OFDM系统中基于迭代和门限理论降低PAPR的改进PTS方法

传统的部分传输序列法（PTS）中,如果要得到最佳的相位旋转因子,需要遍历所有的可选相位,这样的计算量随分割的子序列数按指数增长。本文提出两种减少计算复杂度的改进PTS方法（经迭代的IPTS和经门限的IPTS）。这两种方法都利用快速傅立叶变换（FFT）的时域循环移位特性增加备选信号,改善PAPR降低的性能。首先提出了一种利用反复迭代移位的IPTS方法(经迭代的IPTS),该方法的计算量随子序列数按线性增长。接着提出了一种利用预先设定的门限值以降低计算复杂度的IPTS方法(经门限的IPTS)。最后理论分析、比较了PTS、PTS/CSS方法与本文方法的计算复杂度。Matlab软件仿真显示,当使用相同的分割子序列数和相位旋转因子时,两种IPTS方法能够以较小的运算量达到了PTS/CSS方法性能;两种IPTS方法降低PAPR的性能要优于PTS方法,但计算复杂度并没有增加。      

A New Algorithm for Peak/Average Power Reduction in OFDM Systems

We present a new method (MMSE-threshold) for peak/average power reduction. This technique is derived from a constellation shaping algorithm, where the constellation points with lower average energy are selected from a larger set of points. There are multiple choices available to select the points with lower peak energy for a given sequence of data bits, and this flexibility is used to reduce the peak to average power ratio (PAPR). Subsequently, this selection algorithm, which is formulated in terms of a zero-one quadratic problem, is optimized by the semidefinite programming algorithm (SDPA). Simulation results show that the PAPR of SDPA is noticeably better than MMSE-Threshold, while the complexity of MMSE-Threshold is smaller than that of SDPA. MMSE-Threshold is compared with alternative techniques reported in the literature. We show that, while we obtain a PAPR that is similar or better than those of the compared techniques, the complexity of MMSE-Threshold is low. In addition, the MMSE-threshold method results in about 1 dB shaping gain (reduction in the average energy) with less than 1% rate loss for PAPR reduction.     

OFDM系统降低峰均功率比的研究

正交频分复用技术非常适合高速通信系统,但却存在着高峰均比问题。部分传输序列法因其灵活有效的降低OFDM系统的峰均功率比而被广泛的研究。对以前技术研究之后,提出一种基于相位的搜索算法。仿真结果表明,用这种方法降低系统的计算复杂度,并且其性能和以前的方法相比没有降低。     

OFDM系统基于信道多径时延检测的自适应信道估计

针对宽带正交频分复用(OFDM)系统,提出了一种基于信道多径时延估计的自适应信道估计算法.文章分析了无线信道的特性,随后利用导频频域相关性进行多径时延估计,提出通过动态跟踪信道时延估计参数自适应选择信道估计中适宜的导频数目的方法,同时实现循环前缀(CP)长度的实时调整.仿真结果表明,在时变环境下,本文算法可以有效获知当前信道时延参数的近似统计信息,实现对系统冗余信息的优化.     

OFDM系统在动态环境下的自适应信道估计

分析了OFDM通信系统信道估计采用二维自适应维纳滤波的情形，信道估计根据对不同的信道环境（多径时延，多普勒频移）的实时检测，自适应地选择适宜的滤波参数，动态地跟踪信道的变化特征。仿真分析得到了最优的滤波参数并且证明了在非完全匹配条件下的插值亦有很好的效果。     

A Flexible Peak-To-Average Power Ratio Reduction Scheme for OFDM Systems by the Adaptive Projected Subgradient Method

One of the main issues of the orthogonal frequency-division multiplexing (OFDM) modulation is the high peak-to-average power ratio (PAPR) of the transmitted signal, which adversely affects the complexity of power amplifiers. In this paper, we consider transmitters that reduce the PAPR by slightly disturbing the symbols in carriers used to transmit information and by sending dummy symbols—i.e., symbols not conveying information—in unused carriers. The optimal choice of the data and dummy symbols is determined by the solution of a convex optimization problem. To reduce the PAPR with low complexity, we apply a modified version of the adaptive projected subgradient method to a sequence of convex cost functions closely related to the original optimization problem. The resulting algorithm achieves near-optimal PAPR in practical scenarios, generalizes existing algorithms based on Polyak's method, and can easily handle multiple constraints.      

OFDM系统信道估计算法研究

在OFDM技术应用于移动通信领域时,其信道估计技术是一项关键技术。分析了级联的一维的信道估计方法,采用加窗的方式改进了基于FFT一维级联信道估计算法,在此基础上提出一种新的基于DCT（EIDCT）变换的信道估计算法。最后给出了改进算法的性能仿真,并对仿真结果进行分析。     

Optimization of Peak to Average Power Reduction in OFDM

Orthogonal frequency division multiplexing (OFDM) a multicarrier system [1, 2] provides base for all advanced wireless communication system. The performance of OFDM is degraded by peak-to-averagepower ratio (PAPR). High PAPR requires high power amplifiers (HPAs). The nonlinearity of the HPA exhibits amplitude and section distortions, that cause loss of orthogonality among the subcarriers, and hence, intercarrier interference (ICI) is introduced inside the transmitted signal. Not only that, high PAPR put together lands up in in-band distortion and out-of-band radiation. Rather than using HPA’s, the only way to improve performance of OFDM system is to reduce PAPR. The PAPR reduction of OFDM system gives fair reduction in PAPR under partial transmits sequence (PTS) and DCT-SLM techniques. Here in this paper we proposed a combination of PTS and DCT-SLM and an algorithm to cut back the PAPR. This hybrid combined technique reduces PAPR effectively and minimizes the complexity of PTS technique.     

Adaptive Channel Estimation for Multiple Antenna OFDM Systems

We propose a simple adaptive algorithm for channel estimation of multiple antenna OFDM systems to provide competitive performance with low computational complexity and tracking ability. The derivation of the mean-squared error (MSE) for the proposed scheme is presented and the optimum selection of the parameter in the scheme is analyzed by utilizing the derived MSE expression.     

基于可变遗忘因子RLS在OFDM系统的载波频偏估计

在时变信道下正交频分复用（OFDM）系统中,通过导频辅助,提出基于可变遗忘因子RLS（VFF-RLS）的载波频偏（CFO）估计改进算法。针对传统RLS（CFF-RLS）算法中遗忘因子无法同时满足CFO估计收敛速度和收敛精度的缺陷,本文设计了线性变化遗忘因子(LFF)和非线性变化遗忘因子(NLFF) 两种可变遗忘因子方案来提升CFO估计性能。仿真结果显示：在低信噪比的情形下,基于VFF-RLS算法的CFO估计性能明显优于基于CFF-RLS算法的CFO估计性能。     

一种基于PTS技术降低OFDM系统峰均比的改进算法

该文在研究部分传输序列(PTS)技术降低OFDM系统峰均功率比(PAPR)的基础上,提出了多级寻优的改进PTS方法。该方法通过增加寻优的级数,而减少每级优化所预设的相位集合中的元素个数来寻找最优相位旋转因子,从而改变传统PTS技术中的计算量随分割子序列数和待选相位集合元素的个数的增加而呈指数增长的趋势,并使该方法与次优搜索方法相结合。通过计算复杂度的分析和仿真结果表明,同传统的方法相比,该方法不仅能较大地降低计算复杂度,且能改善降低峰均功率比的性能。     

一种基于压扩变换降低OFDM系统峰均比的方法

对一种用于减小OFDM信号峰均比的压缩扩展技术进行了分析,该技术能够以相对较小的计算复杂度大大降低OFDM信号的PAPR。作者采用μ率压扩算法,其中的压扩拐点值取为均值而不是传统方法中的峰值。计算机仿真表明这种方法能大大降低OFDM信号的PAPR,并且与PTS技术相比这种方法更有效,计算复杂度也比PTS技术低。     

基于幅度预矫正降低OFDM系统PAPR的方法

本文针对幅度预矫正技术在降低OFDM系统PAPR(peak-to-average power ratio)的同时增加了系统平均功率的缺点,采用对部分输入符号进行两个相反方向的放大和缩小,以达到进一步降低系统PAPR,并且降低系统平均功率的目的.分析及仿真结果表明,在保证系统在AWGN信道中的误比特率性能的前提下,在概率为10-4处,改进后的方法比原方法PAPR降低了0.6dB,系统平均功率也有明显降低.本文方法具有算法简单、不产生失真,不发送边带信息及灵活性高,且易于与其他降低PAPR技术结合使用等特点,具有较好的实用价值.     

用部分传输序列方法降低实数OFDM信号峰均值比

该文分析了部分传输序列方法(PTS)用于降低实数OFDM信号峰均值比(PAPR)时存在的问题,分别研究并提出用于降低实数OFDM信号PAPR的相邻分割PTS(APTS)及交织分割PTS(IPTS)具体实现方法。仿真结果表明,APTS和IPTS能够有效降低信号的PAPR;在相同待选传输序列情况下,APTS算法性能与IPTS算法基本相当;APTS算法需要多次的IDFT运算,而IPTS仅需要一次IDFT运算。     

一种新的基于判决反馈的OFDM系统信道估计方法

提出了一种基于判决反馈机制的信道估计方法,采用少量导频估计出初始信道信息,再从接收端检测出的符号中选取一组包含了少量错误的符号作为准导频,反馈到信道估计器,与初始的导频一起构成数量更多的导频序列,经过几次迭代,达到提高信道估计精度的目的。为提高初始估计的精度。信道估计采用了一种频域分集的方法。计算机仿真表明,在多径瑞利衰落信道下,本文的方法可以在较低的复杂度下有效地提高信道估计的精度,在MMSE接收条件下,系统具有良好的误比特率（BER）性能。     

OFDM通信系统中自适应信道估计方法

提出了OFDM通信系统中一种新型自适应信道估计方法。利用信道冲激响应的最重要分量，该自适应信道估计方法能有效降低子载波间干扰和高斯白噪声的影响。根据性能仿真结果比较，本方法的MSE和BElK结果优于变换域插值、时域插值。     

一种基于DFT的低复杂度虚子载波OFDM信道估计算法

该文针对采用虚子载波的OFDM通信系统,提出了一种基于DFT的低复杂度信道估计算法,并与最小二乘(Least Square,LS)估计算法和线性最小均方误差(Linear Minimum Mean Square Error,LMMSE)算法进行了详细的性能和复杂度比较。所提DFT算法较好地降低了高斯白噪声的影响,相对于LS算法获得了较大的性能增益。该算法在复杂度与性能之间取得了较好的折衷,具有很好的实用价值。     

基于导频辅助的低压电力线通信系统信道估计算法

针对低压电力线信道噪声强、衰减大的问题,基于OFDM技术的电力线通信系统采用时域扩充结构,提高导频数据的可靠性.确定了在OFDM时域扩充结构中应用LMMSE准则时最大多径时延的最优取值.在此基础上提出一种更适合时域扩充系统的信道估计优化算法,通过利用OFDM符号内所有导频参与信道估计从而在低信噪比条件下获得更低的误码率.仿真结果表明提出的改进算法与传统LMMSE算法相比具有更好的信道估计性能,在信噪比低于0dB时改进算法的性能优势更加明显.     

Joint Estimation of Power and DC Offset for OFDM Systems Based on Distribution Function

In orthogonal frequency division multiplexing (OFDM) systems, the estimate errors of automatic gain control (AGC) and DC estimation (DCE) are larger than in single carrier systems. It is due to the fact that OFDM signals have Gaussian distributions. Consequently, estimate errors will occur in the AGC and the DCE because of the truncating effect generated by the analog-to-digital converter (A/D). In addition, the AGC and the DCE will interfere with each other when operated independently. In this paper, we analyze the estimate errors of the conventional AGC and DCE method in OFDM systems. We then propose the joint AGC-DCE method which performs the AGC and the DCE at the same time. The proposed method uses the distribution function of OFDM signals, and overcomes the truncating effect of the A/D and the interference between the signal power and the DC offset. In addition, we propose an enhanced AGC-DCE method which combines the proposed method with the conventional DCE so as to further reduce estimate errors. Simulation results verify that, the enhanced method estimates the signal power and the DC level more accurately than the conventional method. If the same estimate errors are reached, the estimate delay of the enhanced method can be only 1/4 of that of the conventional one.     

一种降低OFDM系统PAPR的改进压扩方法

正交频分复用（OFDM）技术是一种多载波调制方式。作为第四代移动通信系统的核心技术,OFDM己成为当今高速无线通信领域的研究热点。正交频分复用（OFDM）系统存在高峰值平均功率比（PAPR）问题,这一问题会导致系统性能降低,为其实用化设置了障碍。文中研究了一种改进的降低PAPR的非线性压缩与扩展算法,可以得到更好的PAPR性能。该方法的特点在于采用一种计算复杂度较低的非线性压扩函数,同时融合了叠加训练序列方法的改进非线性压扩算法,从而有效地降低系统PAPR。理论研究和仿真结果表明,与传统方法相比,所提出的改进压缩与扩展方法可以在降低系统复杂度的同时得到更好的PAPR性能。