Interference cancellation with an array processing MLSE receiver

the capacity and transmission quality in cellular systems can be improved by using receivers that mitigate cochannel interference and multipath propagation. In this paper, a class of multichannel maximum likelihood sequence estimation (MLSE) receivers is developed for this purpose. Interference mitigation is accomplished via adaptive antenna arrays, while multipath propagation is combated via MLSE. Practical considerations are included, such as fixed front-end filtering, sampling, and estimation of parameters from received samples. Maximal ratio combining, conventional array processing and metric combining (MC) are shown to be special cases of the proposed receivers. Performance is evaluated for π/4-shift DQPSK, using the parameters and transmission format of the time-division multiple-access (TDMA)-based IS-136 (D-AMPS) digital cellular standard. Semi-analytical performance predictions are developed which confirm simulation trends. The results show that these receivers can operate at significantly lower carrier-to-interferer (C/I) levels than conventional MC receivers     

Adaptive channel estimation for multichannel MLSE receivers

The performance of multichannel coherent maximum-likelihood sequence estimation (MLSE) reception in the presence of co-channel interference is limited by the channel estimation accuracy. An adaptive channel estimation approach is developed which improves the performance through interference cancellation. Significant performance gains (up to 8 dB) are demonstrated for the Digital Advanced Mobile Phone Service (D-AMPS) (IS-136) digital cellular system     

Performance of antenna array systems with optimum combining in aRayleigh fading environment

The effect of cochannel interference on the performance of digital mobile radio systems in a Rayleigh fading environment is studied. The average bit error rate (BER) of an antenna array system with an optimum combining scheme that maximizes the output signal-to interference-plus-noise ratio is analyzed. BER expressions which are easy to evaluate numerically are derived for coherent binary phase-shift keying schemes in an environment with cochannel interference and noise     

Performance of antenna array systems with optimum combining in aRayleigh fading environment

The effect of cochannel interference on the performance of digital mobile radio systems in a Rayleigh fading environment is studied. The average bit error rate (BER) of an antenna array system with an optimum combining scheme that maximizes the output signal-to-interference-plus-noise ratio (SINR) is analyzed. BER expressions which are easy to evaluate numerically are derived for coherent binary phase shift keying (BPSK) schemes in an environment with cochannel interference and noise     

Digital error rate performance of active phased array satellite systems

A model for a multiple spot beam digital satellite employing an active phased array antenna with frequency reuse is proposed and applied to study the power limited down-link bit error rate performance. Since the power amplifier preceding each antenna element is generally nonlinear, intermodulation distortion is produced among the various spot beams, degrading performance. Expressions for the bit error rate are derived, averaged over an ensemble of random arrays. Typical results indicate a degradation of 1-5 dB relative to a horn-reflector antenna of the same total effective aperture with a single channel per transponder, as the number of cochannel signals and the number of antenna elements are respectively varied between 10 and 30, and 500 and 10 000. The degradation is shown to result from nonlinearly induced signal suppression and cochannel interference.     

Blind multipath separation and combining technique for signal recovery

To achieve better mitigation of both cochannel interference (CCI) and intersymbol interference, a new structure using generalized estimation of multipath signals in conjunction with maximal-ratio combining diversity for wireless communications over multipath channels is introduced. In this structure, the signal replicas received from multiple paths are first independently produced by a bank of blind spatial filters and then constructively combined by a diversity combining receiver for final signal estimate. The new scheme can be applied on single antenna array or between multiple antenna subarrays. It will be shown, from both theoretical analysis and numerical experiments, that the new scheme provides both space diversity gains and path diversity gains while suppressing the CCIs.     

Exact outage probability for equal gain combining with cochannel interference in Rayleigh fading

Equal gain combining (EGC) diversity has performance close to that of maximal ratio combining but at lower implementation complexity. We present a new outage performance analysis for EGC in mobile cellular radio systems that are limited by cochannel interference and undergo Rayleigh fading. We utilize a new model where interfering signals add in amplitude and phase across antenna array elements. In addition, the interfering signals may each have a different power. In comparing our analysis to an existing method, we find that: 1) as much as 1.5 dB difference in signal-to-interference ratio may exist at the same probability of outage; 2) the existing method can lead to overly optimistic outage performance prediction in certain situations.     

Mean-square crosstalk approach to CPM

The maximum-likelihood sequence estimator (MLSE) for continuous phase modulation (CPM) signals in an additive white Gaussian noise (AWGN) channel is a very efficient method of detection. This paper describes an extension of a relatively simple crosstalk approach for the performance analysis of linear quadrature receivers with cochannel interference (CCI) and adjacent channel interference (ACI) present to the MLSE receiver. Many CPM signals are analyzed, including those using new baseband modulating pulses. One of the new schemes allows an ACI signal to be 62 dB greater than the desired user signal at a frequency separation of one-and-a-half times the bit rate, with just a 2-dB degradation in required E_b/N₀     

天线阵CDMA系统中基于神经网络的盲空时信道估计

提出了天线阵ＣＤＭＡ系统中盲空时信道估计的约束优化神经网络模型,对其全局收敛性进行了分析,并对其性能进行了数值模拟。     

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     

Maximal-ratio combining architectures and performance with channel estimation based on a training sequence

Maximum-ratio combining (MRC) is a simple and effective combining scheme for adaptive antenna arrays to combat noise, fading, and to a certain degree, cochannel interference. However, it requires estimation of the spatial signature (i.e., the channel gain and phase at each antenna element) of the desired signal across the array. Assuming that this estimate is obtained by correlation using a known training sequence of K symbols embedded in the useful signal, we proceed to develop a fully analytical assessment of the impact of estimation error on the output signal-to-noise ratio (SNR) of the array. The originality of the approach revolves around the derivation of the distribution of the normalized SNR, that is the real SNR normalized to the ideal (i.e., perfect estimation) SNR. The end result is a set of distributions which can potentially reduce or in certain cases eliminate the need for simulation to determine certain design parameters such as array size, training sequence length, etc. These are then applied to find closed-form expressions for the outage probability and the error probability in differential phase-shift keying and quarternary phase-shift keying after training in uncorrelated Rayleigh fading.     

An investigation of the design of a log-periodic dipole array withlow side-lobe levels for broadcast applications

The problem of cochannel interference is addressed. The ability of the Numerical Electromagnetics Code (NEC2) computer program to predict the radiation pattern of an antenna array on a mast is investigated. The evaluation and validation of NEC2 is undertaken by the comparison of measured and computed radiation patterns of log-periodic dipole antenna arrays. Emphasis is placed on the correspondence of the side-lobe levels -30 to -40 dB down from the main lobe. This is important in the synthesis of an array with predictable null positions and depths     

On MMSE real-time antenna array processing using fourth-orderstatistics in the US cellular TDMA system

The antenna array processing problem in the reverse link of the current US digital cellular communication system is studied and higher-than-second-order-statistics (HOS) baseband processing is proposed as a possible candidate solution. The remarkable difference of our approach as compared to other existing similar techniques is the idea of the minimization of the mean squared error using fourth-order cumulants alone and nonblind criteria. A recursive Jacobi total least squares algorithm is used in the adaptive implementation to mitigate the effects of high error variance in the estimates of the cumulants based on sample statistics. The method is shown to be very effective in a fast fading environment with multiple cochannel interferers     

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.     

Combinations of an adaptive array antenna and a canceller ofinterference for direct-sequence spread-spectrum multiple-access system

In the realization of code-division multiple access based on a spread-spectrum communication system, i.e. spread-spectrum multiple access (SSMA), reduction of cochannel interference is an important problem. An adaptive array antenna system is proposed that includes a cancellor of cochannel interference, which can improve performance by a combination of temporal and spatial filtering. While the adaptive array suppresses interference sources with arrival angles different from those of the desired user, the adaptive digital filter-canceller rejects those whose arrival angles are the same as those of the desired user. The proposed system can achieve stable acquisition and low error rate of demodulated data even in a heavy-interference channel where a conventional array antenna system cannot achieve satisfactory acquisition     

Field measurements of an indoor high-speed QAM wireless systemusing decision feedback equalization and smart antenna array

This paper reports on field measurements of point-to-point indoor high-speed (10 Mbit/s to 30 Mbit/s at 5 Mbaud) wireless communications realized using a flexible multilevel quadrature amplitude modulation (M-QAM) testbed that features real-time equalization and smart antenna-array technology. The results from an extensive set of measurements, 59262 trials in all, performed without cochannel interference under various receiver configurations and wireless environments are presented and analyzed. The results underscore the dramatic potential for a system that optimally combines equalization and a smart antenna array. For example, using only 10 mW of transmit power, the system delivered 30 Mitts at an uncoded bit error rate (BER) of 10 ^-3 with 5% outage at a coverage radius of 20 in. For a lower data rate of 10 Mbitts, the coverage radius was increased to 32 in, the uncoded BER dropped below 10^-7, and the outage improved to 1%. The field measurements indicate that a 4-tap feedforward-filter decision-feedback equalizer with eight feedback-filter taps is sufficient to mitigate the intersymbol interference for typical indoor environments. They also show a significant gain when using a smart antenna array. For example, when transmitting between rooms at a 2% outage probability, the signal-to-noise ratio (SNR) improves by 8.3 dB when using two antennas instead of one antenna. Doubling the number of antennas to four provided an additional SNR improvement of 5.2 dB. The paper also presents simulation results that confirm the performance trends observed from the field measurements     

Modified pre-FFT OFDM adaptive antenna array with beam-space channel estimation

A modified pre-FFT OFDM adaptive antenna array with max-SNIR criterion is proposed, in which the CSI estimation is made on the beam-space. Simulation results show that the proposed scheme can outperform the element-space scheme, especially when strong cochannel interference is present.     

Outage probability of cellular radio systems using maximal ratiocombining in the presence of multiple interferers

In mobile radio systems, antenna diversity is used to combat fading and reduce the impact of cochannel interference. We derived a new expression for probability density functions of the signal-to-interference-plus-noise ratio and apply it to analyze the outage probability (OTP) for a maximal ratio combining diversity system when multiple cochannel interferers are present. Numerical results showing the impact of the number of antenna elements, the number of cochannel interferers, and signal-to-noise ratio on the OTP are presented. Simulation results validating the analytical results are also presented     

Effects of correlated shadowing signals on channel reuse in mobileradio systems

The authors present a study to evaluate the cochannel interference probability for the desired and interference signals which are correlated due to shadowing. The effects of correlation on the normalized reuse distance are investigated. A generalized expression for the cochannel interference probability is derived by combining uncorrelated (fast) Rayleigh fading and correlated (slow) log-normal signals. The cochannel interference probability for sectorized cell layouts is compared with the lower bound of cochannel interference probability for omnidirectional antenna systems. It is found that omnidirectional cell layouts cause the highest level of cochannel interference probability. An omnidirectional system requires a higher cluster size than a directional system to maintain acceptable cochannel interference     

Grating Lobe Reduction in a Phased Array of Limited Scanning

Amplitude and phase weighting at the subarray outputs alone causes grating lobes (GLs) in the array factor. A combined approach to disrupt the periodicity in the array is proposed to reduce the GLs. In this approach, three measures are simultaneously used. They are: (1) the optimized amplitude weighting at the subarray ports; (2) using the random subarray; and (3) the random staggering of the rows. The optimization was carried out by using genetic algorithms (GAs). The 24 (along X) times 32 (along Y) phased array was designed to verify the proposed approach. Along Y, the scan range is (-10deg, +10deg) and the subarrays are used. Along X, the scan range is (-45deg, +45deg) without using the subarrays. The comparison was done through array factor. In the whole two dimensional scan space, the simulated results show that the GL is -3.77 dB when using the conventional array, -4.28 dB when using (1) alone, -11 dB when using (2) alone, -14 dB when using (3) alone, -20 dB when using the combined approach proposed in this paper. This approach can be used for a phased array with limited scanning and for the digital beamforming antenna array with adaptive nulling.