Acquisition for DS/CDMA systems with multiple antennas in frequency-selective fading channels

A generalized code acquisition scheme for direct-sequence code-division multiple-access systems with multiple antennas is proposed over frequency-selective fading channels. The proposed scheme is developed on the framework of a generalized configuration of multiple antennas and correlators. The nonconsecutive search method is generalized and extended to multiple antenna systems to exploit multipath signals in improving acquisition performance over frequency-selective fading channels. The proposed scheme also adopts a hybrid decision strategy to make effective decisions on acquisition. The mean acquisition time performance of the proposed acquisition scheme is analyzed and evaluated in frequency-selective Rayleigh-fading channels with general multipath delay profiles and spatial-fading correlations. The effects of nonconsecutive search on mean acquisition time are investigated for various channel environments, and the optimal choice of decision strategy is discussed. Furthermore, effects of various configurations of multiple antennas and correlators, decision thresholds, and correlation interval on the performance are also investigated.     

Differentially coherent combining for double-dwell code acquisition in DS-CDMA systems

The use of differentially coherent combining is proposed to improve the performance of a double-dwell acquisition system by increasing the reliability of a decision in the verification stage. The detection and mean acquisition time performance of the acquisition scheme with the proposed combining scheme is analyzed in frequency-selective Rayleigh fading channels, and compared with that of two previously published double-dwell acquisition schemes based on long correlation intervals and noncoherent combining. It is shown that the proposed acquisition scheme outperforms the previous ones, and that the performance improvement increases as the frequency offset increases.     

Direct-sequence spread-spectrum parallel acquisition innonselective and frequency-selective Rician fading channels

This work presents the performance of the direct-sequence spread-spectrum (DS-SS) parallel acquisition system, previously proposed by the authors (1989, 1990), for nonselective and frequency-selective Rician (i.e. specular plus Rayleigh) fading channels. The acquisition system utilizes a bank of parallel I-Q noncoherent matched filters for the search mode, and a coincidence detector for the verification mode. The probabilities of detection and false alarm are derived and the mean and variance of the acquisition time are evaluated as a measure of the system performance. The nonselective channel is a Rayleigh fast fading channel, while the frequency-selective channel model is the so-called wide sense stationary uncorrelated scattering (WSSUS), selective only on frequency. These channels are typical for aircraft-satellite and line-of-sight (LOS) communications     

Direct sequence spread spectrum matched filter acquisition infrequency-selective Rayleigh fading channels

This paper presents the performance of a matched filter acquisition scheme in nonselective and frequency-selective Rayleigh fading channels. This acquisition scheme employs a reference filter to estimate the variance of the interference at the output of the detecting matched filter. Expressions for probability of detection and probability of false alarm are derived and the mean acquisition time is evaluated for various channel and system parameters. The results presented show the relationship between channel and system parameters and the system performance     

Use of multiple antennas for DS/CDMA code acquisition

A generalized acquisition scheme is proposed for direct sequence code-division multiple-access systems with multiple antennas. The proposed scheme employs grouping of multiple antennas as a means of a tradeoff between two important factors determining the mean acquisition time, combining gain and search time. The performance of the proposed acquisition scheme is analyzed in frequency-selective Rayleigh-fading channels with consideration of spatial correlations. Numerical results show that the use of the largest number of antenna groups is preferable to reducing the mean acquisition time at low signal-to-interference ratio (SIR) values. At high SIR values, on the contrary, the mean acquisition time is found to increase in proportion to the number of antenna groups. In a typical environment, the presence of spatial correlation is shown to increase or decrease the mean acquisition time within 50% compared with the uncorrelated fading case.     

Adaptive Hybrid Acquisition of PN Sequences Based on Automatic Multipath Cancellation in Frequency-Selective Rayleigh Fading Channels

A hybrid PN-code acquisition scheme is proposed and studied for direct sequence code division multiple access communications in frequency-selective Rayleigh fading channels. The most important feature of the proposed system is its ability to dynamically estimate the number of multipath signals that may lie in the reference channel. The basic idea of the considered algorithm is to sort the partial correlations and carry out successive censoring tests by updating the noise estimation until the first path synchronization. Exact expressions for the probability of false alarm and the mean acquisition time are derived. The performances of the proposed system are then studied and compared with those of the adaptive hybrid acquisition processor (AHAP) and the ordered statistic hybrid acquisition processor (OSHAP). The effects of various channel parameters on the acquisition performance, namely the number of resolvable paths, the multipath intensity profile and the signal-to-noise ratio are also investigated. Numerical results show that the proposed acquisition scheme outperforms the conventional ones which are based on AHAP and OSHAP algorithms.     

一种基于自动删除算法的PN码捕获技术研究

针对多径干扰对频率选择性信道下伪码捕获性能的影响,提出了一种基于自动删除算法的PN码捕获方法。该串行相关捕获系统通过一个基于排序数据方差的自动删除单元平均恒虚警检测器来删除参考滑窗中的干扰样本,并且不需要多径干扰数目的先验信息。对系统的平均捕获时间及检测性能进行了计算与仿真,与传统的自适应捕获方法相比,改进后捕获算法的性能得到较大的改善。     

Iterative Channel Estimators in V-BLAST OFDM Systems

An iterative pilot-symbol aided modulation (PSAM) channel estimation approach is proposed for vertical Bell Laboratories layered space-time (V-BLAST) orthogonal frequency division multiplexing systems operating on frequency-selective fading channels. Since the signals at the receive antennas are the superposition of signals from multiple transmit antennas, accurate channel estimates are crucial for good error performance. Furthermore, the time selectivity of the fading channels leads to inter-carrier interference (ICI). While ICI can be ignored for slow fading channels, it should be mitigated for fast fading channels. This paper proposes an ICI mitigation scheme for time-varying channels. We also propose an iterative channel estimator with low-complexity. Simulation results demonstrate the usefulness of the proposed algorithm on frequency-selective fading channels.     

Serial acquisition performance of single-carrier and multicarrier DS-CDMA over Nakagami-m fading channels

We investigate the issue of pseudo noise (PN) code acquisition in single-carrier and multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) systems, when the channel is modeled by frequency-selective Nakagami-m (1960) fading. The PN code acquisition performance of single-carrier and MC DS-CDMA systems is analyzed and compared when communicating over Nakagami-m fading channels under the hypothesis of multiple synchronous states (H/sub 1/ cells) in the uncertainty region of the PN code. In the context of MC DS-CDMA, the code acquisition performance is evaluated, when the correlator outputs of the subcarriers associated with the same phase of the local PN code replica are noncoherently combined by using equal gain combining (EGC) or selection combining (SC) schemes. The performance comparison of the above mentioned schemes shows that the code acquisition performance of the MC DS-CDMA scheme, especially when using the EGC scheme, is more robust, than that of single-carrier DS-CDMA schemes communicating over the multipath Nakagami-m fading channels encountered. However, our code acquisition performance comparison also shows that if the detection threshold was set inappropriately, the performance might be degraded, even if the channel fading becomes less severe.     

DSSS-MIMO结构的标签延迟发射分集方法

针对频率选择性衰落多输入多输出(Multiple-Input Multiple-Output,MIMO)结构,提出在各发射天线通道引入直接序列扩频(Direct-Sequence Spread-Spectrum,DSSS)操作以便实现信道的矢量化,然后提出了一种标签延迟发射分集方法和相应的实现频率选择性衰落无线信道估计的盲方法.仿真结果表明了上述标签延迟发射分集方法的有效性及其信道盲估计方法的性能.     

二维扩频系统中时频码片级差分检测的性能分析

时变频率选择性衰落信道中,针对二维扩频系统提出了一种时频码片级差分检测方法,并进行了误码率分析,同时进行了仿真验证.分析和仿真结果表明,时频码片级差分检测同时具有抵抗信道频率选择性和时间选择性衰落的能力,在选择性衰落信道中的性能优于平坦衰落信道中的性能.通过性能对比发现,时频码片级差分检测相对多载波扩频码片级差分检测能够更加有效地降低了载波间干扰对系统性能的影响.     

Intercarrier Interference Suppression for OFDMA Uplink in Time- and Frequency-Selective Fading Channels

This paper investigates intercarrier interference (ICI) suppression and channel estimation for the uplink of an orthogonal frequency-division multiple-access (OFDMA) system in a time- and frequency-selective fading channel. In such a doubly selective channel, channel variations within each OFDMA block disrupt the orthogonality among subcarriers and leads to ICI. We develop an appropriate signal model for the OFDMA uplink in a doubly selective fading channel and propose a minimum mean square error (MMSE) scheme and an MMSE successive detection (MMSE-SD) scheme to suppress ICI. It is shown that the MMSE scheme is the optimal linear scheme in terms of maximizing achievable data rate and that the MMSE-SD scheme is able to further remove ICI and exploit the Doppler diversity embedded in time-varying channels. As an essential component in ICI suppression, channel estimation is also considered. A basis expansion model (BEM) is formulated for the OFDMA uplink channel, and a pilot-aided channel-estimation algorithm is developed to track users' channels in the time domain. Simulation results are presented to illustrate the overall performance improvements that can be obtained from using the proposed ICI suppression and channel-estimation schemes.      

A frame synchronization method for adaptive array antennas in digital mobile communications

This paper proposes a frame synchronization method for an adaptive array antenna (AAA) used in digital mobile communications. The proposed scheme, which is based on the maximum likelihood estimation (MLE), calculates error signals by subtracting a training sequence convolved with an estimated channel impulse response from the AAA outputs and searches for the timing that minimizes the mean squared errors. Because the proposed scheme can effectively exploit delayed paths of the desired signal, it can improve the synchronization performance on frequency-selective fading channels. Computer simulations show that the proposed scheme can operate properly even in interference-rich fading environments and that it can significantly improve the synchronization performance.     

Code acquisition of DS/SS signals in fading channels using an LMSadaptive filter

The mean acquisition time performance for a direct-sequence spread-spectrum (DS/SS) code acquisition technique utilizing a finite impulse response (FIR) least-mean-square (LMS)-adaptive filter is investigated in fading conditions. The optimum tap-weight vector for the adaptive filter is derived and computer simulation results are presented to assess the acquisition performance for the system. It is shown that fading causes a slight degradation to the mean acquisition time (less than 3-dB signal-to-noise ratio (SNR) degradation) which is far less than that incurred by other systems utilizing matched filters (about 10 dB). In frequency selective fading channels, the adaptive nature of the proposed scheme is exploited to acquire the strongest received path and as the paths fade the new strongest available path is acquired     

Noncoherent hybrid parallel PN code acquisition for CDMA mobilecommunications

This paper proposes a noncoherent hybrid parallel pseudonoise (PN) code acquisition scheme for code-division multiple access (CDMA) mobile communication systems and analyzes the effect of multiple access interference (MAI) on the code acquisition performance for Rayleigh and Rician fading channels. The hybrid acquisition scheme combines parallel search with serial search to cover the whole uncertainty region of the input code phase. It has a much simpler acquisition hardware structure than the total parallel acquisition and can achieve the mean acquisition time slightly inferior to that of the total parallel acquisition in the case of severe MAI; on the other hand, it provides the flexibility in the tradeoff between the mean acquisition time and system complexity if no MAI is considered. The closed-form expressions of the detection and false-alarm probabilities and mean acquisition time are derived. Numerical analysis quantifies the severe performance degradation of code acquisition due to both MAI and channel fading, and demonstrates the dependence of the increase of mean acquisition time (due to MAI) on the number of users in the CDMA system, system design parameters, and channel fading statistics     

Parallel acquisition of multicarrier direct-sequence spread-spectrum signals in fading and partial-band interference

The frequency diversity of multicarrier direct-sequence signaling can potentially offer robust performance in frequency-selective channels. The paper focuses on the acquisition of multicarrier signals in channels containing fading and partial-band interference. The maximum-likelihood decision rule for parallel acquisition in frequency-selective fading and partial-band interference is derived. Several simpler, near-optimal decision rules are also discussed. The performance of these decision rules is compared to that of equal-gain combining for multicarrier acquisition. Results show that the decision rules designed specifically for partial-band interference give significantly better performance. Methods of acquisition with a limited number of correlators are also discussed. Finally, the potential benefits of estimating the signal strength on each subcarrier prior to acquisition are examined.     

Acquisition performance of CDMA systems with multiple antennas

This paper discusses the code-acquisition problem for preamble search in the code-division multiple-access (CDMA) uplink when multiple antennas are employed for signal reception. The multiple antennas are used either in the form of antenna array for beamforming or in the form of antenna diversity. The performance of a maximum-likelihood (ML) code-acquisition technique utilizing beamforming is analyzed by considering the detection, miss, and false-alarm probabilities. This analysis is based on the statistics of the CDMA noncoherent demodulator and beamformer output. In the case of a code-acquisition scheme utilizing antenna diversity, the probability density function of a decision variable is analytically derived for multiple antennas. The expressions for the probabilities of detection, miss, and false alarm are also presented. The acquisition performance and mean acquisition time for both code-acquisition techniques are numerically evaluated and compared. The code-acquisition approaches with multiple antennas are shown to provide much better performance than the corresponding scheme with a single antenna in a frequency-selective fading channel.     

空时旋转不变性编码技术及其频率选择性信道盲估计方法

针对处于频率选择性衰落环境中的垂直型贝尔实验室分层空时(Vertical Bell iabs LAyered Space-Time,V-BLAST)结构,该文首先对其进行联合空时符号分组,然后提出了一种新颖的空时旋转不变性编码技术(Space-Time Rotational-Invariance Coding Technique,STRICT)。通过巧妙利用上述编码技术中的旋转不变性性质,该文继续为V-BLAST结构提出了一种多输入多输出(Multiple-Input Multiple-Output,MIMO)频率选择性无线信道的盲估计方法。仿真结果表明了该文新颖STRICT的有效性及其信道盲估计方法的性能。     

LS—CMA在MIMO—OFDM频域盲信源分离中的应用

源分离(SS)是M IMO系统接收的关键技术,而盲源分离(BSS)又是其中的一个研究热点。目前最小二乘常模(LS-CM)BSS算法[1~3]都是针对频率非选择性衰落的时域M IMO信道。本文提出了LS-CMA在M IMO-OFDM系统频选信道频域盲源分离中的应用,计算机仿真表明,M IMO-OFDM系统频域盲信源分离的最小二乘常模算法能够获得很好的性能以及较快的收敛性。     

Space-Time Coded Beamforming for DS CDMA Downlink

1　Introduction　 In 3G mobile system, Code Division Multiple Access(CDMA) is a promising multiple access techniques. It iswell know that BeamFormin ( BF), Space Time Coding(STC) and Multiuser Detection(MUD) techniques in CDMAsystem can effectively resist Multi Path Interference (MPI),MAI and near far effects. So the system capacity will be im proved greatly by employing these techniques[1,2,8]. In Ref.[3] introduces Water Filling(WF) theory into STC to assignth…