Adaptive array processing for multipath fading mitigation viaexploitation of filter banks

We propose subband adaptive array processing for mitigation of both intersymbol interference (ISI) and cochannel interference (CCI) in digital mobile communications. Subband adaptive array processing employs filter banks in a front end to an adaptive array receiver. By decomposing the signals into a set of subband signals, the analysis filters enhance the correlation of multipath rays in each subband. This enhancement is blind in the sense that no a priori knowledge of the temporal characteristics or spatial signatures of arriving signals is required. With the increased coherence, the desired signal can be effectively equalized by subsequent spatial processing. Further, the CCI signals and their multipaths can be suppressed with fewer degrees of freedom. The effects of quadrature mirror filter and discrete Fourier transform filter banks on multipath correlation are delineated     

Joint equalization and interference suppression for high data ratewireless systems

Enhanced Data Rates for Global Evolution (EDGE) is currently being standardized as an evolution of GSM in Europe and of IS-136 in the United States as an air interface for high speed data services for third generation mobile systems. In this paper, we study space-time processing for EDGE to provide interference suppression. We consider the use of two receive antennas and propose a joint equalization and diversity receiver. This receiver uses feedforward filters on each diversity branch to perform minimum mean-square error cochannel interference suppression, while leaving the intersymbol interference to be mitigated by the subsequent equalizer. The equalizer is a delayed decision feedback sequence estimator, consisting of a reduced-state Viterbi processor and a feedback filter. The equalizer provides soft output to the channel decoder after deinterleaving. We describe a novel weight generation algorithm and present simulation results on the link performance of EDGE with interference suppression. These results show a significant improvement in the signal-to-interference ratio (SIR) performance due to both diversity (against fading) and interference suppression. At a 10% block error rate, the proposed receiver provides a 20 dB improvement in SIR for both the typical urban and hilly terrain profiles     

A novel hybrid of adaptive array and equalizer for mobilecommunications

The combination of the adaptive array and equalizer (AE) has been developed for suppressing the cochannel interference and the intersymbol interference (ISI) in mobile communications. In this paper, a novel hybrid of the adaptive array and equalizer (NHAE) system is proposed to combat the problems of insufficient degrees of freedom and mainbeam multipath interferers. The NHAE utilizes a modified training sequence to adjust the weight vector of the array that leads the array to cancel only the cochannel interferers. The ISI which is caused by the multipath interferers and the transmission system is removed by the equalizer following the array. Therefore, the array in the NHAE may need a fewer number of the elements than the conventional array which cancels both the cochannel interferers and multipath interferers. Besides, the presence of the mainbeam multipath interferers, which may seriously degrade the performance of the AE, has much less effect on the NHAE since it is suppressed by the equalizer instead of by the array. Simulation results are presented to demonstrate the merits of the NHAE     

Adaptive equalization and interference cancellation for wirelesscommunication systems

An adaptive equalization and interference cancellation method is proposed. The proposed scheme can cancel both intersymbol interference and cochannel interference, and is blind in the sense that no knowledge of the training sequences of the interfering users is required. In particular, it is a maximum likelihood sequence estimation (MLSE) equalizer that is implemented by the generalized Viterbi algorithm (GVA) with an RLS-based channel estimator. To demonstrate the potential of the proposed method, various simulation results over a frequency selective Rayleigh fading environment in the presence of cochannel interference are presented. In addition, a sequential algorithm is introduced to reduce the computational complexity of GVA     

An adaptive canceller of cochannel interference for spread-spectrummultiple-access communication networks in a power line

The authors propose and investigate an adaptive canceller of intersymbol and cochannel interference due to channel distortion and cross-correlation among pseudonoise sequences assigned to individual users of a DS-SSMA (direct-sequence spread-spectrum multiple-access) system. In order to implement a local area network (LAN) by using a power line installed in a building wall as a transmission channel, the authors have investigated utilization of DS-SSMA which has advantages such as robustness against narrow-band interference and noise and realization of asynchronous code division multiple access. In a power line, however, restriction of transmission bandwidth for communications makes it difficult to suppress cochannel interference and the channel is also time-varying due to fluctuation of loads. Since the proposed canceller adaptively eliminates cochannel interference as well as intersymbol interference, it can facilitate synchronization and increase the number of the simultaneously accessing users on a power line with restricted processing gain. The error probability in the output of the canceller is theoretically calculated for the steady-state case by using a Markov model. Computer simulations illustrate stable convergence properties of the canceller     

Suppression of adjacent-channel, cochannel, and intersymbolinterference by equalizers and linear combiners

We describe the ability of a linear equalizer/combiner or decision-feedback equalizer to suppress all received adjacent-channel, intersymbol, and cochannel interference. The emphasis is on values among transmitter bandwidth, receiver bandwidth, carrier spacing, and antenna diversity which provide the best opportunities for interference suppression. Through analyses of the number of degrees of freedom and constraints in generalized zero-forcing equalizers, and partial comparisons to calculations of equalizer minimum-mean-square performance, four results are obtained. First, with one antenna and a linear equalizer, arbitrarily large receiver bandwidths allow for marginal improvements in spectral efficiency through decreased carrier spacing, because the carrier spacing cannot be reduced to a value below the symbol rate without incurring unsuppressible interference. Second, large receiver bandwidths assist multiple antennas in improving the spectral efficiency in that carrier spacing values may go below the symbol rate, even in the presence of cochannel interference. Third, the use of equalizers and linear combiners, together with large receiver bandwidths, allows large transmitter bandwidths to be used. Fourth, for cochannel interference and intersymbol interference, the number of interferers that may be suppressible by a generalized zero-forcing linear equalizer/combiner increases linearly with the product of the number of antennas and the truncated integer ratio of the total bandwidth to the symbol rate     

A two-stage hybrid approach for CCI/ISI reduction with space-timeprocessing

We present a hybrid approach for separate cochannel interference (CCI) reduction and intersymbol interference (ISI) equalization in a slow Rayleigh fading channel. In this hybrid approach, a space-time filter is designed to maximize signal-to-interference-plus-noise ratio (SINR) by jointly optimizing the weight vector and the modified channel vector. A Viterbi equalizer then follows to equalize ISI and demodulate data symbols without noise enhancement. We derive an eigenvector solution for the joint optimization of the weight vector and the modified channel vector. Simulation results show good performance even at low carrier-to-interference-ratio (CIR)     

基于子带滤波器组的宽带自适应天线旁瓣相消技术

文[6]提出在数字移动通信中子带滤波器组处理可以提高不同阵元信号的相关性,从而能改善自适应阵列抑制码间干扰(ISI)和共信道干扰(CCI)的能力.在文[6]的基础上,本文研究了子带滤波器组在宽带自适应天线旁瓣相消中的应用,对其原理进行了理论分析,提出了有效的子带处理方法.经研究表明,子带滤波器组处理能有效增加主、辅助天线信号的相关性,从而提高系统干扰相消比.而且适当的过采样能使系统干扰相消比进一步提高.计算机仿真结果和实测雷达数据处理结果证实了子带处理方法的有效性和理论分析的正确性.     

A space-time-filtered Viterbi receiver for CCI/ISI reduction in TDMA systems

In this paper, we present a hybrid space-time-filtered Viterbi receiver using multiple antennas for co-channel interference (CCI) reduction and intersymbol interference (ISI) equalization in a slow Rayleigh fading channel. In this approach, a space-time filter is first applied at the antenna outputs to maximize the signal-to-interference-plus-noise ratio (SINR), and the scalar output is then sent to a Viterbi equalizer. We propose a closed-form solution to jointly determine the weight vector for the space-time filter and the channel vector for the Viterbi equalizer. We also examine the need for a whitening filter prior to the Viterbi equalizer and show that it only marginally improves the performance. Simulation results are provided to validate our approach and to compare the performance of our receiver with that of different existing receivers.     

Adaptive array processing MLSE receivers for TDMA digitalcellular/PCS communications

Array processing is a promising approach for improving quality, coverage, and capacity in digital cellular communication systems. By combining array processing with maximum likelihood sequence estimation (MLSE), intersymbol interference (ISI) introduced by multipath propagation can be mitigated as well. Novel symbol-spaced and fractionally spaced adaptive array processing MLSE receivers are developed for both diversity and phased array antenna configurations. The practical issues of synchronization and channel estimation are addressed. A novel approach to automatic frequency error correction (AFC) is proposed and is shown to be critical when cancelling cochannel interference. Performance is evaluated for the reverse link of the IS-136 TDMA-based digital cellular system. Substantial improvements are obtained over conventional antenna configurations for receiver sensitivity (2.5-4 dB) and over traditional antenna combining when cochannel interference is present (0.5-25 dB)     

Equalizers for transmultiplexers in orthogonal multiple carrier data transmission

Orthogonal multiple carrier data transmission systems are efficiently realized using modified dft transmultiplexer filter banks. In data transmission applications, a non-ideal transmission channel causes distortions such as intersymbol interference and crosstalk between the subrate bands of the transmultiplexer. Hence, in order to equalize these distortions, subband equalizers, which affect the intersymbol interference and crosstalk behavior, are considered for implementation. The special structure of modified dft transmultiplexers requires a discussion concerning the various possibilities of placing the subband equalizers at the receiver. Wiener solutions and lms adaptive algorithms for various new subband equalizer structures are derived and compared by means of simulation results.     

Kalman Filter Equalization for QPSK Communications

The discrete complex Kalman filter is considered as an equalizer for quadrature phase-shift keyed (QPSK) systems in the presence of additive noise and intersymbol interference (ISI). When the channel is unknown, an adaptive Kalman equalizer is used in which the channel complex tap gains are estimated by decision feedback.     

Separation of cochannel signals in TDMA mobile radio

We propose a sequential algorithm that separates cochannel time-division multiple-access (TDMA) signals that encounter multipath interference and noise. The receiver employs a multistage architecture where each stage consists of a beamformer and an equalizer that isolates one source, compensates for intersymbol interference (ISI), and demodulates the data. A problem encountered with such bursty sources is that the beamformer/equalizer trained for a particular time slot may not be appropriate for all the data contained in that slot. This occurs because a cochannel source typically overlaps only part of the time slot of interest and may not overlap the training sequence at all. The algorithm presented overcomes this problem by processing the data forward and backward in a sequential noncausal manner. Computer simulations using signals with the IS-54 format are presented to demonstrate the properties of the sequential algorithm     

Blind Multiuser Detection with Array Observations

Cochannel interference is usually a major limitation to the performance of mobile wireless systems. Examples of different forms of cochannel interference include multi-access interference in CDMA systems and cochannel interference resulting from frequency reuse in TDMA systems. In order to mitigate the interference from other users we present a blind multiuser receiver which utilizes array observations and performs both spatial and temporal processing of the received signal. The presented technique is completely blind in the sense that no signature sequences, channel state or spatial location needs to be known a priori, nor use of a training sequences, channel state or spatial location needs to be known a priori, nor use of a training sequence is required for the adaptation. The diversity introduced by the array observations can be efficiently combined with the use of CDMA signature sequences. After initial convergence, a reliable estimate of the combined temporal and spatial signature for each user is provided that can be employed by a multiuser receiver of lower complexity.     

大容量散射调制解调技术研究

简要介绍了时域自适应判决均衡器的原理,并根据对流层散射传播特点,利用自适应均衡技术以提高散射系统的抗码间干扰能力。采用四重显分集加自适应均衡的接收技术不仅可以减小码间干扰,收集隐分集能量,而且与显分集结合能够平滑信道选择性深衰落以及空间平坦衰落,大大提高设备通信能力。详细论述了自适应均衡器的技术特点和检测技术原理,并给出了对流层散射自适应均衡解调器的实现方案和抗多径测试结果。     

MSK-type signals in the presence of adjacent channel interferenceand white noise

In this letter, the problem of adjacent channel interference (ACI) caused by the close packing of constant envelope MSK-type users in a given frequency band is considered. An optimal receiver filter, based on the theory of matched filtering, is found, and it serves as an upper bound on the signal-to-interference ratio. The intersymbol interference (ISI) caused by the time response of the matched filter is eliminated by a decision feedback equalizer (DFE) which, however, degrades performance. It was found that the matched-filter upper bound allows about 3-15 dB more ACI than the performance of a classical correlation detector (for additive white Gaussian noise only), depending on the frequency separation between channels. The DFE performance is only a little bit worse than that of the matched filter     

Separation of cochannel GSM signals using an adaptive array

The Global System for Mobile communications (GSM) is a digital cellular radio network that employs time division multiple access (TDMA). In such a cellular system, frequencies are reused in different regions for spectral efficiency, and thus, the transmissions in a given cell can interfere with those in distant cells. This cochannel interference can be a major impairment to the signal of interest. In this paper, we describe a beamformer and equalizer system that is capable of separating and demodulating several cochannel GSM signals. The signal model includes intersymbol interference (ISI) induced by the Gaussian transmit filter, and the channel model incorporates multipath propagation and additive white Gaussian noise. The GSM synchronization sequences are used to compute the beamformer weights and achieve frame synchronization simultaneously. Decision-feedback equalization is employed to compensate for the transmit filter ISI and to demodulate the data     

Combined channel estimation and data detection using softstatistics for frequency-selective fast-fading digital links

A novel adaptive nonlinear equalizer for fast time-varying multipath channels that combines the channel estimation and data detection tasks is presented. The a posteriori probabilities (APPs) of the states of the intersymbol interference (ISI) channel are recursively computed from the received data by a symbol-by-symbol (SbS) detector and are then employed by a Kalman-type nonlinear channel estimator. Robust channel tracking and good data-detection performance are obtained, with a reasonable receiver complexity     

Performance Analysis of the Spectral Correlation Discriminator Array

Mobile communications suffer from cochannel interference, adjacent channel interference and fading. The cell capacity or the number of users per cell is limited by the interference. In this paper we analyze a new blind adaptive array structure called the spectral correlation discriminator array (SCDA), designed to reject cochannel interference for advanced mobile phone service (AMPS) signals. The blind adaptive array exploits the spectral redundancy in the AMPS signals which arises due to the presence of the supervisory audio tone (SAT). SCDA is shown to provide an improvement of 25–30 dB in signal-to-interference plus noise ratio (SINR) in static multipath channels and 10–15 dB improvement of SINR in Rayleigh fading channels. The SCDA is compared to the least squares constant modulus array (LSCMA) and is shown to have better signal selectivity properties. The reduction in interference power can significantly reduce the frequency reuse factor.     

时域自适应均衡技术的分析与应用

概述了频率选择性衰落信道的传输特性,论述了采用均衡技术的必要性。通过对各种均衡器结构和自适应均衡算法在抵抗符号间干扰能力、收敛速度以及运算复杂度等方面的分析与比较,选择了判决反馈作为均衡器结构、最小均方自适应算法作为自适应准则的均衡器方案。仿真及试验结果证实了设计的时域自适应均衡器不仅具有较强的抵抗符号间干扰能力,而且能够获得隐分集增益,在频率选择性衰落信道中具有良好的应用效果。