Combined ML and QR Detection Algorithm for MIMO‐OFDM Systems with Perfect ChanneI State Information

An effective signal detection algorithm with low complexity is presented for multiple‐input multiple‐output orthogonal frequency division multiplexing systems. The proposed technique, QR‐MLD, combines the conventional maximum likelihood detection (MLD) algorithm and the QR algorithm, resulting in much lower complexity compared to MLD. The proposed technique is compared with a similar algorithm, showing that the complexity of the proposed technique with T=1 is a 95% improvement over that of MLD, at the expense of about a 2‐dB signal‐to‐noise‐ratio (SNR) degradation for a bit error rate (BER) of 10⁻³. Additionally, with T=2, the proposed technique reduces the complexity by 73% for multiplications and 80% for additions and enhances the SNR performance about 1 dB for a BER of 10⁻³.     

Computationally Efficient Lattice Reduction Aided Detection for MIMO‐OFDM Systems under Correlated Fading Channels

We analyze the relationship between channel coherence bandwidth and two complexity‐reduced lattice reduction aided detection (LRAD) algorithms for multiple‐input multiple‐output (MIMO) orthogonal frequency division multiplexing (OFDM) systems in correlated fading channels. In both the adaptive LR algorithm and the fixed interval LR algorithm, we exploit the inherent feature of unimodular transformation matrix P that remains the same for the adjacent highly correlated subcarriers. Complexity simulations demonstrate that the adaptive LR algorithm could eliminate up to approximately 90 percent of the multiplications and 95 percent of the divisions of the brute‐force LR algorithm with large coherence bandwidth. The results also show that the adaptive algorithm with both optimum and globally suboptimum initial interval settings could significantly reduce the LR complexity, compared with the brute‐force LR and fixed interval LR algorithms, while maintaining the system performance.     

Improved Wideband Precoding with Arbitrary Subcarrier Grouping in MIMO‐OFDM Systems

Precoding in the multiple‐input multiple‐output orthogonal frequency division multiplexing system is investigated. In conventional wideband precoding (WBP), only one precoder, obtained from the decomposition of the subcarrier independent channel matrix, is used for all subcarriers. With an investigation of the relationship between the subcarrier independent channel matrix and the temporal/frequency channels, an improved WBP scheme is proposed for practical scenarios in which a part of subcarriers are allocated to a user. The improved WBP scheme is a generalized scheme of which narrow‐band precoding and conventional WBP schemes are special modes. Simulation results demonstrate that the improved WBP scheme almost achieves the optimum performance of a single precoder and outperforms the conventional WBP scheme in terms of the bit error ratio and ergodic capacity with slight complexity increase. The largest advantage of the improved WBP scheme on signal‐to‐noise ratio in simulation results is over 2.1 dB.     

Superimposed Pilot Aided Multiuser Channel Estimation for MIMO‐OFDM Uplinks

This letter addresses the superimposed pilot aided multiuser channel estimation for the uplinks of multi‐input multi‐output orthogonal frequency‐division multiplexing systems. To mitigate the embedded‐data effects on the performance of channel estimation, a novel combining algorithm is proposed. Optimal pilot symbols are developed with respect to the least square channel estimate's mean square error. The averaged sum‐capacity lower bound is derived and simulated. Simulation results show that on a low signal‐to‐noise ratio regime, our proposed scheme achieves better performance and higher capacity than the conventional pilot aided approach.     

Improved Design Criterion for Space‐Frequency Trellis Codes over MIMO‐OFDM Systems

In this paper, we discuss the design problem and the robustness of space‐frequency trellis codes (SFTCs) for multiple input multiple output, orthogonal frequency division multiplexing (MIMO‐OFDM) systems. We find that the channel constructed by the consecutive subcarriers of an OFDM block is a correlated fading channel with the regular correlation function of the number and time delay of the multipaths. By introducing the first‐order auto‐regressive model, we decompose the correlated fading channel into two independent components: a slow fading channel and a fast fading channel. Therefore, the design problem of SFTCs is converted into the joint design in both slow fading and fast fading channels. We present an improved design criterion for SFTCs. We also show that the SFTCs designed according to our criterion are robust against the multipath time delays. Simulation results are provided to confirm our theoretic analysis.     

基于PIC的多载波系统联合检测译码算法

针对多入多出—正交频分复用（MIMO-OFDM）信号检测问题,基于并行干扰抵消技术提出了一种编码MIMO-OFDM系统联合检测与译码算法,通过在干扰抵消检测器和信道译码器之间传递判决信息提高系统的误比特率性能。计算机仿真表明,该算法获得的误码率性能较传统的迫零检测以及V-BLAST检测算法有了较大的提升。     

Iterative Soft-Kalman Filter-based Data Detection and Channel Estimation for Turbo Coded MIMO–OFDM Systems

MIMO channels are often assumed to be constant over a block or packet. This assumption of block stationarity is valid for many fixed wireless scenarios. However, for communications in a mobile environment, the stationarity assumption will result in considerable performance degradation. In this paper, we focus on a new channel estimation technique for Turbo coded MIMO systems using OFDM. In the proposed MIMO–OFDM system, pilots are placed on selected subcarriers and used by a pair of Kalman filter (KF) channel estimators at the receiver. The KF channel estimates are then utilized by a MIMO–OFDM soft data detector based on the computationally efficient QRD-M algorithm. The soft detector output is fed back to the Kalman filters to iteratively improve the channel estimates. The extrinsic information generated by the Turbo decoder is also used as a priori information for the soft data detector. The overall receiver thus combines MIMO data detection, KF-based channel estimation, and Turbo decoding in a joint iterative structure yielding computational efficiency and improved bit-error rate (BER) performance. Parts of this paper were presented at ICC’2005, Seoul, Korea. This work was supported in part by NSF Grant No. CCF-0429596. This work was done when he was with the Nokia Research Center in Dallas, USA.     

OFDM系统中低复杂度联合译码和信道估计方法研究

本文针对OFDM(Orthogonal Frequency Division Multiplexing)系统提出了一种低复杂度联合译码和信道估计算法的接收机。接收机首先通过导频对信道进行估计，然后使用Turbo码译码输出判决信息对信道参数进行稳健的估计。仿真结果表明，即使系统误码率大于10^-2，经过3次迭代估计，系统性能提高仍有0．5-1dB；在高信噪比下，可显著地降低系统的误码底限。本文同时还给出了译码迭代次数对系统性能的影响。     

An Efficient Scheme to Achieve Differential Unitary Space‐Time Modulation on MIMO‐OFDM Systems

Differential unitary space‐time modulation (DUSTM) has emerged as a promising technique to obtain spatial diversity without intractable channel estimation. This paper presents a study of the application of DUSTM on multiple‐input multiple‐output orthogonal frequency division multiplexing (MIMO‐OFDM) systems with frequency‐selective fading channels. From the view of a correlation analysis between subcarriers of OFDM, we obtain the maximum achievable diversity of DUSTM on MIMO‐OFDM systems. Moreover, an efficient implementation strategy based on subcarrier reconstruction is proposed, which transmits all the signals of one signal matrix in one OFDM transmission and performs differential processing between two adjacent OFDM blocks. The proposed method is capable of obtaining both spatial and multipath diversity while reducing the effect of time variation of channels to a minimum. The performance improvement is confirmed by simulation results.     

MIMO OFDM系统中同步和信道的联合估计

提出了一种MIMO OFDM系统的定时恢复、频率同步和信道估计的联合算法。为了减少算法的复杂度，算法分两步完成：首先利用接收信号的自相关函数进行粗同步和信道估计，得到时延和频偏的粗估计，然后在粗估计基础上采用最大似然准则进行精确的同步和信道估计。仿真结果表明，该算法能够达到很好的效果，系统误码率接近已知信道响应时的情况。     

Turbo编码MIMO/OFDM系统中的Turbo均衡

针对Turbo编码MIMO/OFDM系统,本文提出一种低复杂度的Turbo均衡算法,均衡器采用性能近于最优检测的概率数据辅助（Probabilistic Data Association）算法,与软输入软输出的Turbo信道解码器之间迭代交换外信息,实现信道均衡与信道解码的迭代更新,以充分利用已获得的信息,克服传统判决反馈均衡器误差传播的缺陷。仿真表明,该均衡算法性能要比MMSE＋MF线性检测算法提高约1dB,在Eb/No为4dB时误比特率达到10-6,且算法复杂度仅为O（N3）,经两次迭代就可获得较为满意的码间干扰消除效果。     

Blind Joint Symbol Detection and DOA Estimation for OFDM System with Antenna Array

An analysis of the received signal of Orthogonal frequency division multiplexing (OFDM) system with array antennas shows that the received signal has trilinear model characteristics. Trilinear decomposition-based joint symbol detection and direction of arrival (DOA) estimation for OFDM system with antenna array is proposed in this paper. The simulation results reveal that the symbol detection performance of the proposed algorithm is very close to the post-FFT receiver with perfect channel state information; DOA estimation performance is very close to least squares method, and even this algorithm supports small sample sizes. Finally this algorithm does not require the channel fading information, DOA and training sequence or pilot information, so it has blind characteristics.

A Full Rate Quasi‐orthogonal STF‐OFDM with DAC‐ZF Decoder over Wireless Fading Channels

In this letter, we propose a quasi‐orthogonal space‐time‐frequency (QOSTF) block coded orthogonal frequency division multiplexing (OFDM) that can achieve full symbol rate with four transmit antennas. Since the proposed QOSTF‐OFDM cannot achieve full diversity, we use a diversity advantage collection with zero forcing (DAC‐ZF) decoder to compensate the diversity loss at the receiving side. Due to modulation advantage and collected diversity advantage, the proposed scheme exhibits a better bit‐error rate performance than other orthogonal schemes.     

SMLD: Enhanced MIMO‐Signal Detection for Wireless MIMO Communication Receivers

This letter proposes a simplified maximum likelihood detection (SMLD) scheme to improve the detection performance of multiple‐input multiple‐output receivers. The SMLD detects V streams according to the first detected V sub‐streams. Through an ML test, the most probable stream is selected. Moreover, to detect the layer with the worst post‐detection SNR accurately, reverse ordering is applied to the SMLD. Simulation results show that the performance of the Vertical Bell Laboratories layered space‐time (V‐BLAST) system can be improved by adopting the SMLD technique. In the case of reverse ordering, the SMLD can achieve a similar ML performance with significant reduction in computational complexity.     

Joint Semi-Blind Channel Estimation and Data Sequence Detection of OFDM Systems

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多天线系统及其在OFDM系统中的应用

利用配置多个天线实现高速可靠的通信是当今无线通信领域研究的热点之一。本文介绍了具有多个天线系统的几种不同构成方式，从系统误码率和频带利用率两个方面讨论了不同构成方式对系统的改善。同时，对多天线技术应用于正交频分复用(OFDM)系统的性能进行了分析，并给出了频率选择性瑞利衰落信道中最优译码算法下的系统性能比较。仿真结果表明，多天线OFDM系统既保持了OFDM抗频率选择性衰落的特点，又能够改善系统的误码率性能，提高频带利用率。     

Power distribution methods for MIMO‐OFDM systems and field experimentations

In this paper, we propose several power allocation schemes for multi‐input multi‐output (MIMO) orthogonal frequency division multiplexing (OFDM) transmission based on the minimization of an approximated bit error rate (BER) expression, and we evaluate the different solutions via field trial experimentations. The methods illustrated in this paper, serve to allocate power among the different transmit antennas and the different subcarriers which compose the MIMO OFDM transmitted signal. Several solutions are available to perform power allocation. Frequency domain power allocation, spatial domain power allocation and combined spatial and frequency power allocation are evaluated. We first review and describe the analytical solution for each power allocation scheme and then evaluate the complexity in terms of both computational operations and BER performances. Simulation results show the performance in term of BER and link the advantage of each possibility of power distribution with the associated complexity. Copyright © 2009 John Wiley & Sons, Ltd.     

Joint Time-Domain Tracking of Channel and Frequency Offsets for MIMO OFDM Systems

In this paper we address the problem of joint channel and frequency offset estimation and tracking in multiple-input multiple-output (MIMO) OFDM systems for mobile users. The proposed method stems from extended Kalman filtering and is suitable for time-frequency-space selective channels. Separate offset for each MIMO channel branch is considered because of the mobility and rich scattering. The channel taps and the frequency offsets are estimated in time-domain while the equalization is performed in frequency domain. Simulation results demonstrate that the proposed method tracks time-varying channels and frequency offsets with high fidelity. Realistic channel models are used in mobile scenarios. The proposed time-domain approach has improved performance and robustness in comparison to purely frequency domain processing. Computational complexity is lower as well.     

OFDM分组检测算法的研究及其VLSI实现

基于OFDM系统中短训练符号的相关性,对已有分组检测算法加以改进,提出了一种适用于正交频分复用(orthogonal frequency division multiplexing,OFDM)系统的分组检测算法.仿真结果表明,该算法在各种信噪比下均具有良好的检测能力.此外,对此算法进行了VLSI的优化实现,计算部分的实现方式采用流水线设计的思想,通过优化结构和分时复用内部电路单元,降低了所需电路的面积,提高了系统工作频率.在0.18μmCMOS工艺下,电路可稳定工作在155MHz频率,电路总面积为1034486μm2.     

Joint Frequency and Symbol Synchronization Schemes for an OFDM System

This paper proposes two multi-stage joint symbol timing and carrier frequency synchronization schemes for an orthogonal frequency division multiplex (OFDM) system. Simultaneous estimation of symbol timing and frequency offset is derived from the maximum likelihood (ML) principle, assuming a cyclic prefix (guard interval) is inserted in each OFDM symbol. One of the proposed algorithms derives an initial frequency estimate in the first stage that reduces the frequency uncertainty to only two or three sub-carrier spacings. The timing information and a finer frequency estimate that has a resolution of a sub-carrier spacing are obtained in the second stage. The third stage provides an estimation for the residual fractional frequency error. The other algorithm bypasses the first stage for one can use the second stage alone to search for the timing and frequency offsets. However, the computing complexity of the second stage is higher than that of the first stage, thus the three-stage algorithm is a preferred choice unless the frequency uncertainty is small. Simulation results show that both algorithms yield excellent performance not only in white Gaussian channels but also in multipath fading channels.