Peak-to-average power ratio in MIMO-OFDM systems using selective mapping

This letter evaluates the peak-to-average power ratio (PAR) performance in a space-time block coded (STBC) multi-input multi-output orthogonal frequency-division multiplexing (MIMO-OFDM) system using the selective mapping (SLM) approach. The investigated SLM-based MIMO-OFDM system selects the transmitted sequence with the lowest average PAR over all transmit antennas and retrieves the side information very accurately at the expense of only 0.5-dB degradation of the PAR, which can improve the overall detection performance of the MIMO-OFDM system with erroneous side information, compared to the individual SLM approach.     

MIMO-OFDM系统同步器的硬件结构设计

由于在MIMO-OFDM系统中有多个发射和接收天线,并且每个接收天线都能接收到所有发射天线的信号,当各个收发天线之问的子信道延迟不同时,在每个接收天线端的帧同步检测就会受到其他天线发射同步信号的干扰。为了解决这个问题,我们提出了一种MIMO-OFDM系统的帧同步的设计方法及相应同步器的硬件结构设计。该同步单元可以实现时域和频域的同步,其中时域的同步是通过一组ZCZ码的匹配滤波器实现的,而频域的同步是通过同步帧的训练符号实现的。并且在Altera＇s公司的FPGA开发板上实现了同步器的硬件设计。     

Broadband MIMO-OFDM wireless communications

Orthogonal frequency division multiplexing (OFDM) is a popular method for high data rate wireless transmission. OFDM may be combined with antenna arrays at the transmitter and receiver to increase the diversity gain and/or to enhance the system capacity on time-varying and frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) configuration. The paper explores various physical layer research challenges in MIMO-OFDM system design, including physical channel measurements and modeling, analog beam forming techniques using adaptive antenna arrays, space-time techniques for MIMO-OFDM, error control coding techniques, OFDM preamble and packet design, and signal processing algorithms used to perform time and frequency synchronization, channel estimation, and channel tracking in MIMO-OFDM systems. Finally, the paper considers a software radio implementation of MIMO-OFDM.     

A road to future broadband wireless access: MIMO-OFDM-Based air interface

Orthogonal frequency-division multiplexing is a popular method for high-data-rate wireless transmission. OFDM may be combined with multiple antennas at both the access point and mobile terminal to increase diversity gain and/or enhance system capacity on a time-varying multipath fading channel, resulting in a multiple-input multiple-output OFDM system. In this article we give a brief technical overview of MIMO-OFDM system design. We focus on various research topics for the MIMO-OFDM-based air interface, including spatial channel modeling, MIMO-OFDM transceiver design, MIMO-OFDM channel estimation, space-time techniques for MIMO-OFDM, and error correction code. The corresponding link-level simulation results are encouraging, and show that MIMO-OFDM is a promising road to future broadband wireless access.     

Effects of Carrier Frequency Offset and Channel Estimation Errors on the Performance of MIMO-OFDM Systems in Spatially Correlated Channels

As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency division multiplexing (OFDM) is extensively used in high-rate wireless communication systems, such as, the wireless local area network (WLAN) and the digital television terrestrial broadcasting (DTTB) systems, to support high performance bandwidth-efficient multimedia services. Multiple antennas and transmit or receive diversity, multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM), can be used to improve error performance and capacity of wireless systems. In this paper, we consider the effects of carrier frequency offset and channel estimation errors on the performance of MIMO-OFDM systems in spatially correlated channels. Theoretical calculations and computer simulations are done to analyze the performance degradation of MIMO-OFDM systems in spatially correlated channels due to carrier frequency offset and channel estimation errors, and the theoretical and simulated results match well.     

Training sequence design and channel estimation of OFDM-CDMA broadband wireless access networks with diversity techniques

In this paper, an OFDM-CDMA system employing various diversity schemes is considered for a possible candidate of broadband wireless access networks and broadcasting applications. With an emphasis on a preamble design for multi-channel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in OFDM-based multiple-antenna transmission systems. By properly designing each preamble for multiple antennas to be orthogonal in the time domain, the channel estimation can be applied to the HIPERLAN/2 and IEEE 802.11a standards in the case of more than two transmit antennas. Also, the effect of diversity techniques on the performance of OFDM-CDMA based broadband wireless access networks is investigated and the maximum achievable diversity gain for a two-path Rayleigh fading environment is evaluated. Simulation results show that the OFDM-CDMA system applying a space-time-frequency diversity with a full-rate full diversity code can give the diversity of D=4 and D=8 for both multi-user cases of maximum user and half user capacities, respectively.     

MIMO-OFDM系统定时同步算法

应用MIMO—OFDM无线通信系统的空间信号资源，提出了基于单个前导符号的MIMO--OFDM系统帧定时和符号定时同步的分集算法，以克服高速无线多径信道中深衰落对MIMO—OFDM系统定时同步性能的影响，给出了具体的帧定时、符号定时同步的分集算法以及在高速无线多径信道COST207模型下帧定时和符号定时同步的仿真结果。     

Comparing Selected Mapping and Partial Transmit Sequence for PAR Reduction

Selected mapping (SLM) and partial transmit sequence (PTS) are two existing distortionless peak-to-average power ratio (PAR) reduction schemes that have been proposed for orthogonal frequency division multiplexing (OFDM). Previously, it was argued that SLM and PTS have comparable PAR reduction performance but that the latter has lower computational complexity because it uses fewer IFFTs. In this paper, we show that the overall computational complexity of PTS is only lower than that of SLM in certain cases, and that SLM always has better PAR reduction performance. We compare the two schemes using three different performance metrics by assuming a given amount of computational complexity that can be afforded. Using the metrics, we show that SLM outperforms PTS for a given amount of complexity.     

Multi-antenna scheme for high capacity transmission in the digital audio broadcasting

In this paper, we apply multi-antenna scheme to DAB system for high-rate transmission. In the MIMO-DAB system with a number of transmitting and receiving antennas increases the transmission rate efficiently with low multiplication operations. For channel estimation in the MIMO-DAB system, we propose a new PRS structure and evaluate the channel estimation performance. The proposed PRS structure can estimate multi-channel information based on the semi-blind processing. The performance of proposed PRS is compared with that of conventional training sequence employing Tarokh's transmission matrix. And also we investigate the performance limitation due to the imperfect channel estimation in a MIMO-DAB receiver. It has been shown that the MIMO-DAB system with multi-antennas can achieve the high-rate transmission for multimedia broadcasting and the performance of MIMO-DAB system is impaired by imperfect channel information.     

一种改进的MIMO-OFDM系统Steiner估计算法

鉴于MIMO-OFDM（多输入多输出-正交频分复用）系统中时域信道方法的灵活性和低带宽开销的特点,本文采用Steiner训练符号设计方法设计MIMO-OFDM系统中的时域训练符号,给出传统CDMA系统的Steiner信道估计器在MIMO-OFDM系统中的实现过程,解决了MIMO-OFDM时域信道估计矩阵求逆的问题,降低了时域信道估计的实现复杂度。并进一步提出了采用接收天线DOA（波达方向）改进的Steiner信道估计方法,提高了信道估计的精度,获得了一定程度的性能增益,增强了时域信道估计方案的实际可行性。     

MIMO-OFDM快衰落信道的稀疏自适应感知估计

在多输入多输出（MIMO）-正交频分复用（OFDM） 系统中,怎样在较高频谱利用率的情况下对快时变信道进行较为准确的估计是一个具有挑战性的课题。该文在利用压缩感知理论可提高系统频谱利用率的基础上,提出了一种适合于快时变环境下MIMO-OFDM 系统的稀疏自适应信道估计方法。该方法不再受到奈奎斯特采样频率条件约束,避免了传统导频辅助信道估计方法频谱利用率低的缺点。该文方法通过构建多天线群时频结构特征稀疏基,利用多天线间和群时变OFDM符号内信道冲激响应具有更强稀疏性的特点,对MIMO-OFDM快衰落信道进行稀疏变换。由于实际MIMO-OFDM快衰落信道往往处于频率选择性、时变性和多种干扰并存的复杂环境,受到干扰的信道参数对系统而言是未知,采用该方法克服了现有基于压缩感知理论的信道估计方法需要预先知道信道冲激响应稀疏度才能重构信道参数的不足,在信道稀疏度未知道的情况下,运用稀疏自适应的方法来对不同时频结构特征的信道参数进行估计。仿真结果表明所提估计方法具有对快时变信道参数估计的鲁棒性和较高频谱利用率,且均方误差小。      

降低MIMO-OFDM系统峰均比的改进PTS算法

多输入多输出频分复用技术是将OFDM与MIMO技术相结合的无线通信系统,也具有OFDM系统存在的峰均比较高的问题。分析了MIMO-OFDM系统模型及PAPR问题,在次优PTS算法的基础上,提出了迭代限幅PTS联合算法。该算法先将MIMO-OFDM信号经过迭代PTS算法进行处理,然后再将其经过限幅处理进一步降低系统的PAPR。最后在自适应PTS算法的基础上进行了改进,仿真结果表明系统复杂度和PAPR性能有所降低,但可以通过调整门限来提高性能。     

HTRCI and Channel Ranking Based Joint Symbols Detection for MQRD-PCM/MIMO-OFDM

MIMO-OFDM is considered a key technology in emerging high-data rate systems. In MIMO-OFDM systems, channel estimation and signal detection are important to distinguish transmit signals from multiple transmit antennas. Previously, we have proposed a parallel detection algorithm using multiple QR decompositions (Q is an orthogonal square matrix, R is upper triangular matrix) with permuted channel matrix (MQRD-PCM) to reduce the system complexity of MIMO-OFDM. This method achieves a good BER performance with a low system complexity. However, since MQRD-PCM is a kind of parallel detection method, the wrong detection probability is increased due to the bad channel signal-to-interference plus noise power ratio (SINR) of the transmitted signals. As a result, the average BER performance is influenced by the wrong detection probability of the bad channel SINR. To overcome the above-mentioned problems, in this paper, we propose the high time resolution carrier interferometry and channel ranking based joint signal detection for MQRD-PCM/MIMO-OFDM.     

MIMO-OFDM系统中基于循环移位和信号联合的改进SLM算法

为降低多输入多输出正交频分复用(MIMO-OFDM, multiple input multiple output orthogonal frequency division multiplexing)系统中传统选择性映射(SLM, selected mapping)算法的计算复杂度,提出了通过信号时域循环移位和天线间信号联合产生更多具有不同峰均功率比(PAPR, peak to average power ratio)的备选序列集合的方法。接收端先根据发射端序列选取情况恢复出频域旋转信号,再比较反向旋转序列与最近星座点的距离来恢复原始序列。仿真结果表明,所提出的方法能有效地抑制MIMO-OFDM信号的PAPR。另外,与传统SLM算法相比,提出方法明显降低了计算复杂度,而且可以获得传统SLM方法在已知边带副信息情况下近似的比特误码率性能。     

Robust and Improved Channel Estimation Algorithm for MIMO-OFDM Systems

Multiple-input multiple-output (MIMO) system using orthogonal frequency division multiplexing (OFDM) technique has become a promising method for reliable high data-rate wireless transmission system in which the channel is dispersive in both time and frequency domains. Due to multiple cochannel interferences in a MIMO system, the accuracy of channel estimation is a vital factor for proper receiver design in order to realize the full potential performance of the MIMO-OFDM system. A robust and improved channel estimation algorithm is proposed in this paper for MIMO-OFDM systems based on the least squares (LS) algorithm. The proposed algorithm, called improved LS (ILS), employs the noise correlation in order to reduce the variance of the LS estimation error by estimating and suppressing the noise in signal subspace. The performance of the ILS channel estimation algorithm is robust to the number of antennas in transmit and receive sides. The new algorithm attains a significant improvement in performance in comparison with that of the regular LS estimator. Also, with respect to mean square error criterion and without using channel statistics, the ILS algorithm achieves a performance very close to that of the minimum mean square error (MMSE) estimator in terms of the parameters used in practical MIMO-OFDM systems. A modification of the ILS algorithm, called modified ILS (MILS), is proposed based on using the second order statistical parameters of channel. Analytically, it is shown that the MILS estimator achieves the exact performance of the MMSE estimator. Due to no specific data sequences being required to perform the estimation, in addition to the training mode, the proposed channel estimation algorithms can also be extended and used in the tracking mode with decision-aided method.     

MIMO-OFDM系统信道估计算法综述

MIMO-OFDM技术是近年来通信信号处理领域的研究热点,无线信道不仅是频率选择性的,而且是时变的,为了获得较高的系统性能,信道估计就显得尤为重要.对MIMO-OFDM系统中现有的信道估计算法进行了综述,重点介绍了基于训练序列的、导频符号的以及盲和半盲信道估计算法.比较了各种算法的优缺点,提出了未来信道估计的发展方向.     

OFDMA Uplink Performance for Interactive Wireless Broadcasting

A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called OFDMA is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This work focuses on the performance of OFDM when used as a modulation and access technique for the uplink of an interactive wireless broadcasting system. In such an application, we present our investigations of the effects of symbol timing misalignments and peak-to-average power ratio (PAR). Under the condition of the same side information bits, the selective mapping (SLM) approach can be implemented by decimating OFDM samples, which is less complex compared to the ordinary SLM incurring a slight degradation of the PAR performance. Taking into account the effects of clipping operation and symbol timing misalignment, the bit error rate (BER) and throughput performances are investigated for a typical OFDMA uplink scenario.     

Throughput Performance Analysis of AMC Based on a New SNR Estimation Algorithm Using Preamble

The rapid growth in mobile communication users necessitates the development of reliable communication systems that provide higher data rates. To meet these requirements, techniques such as multiple input multiple output (MIMO) and orthogonal frequency division multiplexing (OFDM) have been developed in recent years. Current research activity is focused on developing MIMO-OFDM systems that combine the benefits of both techniques. In addition, for a fast wireless channel environment, the data rate and reliability can be optimized by setting the modulation and coding adaptively according to the channel conditions, as well as by using sub-carrier frequency and power allocation techniques. The overall system performance depends on how accurately the feedback-based system obtains the channel state information and feeds it back to the transmitter without delay. In this paper, we propose a signal-to-noise ratio (SNR) estimation algorithm in which the preamble is known for both sides of the transceiver. Also, we applied AMC on several channel environments using the parameters of IEEE 802.11n and compared throughput performance using each of the different SNR Estimation Algorithm. The results obtained prove that our proposed algorithm is more accurate than traditional algorithms.     

A Simulation Study on Channel Estimation for MIMO-OFDM Based Beyond 3G Mobile Systems

1 Introduction Future wireless cellular communication networks areexpected to fully support multi-media services on a real-ti me basis ,therefore pose a challengingissue of develop-ing high-speed access and high data-rate transmissiontechnologies , which are not satisfied by current 2nd-and3rd-generation cellular mobile communications net-works[1]. Orthogonal Frequency Division Multiplexing(OFDM) has become a popular technique for high datarate wirelesstransmission duetoits many advantagesfo…     

STBC MIMO-OFDM peak-to-average power ratio reduction by cross-antenna rotation and inversion

Multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) has become a promising candidate for high performance 4G broadband wireless communications. However, like OFDM, one main disadvantage of MIMO-OFDM is that the signals transmitted on different antennas might exhibit a prohibitively large peak-to-average power ratio (PAPR). We will show that the PAPR reduction for MIMO-OFDM needs a more efficient solution than applying existing schemes for OFDM systems separately on each antenna. Further, we suggest a scheme of cross-antenna rotation and inversion (CARI), which utilizes additional degrees of freedom by employing multiple antennas. Two suboptimal schemes, termed successive suboptimal CARI (SS-CARI) and random suboptimal (RS-CARI) show significant performance advantages and lower computational complexity compared to the concurrent selective mapping (SLM) scheme proposed in Y. -L. Lee et al. (2003).