基于LS的OFDM零中频接收机IQ不平衡数字补偿技术

零中频接收机已成为未来无线终端发展潮流,但是零中频的结构会引入较大的射频损伤;本文首先介绍了射频I/Q不平衡时对高斯以及频率选择性信道下OFDM接收机性能的影响;通过特殊导频设计,解耦合I、Q路的相互影响;如此可以方便地估计和补偿射频I/Q不平衡对高斯以及频率选择性信道的影响。仿真表明本文所示方法大大提高了OFDM零中频接收机的性能。     

正交通道中I/Q不平衡估计与补偿研究

I/Q通道普遍存在于信号处理的正交调制和解调过程中。严格的正交过程要求I/Q两个通道的增益和时延完全一致且I/Q两路中都不存在直流分量,同时还要求I路和Q路的本振信号的相位精确的相差90°。然而,在实际的信号处理过程中,难免会存在误差和不匹配的情况。基于I/Q通道不平衡的基础,研究各种不平衡因子对I/Q通道性能的影响。利用提出的补偿算法,在对存在不同幅度和相位不平衡的情况进行仿真,仿真结果表明在不同的不平衡条件下,该算法都能够有效地补偿不平衡,改善了系统性能。     

宽带无线通信系统的零中频接收机设计

零中频(ZIF)接收机具有体积小、成本低、易于集成和扩展等优点,是宽带无线通信接收机的重要研究方向.本文首先论述了零中频接收机的系统架构,并分析了零中频接收架构所引入的本振泄漏、直流偏差、I/Q不平衡等问题及补偿算法,提出了一种可行的LTE零中频接收机设计方案.仿真结果表明,该方案能够很好地解决现有零中频接收机技术存在的问题,显著提高LTE宽带接收机的性能.     

基于统计特征的 ＩＱ 不平衡高效校正算法

针对零中频接收机的I/Q不平衡问题,提出了一种数字域的基于统计特征的盲估计IQ不平衡校正算法,该算法利用具有循环对称特性的信号的统计特征,直接估计出系统的不平衡参数,进而完成IQ不平衡校正,相比于传统的算法,本算法具有计算复杂度低,校正精度高且不容易受到加性噪声的干扰等优点。仿真结果表明,所提算法不仅可以准确估计零中频接收机的IQ不平衡参数,还能对IQ不平衡进行有效的校正,从而提高系统的镜像抑制比。     

基于信道化架构的宽带I/Q不平衡校准技术

零中频接收机凭借其架构简单、易于集成等特点已被广泛应用于通信系统和雷达系统,为未来雷达通信一体化技术发展奠定了基础。然而,零中频接收机存在I/Q不平衡问题,这不仅会造成通信星座图的偏移,还会引入雷达虚假目标。现有宽带I/Q补偿方法的精度不高,且都集中于后处理,无法做到实时。因此,本文首先建立了宽带I/Q不平衡模型,并提出了一种融合信道化架构和盲估计补偿算法的宽带I/Q失衡校准技术。该技术利用信道化架构将宽带信号划分为窄带信号,并利用盲估计算法对带有镜像信号的子信道进行在线补偿。实验表明,该方法在获得高精度补偿参数的同时,完成了对宽带I/Q失衡的实时补偿。镜像抑制比达到55 dB。     

低中频接收机I/Q不平衡的数字估计和补偿

相对于传统的超外差式接收机,低中频接收机由于不需要镜像抑制滤波器、中频滤波器等片外大体积昂贵器件,因而系统设计可以更加灵活,体积更小,成本更低,集成度更高。低中频接收机采用I/Q下变频进行镜像分离,但模拟前端I/Q不平衡导致的镜像抑制不足问题却是其一大缺点。本文提出了一种利用测试信号数字估计和补偿I/Q不平衡的方法来提高镜像抑制能力。仿真结果表明,在幅度不平衡不大于10%和相位不平衡不大于10°时,经补偿后的镜像抑制比能够达到50d B以上。     

基于STBC的MIMO OFDM系统中的I/Q不平衡及CFO的联合均衡策略

基于STBC方案,针对MIMO OFDM通信系统中同时存在发射机和接收机I/Q不平衡、前端滤波器失配、CFO和频率选择性信道失真的组合影响进行了深入研究,并提出了一种适用的联合均衡策略;具体实现是首先通过对MIMO OFDM系统中只存在发射机I/Q不平衡和多径信道干扰的分析,得到一种频域均衡器;然后再考虑同时存在接收机I/Q不平衡和CFO的情况,得到了2个时域均衡器;最后把2个时域均衡器变换到频域,并结合消除发射机I/Q不平衡和多径信道干扰的频域均衡技术,提出了一种全面的联合均衡技术即频域子载波均衡器。仿真结果表明,针对MIMO OFDM系统提出的频域子载波均衡技术不仅能扩展到其他高阶STBC系统,而且使均衡后的系统BER性能得到了明显的提高。     

宽带信号I/Q采样不平衡校正方法研究

针对宽带信号模拟正交下变频时产生的 I/Q 信号不平衡问题，本文提出了一种在数字域对 I/Q 信号不平衡的校正方法，建立了 I/Q 信号不平衡的数字模型，分析了 I/Q 信号不平衡给系统带来的影响，推导了 I/Q 信号不平衡的校正方法，并对校正结果进行了仿真验证，该方法已在某设备中成功应用。     

OFDM联合信道均衡与I/Q失衡补偿算法

分析多径衰落信道下零中频接收机的I/Q失衡对OFDM系统造成的影响,并在此基础上构造一个结合了信道频率响应与I/Q失衡因子的复合信道模型.通过设计OFDM双导频符号进行复合信道均衡,从而能同时补偿多径衰落信道和I/Q失衡对OFDM系统造成的影响.仿真结果表明:这种联合算法大大提高了OFDM系统的性能,而且具有较低的实现复杂度.     

OFDM WLAN接收机IQ不平衡的时域补偿新方法

针对采用高阶调制的正交频分复用(OFDM)系统在前端模拟处理过程中易遭受到同相相位和正交相位(IQ)不平衡影响的问题,提出了一种新的OFDMWLAN系统接收机IQ不平衡估计和补偿方法。该方法利用较少的时域训练序列进行IQ不平衡参数的估计及补偿。仿真结果表明,该方法在降低了计算复杂度的同时,也获得了较好的估计性能。     

Adaptive MAP receiver via the EM algorithm and message passings for MIMO-OFDM mobile communications

This paper proposes two new types of maximum a posteriori probability (MAP) receivers for multiple-input-multiple-output and orthogonal frequency-division multiplexing mobile communications with a channel coding such as the low-density parity-check code. One proposed receiver employs the expectation-maximization algorithm so as to improve performance of approximated MAP detection. Differently from a conventional receiver employing the minimum mean-square estimation (MMSE) algorithm, it applies the recursive least squares (RLS) algorithm to the channel estimation in order to track a fast fading channel. For the purpose of further improvement, the other proposed receiver applies a new adaptive algorithm that can be derived from the message passing on factor graphs. The algorithm exploits all detected signals but one of targeted time, and can gain a considerable advantage over the MMSE and RLS. Computer simulations show that the first proposed receiver is superior in channel-tracking ability to the conventional receiver employing the MMSE. Furthermore, it is demonstrated that the second proposed receiver remarkably outperforms both the conventional and the first proposed ones.     

V-BLAST系统的一种新的分集迭代检测算法

讨论了原V-LAST(垂直Bell实验室分层空间-时间)系统的算法,从获得接收分集的角度给出了一种新的迭代处理算法。该算法利用接收端进行分集接收处理来改善V-BLAST系统每层的接收信号,对进一步抵抗V-BLAST系统的误差传播效应取得很好的效果。该算法可以看做是V-BLAST系统许多算法的一种延伸。仿真结果显示,与原V-BLAST算法相比具有更好的误码性能:在开环2发2收的情况下,信道为瑞利平坦慢衰落,采用QPSK(正交相移键控)调制,误码率为10-2时可获得2 dB的增益。     

WCDMA下行链路接收机设计

根据下行链路的需求设计了可以满足实时性需要的接收机。在下行链路中,只有所希望的用户信号被解调,而抑制其他用户引起的干扰。针对这一特点,下行链路接收主要采用单用户接收机,在本文中对各种单用户接收算法进行了分析、仿真及性能比较,本文的单用户接收算法采用自适应LMMS单用户接收算法。采用的接收机是在自适应LMMSE接收机基础上提出的差分自适应接收机,该接收机运算量比较小,性能比较好,并且能够跟踪同步,能够在用户终端上使用。     

双通道接收机空间谱估计中的理论实现

介绍了双通道接收机在空间谱估计技术中的实现方法,其是在单通道接收机空间谱估计测向理论的基础上发展而来。同时给出了一种易于实现的权微扰方法来获得阵列的协方差矩阵,并通过结合MUSIC算法进行了计算机模拟仿真。通过实验证明,该理论方法可正确和有效地估计信号的波达方向。     

DS—CDMA通信中的一种2D-Rake接收机

Rake接收机在3G系统中已被广泛使用,实际应用证明它能够有效对抗移动无线信道中的多径衰落。传统Rake接收机有其时域一维处理的局限性,为获得更好的系统性能,本文在分析空时信道模型的基础上提出一种新的空时2D-Rake接收机算法,它将波束形成技术与Rake相结合。这种波束形成算法能够在干扰方向上完全陷零,其权重仅与阵列天线的导向矢量有关,与接收信号无关。最后简要分析了所提出的2D-Rake接收机的性能。仿真结果表明,这种空时2D-Rake接收机与传统Rake接收机相比,具有较好的性能,而且该算法对噪声具有鲁棒性。     

A constrained MMSE receiver for DS/CDMA systems in fading channels

The convergence problem of minimum mean square-error (MMSE) receivers is discussed, and to overcome the problem, a constrained MMSE receiver is proposed. In addition, we propose the orthogonal decomposition-based least mean square algorithm to implement the constrained MMSE receiver adaptively. Through computer simulations, it is shown that the proposed receiver provides significant performance improvement in the bit-error rate over the conventional matched filter receiver and currently available MMSE receivers.     

Clock synchronization using a linear process model

In this paper, we present a clock synchronization scheme based on a simple linear process model which describes the behaviors of clocks at a transmitter and a receiver. In the clock synchronization scheme, a transmitter sends explicit time indications or timestamps to a receiver, which uses them to synchronize its local clock to that of the transmitter. Here, it is assumed that there is no common network clock available to the transmitter and the receiver and, instead, the receiver relies on locking its clock to the arrival of the timestamps sent by the transmitter. The clock synchronization algorithm used by the receiver is based on a weighted least‐squares criterion. Using this algorithm, the receiver observes and processes several consecutive clock samples (timestamps) to generate accurate timing signals. This algorithm is very efficient computationally, and requires the storage of only a small number of clock samples in order to generate accurate timing signals. Copyright © 2006 John Wiley & Sons, Ltd.     

Adaptive DFE Multiuser Receiver for CDMA Systems using Two-Step LMS-Type Algorithm: An Equalization Approach

This paper presents an adaptive decision feedback equalizer (DFE) based multiuser receiver for code division multiple access (CDMA) systems over smoothly time-varying multipath fading channels using the two-step LMS-type algorithm. The frequency-selective fading channel is modeled as a tapped-delay-line filter with smoothly time-varying Rayleigh-distributed tap coefficients. The receiver uses an adaptive minimum mean square error (MMSE) multiuser channel estimator based on the reduced Kalman least mean square (RK-LMS) algorithm to predict these tap coefficients (Kohli and Mehra, Wireless Personal Communication 46:507–521, 2008). We propose the design of adaptive MMSE feedforward and feedback filters by using the estimated channel response. Unlike the previously available Kalman filtering algorithm based approach (Chen and Chen, IEEE Transactions on Signal Processing 49:1523–1532, 2001), the incorporation of RK-LMS algorithm reduces the computational complexity of multiuser receiver. The computer simulation results are presented to show the substantial improvement in its bit error rate performance over the conventional LMS algorithm based receiver. It can be inferred that the proposed multiuser receiver proves to be robust against the nonstationarity introduced due to channel variations, and it is also beneficial for the multiuser interference cancellation and data detection in CDMA systems.     

A low complexity reduced-rank MMSE receiver for DS/CDMA communications

The minimum mean squared error (MMSE) receiver is a linear filter which can achieve optimal near-far resistance in direct-sequence code-division multiple-access communications. However, one of the main problems of this receiver is the required number of filter taps, which is typically large. This is especially true in systems with a large processing gain in which case the receiver's computation burden becomes very high. As a result, methods for reducing the complexity of the MMSE receiver have been of great interest in recent years. We propose an efficient partitioned MMSE receiver based on a classification algorithm. It is shown that the computational complexity (in terms of the filter taps) of the proposed receiver can be reduced significantly while good performance is maintained. Based on the special structure of our proposed receiver, we also propose a release-merge adaptive partition algorithm which can update the partition and the receiver's coefficients simultaneously. In particular, it is demonstrated that the proposed receiver can perform much better than previously proposed reduced-rank MMSE receivers, such as the partial despreading MMSE receiver and the cyclically shifted filter bank receiver, with even a smaller number of taps.     

MIMO Receiver Using Reduced Complexity Sequence Estimation With Channel Estimation and Tracking

A sequence-based multiple-input–multiple-output receiver incorporating channel estimation and tracking for a frequency selective fading environment is developed. It employs near-maximum likelihood sequence estimation using the partitioned Viterbi algorithm and offers linearly increasing complexity with the number of transmit antennas. The receiver implements channel estimation and tracking using a vector-polynomial-based generalized recursive least squares algorithm. The resulting integrated receiver can operate in a continuously time-varying Rayleigh or Rician fading environment. Simulation results show that it offers a good tradeoff between complexity and performance. Further complexity reduction can be achieved using tentative decisions as reference signals to the estimator and a reduced complexity form of the estimator.