MIMO-OFDM系统的小区间干扰抑制技术研究

首先简要介绍了MIMO（多输入多输出）系统的特点和缺点,结合OFDM（正交频分复用）技术后MIMO-OFDM系统的原理．并针对一些信号检测技术的优缺点具体阐述了MIMO-OFDM系统小区间干扰抑制技术研究现状。然后对MIMO-OFDM系统小区间干扰抑制技术进行了介绍,对目前MIMO-0FDM系统小区间干扰抑制技术的研究状况进行了综述。最后对LTE（长期演进）小区间干扰抑制技术的研究进行了解释说明,再次对需要进一步研究的问题进行了讨论。     

一种OFDM系统强窄带干扰消除方法

OFDM系统由于具有子载波的正交性从而可以充分利用频谱资源,鉴于当OFDM受到窄带干扰而影响较多的子载波时,会出现系统误码率性能严重恶化的现状。文中提出了一种OFDM系统窄带干扰消除技术,该技术在接收端检测出受干扰子载波,利用反馈信道告知发送端,并在发送端将受干扰的子载波不加调制的信息发送到信道中,而在信道中该部分子载波只受干扰信号的影响,接收端将干扰信息重构并储存,从而完成干扰消除确保正常通信。仿真和分析结果表明,该方法在不浪费频谱资源的情况下,能有效抑制OFDM系统的窄带干扰。     

直扩系统窄带干扰抑制技术研究

在介绍扩频通信原理基础上,分析了直扩系统干扰的机理.讨论了直扩系统抑制窄带干扰的主要方法.     

基于变换域通信的OFDM系统窄带干扰抑制方法

为解决宽带OFDM系统易受传统窄带信号干扰的问题,采用基于变换域通信(TDC)的OFDM系统,通过在变换域中的电磁环境采样结果,对OFDM信号进行设计,避免使用受干扰频谱传送信息。该方法在强窄带干扰得到抑制的同时,信噪比也不容易受到变换域滤波的影响。在加性高斯白噪声信道中的仿真结果表明,基于TDC的干扰抑制方法能够有效地抑制宽带OFDM系统的窄带干扰。     

DSSS中的窄带干扰抑制技术

直接序列扩频(DSSS)通信具有抗干扰能力强的特点。采用窄带干扰抑制技术,可以有效提高DSSS系统的性能和顽存力。介绍了DSSS通信中的基于时域和变换域的干扰抑制技术。     

基于DFT的自适应窄带干扰抑制算法研究

介绍了一种频域自适应窄带干扰抑制的方法,该方法基于离散DFF（傅里叶变换）技术,在频域采用N-∑算法自适应确定干扰抑制门限,可以有效地去除窄带干扰对宽带通信系统解调性能的影响,适用于DSSS（直接系列扩频）信号中去除强窄带干扰信号的影响。该算法简单高效,非常适合于数字硬件实现,实验和实测结果表明了该算法的有效性。     

低信噪比下基于自适应门限的窄带干扰抑制研究

对一种基于自适应门限的变换域窄带干扰抑制技术进行了研究。该技术根据在低信噪比下接收信号经过快速傅立叶变换后数据的统计特性,估计出高斯白噪声和干扰之间的门限值,然后将高于门限的干扰谱线幅度值抑制到与噪声相近的程度,大大减少了干扰信号的功率,从而提高信噪比。理论分析和仿真结果表明,该技术改善了系统性能且便于硬件实现。     

窄带干扰下MIMO系统的特征域传输方案

该文提出一种特征域的传输方案(ETS),用以提高窄带干扰下MIMO系统的容量。首先,发射端在特征域产生N路发射信息,然后根据窄带干扰信号的统计特征,通过预编码将发射信息从特征域变换至时域进行传输。在接收端,时域的接收信号再次被变换为N路特征域信号。据此,期望信道被划分为N路并行子信道,根据各子信道的干扰分布采用相对应的传输方案,以最大化干扰场景下信道的利用率。由计算机仿真可得,当系统容量为15 bit/(s?Hz)时,该文所提特征域传输方案相对于传统干扰抑制算法在22 MIMO系统中取得了约10 dB的性能增益。     

变换域窄带干扰抑制算法的仿真与实现

本文主要针对变换域窄带干扰抑制算法进行了研究。文中详细介绍了窄带干扰抑制的关键性算法。通过对算法的 Matlab 仿真和在 FPGA 中的实现,证明了算法的有效性。     

电力线通信系统中的脉冲干扰抑制研究

提出了一种基于最大似然(ML)准则的脉冲干扰抑制方法。该方法是在接收端根据接收信号的循环平稳特性,首先估计出信号功率及噪声加脉冲干扰功率,然后将估计出的值应用于门限值的计算中,实时地更新门限值,因此能够更准确地去除脉冲干扰。最后通过计算机的仿真结果表明,本文的方法相较于固定的门限值滤除方法具有更低的误比特率。     

Per-tone equalization for MIMO OFDM systems

This paper focuses on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with channel order larger than the cyclic prefix (CP) length. Writing the demodulating fast Fourier transform (FFT) as a sliding FFT followed by a downsampling operation, we show in this paper that by swapping the filtering operations of the MIMO channel and the sliding FFT, the data model for the temporally smoothened received signal of each individual tone of the MIMO OFDM system is very similar to the data model for the temporally smoothened received signal of a MIMO single-carrier (SC) system. As a result, to recover the data symbol vectors, the conventional equalization approach for MIMO SC systems can be applied to each individual tone of the MIMO OFDM system. This so-called per-tone equalization (PTEQ) approach for MIMO OFDM systems is an attractive alternative to the recently developed time-domain equalization (TEQ) approach for MIMO OFDM systems. In the second part of this paper, we focus on direct per-tone equalizer design and adapt an existing semi-blind equalizer design method for space-time block coding (STBC) SC systems to the corresponding semi-blind per-tone equalizer design method for STBC OFDM systems.     

A Dual-Linearly-Polarized MEMS-Reconfigurable Antenna for Narrowband MIMO Communication Systems

The design and characterization is described of a compact dual-linearly-polarized reconfigurable 2-port antenna. The antenna can operate in two different selectable linear polarization bases, thus being capable of reconfiguring/rotating its polarization base from vertical/horizontal $(0^{circ}/90^{circ})$, to slant $pm 45^{circ}$. The antenna has been implemented on a Quartz substrate, and uses monolithically integrated micro-electromechanical (MEM) switches to select between the two aforementioned polarization bases. The antenna operates at 3.8 GHz and presents a fractional bandwidth of 1.7%. The interest of the proposed antenna is two-fold. First, in LOS scenarios, the antenna enables polarization tracking in polarization-sensitive communication schemes. Second, there are the gains of using it in a multiple-input multiple-output (MIMO) communication system employing orthogonal space-time block codes (OSTBC) to improve the diversity order/gain of the system in NLOS conditions. These benefits were verified through channel measurements conducted in LOS and NLOS propagation scenarios. Despite the simplicity of the antenna, the achievable polarization matching gains (in LOS scenarios) and diversity gains (in NLOS scenarios) are remarkable. These gains come at no expenses of introducing additional receive ports to the system (increasing the number of Radio-Frequency (RF) transceivers), rather as a result of the reconfigurable capabilities of the proposed antenna.      

短波窄带OFDM调制解调器设计与实现

在技术和参数上设计实现了应急通信系统中短波窄带正交频分复用(Orthogonal Frequency DivisionMultiplexing,OFDM)调制解调器。依据短波多径信道传输特性和应急通信分组突发传输的特点,研究了适合系统传输的延时相关帧检测、载波频率同步、符号定时同步和信道估计等算法。在3 kHz话音带宽上按照系统速率要求研究设计了短波OFDM调制解调技术参数和技术方案,分析了接收信号帧检测和符号定时同步响应,仿真实现了多径信道下短波窄带数据的高速传输。     

Intercarrier interference in MIMO OFDM

In this paper, we examine multicarrier transmission over time-varying channels. We first develop a model for such a transmission scheme and focus particularly on multiple-input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM). Using this method, we analyze the impact of time variation within a transmission block (time variation could arise both from Doppler spread of the channel and from synchronization errors). To mitigate the effects of such time variations, we propose a time-domain approach. We design ICI-mitigating block linear filters, and we examine how they are modified in the context of space-time block-coded transmissions. Our approach reduces to the familiar single-tap frequency-domain equalizer when the channel is block time invariant. Channel estimation in rapidly time-varying scenarios becomes critical, and we propose a scheme for estimating channel parameters varying within a transmission block. Along with the channel estimation scheme, we also examine the issue of pilot tone placement and show that in time-varying channels, it may be better to group pilot tones together into clumps that are equispaced onto the FFT grid; this placement technique is in contrast to the common wisdom for time-invariant channels. Finally, we provide numerical results illustrating the performance of these schemes, both for uncoded and space-time block-coded systems.     

Optimal training signals for MIMO OFDM channel estimation

This paper presents general classes of optimal training signals for the estimation of frequency-selective channels in MIMO OFDM systems. Basic properties of the discrete Fourier transform are used to derive the optimal training signals which minimize the channel estimation mean square error. Both single and multiple OFDM training symbols are considered. Several optimal pilot tone allocations across the transmit antennas are presented and classified as frequency-division multiplexing, time-division multiplexing, code-division multiplexing in the frequency-domain, code-division multiplexing in the time-domain, and combinations thereof. All existing optimal training signals in the literature are special cases of the presented optimal training signals and our designs can be applied to pilot-only schemes as well as pilot-data-multiplexed schemes.     

Block-Based Performance Measures for MIMO OFDM Beamforming Systems

In this paper, we consider an adaptive modulation system with multiple-input–multiple-output (MIMO) antennas in conjunction with orthogonal frequency-division multiplexing (OFDM) operating over frequency-selective Rayleigh fading environments. In particular, we consider a type of beamforming with a maximum ratio transmission/maximum ratio combining (MRT-MRC) transceiver structure. For this system, we derive a central limit theorem for various block-based performance metrics. This motivates an accurate Gaussian approximation to the system data rate and the number of outages per OFDM block. In addition to the data rate and outage distributions, we also consider the subcarrier signal-to-noise ratio (SNR) as a process in the frequency domain and compute level crossing rates (LCRs) and average fade bandwidths (AFBs). Hence, we provide fundamental but novel results for the MIMO OFDM channel. The accuracy of these results is verified by Monte Carlo simulations, and applications to performance analysis and system design are discussed.      

Turbo编码MIMO OFDM系统多径干扰抵消

正交频分复用(OFDM)利用保护间隔(GI)消除因多径时延而产生的符号间干扰(ISI)和载波间干扰(ICI),但是太长的GI会占用过多的信道资源。为此,在保持一定GI长度的情况下,提出了利用减法抵消法消除ISI和加法抵消法抵消ICI的多径干扰抵消(MPIC)方法,使得当多径时延大于GI时,仍然可以保持系统的低误码率以及较强的抗多径干扰的优异性能。蒙特卡洛仿真结果表明,有多径干扰的情况下,在Turbo码编码的MIMO OFDM系统中使用提出的多径干扰抵消方法可以更有效地减少ISI和ICI。     

一种改进的MIMO OFDM系统信道估计算法

本文对MIMO OFDM系统中基于训练序列的信道估计问题进行了研究，针对信道冲击响应的最大抽头数大于每个OFDM符号中导频数的情况，提出一种有效的结合前后若干训练序列进行信道估计的算法和结合方式。仿真结果表明，在基于无线局域网（WLAN）中打包传送的MIMO OFDM系统里，本文的方法比采用块状训练序列的估计算法有着更小的归一化均方误差。     

MIMO-OFDM系统的两步符号定时同步方法

多天线正交频分复用（MIMO-OFDM,Multiple-Input and Multiple-Output Orthogonal Frequency Division Multiplex）系统中,精确的符号定时同步可使接收端解调数据时消除载波频率偏移和符号间干扰。而由于MIMO空间信道复杂度较高,使得单天线OFDM系统的同步方法不能直接应用于MIMO-OFDM系统中。提出的两步同步方法首先利用时域导频作为第一步同步进行粗同步,然后利用循环交织前缀作为第二步同步对每一个MIMO-OFDM符号进行同步偏差调整。需要指出的是,两次同步过程均可以产生尖锐的相关峰,使得同步精度大为提高。     

MIMO OFDM Receivers for Systems With IQ Imbalances

Orthogonal frequency division multiplexing (OFDM) is a widely recognized modulation scheme for high data rate communications. However, the implementation of OFDM-based systems suffers from in-phase and quadrature-phase (IQ) imbalances in the front-end analog processing. Such imbalances are caused by the analog processing of the received radio frequency (RF) signal, and they cannot be efficiently or entirely eliminated in the analog domain. The resulting IQ distortion limits the achievable operating SNR at the receiver and, consequently, the achievable data rates. The issue of IQ imbalances is even more severe at higher SNR and higher carrier frequencies. In this paper, the effect of IQ imbalances on multi-input multioutput (MIMO) OFDM systems is studied, and a framework for combating such distortions through digital signal processing is developed. An input–output relation governing MIMO OFDM systems is derived. The framework is used to design receiver algorithms with compensation for IQ imbalances. It is shown that the complexity of the system at the receiver grows from dimension$(n_Rtimes n_T)$for ideal IQ branches to$(2n_Rtimes 2n_T)$in the presence of IQ imbalances. However, by exploiting the structure of space-time block codes along with the distortion models, one can obtain efficient receivers that are robust to IQ imbalances. Simulation results show significant improvement in the achievable BER of the proposed MIMO receivers for space-time block-coded OFDM systems in the presence of IQ imbalances.