MIMO-OFDM系统中信道估计技术的性能评估

本文给出了MIMO-OFDM系统中的三种信道估计算法,详细的分析与比较了这几种算法在估计均方误差、实现复杂度、跟踪速度以及频带效率等方面的性能,最后通过MATLAB仿真试验对其进行了验证.     

Performance of LDPC coded FMT systems over frequency selective fading channel

This paper proposes the Low Density Parity Check （LDPC） coded Filtered MultiTone （FMT） systems with high-order modulation for the high data rate reliable transmission over frequency selective fading channel. For the purpose of accomplishing soft input soft output iterative decoding of LDPC codes, a new soft decision metric generation method is proposed, which obviates the need of the noise variance estimation, for M-PSK/M-QAM-type high-order modulation over frequency selective fading channel. Computer simulation indicates that, there is no performance loss with our new metric, but the complexity of implementation is reduced, and that the LDPC codes are effective to improve the Bit Error Rate （BER） of FMT in frequency selective fading channel.     

Iterative receiver for OFDM systems based on semi-blind channel estimation

In this paper we propose two iterative algorithms of joint channel estimation and symbol detection for Orthogonal Frequency Division Multiplexing （OFDM） systems. In which, superimposed pilot scheme is adopted and an initial Channel State Information （CSI） is obtained by employing a first-order statistic. In each subsequent iteration, we propose two algorithms to update the CSI. The Mean Square Error （MSE） of channel estimation and Bit Error Rate （BER） performance are given and simulation results demonstrate that the iterative algorithm using method B has good performance approaching the ideal condition.     

Block adaptive subcarrier allocation for wireless multiuser OFDM system

In this paper, a blockwise adaptive subcarrier allocation algorithm for multiuser Orthogonal Frequency Division Multiplexing (OFDM) system is introduced. Assuming the knowledge of channel information for all users, the algorithm minimizes the total transmit power while satisfying the total power and users‘ rate constraints. The result of simulation shows that the proposed algorithm reduces the average bit Signal-to-Noise Ratio (SNR) by approximately 4 dB compared with OFDM-Frequency Division Multiple Access (OFDM-FDMA), and supports more users in a multiuser Rayleigh fading channel. As assigning subcarriers in block, the computational complexity of the algorithm is much lower than that of the derivation of “water-filling” algorithm.     

A comparison of two 2D channel estimators for OFDM system

The paper deals with two-dimensional （2D） channel estimation of Orthogonal Frequency Division Multiplexing （OFDM） system ill slow fading wireless channel. We concentrate on two channel estimation schemes： Least Square （LS）＋Weighted BiLinear （WBL） and LS＋Linear Minimum Mean-Squared Error （LMMSE） where the first method is proposed in this paper. After theory analysis and simulation in Typical Urban （TU） channel, we find that LS＋LMMSE achieves the optimal perform- anee by exploiting prior knowledge of channel whereas LS＋WBL, without requiring channel knowl- edge and with only half of the computational amount of LS＋LMMSE, approaches LS＋LMMSE in Bit Error Ratio （BER） performance when the distance of two adjoining pilot symbols along frequency direction is sufficiently small. This makes LS＋WBL very suitable for wideband wireless applications.     

Cyclostationarity-based OFDM signal sensing over doubly-selective fading channels

In this paper, using cyclostationarity-based sensing method to detect the presence of Or-thogonal Frequency Division Multiplexing (OFDM) signal over doubly-selective fading channels is studied. By approximating the channel with Basis Expansion Model (BEM), we derive the second-order cyclostationary statistics of the received OFDM signal over doubly-selective fading channels. Theo-retical analysis indicates that new cyclostationary signatures produced by Doppler spread and mul-tipath delay can be further exploited in the detecting process. Simulation examples demonstrate that the sensing methods using channel-induced cyclostationary features provide substantial improvements on detection performance.     

Time-frequency 2-D LMS based long-range channel prediction for wireless OFDM systems

Adaptive modulation can optimize the spectrum efficiency and system performance with the channel state information achieved by the long-range channel prediction.To avoid re-estimating channel correlation function as the channel statioharity varies and to track the channel adaptively, LMS (Least-Mean-Square) based long-range channel prediction is discussed in the existing literature, but it needs long observation interval to reach the convergence.Given that all OFDM (Orthogonal Frequency Division Multiplexing) subcarriers have the identical time-domain correlation and sta- tionarity during the same time interval,this paper proposed a 2-D LMS based predictor which updates the filter weights in both time and frequency domain.The proposed scheme can effectively decrease the observation intervals and significantly speed up the convergence than the conventional LMS and Parallel LMS (PLMS).Complexity analysis and simulation results prove that the proposed scheme can improve the BER (Bit Error Rate) performance and spectrum efficiency with negligible complexity increase.     

GF(2p)LDPC编码的相位调制OFDM系统在瑞利衰落信道的性能

OFDM是一项能有效对抗高速无线通信中多径衰落的关键技术，为了进一步提高OFDM系统的误码性能，许多信道编码技术已被应用于OFDM系统中，二元域LDPC码以其近香农限的误码性能和较低的译码复杂度成为研究的热点。在AWGN信道下，多元域LDPC码比等效码长的二元域LDPC码有更好的纠错性能。本文提出了一种将多元域LDPC码经过MPSK调制后用于OFDM系统的新方法。仿真结果表明，在多径衰落信道下，通过合理选择多元LDPC码域的阶数和调制的方法，多元域LDPC编码的高阶调制OFDM系统比等效码长的二元域LDPC编码OFDM系统具有更好的性能，并且由于采用了多元域LDPC的快速BP译码，译码复杂度只是稍有增加。     

Robust and fast frequency offset estimation for OFDM based satellite communication

A pilot-aided Orthogonal Frequency Division Multiplexing (OFDM) frequency offset estimator designed for satellite communication system is proposed in the paper. The estimator focuses on the acquisition of the integer part of carrier frequency offset and the acquisition range is as large as the whole signal bandwidth. Making full use of the phase difference between received pilot and local referential pilot, a fast estimation is obtained. Compared with existing method, our method can also work well even in SNR as low as 0dB. Simulations verify the good performance of our method.     

空间通信中调制与编码热点技术探析

深空探测、卫星导航及卫星遥感技术的迅猛发展对空间通信的数据处理速率和抗干扰性能提出了更高要求。因此,更高的频谱利用率和抗强干扰能力是未来空间通信的发展方向。本文分析了空间通信中制约信息传输性能的主要因素,着重对近年来空间通信中调制及编码热点技术的工作原理、性能及应用情况进行描述,并针对复杂电磁环境下新型调制与编码技术的联合应用提出了解决方案。分析表明,联合应用方案可以同时提高频谱利用率和抗干扰能力。     

基于802.16e的OFDM调制解调技术的研究

随着正交频分复用技术（OFDM）的日益成熟,它在现实生活中的应用的范围也越来越广泛了。OFDM技术现在不仅广泛的应用在数字广播、数字电视中,而且由于其具有抗多径衰落,频谱利用率高,传输速度快的优点,被普偏认为是第四代移动通信物理层的关键技术之一,它能够同时为多个用户提供高数据速率的视频和语音服务。主要是针对基于802.16e标准的OFDM的不同调制解调方式在不同信道中传输的优缺点进行研究比较。     

PILOT－BASED FREQUENCY OFFSET DETECTION SCHEME IN OFDM SYSTEM

The frequency offset information is extracted from local pilot amplitude characteristics, which suffer much less distortion in frequency-selective fading channels than those utilizing frequency domain correlation techniques. Simulation shows that the performance of this scheme has better performance than the existing frequency domain pilot-based frequency offset detection scheme.     

使用接收分集的确定性OFDM盲信道估计算法

本文针对无循环前缀的OFDM系统研究了非数据辅助的信道估计方法.借助于2个接收天线实现的信道分集,提出了一个确定性信道盲估计算法,并给出了信道可辨识性条件.该算法使用1个OFDM块便可在一个标量模糊因子的意义上确定信道冲激响应.算法计算量小,而且对输入信号的星座和统计特性没有约束.仿真结果证实了算法的有效性.     

多径瑞利衰落信道中的盲信噪比估计

提出了一种在多径瑞利衰落信道中的信噪比盲估计算法。采用自相关检测和多项式拟合的方法，在保证接收端参数符合降噪原理的条件下．利用接收信号同相分量的自相关函数实现了较精确的盲信噪比估计。算法分析和计算机仿真结果表明．该算法适用于低信噪比情况，在信噪比为0-20dB时估计误差小于1dB。     

A novel estimation method of timing offset for OFDM based WLAN systems

The conventional timing synchronization methods based on time domain correlation have the problems of timing metric plateau in Additive White Gaussian Noise(AWGN) channel and estimation error in multipath fading channel. To resolve the problems, this paper proposes a novel timing metric using the characteristics of long training symbols in IEEE802.11a and a new timing recovery method based on the new timing metric for Orthogonal Frequency Division Multiplexing(OFDM)-based WLAN systems. The proposed timing metric is defined as a sum of absolute values of the imaginary parts of all the subcarrier samples. It exhibits a unique characteristic that is very sensitive to the true synchronization point since it has minimum value at the true synchronization point and maximum around the true synchronization point. The simulation results show that the performance of timing synchronization is significantly improved, as a result, the probability of error estimation is lower than 10^-4 when Signal-to-Noise Ratio(SNR) is more than 10dB.     

OFDM系统信道估计算法及其FPGA实现

文章深入分析研究了欧洲数字高清晰度电视地面广播(DVB-T)系统的COFDM调制方案。通过对大量算法的仿真，总结出适用于该方案且易于实现的信道估计算法，结合Matlab、Quartus仿真及工程应用，提出一种基于FPGA的电路设计，并完成了相应的硬件电路。     

A SYNCHRONIZATION ALGORITHM FOR HF （HIGH FREQUENCY） BROADBAND OFDM SYSTEM

In this letter, a kind of associated synchronization algorithm which is suitable for HF （High Frequency） broadband OFDM （Orthogonal Frequency Division Multiplexing） system is presented based on describing and constructing the GMW （Gorden, Mills and Welch） sequence. The algorithm is based on the Schmidl and Minn＇s symbol timing principle, the constructed GMW sequence is transmitted and disposed, and the synchronization is adjudicated using the correlation of GMW sequence. The simulation result indicates that this algorithm has high performance synchronization ability under the low SNR （Signal to Noise Ratio） at two different kinds of channel models.     

Propagator based closed-form blind carrier frequency offset estimation for OFDM systems

Most of the blind Orthogonal Frequency Division Multiplexing （OFDM） Carrier Frequency Offset （CFO） estimators necessitate large number of samples to ensure the estimation accuracy. However, the number of samples can not be selected too large because of the carrier offset drift. In this letter, a new closed form algorithm for blind OFDM CFO estimation in frequency-selective channel is proposed. This method utilizes the propagator obtained from data matrix and the diagonal loading technique, thus it has better performance even only using one or two OFDM blocks. Furthermore, the range of the CFO estimation which can be handled is overall transmission spectral. Simulation results confirm its effectiveness.     

A NOVEL INTEGER FREQUENCY OFFSET ESTIMATOR FOR OFDM

One of the principal disadvantages of Orthogonal Frequency Division Multiplexing （OFDM） is very sensitive to carrier frequency offset. The integer frequency offset has no effect on the orthogonality among the subcarriers, but it causes a circular shift and phase rotation of the received data symbols sequence, resulting in a Bit Error Rate（BER） of 0.5. In this paper, a novel integer frequency offset estimator for OFDM is derived based on maximum likelihood estimation technique and exploration of the differential relation between two consecutive OFDM data symbol sequences in frequency domain. Its performance is compared with the conventional method by computer simulations for the additive white Gaussian noise channel and a multipath fading channel. Simulation results show that the performance of the proposed estimator is better than the conventional estimator.