OFDM系统中基于压缩感知的非线性失真恢复研究

正交频分复用系统(OFDM)的高峰均比(PAPR)问题降低了系统传输效率,增加了解调难度,使高功率放大器(HPA)满负载工作。该文针对削波与HPA引入的非线性失真问题,提出在接收端采取基于泰勒级数的预处理方式对非线性干扰进行处理,最小化信号畸变影响。在发送端通过削波降低信号的PAPR,并利用削波噪声在时域上的稀疏性特性,将传输过程作为整体考虑,最后利用正交匹配追踪算法(OMP)恢复总的非线性失真信号。仿真结果表明,该文所提方法能够有效抑制系统的干扰信号,减少了有用信号受到的非线性影响,验证了该方案的正确性。     

OFDM系统中基于限幅和压缩感知的非线性失真补偿算法

正交频分复用（Orthogonal Frequency Division Multiplexing,OFDM）信号具有较高的峰均功率比（Peak to Average Power Ratio,PAPR）,不仅影响功率放大器（High Power Amplifier,HPA）的工作效率,而且HPA使得OFDM信号产生严重的非线性失真,导致系统的误比特率（Bite Error Rate,BER）增大.本文基于限幅和压缩感知（Compressive Sensing,CS）提出了改进的补偿算法,发送端采用限幅降低信号的PAPR,接收端首先采用改进的逆模型方式减小HPA引入的非线性失真,再采用CS抵消由限幅引入的信号失真.仿真表明,所提方法不仅明显降低了OFDM信号的PAPR,而且有效提高了系统的BER性能.     

LED clipping distortion compensation in optical wireless communication via multiple transmit LEDs

A method to reduce signal clipping distortion in indoor optical wireless communication systems based on orthogonal frequency division multiplexing (OFDM) modulation is presented in this paper. Compensating the resulting clipping distortion due to LED operational constrains is achieved through the use of iterative signal clipping (ISC) and multiple light emitting diodes (LED) at the transmitter. ISC technique is based on iterative clipping of the time domain OFDM signal and transmission from a multiple LED transmitter. Transmit LEDs are synchronized, located close to each other, and placed to emit light in the same direction. Hence, the channel path gains from each LED to the receiver photo diode are similar. The received signals from the different LEDs add coherently at the receiver. Reported results demonstrate that the effect of distortion due to clipping is eliminated or significantly reduced based on the considered number of LEDs.     

Performance analysis of deliberately clipped OFDM signals

We analyze the performance of the clipped orthogonal frequency division multiplexing (OFDM) system in terms of peak power reduction capability and degradation of channel capacity. The clipping is performed on the baseband OFDM signals with and without oversampling, followed by the ideal low-pass filter. First, the effect of the envelope clipping on the peak-to-average power ratio (PAPR) and the instantaneous power of the band-limited OFDM signal is studied. We then discuss the channel capacity of the oversampled and clipped OFDM signals over the additive white Gaussian noise and ideally interleaved Rayleigh fading channels. The capacity is calculated based on the assumption that the distortion terms caused by the clipping are Gaussian. It is shown that the SNR penalty due to the clipping can be considerably alleviated by using optimal coding and reducing the information data rate. The results are justified by the simulation results using near optimal turbo codes     

PAPR Reduction of OFDM Signal via Custom Conic Optimized Iterative Adaptive Clipping and Filtering

High peak to average power ratio (PAPR) of OFDM reduces the transmitter power efficiency and increases the complication in hardware implementation. A number of techniques have been developed to minimize PAPR of OFDM. Among these techniques, iterative clipping and filtering (ICF) is an effective recent technique that produces significant results in minimizing PAPR. But, the major limitation of this approach is that it requires several iterations for minimizing PAPR. In order to overcome the above limitation, an optimization based ICF is used in this research work. An efficient custom optimized adaptive clipping and filtering technique is proposed. The proposed method shows a greater reduction in PAPR in lesser iterations with reduced out-of-band distortion and bit error rate when compared over classical clipping techniques. Moreover, the effects of oversampling on the performance of the proposed OFDM system are also evaluated.     

最高功放效率PAPR抑制评估标准和迭代准则

高峰均功率比一直是掣肘OFDM系统性能的最大障碍。为了更精确地评估峰均比抑制对功放失真的影响,提出了最优CCDF(OCCDF)迭代收敛准则。新准则考虑放大器中每个子载波的失真,限定MER条件下得到尽可能低的放大器输入功率回退(IBO),获得最高功放效率所匹配的OFDM整体信号分布。基于此,在CMMB系统中给出了一种高效的时域削波结合星座图扩展(ACE)的峰均比抑制技术。采用OCCDF迭代峰均比抑制方案实验结果:MER=40 dB条件下,基于ACE峰均比抑制的OFDM系统相比抑制前获得了3.05 dB的功放IBO性能改善。对应到功率放大器的性能改善:功放效率提升了15.74%,总功耗和管耗分别降低了22.2%和40.19%。     

Effects of clipping and filtering on the performance of OFDM

Orthogonal frequency division multiplexing (OFDM) is an attractive technique for wireless communication applications. However, an OFDM signal has a large peak-to-mean envelope power ratio, which can result in significant distortion when passed through a nonlinear device, such as a transmitter power amplifier. We investigate, through extensive computer simulations, the effects of clipping and filtering on the performance of OFDM, including the power spectral density, the crest factor, and the bit-error rate. Our results show that clipping and filtering is a promising technique for the transmission of OFDM signals using realistic linear amplifiers     

An Adaptive Clipping and Filtering Technique for PAPR Reduction of OFDM Signals

In recent years, many peak-to-average power ratio (PAPR) reduction techniques have been proposed for orthogonal frequency division multiplexing (OFDM) signals. Among various techniques, the iterative clipping and filtering (ICAF) technique has been considered as a practical scheme, and widely used owing to its non-expansion of bandwidth, low computational complexity, and simplicity in implementation without receiver-side cooperation. However, the performance of conventional ICAF technique is degraded, because the same signals are iteratively clipped with a fixed clipping threshold (CT) in every clipping operation. In this paper, we analyze the performance of conventional ICAF technique, and then propose an adaptive ICAF scheme, which clips the signal with an adaptively modified CT in every clipping operation to achieve enhanced PAPR reduction of OFDM signals. Simulation results show that the proposed scheme significantly outperforms the conventional scheme, in PAPR reduction of OFDM signals at the same number of iterations.     

限幅OFDM系统中的判决辅助信道估计

在正交频分复用(OFDM)系统中，对基带时域信号限幅是通常采用的一种降低信号峰均比的方法，但是由限幅产生的非线性干扰使系统的信道估计和信号检测性能降低。本文针对OFDM系统的频域插值信道估计方案，分析了限幅对信道估计性能的影响，提出一种利用判决的信号辅助信道估计的方法，并分析了该方法为信道估计所能提供的最大信干比增益。理论分析和仿真结果表明这种方法能够提高信道估计的精度，从而提高信号检测的性能，降低接收机的误符号率。     

OFDM系统中限幅非线性失真迭代对消法性能分析

限幅技术常用于正交频分复用系统中以抑制高峰值平均功率比,为抑制限幅造成的带内失真和带外辐射,可以在接收端进行限幅噪声的迭代估计和对消。该文提出对限幅带来的非线性失真进行迭代估计和对消的效果要比仅对限幅噪声进行估计对消好。前者在理想情况下能完全恢复出发端未限幅前的信号,但后者将始终受到由限幅带来的信号衰减的影响。理论分析和仿真结果验证了该结论及性能,同时数字结果表明,该算法能有效改进系统性能。在归一化限幅门限为1.2时,只通过一次迭代就能提高2dB。     

Novel Peak‐to‐Average Power Ratio Reduction Methods for OFDM/OQAM Systems

The tone reservation method is one of the most effective pre‐distortion methods for peak‐to‐average power ratio reduction in orthogonal frequency division multiplexing (OFDM) systems. Its direct application to OFDM systems with offset quadrature amplitude modulation (OQAM) is, however, not effective. In this paper, two novel TR‐based methods are proposed, specifically designed for OFDM/OQAM systems by taking into consideration the overlapping nature of OQAM signals. These two methods have different approaches to the generation of the peak‐cancelling signal. The first one (overlapped scaling tone reservation) generates the peak‐cancelling signal using a least squares approximation algorithm with possible adjacent symbol overlap; the second one (multi‐kernel tone reservation) generates the peak‐cancelling signal by using multiple impulse‐like time domain kernels. It is shown by simulation that, when used in OFDM/OQAM systems, the proposed methods can provide better performance than the direct application of the existing controlled clipping tone reservation method, and even outperform the multi‐block tone reservation method.     

Clipping noise cancellation in OFDM systems using oversampledsignal reconstruction

Clipping the OFDM signals in the digital part of the transmitter is one of the simplest methods to reduce the peak factor. However, it suffers from additional clipping distortion, peak regrowth after digital to analog conversion, and out-of-band radiation in the case of oversampled sequence clipping. We use oversampled sequence clipping to combat the effect of peak regrowth and propose a method to reconstruct the clipped samples and mitigate the clipping distortion in the presence of channel noise at the expense of bandwidth expansion. We show through extensive simulations that by slightly increasing the bandwidth of the system, we can significantly improve the performance while limiting the maximum amplitude of the analog signal     

面向5G三大应用场景的F-OFDM系统PAPR抑制算法研究

曾作为5G候选波形技术的基于子带滤波的正交频分复用(Filtered-Orthogonal Frequency Division Multiplexing,F-OFDM)是基于正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)发展而来的,和OFDM一样存在峰值平均功率比(Peak to Average Power Ratio,PAPR)的问题。高PAPR的信号在发送时对系统功率放大器的性能要求很高,容易发生信号失真,并且会直接影响整个系统的运行成本和效率。为了提高F-OFDM系统在5G三大应用场景中的PAPR抑制性能,引入一种基于迭代限幅滤波的次优PTS抑制算法。该算法主要分为两个过程,先对F-OFDM系统的各子带的高PAPR值信号进行线性抑制,然后待各子带的信号经过各自的数字子带滤波器合并后再进行迭代限幅滤波处理,从而完成对F-OFDM信号PAPR的抑制。仿真表明,该算法相对于单独使用次优PTS算法或迭代限幅滤波算法,能提高F-OFDM系统的PAPR抑制性能。     

可见光通信光叠加正交频分复用技术研究

针对可见光通信(VLC)正交频分复用(OFDM)系统 发光二极管(LED)动态范围窄、非线性和限幅失真严重等问题,利用光 源具有多个灯芯的特点,将OFDM频带划分为多个子带,每个子带内数据通过傅里叶反变换(IFFT)产生一路独立信号输入LED 灯芯,所有LED灯芯发出的光信号在空间上叠加形成光叠加OFDM(OSM-OFDM) 信号。在相同直流偏置情况下, OSM-OFDM系统每一路信号的调制深度较低,能够有效地避免LED非线性和限幅影响,特别是 在输入信号功率较大时,系统 误码率(BER)降低了至少1个数量级。根据有效信噪比(SNR) 最大原则能够得到OSM-OFDM系统最优直流偏置,而且在系统设计时可以根据 系统有效信噪比公式选择合适的LED灯芯数。实验表明,OSM-OFDM系统BER 性能要比直流偏置光OFDM(DCO-OFDM)系统提升了1个数量 级。     

OFDM performance in amplifier nonlinearity

The activities of the current European RACE and ACTS projects have led to an increasing interest in OFDM (orthogonal frequency division multiplexing) as a means of combating impulsive noise and multipath effects and making fuller use of the available bandwidth of the system. This paper analyses the performance of OFDM signals in amplifier nonlinearity. In particular, bit error rate (BER) degradation as a result of amplitude limiting or clipping are analysed. In the presence of both nonlinear distortion and additive Gaussian noise, optimized output power back off is provided to balance the requirements of minimum BER and power amplifier efficiency. For this purpose, an OFDM system has been built using the SPW (Signal Processing Worksystem) simulator     

OFDM系统中基于压缩感知恢复由限幅和HPA产生的非线性失真研究

正交频分复用（Orthogonal Frequency Division Multiplexing,OFDM）系统的主要缺点之一就是有较高的峰均功率比（Peak to Average Power Ratio,PAPR）,降低了功率放大器（High Power Amplifier,HPA）的工作效率,同时HPA引入的非线性失真,恶化了系统的误比特率（Bite Error Rate,BER）性能.本文所提算法将限幅和HPA引入的非线性失真视为一个整体来考虑,利用与限幅噪声在时域上的近似稀疏性,对整个非线性过程进行建模.发送端通过限幅降低了OFDM信号的PAPR,在接收端,选取受噪声干扰小的可靠性观测向量,最小化信道噪声的影响,基于非线性模型计算得到的参数,利用压缩感知（Compressive Sensing,CS）算法能有效地恢复总的非线性失真信号,提升了系统的BER性能.     

OFDM系统中抗非线性失真的迭代信道估计算法

针对OFDM系统中信号限幅滤波造成的信道估计性能下降,提出了一种基于梳状导频的迭代估计算法。该算法迭代估计信号中的非线性噪声,并据此补偿信号的非线性失真,提高信道再估计的性能。推导出了该算法在限幅预畸变系统中所能达到的理论均方误差下限,同时给出了初始信道估计的理论均方误差。仿真表明,该算法克服了由限幅滤波造成的性能瓶颈,且收敛速度快。与文献迭代信道估计算法相比,具有更强的抵抗峰均比抑制的能力。     

基于信道估计的限幅OFDM信号检测性能分析

大峰均比是正交频分复用(OFDM)系统的主要缺陷之一，对时域OFDM信号限幅是通常采用的一种抑制发送信号峰值功率的方法，但是限幅产生的非线性干扰，使系统的信道估计和信号检测性能降低。本文分析了限幅干扰对信道估计以及基于估计信道信息的OFDM数据信号检测性能的影响，推导出了基于信道估计信号检测的信噪比与限幅参数之间的关系。利用信噪比和高斯信道的误符号率及误比特率公式，本文对频率选择性瑞利衰落信道环境限幅OFDM系统中信号的误符号率和误比特率进行估算。计算机仿真和估算的结果相吻合。     

Clipping noise mitigation for OFDM by decision-aided reconstruction

Clipping is often used to reduce the large peak-to-mean envelope power ratio of orthogonal frequency-division multiplexing (OFDM) signals. However, it introduces additional noise that degrades the system performance. A technique called decision-aided reconstruction (DAR) is proposed for mitigating the clipping noise. The performance of the proposed technique is evaluated for additive white Gaussian noise and static intersymbol interference channels. The effect of using a realistic channel estimate is also examined. Results show that the DAR technique can mitigate the clipping noise significantly for OFDM systems that have large block sizes     

可见光通信自适应直流偏置光正交频分复用系统设计与性能研究

传统的直流偏置光正交频分复用(DCO-OFDM)系统通过添加直流偏置(DC)和限幅获得单极性信号,由于傅里叶逆变换(IDFT)输出时域信号的随机性,固定的直流偏置会产生较大的非线性限幅失真。该文提出设计自适应直流偏置光正交频分复用(ADCO-OFDM)系统,以双边限幅前、后符号之间的均方误差(MSE)最小化为优化目标,求解使非线性限幅失真最小的DC。介绍了ADCO-OFDM系统原理、推导了子载波比特信噪比(SNR),建立了均方误差最小化优化模型,并给出了最优直流偏置的求解过程。采用蒙特卡罗(Monte Carlo)仿真方法分析了误码率(BER)和光功率消耗,验证了系统设计和性能分析的正确性。