithm, the desired users’ signal in an arbitrary path is passed and the inter-path interference is canceled in other paths in each RAKE finger. Also in this stage, the multiple access interference (MAI) from other users is reduced. Thus, the matched filter (MF) can be used for the MAI reduction in each RAKE finger in the second stage. In the third stage, the output signals from the MFs are combined according to the conventional maximal ratio combining principle and then are fed into the decision circuit for the desired user. How the Nakagami fading parameters, power control imperfections, or the number of resolvable paths affect the reverse link capacity of the system is discussed in detail. Analytical and simulation results are also given for systems with different processing gains and number of BSs in the cell-selection process with various Nakagami fading parameters.     

Adaptive detection in asynchronous code-division multiple-accesssystem in multipath fading channels

In code-division multiple-access systems transmitting data over time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) arise. In this paper, we address interference suppression, multipath diversity and processing gain protection for multiuser detection with less noise enhancement by using a parallel cancelling scheme. The proposed detector consists of a RAKE filter, forward filter, and feedback filter with different functions for each filter. The RAKE filter increases the signal-to-noise ratio by taking the advantage of multipath and code diversities. The forward filter is proposed, in combination with the feedback filter, to remove the effects of MAI and ISI by parallel cancellation. In order to avoid performance deterioration due to unreliable initial estimation in the parallel cancellation, a cost function with proper weighting is introduced to improve the performance of the proposed detector. In the proposed design method, a recursive least square algorithm is employed to update the tap-coefficients of all filters for MAI and ISI cancellation. Finally, the performance of the proposed detector is analyzed and compared with other detectors     

Impact of spreading bandwidth on RAKE reception in dense multipathchannels

Spread spectrum (SS) multiple access techniques have been proposed for third generation broadband wireless access. We develop an analytical framework to quantify the effects of spreading bandwidth on SS systems operating in dense multipath environments in terms of the receiver performance, receiver complexity, and multipath channel parameters. In particular, we consider wide-sense stationary uncorrelated scattering (WSSUS) Gaussian channels with frequency-selective fading. The focus of the paper is to characterize the combined signal of the RAKE receiver fingers tracking the strongest multipath components. Closed form expressions for the mean and the variance of the total RAKE receiver output signal-to-noise ratio (SNR) are derived in terms of the number of RAKE fingers, spreading bandwidth, and multipath spread of the channel. The proposed problem is made analytically tractable by transforming the physical RAKE paths into the virtual path domain. A representative result indicates that for SS systems with 5 MHz signal bandwidth operating in a channel with constant power delay profile having 5 μs spread, the average SNR gain from increasing the number of RAKE fingers from one to three is 3.8 dB and from three to five is 1.5 dB. Furthermore, the reduction in the variation of SNR is 1.1 dB and 0.4 dB for the same increments in the number of fingers     

Adaptive parameter estimation using parallel Kalman filtering forspread spectrum code and Doppler tracking

We present a new digital direct-sequence (DS) receiver with joint estimation of code delay, multipath gains and Doppler shift. A parameter estimator consisting of a parallel bank of extended Kalman filters (EKF's) extracts estimates of the timing, τ and the multipath coefficients, f_l distorting the received signal. A “detected” estimate of the Doppler shift, v_r distorting the received signal is also provided by the estimator. We compute the bit error rate that results when a RAKE matched filter uses the estimated parameters to detect the DPSK encoded binary data in the received signal. The bit-error rate (BER) is evaluated, and successful performance of the proposed receiver in the presence of Doppler shift distortion is observed in many cases. We demonstrate that the receiver can operate when the multipath coefficients vary in time (Doppler spread)     

一种新的用于DS/CDMA系统的多级盲空时多用户接收机

本文讨论的接收机是用于DS/CDMA系统的多级自适应阵列和串行干扰消除联合接收机(AA-SIC),介绍的接收机同时使用了空域和时域处理以及多用户联合检测技术.一般的联合接收机要使用RAKE单元,本文所讨论的接收机没有使用RAKE单元,而利用智能天线的自适应算法能起到分集接收的功能,不仅能抑制多址干扰信号,而且构成简便,计算时间较短,有利于实际应用.     

Pilot-added signal detection algorithm and application in OCC-CDMA systems under multipath interference

A RAKE receiver is found not useful in an OCC-CDMA system based on orthogonal complementary codes, and thus other alternatives have to be found to replace RAKE for signal detection in multipath channels. Thus, a pilot-aided detection scheme is introduced, particularly for its application in an OCC-CDMA system. Simulation results show that the scheme works well for an OCC-CDMA system under multipath interference.     

A RAKE Receiver With an ICI/ISI Equalizer for a CCK Modem

In this paper, we first derive the theoretical performance of a complementary code keying (CCK) code on an additive white Gaussian noise (AWGN) channel and over a multipath channel. To derive the error performance, we use the weight and cross-correlation distributions of the CCK code for optimal and suboptimal decoding, respectively, based on union bound. In addition, we propose a RAKE receiver for a CCK modem, which is suitable for a multipath environment with a large delay spread. The RAKE receiver principle is acceptable for modest multipath because it can coherently combine multipath components to provide signal-to-noise ratio (SNR) enhancement. However, as the delay spread is larger and the data rate of systems goes higher, intersymbol interference (ISI) generated due to multipath environments are increased. To handle the increasing ISI, the CCK modem needs an equalization technique to remove the ISI, together with RAKE processing. Thus, our proposed system is based on a channel matched filter (CMF) with a decision feedback equalizer (DFE). The CMF is applied for RAKE processing, whereas the DFE structure is used for ISI cancellation. In our system, ISI is calculated and removed by using a decoded CCK codeword.     

A robust PN code tracking algorithm for frequency selective Rayleigh-fading channels

In this letter, we propose a new tracking scheme that is robust against multipath fading for pseudonoise (PN) code tracking in direct-sequence-spread spectrum systems. The proposed scheme employs an adaptive filter whose taps are adapted using a block least-mean square algorithm and it results in minimizing the effect of multipath interference on the tracking performance. We show that the mean-squared tracking error performance of the proposed scheme is not affected by the presence of closely spaced paths (e.g., one to three chips), unlike that of conventional delay locked loops. We also show that the tap-weight distribution of the filter provides accurate estimates of the multipath delays. For example, at E/sub b//N/sub 0/=5 dB, 98% of the time the path estimates lie within one sample (1/5 of a chip) from the actual delays. Furthermore, simulation results suggest that multipath delays over a wide range of terminal speeds can be tracked successfully. The proposed scheme is well suited for wideband code-division multiple-access systems where a large number of closely-spaced multipath components need to be tracked and used in RAKE combining.     

Blind RAKE receiver with joint diversity combining and codetracking for DS/SS communication

A RAKE receiver achieving joint blind multipath diversity combining and code tracking is proposed. An improved known modulus adaptive algorithm is exploited to perform multipath diversity combining and to support the modified code tracking in the blind mode. Computer simulation results have indicated very attractive behaviour of the proposed technique