影响预失真OFDM系统的关键参数IBO阈值的研究

OFDM系统的一个主要缺陷就是峰平比过高引起的高功率放大器(HPA)的非线性失真.该文研究了当使用预失真器进行补偿时,系统的接收误码率BER随HPA的输入回退IBO值变化而产生的性能改变,从而给出系统性能有效改善时,HPA的IBO阈值.     

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.     

A low complexity selected mapping scheme for peak to average power ratio reduction with digital predistortion in OFDM systems

One of the effective methods used for reducing peak‐to‐average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems is selected mapping (SLM). In this paper, a new SLM scheme called DSI‐SLM, which is a combination of dummy sequence insertion (DSI) and conventional selected mapping (C‐SLM) is proposed. Previous techniques have had some drawbacks. In DSI, increasing the number of dummy sequences to have better PAPR degrades transmission efficiency, and in C‐SLM, the complexity rises dramatically when the number of sub‐blocks increases. The proposed DSI‐SLM scheme significantly reduces the complexity because of the reduction in the number of sub‐blocks compared with the C‐SLM technique while its PAPR performance is even better. To enhance the efficiency of the OFDM system and suppress the out‐of‐band distortion from the power amplifier nonlinearity, a digital predistortion technique is applied to the DSI‐SLM scheme. Simulations are carried out with the actual power amplifier model and the OFDM signal based on the worldwide interoperability for microwave access standard and quadrature phase‐shift keying modulation. The simulation results show improvement in PAPR reduction and complexity, whereas the BER performance is slightly worse. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Threshold IBO of HPA in the predistorted OFDM communication system

Orthogonal frequency division modulation (OFDM) system is very useful for the multi-path channel and highly bandwidth-efficient system. However, OFDM signal shows the high peak to average power ratio (PAPR) so that nonlinear distortion can happen in the high power amplifier (HPA). This paper studies BER performance variation according to input back off (IBO) values when the predistorter is used for the compensation for the HPA nonlinearity. In the case of the solid state power amplifier (SSPA) of nonlinearity parameter p=0.5 and IBO=0 dB, the system with predistorter is poorer than the one without predistorter. But this situation is inverted, if IBO is increased from 0 dB to 3 dB. So, we can find that there is a threshold value of IBO for the predistorter to effectively compensate for the nonlinear SSPA when the sub-carrier number and p value are changed. This threshold IBO value is proportional to the nonlinearity parameter p value but it is nearly independent of the sub-carrier number because amplitude and distribution are normalized. The OFDM system with predistorter can be improved only when thee IBO value is higher than the threshold value.     

PAPR reduction for OFDM‐based visible light communication systems using proposed hybrid technique

Orthogonal Frequency Division Multiplexing (OFDM) is utilized with visible light communication (VLC) systems to decrease the impacts of inter‐symbol interference and to achieve communication with high speed of data transmission and huge bandwidth. In any case, the main problem in OFDM‐based VLC systems is high peak‐to‐average power ratios (PAPRs). This paper proposes a hybrid PAPR reduction technique based on signal transformation combined with clipping. The Hadamard transform is used in the proposed technique to reduce the PAPR without affecting the bit error rate (BER) of the VLC systems. The optimum clipping threshold at which the PAPR is reduced simultaneously with the improvement the BER of the VLC systems is also determined. The performance of the proposed system is assessed in terms of complementary cumulative distribution function and the BER. The obtained results demonstrate that the proposed procedure can simultaneously decrease the PAPR and achieve good BER performance compared to the OFDM‐based VLC system.     

基于功放效率的OFDM信号幅度的最佳分布——峰均比抑制

星座图扩展（ACE）技术可以无损地降低OFDM信号的峰均比,本文的论述指出仅追求低峰均比对应的功放效率和效能并不是最好.笔者提出了OFDM信号的最佳互补累积分布函数（OCCDF）峰均比抑制方法,即考虑OFDM信号的幅度分布及出现概率,整体地评估其对放大器失真的影响,以OFDM信号放大器的效率为评估峰均比优劣的标准,在算法上以限定MER条件下得到尽可能低的功放输入功率回退（IBO）为迭代收敛目标,预见了应该存在最高功放效率意义匹配的所谓OFDM信号的最佳幅度分布.实验证实：相比原始OFDM系统,在放大基于OCCDF准则优化后的OFDM信号时,放大器获得了3.15dB的IBO性能改善,对比同等条件下单载波系统仅差0.05dB.     

Security aware hybrid precoding based peak‐to‐average power ratio reduction Approaches for telecommunication systems

In wireless telecommunication, one of the modulation approaches used is the orthogonal frequency division multiplexing (OFDM). Moreover, the high peak‐to‐average power ratio (PAPR) is the one notorious demerit in OFDM systems. Hence, the high power amplifier (HPA) is used in its linear region. Otherwise, the bit error rate (BER) will be increased. Several approaches are proposed in the wireless communications for reducing the PAPR issue. In this paper, we propose a hybrid Discrete Hartley Matrix Transform (DHMT) precoding using both selected mapping (SLM) and partial transmit sequence (PTS) PAPR reduction strategies. For the multicarrier modulation process, instead of Inverse Fast Fourier transform (IFFT) operation, the DHMT operation is used because of its low computational complexity. Based on multi‐chaotic, the time‐frequency domain encryption (TFDE) approach is adopted for physical layer security to confirm the security in data transmission. For enhancing the physical layer security, the proposed encryption system generates chaotic sequences based on Logistic maps and Lozi in the frequency and time domains together. In this study, the implementation of the DHMT‐based OFDM system is processed to reduce the maximum PAPR. Implementation is performed on the MATLAB platform, and the performances are calculated using complementary cumulative distribution function (CCDF), BER regards to signal‐to‐noise ratio (SNR), and the outputs are compared based on the computation time. However, compared with the existing models, the proposed model produced better PAPR minimization regarding SNR.     

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%。     

Hybrid Zadoff‐Chu and multilateral piecewise exponential companding transform–based PAPR reduction technique in OFDM systems

The orthogonal frequency‐division multiplexing (OFDM) is a multicarrier modulation system that is used to transmit the large volume of data to the receiver. Reducing the peak‐to‐average power ratio (PAPR) in OFDM system is one of the demanding and crucial task in recent days. For this reason, various precoding and companding mechanisms are developed in the traditional works, but it remains with the limitations of increased complexity, reduced performance, and nonlinear distortion. The reduction of PAPR is achieved by minimizing the companding distortion with the enhancement of the bit error rate (BER) performance significantly. Then, in order to avoid clipping in OFDM, a multilateral piecewise exponential companding transform (MPECT) method has been utilized rather than using piecewise exponential companding transform (PEC) where PAPR is getting reduced. The OFDM is very sensitive to synchronizing error. To overcome this sensitivity, employ the Zadoff‐Chu sequence to carrier frequency offsets. Zadoff‐Chu matrix transform (ZCMT) has numerous merits among the other ODFM systems such as the improvement in the performance of the channels that are fading away and provides an ideal periodic autocorrelation and a constant magnitude periodic cross correlation. Both of these techniques provide improvement in the ODFM systems. To get more efficiency, this paper aims to develop a hybrid technique by integrating the ZCMT and MPECT techniques for reducing the PAPR in OFDM systems. Further, convolutional encoding is applied for better BER and PAPR. The simulation results of the proposed ZCMT‐MPECT technique are evaluated and compared with the conventional OFDM and other precoding methods.     

Selected mapping with symbol re‐mapping for OFDM/TDM using MMSE‐FDE

Orthogonal frequency division multiplexing (OFDM) signals have a problem with a high peak‐to‐average power ratio (PAPR). A distortionless selected mapping (SLM) has been proposed to reduce the PAPR, but a high computational complexity prohibits its application to an OFDM system with a large number of subcarriers. Recently, we proposed OFDM combined with time division multiplexing (OFDM/TDM) using minimum mean square error frequency‐domain equalization (MMSE‐FDE) to improve the bit error rate (BER) performance of conventional OFDM with a lower PAPR. The PAPR problem, however, cannot be completely eliminated. In this paper, we present an SLM combined with symbol re‐mapping for OFDM/TDM using MMSE‐FDE. Unlike the conventional OFDM, where SLM is applied over subcarriers in the frequency domain, we exploit both time and frequency dimensions of OFDM/TDM signal to improve the performance with respect to PAPR and BER. A mathematical model for PAPR distribution of OFDM/TDM with SLM is presented to complement the computer simulation results. It is shown that proposed SLM can further reduce the PAPR without sacrificing the BER performance with the same or reduced computational complexity. Copyright © 2010 John Wiley & Sons, Ltd.     

SLM‐based data position permutation method for PAPR reduction in OFDM systems

The high peak‐to‐average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems not only increases the complexity of the analog‐to‐digital (A/D) and digital‐to‐analog (D/A) converters but also reduces the efficiency of the radio frequency (RF) power amplifier. In this paper, we present a data position permutation (DPP) method, which is based on a selected mapping (SLM) scheme, for reducing the PAPR in OFDM systems. The candidate signal on each branch of the SLM scheme is generated by permuting the position and rotating the phase of the original data. In addition, a modified DPP method with lower bit error rate (BER) is proposed. The simulation results show that the proposed method provides better performance with regard to complexity, spectrum efficiency, and BER as compared to that of the SLM‐based dummy sequence insertion (SLM‐DSI) method. Copyright © 2008 John Wiley & Sons, Ltd.     

降低OFDM系统中PAPR的次优化PTS算法

正交频分复用(OFDM)系统的主要缺点是峰均功率比(PAPR)增大会严重地降低了发射机中高功率放大器(HPA)的效率.部分传输序列(PTS)算法能有效地降低OFDM信号的PAPR概率,但随着PTS中分割子块的增多也带来了高的计算复杂度.为此,文中提出一种新的PTS算法,该算法比传统的穷尽搜寻法具有更佳的性能.仿真结果表明,文中所提PTS算法在降低PAPR相似的情况下,具有更小的计算复杂度.     

A ZCMT Precoding Based Multicarrier OFDM System to Minimize the High PAPR

This paper presents a Zadoff-Chu matrix transform (ZCMT) precoding based multicarrier orthogonal-frequency-division-multiplexing (OFDM) system to minimize the high peak-to-average power ratio (PAPR). In the proposed system, ZCMT kernel is applied to the constellation symbols which not only reduces PAPR but also improves bit-error-rate (BER) performance. The ZCMT precoded OFDM signals allow the radio-frequency high-power-amplifier (HPA) to operate near its saturation level, thus maximize the power efficiency. Extensive computer simulations have been performed to analyze the PAPR, BER and power-spectral-density (PSD). Simulation results show that the proposed system has excellent PAPR gain and BER performance with no spectral distortion.     

The Impact of Nonlinear HPA on the Determination of the Impulse Sample Power in IP-OFDM Systems

The recently introduced impulse postfix OFDM (IP-OFDM) system, which performs channel estimation in time-domain by exploiting the IP instead of pilot tones, can achieve the enhanced bit error rate (BER) performance compared to that of conventional OFDM systems. Since there is a peak-to-average power ratio (PAPR) issue of using the IP, however, the decision criteria for determining the power boosting factor (PBF) of IP were proposed to resolve the PAPR issue while achieving the optimum BER performance in the IP-OFDM system with an ideal high power amplifier (HPA). In this paper, a practical IP-OFDM system with a nonlinear HPA is considered and the impact of nonlinear HPA on the determination of the PBF of IP as well as the BER performance are analyzed. The BER lower bound is also derived in the case of QPSK and 16QAM. It is shown that the approximated lower bound approaches to the simulation results as the PBF is increased. According to the analytical results, due to the nonlinearity of the HPA, 1–3dB increased PBF than the ideal PBF is required to achieve the optimum BER performance, which is even deteriorated compared to that of IP-OFDM with an ideal soft envelope limiter.     

QAM-OFDM卫星通信系统PAPR抑制方法研究

正交频分复用(OFDM)具有很高的频谱利用率和良好的抗多径衰落能力,是卫星通信备受关注的新技术之一。但OFDM信号峰值平均功率比(PAPR)较高的问题限制了其在卫星通信系统中的应用。针对卫星通信高数据率和高可靠性传输的要求,提出一种新的抑制QAM-OFDM信号PAPR的部分格状(PT)成形技术,分析了QAM-OFDM卫星通信系统在AWGN、频率选择性Rician衰落信道的误比特率性能。仿真实验表明,该技术不仅能有效降低QAM-OFDM信号PAPR,结合纠错编码还可大大改善OFDM卫星通信系统误比特率性能。     

Transforming the Statistical Distribution of Wireless OFDM Signal for PAPR Reduction

To reduce the high peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signal, a novel nonlinear companding transform (CT) scheme is proposed in this paper. This scheme can reallocate both the amplitude/power as well as statistical distribution of the companded signal more reasonably and flexibly than existing CT methods with low computational complexity. By choosing an appropriate companding parameter, it can provide more effective PAPR reduction but at the price of a minimal amount of bit-error-rate (BER) performance degradation caused by the companding distortion. The closed-form expressions including the achievable PAPR gain, signal attenuation factor, and corresponding selection criteria for the companding parameter were derived. Computer simulations demonstrate that the scheme significantly improves the overall performances of OFDM system in terms of PAPR, BER and bandwidth efficiency under the multipath fading channel or with the high power amplifier.

     

基于脉冲成形技术降低OFDM峰均比

正交频分复用（OFDM）利用相互正交的多个子载波来传输信息,具有较高的频谱利用率和良好的抗多径干扰能力,适用于高速数据传输,成为第四代移动通信的核心技术之一。但OFDM系统中一个最主要的缺点在于OFDM信号的峰值平均功率比（PAPR）很大,这容易导致系统线性功放的效率降低。研究了将脉冲成形应用到OFDM系统来降低PAPR的技术,以及脉冲波形的选取。仿真结果表明该方法能有效降低OFDM系统的PAPR,而且不影响系统的BER性能。     

Block coding scheme for reducing PAPR in OFDM systems with large number of subcarriers

The major drawback in Orthogonal Frequency Division Multiplexing (OFDM) system is due to the high Peak-to-Average Power Ratio (PAPR), so the performance of the system is significantly degraded by the nonlinearity of a High Power Amplifier (HPA) in the transmitter. In order to mitigate distortion, a block coding scheme for reducing PAPR in OFDM systems with large number of subcarriers based on complementary sequences and predistortion is proposed, which is capable of both error correction and PAPR reduction. Computer simulation results show that the proposed scheme significantly improves Bit Error Rate(BER) performance as compared to an uncoded system when an HPA is employed or a coded system without predistortion.     

PAPR Reduction in OFDM Systems Based on Autoregressive Filtering

Orthogonal frequency division multiplexing (OFDM) has a very high peak-to-average power ratio (PAPR) that causes a severe nonlinear distortion in practical hardware implementation of high power amplifiers (HPA). In this article, a new PAPR reduction method is proposed based on autoregressive (AR) error filtering. This method proposes the use of signal whitening property of error filtering as a preprocessing step to remove the predictable content of stationary stochastic processes which can reduce the autocorrelation of input data sequences and is shown to be a very effective solution for the PAPR problem in OFDM systems. It is shown that the proposed method can achieve a significant reduction in PAPR without degrading the error performance or power spectral levels. It is also shown that the proposed method is applicable to any modulation scheme and can work for any number of subcarriers under both additive white Gaussian noise and wireless Rayleigh fading channel.