Carrier Frequency Offset Estimation for MIMO over Spatially Correlated Flat-fading Channels

This paper deals with Carrier frequency offset （CFO） estimation for Multiple-input multiple-output （MIMO） systems. A Rayleigh fiat-fading channel model is considered and spatial correlation among channels corresponding to different pairs of transmit and receive antennas is taken into account. A Data-aided （DA） Maximum likelihood （ML）CFO estimator based on the marginal likeli- hood function is proposed. It can exploit spatial diversity and make use of the knowledge of spatial correlation by averaging the conditional likelihood function over all realizations of the channel. The Cramer-Rao bound （CRB） for the problem is derived as a benchmark. Simulation results are given to illustrate the performance of the pro- posed estimator. It is shown that its performance is close to the CRB at high Signal-to-noise ratio （SNR） region and improved with decrease of the spatial correlation and in- crease of the number of receive antennas.     

Joint Maximum-Likelihood Estimation of Frequency Offset and Channel in MIMO Systems

This paper addresses the problem of frequency offsets and channel gains estimation for a Multiinput multi-output （MIMO） system in fiat-fading channels. The general case where the frequency offsets are possibly different for each transmit antenna is considered, The Maximum-likelihood （ML） estimation of the carrier frequency offset for each transmit antenna in a MIMO system is investigated in this paper. The exact solution to this estimation problem turns out to be too complex as it involves a search over a multi-dimensional domain. However, by making use of the Particle swarm optimization （PSO） method, we efficiently solve the above complex problem. So based on the PSO theory, a novel joint estimation algorithm of frequency offsets and channel gains is proposed. Simulation results show that the proposed algorithm has better performance as compared with the correlation-based estimation algorithm and asymptotically achieves the Cramer-Rao lower bound （CRLB）, which provides a new idea to solve the problem of joint frequency offsets and channel gains estimation for MIMO systems.     

Group transmit antenna selection based on GIS technology for V-BLAST system

The conventional transmit antenna selection for Vertical Bell Laboratories Layered Space Time （V-BLAST） system is very complex because it needs to compute the inverse of channel matrices time after time. In this paper, a new group transmit antenna selection scheme for V-BLAST system is proposed. The 1st group transmit antennas are decided according to a certain selection criterion among the available antennas. Then, with Group Interference Suppression （GIS） technology, the interferences of the transmit symbols from the selected antennas can be suppressed. Finally, the 2nd group transmit antennas are decided among the residual available antennas. Simulations show that its performance is lower than that of the conventional selection scheme. However, the new selection scheme has lower complexity than the conventional one.     

Layered MAP algorithm for MIMO ISI channels

The layered maximum a posteriori （L-MAP） algorithm has been proposed to detect signals under frequency selective fading multiple input multiple output （MIMO） channels. Compared to the optimum MAP detector, the L-MAP algorithm can efficiently identify signal bits, and the complexity grows linearly with the number of input antennas. The basic idea of L-MAP is to operate on each input sub-stream with an optimum MAP sequential detector separately by assuming the other streams are Gaussian noise. The soft output can also be forwarded to outer channel decoder for iterative decoding. Simulation results show that the proposed method can converge with a small number of iterations under different channel conditions and outperforms other sub-optimum detectors for rank-deficient channels.     

STATISTICAL APPROXIMATION BASED FINE FREQUENCY SYNCHRONIZATION FOR OFDM SYSTEMS

The paper proposes a novel approach for fine frequency synchronization of OFDM syn- chronization systems in multi-path channels. Maximum Likelihood (ML) function of frequency offsets including integral and decimal parts in frequency domain is developed according to the law of great number to eliminate the noise impact of the signal. When the timing delay close to the actual time, the proposed function produces a deep valley indicating frequency offset when large Valley-Square- Error (VSE) appears. Coarse timing offset can also be detected when function’s Valley-Square-Error (VSE) is maximized. Simulation results shows that the proposed algorithm gives very robust estimation of frequency offset, and a coarse timing offset estimation.     

Performance analysis of frequency synchronization in MIMO-OFDM systems with ST/MRC scheme

In this paper, the performance of frequency synchronization in a multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system is analyzed for the purpose of carrier frequency offset (CFO) estimation and compensation. Speci?cally, a joint transmit antenna selection (ST) and receive maximum ratio combining (MRC) （ST/MRC） method is adopted, that is, only one transmit antenna with the highest channel power is selected while MRC is used at the receiver to maximize the sum of frequency synchronization metric. The mean square error (MSE) closed-form expressions of CFO estimation are derived for several antenna con?gurations. Simulations in both ?at and multipath fading channels validate the theoretical analysis.     

Does Amplify-and-Forward Cooperative Relay Improve OFDM/OFDMA Ergodic Capacity？

The ergodic information rate for Orthogonal Frequency-Division Multiplexing / Orthogonal Frequency-Division Multiple Access with amplify-and-forward （AF） relaying systems in the presence of frequency offsets is evaluated. Unlike previous work, per-subcarrier adaptive power allocation is performed on each relay to optimize the system ergodic information rate. For a given frequency offset and total number of relays M, the AF ergodic information rate is proven to be a monotonically increasing function of α （the ratio of the power allocated to the source node and the total transmit power）, implying that the maximum ergodic information rate can be obtained at α=1 （i.e., there is no cooperative relay）. Furthermore, the proof of ＂cooperative relays cannot improve the AF ergodic information rate in a quasi-static wireless channel＂ is also provided in this letter.     

放大转发协作中继是否提升OFDM/OFDMA遍历容量?(英)

The ergodic information rate for Orthogonal Frequency-Division Multiplexing/ Orthogonal Frequency-Division Multiple Access with amplify-and-forward(AF)relaying systems in the presence of frequency offsets is evaluated.Unlike previous work,per-subcarrier adaptive power allocation is performed on each relay to optimize the system ergodic information rate.For a given frequency offset and total number of relays M,the AF ergodic information rate is proven to be a monotonically increasing function of a(the ratio of the power allocated to the source node and the total transmit power),implying that the maximum ergodic information rate can be obtained at α=1(i.e.,there is no cooperative relay).Furthermore,the proof of "cooperative relays cannot improve the AF ergodic information rate in a quasi-static wireless channel" is also provided in this letter.     

On the MAC-BC duality of multiuser non-regenerative MIMO relay systems

Multiple input multiple output(MIMO) relaying techniques can greatly improve the spectral efficiency and extend network coverage for future wireless systems.This article investigates a multiuser MIMO relay channel,where a base station(BS) with multiple antennas communicates with multiple mobile stations(MS) via a relay station(RS) with multiple antennas.The RS applies linear processing to the received signal and then forwards the processed signal.The dual channel conditions between MIMO relay multiple access channel(MAC) and broadcast channel(BC) are first developed for single-relay scenario with white Gaussian noise.Then the MAC-BC duality for MIMO relay systems is established by proving that the capacity region of MIMO relay MAC is equal to that of dual MIMO relay BC under the same total network transmit power constraint.In addition,the duality is also extended to multi-relay scenario with arbitrary noise.Finally,several simple general numerical examples are provided to better illustrate the effectiveness of the MIMO relay MAC-BC duality.     

Frequency-, radiation pattern- and polarization-reconfigurableantenna for wireless applications

In this communication, a frequency-, radiation pattern-and polarization-reconfigurable antenna employing liquid metal is presented. Two crossed dipole antennas are surrounded by four independent reflectors and directors to realize multi-beam switching. The length of dipole arms can be adjusted by extracting the liquid metal from the needle tube to achieve frequency reconfiguration. The polarization can be switched by injecting liquid metal into different dipole microfluidic channels. It is simpl...     

On parameter estimation of MIMO flat-fading channels with frequency offsets

We address the frequency offsets and channel gains estimation problem for a multi-input multi-output (MIMO) flat-fading channel using a training sequence. The general case where the frequency offsets are possibly different for each transmit antenna is considered. The Cramer-Rao bound (CRB) for the problem at hand is derived. Additionally, we present a simple, closed-form expression for the large-sample CRB and show that it depends in a simple way on the channel parameters. Next, the parameters estimation issue is investigated. First, the maximum likelihood estimator (MLE), which entails solving an n-dimensional maximization problem where n is the number of transmit antennas, is derived. Then, we show that the likelihood function can be written as the product of n one-dimensional (1-D) functions if a suitable choice of the training sequence is made. Based on this fact, we suggest two computationally simpler methods. Numerical examples that illustrate the performance of the estimators and compare it with the CRB are provided.     

基于STFAP的MIMO雷达运动目标参数估计的CRB研究

多发多收(Multiple-Input Multiple-Output, MIMO)雷达在目标检测、参数估计等方面具有显著优势。目标参数估值的CRB被证明是系统设计和空时自适应处理(STAP)性能分析中的有力工具。该文针对采用频分正交信号的共置天线MIMO雷达,首先建立基于MIMO雷达的目标和杂波空-时-频信号模型;在此基础上,研究基于空-时-频自适应处理(STFAP)的MIMO雷达地面运动目标角度和多普勒参数最大似然估值的克拉美-罗界(CRB);最后通过CRB性能仿真分析验证了MIMO雷达STFAP有效消除动目标检测盲速,提高目标参数估计精度的优势。     

Robust frequency offset estimation for pilot symbol assisted packetCDMA with MIMO antenna systems

We propose frequency offset estimation and combining techniques for pilot symbol assisted (PSA) packet downlink code-division multiple access (CDMA) with multiple-input and multiple-output (MIMO) antenna systems. Orthogonal Walsh codes are used for dedicated pilot symbols at the transmit antennas. The discrete Fourier transform (DFT)-based frequency offset estimation is used for simple implementation. In addition, simple interpolation is also used for resolution improvement of the DFT. When identical frequency offset is assumed, the frequency offset estimates for each single-input single-output (SISO) sub-stream (i.e., transmit/receive antenna pair) can be combined at the receiver. Simulation results show that the estimation and combining techniques proposed deliver improvement in the frequency offset estimation performance     

分布式发射天线MIMO OFDM的一种低复杂度频偏校正算法

 在分布式发射天线MIMO OFDM系统中,各发射天线由于使用独立的晶振导致了每根发射天线到接收机间具有不同的频率偏移. 为了降低频偏造成的子载波间干扰,本文提出一种以最大化每根接收天线上子载波的条件平均信干噪比为准则,校正接收信号中频率偏移的算法. 利用多项式近似法对条件平均信干噪比作近似处理,获得了低复杂度的频偏校正值解析式. 与已有的频率偏移校正算法相比,所提算法的性能略有提升,复杂度降低50%或更多.     

分布式MIMO中的最大似然频偏估计

在考虑分布式多入多出(MIMO)系统中各收发天线对之间的频偏均可能不同的一般情况下,推导出了平坦衰落MIMO信道模型下的最大似然频偏估计.针对收端是否采用分布式天线的两种不同情况,分别提出了一种只涉及一维最大化问题的较低复杂度频偏估计方法.仿真结果表明,所提频偏估计方法在平坦衰落MIMO信道下可以获得令人满意的估计性能;而在收端仍采用集中式天线这一特殊情况下,利用所有接收天线上的接收信号进行联合估计,可以进一步提高估计性能.     

分布式残余频偏信道中STBC-OFDM的发射功率优化

在分布式残余频偏信道中,考虑多径瑞利衰落,针对采用判决反馈检测的两发射天线STBC-OFDM链路,提出了一种发射功率优化方法:根据平均信道功率增益、残余频偏方差以及噪声方差的大小,以最小化平均误比特率下界为目标,设置两分布发射天线的发射功率,仿真结果表明:相比于传统的各天线满功率发射方法,所提方法能够在节省发射功率的同...     

Target Velocity Estimation and Antenna Placement for MIMO Radar With Widely Separated Antennas

This paper studies the velocity estimation performance for multiple-input multiple-output (MIMO) radar with widely spaced antennas. We derive the Cramer–Rao bound (CRB) for velocity estimation and study the optimized system/configuration design based on CRB. General results are presented for an extended target with reflectivity varying with look angle. Then detailed analysis is provided for a simplified case, assuming an isotropic scatterer. For given transmitted signals, optimal antenna placement is analyzed in the sense of minimizing the CRB of the velocity estimation error. We show that when all antennas are located at approximately the same distance from the target, symmetrical placement is optimal and the relative position of transmitters and receivers can be arbitrary under the orthogonal received signal assumption. In this case, it is also shown that for MIMO radar with optimal placement, velocity estimation accuracy can be improved by increasing either the signal time duration or the product of the number of transmit and receive antennas.      

MIMO DS/CDMA系统中基于信号周期平稳特性的频偏盲估计算法

MIMO DS／CDMA通信系统巾，每对收发天线问的载波频偏各不相同，这使得频偏估计变得异常困难。该文基于DS／CDMA信号的周期平稳特性提出了一种适用于该系统的频偏盲估计算法。该算法主要特点在于：（1）它能够分别估计每对收发天线间的载波频偏：（2）综合利用了信号在所有非零周期频率处的周期自棚关函数值，（3）它是一种盲估计算法，不需要专门的训练序列，不会降低系统的带宽利用率。仿真结果表明在FIR信道模型下，该算法能够准确地估计出系统巾各个收发天线对间的载波频偏。     

分布式全相参雷达相干参数估计性能

针对部分收发共置的分布式全相参雷达,该文首先建立了全发任意收的混合结构,然后在发射多脉冲条件下推导了相干参数估计的克拉美-罗界(CRB)闭式解,研究了相干参数估计性能与收发天线数及脉冲数之间关系。结论表明:共置天线较分置天线能够获得更低的相干参数估计CRB,增加收发天线数或脉冲数,能够降低相干参数估计CRB。最后的仿真实验验证了研究结论的正确性。     

Achievable sum‐rate analysis of correlated two‐antenna MIMO uplink channels

This paper analyzes the achievable sum‐rate of correlated two‐antenna multiple‐input multiple‐output (MIMO) uplink channels. Most of previous works have considered the case when a single user has multiple transmit antennas (i.e. multi‐antenna single‐user scenario). This paper considers the case when two‐antenna MIMO uplink channels comprise two users with a single transmit antenna (i.e. single‐antenna two‐user scenario). The analytic and simulation results show that the achievable sum‐rate of correlated single‐antenna two‐user MIMO uplink channels highly depends on the angle difference between the receive correlation coefficients of two users. It is also shown that the achievable sum‐rate of correlated single‐antenna two‐user MIMO uplink channels is larger than that of correlated two‐antenna single‐user MIMO uplink channels and can even be larger than that of independent and identically distributed Rayleigh two‐antenna MIMO uplink channels. Copyright © 2009 John Wiley & Sons, Ltd.