A two-stage CMA-based receiver for blind joint equalization and multiuser detection in high data-rate DS-CDMA systems

The paper deals with the problem of blind mitigation of intersymbol interference (ISI) as well as multiple-access interference (MAI) in asynchronous high data-rate direct-sequence code-division multiple-access systems. A blind adaptive multiuser receiver based on the constant-modulus algorithm (CMA) is proposed, which demodulates each desired user by exploiting only the knowledge of its spreading code, without requiring estimation of the users's channels and timings. In order to overcome the CMA interference capture problem, which arises in a multiuser scenario, a two-stage adaptive receiver is adopted: In the first stage, partial MAI and ISI suppression is blindly achieved by exploiting the desired user signature structure properties; in the second stage, the residual MAI and the ISI are removed by using the CMA, and the information symbols of the desired user are reliably recovered. Theoretical analysis and simulation results show that the first stage is an effective blind adaptive strategy which allows the CMA detector in the second stage to lock on the desired-user symbol, at a particular delay. The proposed blind receiver achieves a significant performance gain in comparison with existing blind methods.     

Multiuser detectors with single-user parameter estimation onquasi-synchronous CDMA channels

We present simulated error rate and code delay/amplitude tracking performance for two recently developed detectors: the adaptive whitened matched filter, and the quasi-synchronous decorrelator. Rapid convergence (within 50 symbols) of the estimated amplitude and delay of the desired user is demonstrated based on an approximate joint maximum likelihood approach. The adaptive whitened matched filter exhibits better bit error rate performance than the decorrelator, but requires a complex eigenanalysis stage     

智能天线辅助的PN 码同步捕获新方法研究

为有效解决强干扰环境下长PN 码的同步捕获问题,研究了基于自适应滤波器的PN 码同步捕获方法,给出了基于自适应滤波器权矢量范数的同步捕获判决准则,与传统的基于均方误差的判决相比,这种方法可以大幅提高判决的正确概率。在此基础上,研究了智能天线权值与PN 码同步联合捕获算法,并对其性能进行了计算机仿真验证,结果表明这种空时联合的捕获算法可以有效实现低SINR 环境下的长PN 码捕获。     

Interference suppression to aid acquisition in direct-sequencespread-spectrum communications

The acquisition and tracking systems of a spread-spectrum receiver are probably the most critical components of the receiver, since if they fail to function properly, it is doubtful that the desired signal can be successfully detected. This means that the affect of interference (such as jamming) on the receiver while it is attempting to learn the correct phase position of the incoming code might be especially harmful, since the interference might not allow the receiver to acquire the signal. To address this problem, a narrow-band interference suppression filter is used to enhance the performance of a serial search acquisition scheme for a direct-sequence spread-spectrum receiver. Analytical expressions for the probabilities of error in both the search and lock modes are derived, and numerical results are used to illustrate the sensitivity of the receiver to various system parameters. It is shown that the presence of the rejection filter can be significantly improves the performance of the acquisition system     

Blind asynchronous multiuser CDMA receivers for ISI channels usingcode-aided CMA

A code-aided constant modulus algorithm (CMA) based approach is presented for blind detection of asynchronous short-code DS-CDMA (direct sequence code division multiple access) signals in intersymbol interference (ISI)/multipath channels. Only the spreading code of the desired user is assumed to be known; its transmission delay may be unknown. A linear equalizer is designed by minimizing the Godard/CMA cost function of the equalizer output with respect to the equalizer coefficients subject to the fact that the equalizer lies in a subspace associated with the desired user's code sequence. Constrained CMA leads to the extraction of the desired user's signal whereas unconstrained minimization leads to the extraction of any one of the active users. The results are further improved by using unconstrained CMA initialized by the results of the code-aided CMA. Identifiability properties of the approach are analyzed. Illustrative simulation examples are provided     

Dynamically Partitioned Hierarchical Constant Modulus Algorithm for Variable Delay Spread Channels

The Constant Modulus Algorithm (CMA) adopts the constant modulus (CM) criterion to minimize the deviation of the equalizer output from a fixed value. However it has two drawbacks: (1) slow rate of convergence (2) the likelihood of getting trapped in a local minimum. The problem gets even worse when the channel delay spread varies rapidly as the filter length cannot match the delay spread. If the filter length is significantly longer than the delay spread, the convergence rate can be slow. In this paper, we improve the performance of the standard CMA by using a dynamically partitioned hierarchical structure to organize the taps of a filter. The filter length is dynamically partitioned according to the delay spread such that they are tightly matched. Preferably, the length of a sub-filter is slightly longer than the delay spread of the channel. The performance evaluation is divided into two parts. In Part I, we do simulation runs for both the cost functions and cost surfaces comparing the standard CMA, where the filter length is significantly longer than the delay spread, and the dynamically partitioned CMA, where the filter length matches the delay spread. In Part II, an analysis is provided to show that the proposed partitioned scheme can speed up the convergence rate and reduce the cost function.     

Code timing acquisition for DS-CDMA in fading channels bydifferential correlations

We propose simple and efficient algorithms for the code timing acquisition in the direct-sequence code-division multiple-access communication system. The essential assumption is that a preamble or an unmodulated pilot channel is available for the desired user. Then the correlation matrix R(τ) of the sampled data, where τ is suitably chosen time lag, contains the timing information only of desired user, while the contributions of uncorrelated interferers and noise are suppressed out. Hence, compared to the conventional approach, more interference suppression is achieved. Coarse delay estimates are then obtained by a matched filter (MF) or multiple signal classification-type approaches. In the latter case, only L eigenvectors are computed, where L is the number of resolvable paths. If only one path exists, an additional procedure is proposed to both approaches, by which the estimation accuracy is greatly improved with negligible increase in computation. More precisely, the chip timing offset due to chip-asynchronous sampling can be determined by solving a system of two second-order polynomials for each chip interval. Therefore, only at most 2C hypotheses are needed, where C is the processing gain. All the proposed methods are computationally quite simple, containing mainly MF-operations, or at most computation of only few eigenvectors. Mean acquisition time analysis is carried out semi-analytically. Numerical experiments speaks for the possibility of achieving significant performance gains compared to conventional acquisition, especially in the presence of strong multiple-access interference, making them attractive options to be attached for the next generation mobile receivers     

The Linearly Constrained LSCMA for Blind Multi-user Detection

The least square constant modulus algorithm (LSCMA) is a popular constant modulus algorithm (CMA) because of its global convergence and stability. But the performance will degrade when it is affected by the problem of interference capture in the MC-CDMA system that has several constant modulus signals. In order to overcome this shortage, a linearly constrained LSCMA multiuser detection algorithm is proposed by using the spreading code of the desired user to impose linear constraint on the LSCMA. The proposed algorithm ensures the algorithm convergence to the desired user. Thus the performance of the system is improved. The simulation results demonstrate that the proposed algorithm offers faster convergence rate and provides better output signal-to-interference-plus-noise-ratio (SINR) and bit error rate (BER) performance compared with the traditional LSCMA.     

基于北斗系统的伪码捕获技术的研究

北斗卫星导航定位系统是我国自主研发的有源三维卫星定位与通信系统.北斗卫星信号的捕获是北斗用户接收机的关键技术之一,捕获性能的优劣直接影响着北斗导航系统的精度和实时性要求.本文基于北斗系统,在FFT并行捕获算法的基础上研究分析了一种相位补偿和同步块累加的伪码捕获算法,并对该算法的性能进行了分析.理论分析和仿真结果表明,该方案能够大大提高信噪比,提高信号的捕获效率.     

Low-complexity space-time processor for DS-CDMA communications

A novel wideband beamforming technique for cellular CDMA systems is presented in this paper. The proposed algorithm asymptotically provides the maximum SINR estimate of the signal with the desired code (SDC) by optimally combining desired signals from different paths and canceling strong multiuser access interference (MUAI). A two-dimensional (2-D) matched filter structure is used where not only different temporal samples of the matched filter output are processed but where those from matched filters connected to different antennas are processed as well. In contrast to previously proposed techniques, an exact code synchronization for the SDC is not required. The algorithm presented herein asymptotically provides the arrival time of the multipaths within a bit period and the optimum beamformers for extracting each of them. Space-time filters for combining the fingers across both space and time while canceling the MUAIs are constructed correspondingly. The instrumental property exploited by this technique is the fact that although the respective spectra of the SDC and MUAI components at the matched filter output are statistically identical, the respective spectra of their squared values differ. A simplified RAKE structure-based receiver is also proposed. The 2-D RAKE receiver considerably decreases the computations but requires a coarse SDC code synchronization. A technique to achieve coarse SDC code synchronization is also proposed     

Cell search in W-CDMA

In a CDMA cellular system, the process of the mobile station searching for a cell and achieving code and time synchronization to its downlink scrambling code is referred to as cell search. Cell search is performed in three scenarios: initial cell search when a mobile station is switched on, idle mode search when inactive, and active mode search during a call. The latter two are also called target cell search. This paper presents algorithms and results for both initial and target cell search scenarios for the wideband CDMA (W-CDMA) standard. In W-CDMA, the cell search itself is divided into five acquisition stages: slot synchronization, frame synchronization and scrambling code group identification, scrambling code identification, frequency acquisition, and cell identification. Initial cell search needs all five stages, while target cell search in general does not need the last two stages. A pipelined process of the first three stages that minimizes the average code and time acquisition time, while keeping the complexity at a reasonable level, is considered. The frequency error in initial cell search, which may be as large as 20 kHz, is taken care of by partial symbol despreading and noncoherent combining. Optimization of key system parameters such as the loading factors for primary synchronization channel, synchronization channel, and common pilot channel for achieving the smallest average code and time acquisition time is studied. After code and time synchronization (the first three stages), a maximum likelihood (ML)-based frequency acquisition method is used to bring down the frequency error to about 200 Hz. The gain of this method is more than 10 dB compared to an alternative scheme that obtains a frequency error estimate using differential detection     

基于软件的GPS信号捕获跟踪算法研究

对GPS软件接收机的捕获和跟踪部分进行了研究。对于捕获模块,通过对常用串行搜索捕获算法和并行频域搜索捕获算法的比较和分析,提出了一种新型并行码相位搜索捕获算法;对于跟踪模块,提出采用非相干延迟锁相环对码进行跟踪,同时采用Costas环对载波进行跟踪;最后,从捕获和跟踪的GPS信号中提取出导航电文,并根据电文中的参数计算出了用户的位置坐标,并在Matlab中对用户的位置坐标进行了定位解算验证。     

伪码同步技术研究及其FPGA实现

针对扩频通信系统中的伪码同步问题,分析了基于匹配滤波捕获跟踪算法的伪码数字化同步方案.围绕匹配滤波捕获算法的原理和仿真,对伪码捕获跟踪同步算法在FPGA中的具体实现进行了研究.实验结果表明,该同步算法在FPGA上实现简练,性能稳定,能很好满足通信系统对速度和精度的要求.     

Blind detection of asynchronous CDMA signals in multipath channelsusing code-constrained inverse filter criterion

A code-constrained inverse filter criterion based approach is presented for blind detection of asynchronous short-code direct sequence code division multiple access (DS-CDMA) signals in multipath channels. Only the spreading code of the desired user is assumed to be known; its transmission delay may be unknown. We focus on maximization of the normalized fourth cumulant of inverse filtered (equalized) data with respect to (w.r.t.) the equalizer coefficients subject to the equalizer lying in a subspace associated with the desired user's code sequence. Constrained maximization leads to extraction of the desired user's signal, whereas unconstrained maximization leads to the extraction of any one of the active users. Illustrative simulation examples are provided     

Filterbank-based blind code synchronization for DS-CDMA systems in multipath fading channels

We present a filterbank approach to blind code synchronization for asynchronous direct-sequence (DS) code-division multiple-access (CDMA) systems. The key idea of the proposed scheme is to first pass the received signal through a bank of filters, which are designed to enhance signals of interest and suppress interference/noise, and then to derive the code timing from the filtered data. The only required knowledge by the proposed filterbank scheme is the spreading code of the desired user. It can be used in various environments, including frequency-nonselective and frequency-selective, time-invariant, and time-varying fading channels. It can deal with colored channel noise and unmodeled interference, such as inter-cell interference (ICI) and narrowband interference. It has relatively low complexity and can be readily implemented using standard adaptive algorithms. We show that under mild conditions, the proposed scheme yields statistically consistent [in signal-to-noise ratio (SNR)] code timing estimates, irrespective of the strength of the interference and with only a finite number of data samples. We also derive an unconditional Cramer-Rao bound (UCRB), which serves as a lower bound for all unbiased blind code synchronization schemes. Numerical results indicate that the proposed scheme compares favorably with a popular subspace-based method in terms of user capacity, near-far resistance, and robustness to time-varying fading and unmodeled interference.     

A PREPROCESSING LS-CMA IN HIGHLY CORRUPTIVE ENVIRONMENT

A fast preprocessing Least Square-Constant Modulus Algorithm (LS-CMA) is proposed for blind adaptive beamforming. This new preprocessing method precludes noise capture caused by the original LS-CMA with the preprocessing procedure controlled by the static Constant Modulus Algorithm (CMA). The simulation results have shown that the proposed fast preprocessing LS-CMA can effectively reject the co-channel interference, and quickly lock onto the constant modulus desired signal with only one snapshot in a highly corruptive environment.     

一种抗远近效应的扩频码同步方法

给出了一种直扩通信系统的码同步方法.该方法的基本思想是首先完成扩频码的检测,然后比较将检测向量与期望用户扩频向量的相关值,得到接收信号中期望用户扩频码的相位,继而完成同步.这种方法的运算量较小,且不需要采用训练序列.文中给出了存在强多址干扰时,在高斯信道和瑞利衰落信道下的数值仿真结果.     

Blind adaptive interference suppression for the near-far resistantacquisition and demodulation of direct-sequence CDMA signals

Two key operations required of a receiver in a direct-sequence (DS) code division multiple access (CDMA) system are the timing acquisition of transmissions that are starting up or have lost synchronization, and the demodulation of transmissions that have been acquired. The reliability of both these operations is limited by multiple-access interference, especially for conventional matched filter-based methods, whose performance displays an interference floor and is vulnerable to the near-far problem. Recent work has shown that, provided timing information is available for a given transmission, it can be demodulated reliably using blind or training-sequence-based adaptive interference suppression techniques. These techniques are near-far resistant, unlike the matched filter demodulator, and do not require explicit knowledge of the interference parameters, unlike nonadaptive multiuser detectors. In this paper, we present a blind adaptive interference suppression technique for joint acquisition and demodulation, which has the unique feature that the output of the acquisition process is not simply the timing of the desired transmission, but a near-far resistant demodulator that implicitly accounts for knowledge of the timing and amplitudes of all transmissions to suppress the multiple-access interference. The only knowledge required by the scheme is that of the desired transmission's signature sequence, so that it is amenable to a decentralized implementation. On the other hand, it can be efficiently implemented as a centralized scheme in which the bulk of the computations for the adaptation are common to all transmissions that need to be acquired or demodulated     

Acquisition performance of an adaptive receiver for DS-CDMA

This paper presents a single-user code timing estimation algorithm for direct-sequence code-division multiple access that is based on processing the weight vector of an adaptive filter. The filter weight vector can be shown to adapt in the mean to a scaled time-shifted version of the spreading code of the desired user. Therefore, our algorithm requires very little side information in order to form its estimate. The acquisition performance of the algorithm is investigated when the filter is adapted using the least mean square (LMS) or the recursive least square (RLS) algorithm. The proposed algorithm is shown through experimental results to be resistant to the near-far problem when the RLS adaptation algorithm is used, but not when the LMS algorithm is used. However, the performance of this code-acquisition technique is still substantially better than the traditional correlator-based approach, even when the computationally simple LMS algorithm is used. As an extension to the basic timing estimator algorithm, we consider the effect of frequency synchronization error on the performance of the timing estimate. As expected, frequency-offset error degrades the performance of the timing estimate. However, a modified version of the adaptive filter is presented to combat this effect     

应用LMS自适应滤波器实现直接序列扩频系统伪码同步

在直扩系统中,伪随机序列的同步必须在数据解调判决前完成。介绍了一种新型的同步方法,这种方法利用LMS自适应滤波器来检测收发端的码延迟,具有比传统的相关检测更优良的性能。更重要的是,这种方法根据稳定后的滤波器工作特性,可直接实现PN码的跟踪,不需要额外电路。