CDMA系统智能天线盲自适应波束形成

给出了基于最大化信号对干扰和噪声的功率比(MSINR)准则的盲自适应波束形成在线递推算法,旨在提高DS-CDMA系统中智能天线的性能.将MSINR准则转化为一种新的等价的无约束准则函数,通过最大化此准则函数来在线估计最优波束形成向量.算法直接利用了DS-CDMA系统解扩前和解扩后的观测信号,无须训练序列.通过在线迭代估计,降低了计算波束权值的复杂度.不同通信环境中的仿真结果表明,该算法具有较高的估计精度和很好的抗多址干扰能力.     

Fast adaptive blind beamforming algorithm for antenna array in CDMA systems

In this paper, the maximum signal-to-interference-plus-noise ratio (MSINR) beamforming problem in antenna-array CDMA systems is considered. In this paper, a modified MSINR criterion presented in a previous paper is interpreted as an unconstrained scalar cost function. By applying recursive least squares (RLS) to minimize the cost function, a novel blind adaptive beamforming algorithm to estimate the beamforming vector, which optimally combines the desired signal contributions from different antenna elements while suppressing noise and interference, is derived. Neither the knowledge of the channel conditions (fading coefficients, signature sequences and timing of interferers, statistics of other noises, etc.) nor training sequence is required. Compared with previously published adaptive beamforming algorithms based on the stochastic-gradient method, it has faster convergence and better tracking capability in the time-varying environment. Simulation results in various signal environments are presented to show the performance of the proposed algorithm.     

A novel adaptive beamforming algorithm for antenna array CDMA systems with strong interferers

Blind beamforming based on the maximum signal-to-noise ratio (MSNR) can improve the performance of an array system only when the processing gain of the given code-division multiple-access (CDMA) system is high enough such that the desired signal can become dominant after despreading (see Choi, S. and Shim, D., IEEE Trans. Veh. Technol., vol.49, p.1793-1806, 2000; Choi, S. and Yun, D., IEEE Trans. Antennas Propagat., vol.45, p.1393-1404, 1997). We consider a maximum signal-to-interference-plus-noise ratio (MSINR) beamforming. The MSINR performance criterion is chosen to deal with strong interferers effectively. It is shown that blind MSINR beamforming is possible by directly utilizing the input and output signals of correlators of the CDMA systems. In addition, we propose an adaptive beamforming algorithm at a lower computational complexity - about O(7.5N) - where N is the number of antenna elements of the array system. Simulation results are presented in various signal environments to show the performance of the proposed adaptive algorithm.     

基于自适应拟牛顿法的CDMA天线阵列波束形成

针对于CDMA系统自适应天线阵列,提出了一种新的盲自适应MSINR(Maximum Signal-to-Interference plus noise ratio)波束形成算法.首先,将MSINR准则转化为一种新的无约束损失函数,并且从理论上分析该损失函数的性质.然后,应用自适应拟牛顿方法得到在线迭代波束形成算法.该算法无需训练序,而是利用CDMA信号自身的结构特点,结合空间处理提高了系统性能.最后,给出了仿真结果,表明算法具有较快的收敛速度和良好的动态跟踪能力.     

Adaptive MMSE receiver with beamforming for DS/CDMA systems

The minimum mean-squared error (MMSE) receiver is a linear filter which can suppress multiple access interference (MAI) effectively in direct-sequence code-division multiple-access (CDMA) communications. An antenna array is also an efficient scheme for suppressing MAI and improving the system performance. In this letter, we consider an adaptive MMSE receiver in conjunction with beamforming in CDMA systems employing an antenna array. The proposed structure is featured as a low complexity receiver, which adapts the MMSE filter coefficients and the beamforming weights simultaneously. However, it does require the channel state information and the direction of arrival (DOA) of the desired user signal. As a result, we propose two adaptation methods to perform joint channel estimation and signal detection without any training sequence. It is demonstrated that the two proposed methods achieve similar bit-error-rate performance. More importantly, their performance degradation compared with the case with perfect channel information is small.     

Time slot allocation in CDMA/TDD systems for mobile multimediaservices

An important issue in wireless multimedia communications is to cope with the traffic asymmetry between uplink and downlink. The asymmetrical slot allocation in the code division multiple-access systems with time-division duplex mode (CDMA/TDD systems) can be a good solution for this problem. However, the level of traffic asymmetry can be significantly different from cell to cell. In this letter, we calculate the utilization of a CDMA/TDD system for the multicell model. In addition, we suggest an optimal slot allocation that maximizes the frequency utilization     

Performance analysis of forward link beamforming techniques forDS/CDMA systems using base station antenna arrays

The performance of DS/CDMA systems using forward link beamforming and RAKE diversity combining is investigated analytically in frequency selective fading channels, whereas previous studies have resorted to Monte Carlo simulations in flat fading channels. The capacity of DS/CDMA systems is considerably improved by employing forward link beamforming     

Reverse-link capacity of power-controlled CDMA systems with beamforming

In this paper, reverse-link capacity, in terms of user capacity and Erlang capacity, of a direct-sequence code-division multiple-access (DS-CDMA) system with the use of beamforming is investigated. Signal-to-interference ratio (SIR)-based power control is assumed and both transmit and receive beamforming are considered. Instead of using tedious iterative methods to evaluate user capacity, a simple closed-form capacity expression with respect to antenna gains, a target SIR, and the CDMA processing gain is derived. Numerical results indicate significant capacity improvement with beamforming. The impact of the estimation errors of arrival angles on the capacity is examined. The joint use of a RAKE receiver and beamforming is investigated and the capacity expression for CDMA systems with multiclass services is also derived.     

Comparison of time slot allocation strategies for CDMA/TDD systems

The traffic (load) asymmetry between uplink and downlink is a remarkable traffic characteristic in cellular mobile multimedia communications. The code division multiple access system with time division duplex mode (CDMA/TDD system), adopting unbalanced slot allocation between uplink and downlink, is a good solution for this traffic asymmetry. However, the level of traffic asymmetry may be significantly different from cell to cell. In this paper, we investigate a slot allocation strategy (DA strategy), by which each cell has its own slot allocation according to the level of traffic asymmetry. We compute the system capacity with DA strategy and find out the optimal slot allocation for the system. We also compare the maximum capacity to that with another strategy (SA strategy), by which all cells have the same slot allocation. As a result, this paper shows that the system with DA strategy outperforms the system with SA strategy in the aspect of capacity     

Hot-spot traffic relief with a tilted antenna in CDMA cellularnetworks

Direct-sequence code division multiple-access (DS-CDMA) cellular networks are highly promising in terms of their potential to provide more capacity than an advanced mobile phone system (AMPS). However, heterogeneous traffic loading causes traffic congestion in a CDMA hot-spot. This paper presents a tilted antenna mechanism for sectored cells in CDMA cellular networks to relieve the congestion in a hot-spot sector. The fixed antenna-tilted mechanism, which only tilts the hot-spot antenna, can provide the merit of traffic balancing. Besides, we design a dynamic antenna-tilted mechanism in which tilting the antennas of the hot spot and its adjacent cell sectors is based on varying the signal-to-noise ratio (SNR). The dynamic mechanism can automatically tilt the antenna corresponding to the variation of traffic. Consequently, more capacity can be provided than in a fixed tilting mechanism, which only tilts the hot-spot antenna. Another benefit is the traffic-balancing effect with a tilted-antenna mechanism that reduces the transceivers of a hot-spot base station. Therefore, extra facilities are unnecessary for the hot spot than for a normal or light traffic sector     

Base-station antenna array receiver with interference cancellationfor wideband CDMA

The authors present a combining technique of adaptive array antenna and multiple access interference (MAI) cancellation for increasing system capacity in wideband code division multiple access (W-CDMA) systems. From the simulation results, an improvement in system performance proportional to the number of antennas is observed and additional improvement is obtained due to the interference cancellation     

时分复接码分多址系统中预瑞克与空时编码技术

提出了一种适用于时分复接码分多址系统的发射分集，即预瑞克与空时格状编码调制相结合方案。采用预瑞在代替瑞克，在接收机中采用单径瑞克就可实现多径分集，可以简化接收机结构和节约费用。计算机仿真结果表明，在多径衰落下，所提出方案的性能比仅采用空时码好。在最小均方误差意义下预瑞克抽头数应与多径数相同。所提出新型星座图－8APK（振幅相位键控）可以用来构造了一个空时码，此空时码性能比由8PSK构造的具有相同结构的空时码好。     

TDD CDMA通信系统中二维角度、时延和极化参数的可分离估计方法

在研究一维角度、时延和极化联合估计(JADPE)方法基础上,提出一种多径信道下的二维角度、时延和极化参数联合估计方法.该方法将极化敏感阵列天线和TDD CDMA系统结合起来,研究了TDD CDMA系统多用户、多径情况下联合参数估计问题,有效实现了TDD CDMA系统二维角度/时延/极化参数的高分辨联合估计.该方法通过引...     

Adaptive transmitted distributed antenna selection strategy in distributed antenna systems with limited feedback beamforming

An adaptive transmitted distributed antenna (DA) selection strategy in distributed antenna systems (DAS) with limited feedback (LFB) beamforming is proposed. By maximising the upper bound of ergodic capacity in DAS with LFB beamforming, the proposed strategy can adaptively adjust the number of transmitted DAs. Simulation results show that the proposed strategy achieves greater ergodic capacity than the strategy with a fixed number of transmitted DAs.     

Lower bound on normalised interference mean and standard deviationfor CDMA systems using antenna arrays

A lower bound on the normalised mean and standard deviation of the interference for the reverse link of CDMA systems using linear antenna arrays is obtained. The PDF of the normalised interference is also obtained for some cases     

Blind equalization in antenna array CDMA systems

Multipath induced interchip-interference (ICI) alters waveforms of transmitted signals and presents a major obstacle to direct-sequence (DS) code-division-multiple-access (CDMA) communications. For systems with aperiodic pseudorandom (PN) spreading sequences, the primary way to counter fading is through employing RAKE receivers that enhance the signal-to-interference ratio (SIR) by combining multipath signals from the desired user. In this paper, we formulate a discrete-time model for antenna array CDMA systems and study the 2-D RAKE receiver problem by casting it into an optimum vector FIR equalizer design and estimation framework. A novel aspect of the present work is the full exploitation of the potential of 2-D RAKE receivers without requiring any detailed knowledge of the multipath channels     

Capacity analysis in CDMA distributed antenna systems

In this letter, the effect of maximal ratio combining (MRC)-based macrodiversity on the reverse-link and forward-link capacity in code division multiple access (CDMA)-distributed antenna systems is analyzed. The concept of virtual cell is illustrated, and the analytical outage probability expressions are derived. The present investigation shows that on the reverse link, the interference can be suppressed greatly with macrodiversity, which leads to a significant increase in capacity. However, on the forward-link, it is proven that if simulcasting is used in CDMA-distributed antenna systems, the forward-link capacity cannot increase with macrodiversity whatever power allocation scheme is adopted. Based on the analysis of the cause of capacity loss, a new transmission scheme is further presented and the optimal power allocation scheme is derived. It is shown that, in this case, the forward-link capacity increases rapidly with the number of involved distributed antennas.     

Active interference suppression in CDMA overlay systems

Communication networks involving the overlay of spread-spectrum systems an narrower band services are of increasing interest as a means of producing greater efficiencies and flexibility in the use of the radio spectrum. Although spread-spectrum systems enjoy a natural immunity to interference from narrowband sources, their performance in the presence of such interference can be significantly enhanced by active suppression techniques. The study of this problem has elicited a very rich body of methodology, which has progressed over nearly 25 years from some of the simplest signal processing paradigms to some of the most advanced. This paper provides an overview of a number of these techniques, most of which have been developed over the past decade. In particular, a progression of techniques is described, in which successively more information about the spread-spectrum signal and interference is used to make improvements on the interference suppression capability via more advanced signal processing methods. These include linear predictive methods that make use of the spectral properties of the spread-spectrum and narrowband signals, nonlinear predictive methods that make use of the spectra and first-order probability distribution of these signals, linear code-aided methods that make use of the spreading codes of the signals of interest and the second-order statistics of the narrowband interference, and finally, a maximum-likelihood code-aided technique that makes use of essentially all that is known about the useful signals and interference. Performance comparisons show that moving up this progression of improved modeling is rewarded with performance gains that can be quite significant     

Performance analysis of beamforming in two-hop AF relay networks with antenna correlation and interference

This paper investigates the performance of a two-hop amplify-and-forward (AF) relay network with antenna correlation and co-channel interference (CCI) over Rayleigh fading channels. In this network, the relay has multiple antennas while both the source and destination are equipped with a single antenna. By assuming that the statistical channel state information (SCSI) is available at the relay, the maximal output signal-to-interference-plus-noise ratio (SINR) is first obtained. Then, with the help of the specialized functions, the analytical expressions for the outage probability (OP), Ergodic capacity and average symbol error rates (ASERs) of the considered AF relay network are all derived. Moreover, the asymptotic analysis at high signal-to-noise ratio (SNR) is also presented to reveal the diversity order and array gain of the relay system. Finally, computer simulations are given to confirm the validity of the theoretical analysis and indicate the effects of antenna correlation and interference on the system performance.