与《降低OFDM系统峰均功率比的载波干涉技术》作者商榷

拜读了何剑辉博士等撰写的<降低OFDM系统峰均功率比的载波干涉技术>一文后,笔者根据其提供的思路对载波干涉OFDM(CI/OFDM)的发送信号作了理论推导和仿真实验,对CI/OFDM降低峰均功率比(PAPR)的实质感到困惑,并提出自己的看法与<降低OFDM系统峰均功率比的载波干涉技术>一文作者商榷.     

降低OFDM系统峰均功率比的载波干涉技术

介绍了应用载波干涉技术的OFDM系统,即CU/OFDM系统,说明了通过CI技术降低OFDM系统峰均功率比的基本原理和特点,并通过计算机仿真展示了系统峰均功率比改善的具体效果.仿真表明,载波干涉技术应用灵活,实施复杂度较低,能有效地解决OFDM系统的高峰均功率比问题.     

对《与<降低OFDM系统峰均功率比的载波干涉技术>作者商榷》的回应

指出了<与<降低OFDM系统峰均功率比的载波干涉技术>作者商榷>文中对载波干涉(CI)技术存在的理解上的错误,强调了CI信号和CI/OFDM系统的主要特点.     

降低OFDM系统高峰均功率比的方法研究

针对OFDM技术具有较高的峰均功率比（peak to average power ration,PAPR）这一主要缺点,提出一种控制OFDM载波信号衍生频谱的迭代算法,在降低系统高峰均功率比的同时,也降低系统的误码率.利用该算法对数据序列长度为64的OFDM信号进行仿真验证,随着迭代次数的增加,衍生频谱系数增大,系统平均功率增加,OFDM信号的峰值功率比得到一定的降低.     

一种基于信号分组平均降低OFDM系统PAPR的方法

介绍了OFDM是一多载波调制技术,具有抗多径信道干扰和频谱利用率高等特点根据OFDM信号的统计特性提出一种降低OFDM系统PAPR(峰均功率比)值的方法,即将IFFT变换后的个信号按一定的规律分成V组,计算分组平均值,用分组平均值代替相应的原信号值。仿真结果表明信号分组平均方法可非常有效地降低系统的峰均功率比值。     

对《与<降低OFDM系统峰均功率比的载波干涉技术>作者商榷》的回应

指出了<与<降低OFDM系统峰均功率比的载波干涉技术>作者商榷>文中对载波干涉(CI)技术存在的理解上的错误,强调了CI信号和CI/OFDM系统的主要特点.     

OFDM技术及其峰均功率比降低的一种有效方法的研究

正交频分复用（OFDM）是一种多载波调制技术，其特有的调制特性决定了它有较高的峰均功率比值。高峰均功率比信号会对实际放大器提出很高的线性要求，因此它是限制OFDM技术实用化的主要障碍。目前虽然有很多降低OFDM信号的峰均功率比（PAPR）的方案，但大多是以增加系统的复杂程度为代价的。本文提出了一种降低OFDM信号PAPR的方法，可以在不过分增加系统复杂性的基础上解决其PAPR在一定程度上降低的问题。     

利用星座扩展来降低OFDM系统中峰均比的新方法

正交频分复用（OFDM）系统有很高的峰均比,这会导致发送端功率放大器的低功率效率和非线性失真.文中提出了一种利用峰值抵消信号来降低OFDM的峰均比的新方法.峰值抵消信号利用未用的或保留的子载波以及星座的可扩展部分通过反馈的方式产生.这种方法可以显著的降低峰均比而不需要边信息.     

一种降低OFDM系统峰均功率比的压扩变换方法

为了降低OFDM系统中的峰均功率比,提出利用反正切变换对OFDM信号进行压扩处理.反正切压扩可以在压缩大信号的同时,增强小信号,从而提高了OFDM系统的误码率性能.仿真结果表明,与著名的μ律压扩方法相比,反正切压扩可以获得更好的峰均功率比性能和更低的误码率.该方法应用简单,不受载波数量和调制方式的限制.     

一种降低OFDM峰均功率比的新方法-交织限幅法

虽然有许多方案能降低OFDM信号的峰均功率比,但都大大增加了系统的复杂性,为此,提出通过交织限幅法来降低OFDM信号峰均功率比,效果很好.     

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.     

基于分组格雷编码的OFDM 雷达通信一体化系统峰均功率比抑制

基于OFDM 共享信号的雷达通信一体化系统具有较高的峰均功率比,由于编码率的限制,采用格雷序列编码抑制系统峰均功率比(PAPR)的算法只适合应用于子载波数较少的场合。针对子载波数较多的系统,该文提出一种利用分组并行格雷编码结合系数加权优化的PAPR 抑制算法,通过将输入比特流分为若干组并行比特,分别进行格雷序列编码、数据符号调制以及逆傅里叶变换,将结果合并就可以得到一个OFDM 符号。在合并之前对每一组引入若干加权系数,这样就构成了多个可供选择的OFDM 符号,通过选择具有最小PAPR 值的OFDM 符号作为发射信号,实现系统PAPR 抑制。最后,仿真了3 种不同分组方案下PAPR 抑制能力、通信误码率以及雷达宽带模糊函数,仿真结果表明,该算法可有效抑制系统PAPR,降低系统误码率,其雷达宽带模糊函数仍为近似图钉型,具有良好的距离和速度分辨能力以及测距、测速精度。      

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.     

Peak‐to‐average power ratio reduction of orthogonal frequency division multiplexing systems using modified tone reservation techniques

This paper proposed a modified tone reservation (TR) technique that can reduce the peak‐to‐average power ratio (PAPR) of the orthogonal frequency division multiplexing (OFDM) system and is able to correct errors to avoid channel interference. The TR technique is a widely used PAPR reduction technique, which divides subcarriers of the OFDM system into two sets to generate peak‐canceling signals and transmit modulated data. The subcarriers used to reduce the PAPR are called the peak reduction tone sets. The mechanism of peak‐canceling signal generation is a primary factor in determining the quality of the PAPR reduction performance of the TR technique. Currently, two signal generation mechanisms exist: TR‐gradient‐based and TR‐clipping‐based techniques. Although TR techniques can effectively reduce the high PAPR in the OFDM system, TR techniques lack the ability to correct errors. Therefore, this paper combined block coded modulation codes and TR techniques to provide the modified TR techniques with error correction abilities. From the simulation results, the modified TR techniques had a superior effect on PAPR reduction performance compared with the conventional TR technique. The modified TR technique also possessed the ability to correct errors during signal transmission. Copyright © 2015 John Wiley & Sons, Ltd.     

PAPR Reduction for Repetition Space-Time-Frequency Coded MIMO-OFDM Systems Using Chu Sequences

Space-time/frequency coding (SFC) can achieve the spatial and multipath diversities for a MIMO-OFDM system by coding across subcarriers, multiple antennas, and/or multiple OFDM sysmbols, where an interesting method to achieve the multipath diversity is repeating across subcarriers proposed by Su et al. While most of the existing space-time/frequency codes do not have the fast ML decoding, a family of space-time-frequency codes with single-symbol ML decoding have been proposed lately by Zhang et al to achieve both full spatial and multipath diversities by using orthogonal space-time block codes (OSTBC) across multiple antennas and OFDM symbols and in the meantime repeating across the subcarriers. In this paper, we first generalize the above OSTBC to linearly transformed quasi OSTBC (QOSTBC) in a straightforward way. The main goal of this paper is to modify the repeating process and adjust their phases so that the peak-to-average power ratio (PAPR) of the OFDM system is reduced. In particular, we propose to use Chu sequences and show that the discrete PAPR can be reduced by Gamma times where Gamma is the times of the repeating across subcarriers for any SFC from the repeating.     

Selected mapping without side information for PAPR reduction in OFDM

Selected mapping (SLM) is a technique used to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency-division multiplexing (OFDM) systems. SLM requires the transmission of several side information bits for each data block, which results in some data rate loss. These bits must generally be channel-encoded because they are particularly critical to the error performance of the system. This increases the system complexity and transmission delay, and decreases the data rate even further. In this paper, we propose a novel SLM method for which no side information needs to be sent. By considering the example of several OFDM systems using either QPSK or 16- QAM modulation, we show that the proposed method performs very well both in terms of PAPR reduction and bit error rate at the receiver output provided that the number of subcarriers is large enough.     

Performance analysis of broadband satellite communication system based on OFDM/TDM

Orthogonal frequency division multiplexing （OFDM） is attracting more attention for its capability of high speed transmission. However, the OFDM possesses an obvious shortage in its high ratio of the peak power to the average power （PAPR）, which has become the main issue holding it back to be applied to the broadband satellite communication system. OFDM combined with time division multiplexing （TDM）, dividing the subcarriers of OFDM into some blocks in time tune, can decrease the high PAPR of OFDM. Meanwhile, the advantages of OFDM can be preserved. In this paper, OFDM/TDM is applied to the broadband satellite communication system. This paper theoretically analyses OFDM/TDM system model as well as its restraining effect on PAPR, and proposes frequency domain multiplexing-pilot （FDM-Pilot） channel estimation algorithm. Simulation results show OFDM/TDM in broadband satellite communication system has approving performance and decreased the PAPR.     

Constant modulus and reduced PAPR block differential encoding for frequency-selective channels

Frequency-selective channels can be converted to a set of flat-fading subchannels by employing orthogonal frequency-division multiplexing (OFDM). Conventional differential encoding on each subchannel, however, suffers from loss of multipath diversity, and a very high peak-to-average power ratio (PAPR), which causes undesirable nonlinear effects. To mitigate these effects, we design a block differential encoding scheme over the subchannels that preserves multipath diversity, and in addition, results in constant modulus transmitted symbols. This property is shown to ensure that the PAPR of the continuous-time transmitted waveform is reduced by a large factor. The maximum-likelihood decoder for the proposed scheme, conditioned on the current and previous received block, is shown to have linear complexity in the number of subcarriers. The constant modulus scheme will yield good bit-error rate performance with full rate only if short blocks are used. However, one may mitigate this problem by relaxing the constant modulus requirement. We show that in a practical OFDM system, we can group the subcarriers into shorter subblocks in a certain manner, and apply the constant modulus technique to each subblock. Thus, we improve diversity at a very low decoder complexity, and at the same time, we introduce an upper bound on the discrete-time PAPR, which, in turn, may lead to appreciable reduction in continuous-time PAPR, depending on the system parameters. Finally, in situations where we can sacrifice rate, additional complex field coding may be used to exploit the multipath diversity provided by channels longer than those the simple scheme can handle.     

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.     

A Modified SLM Scheme With Low Complexity for PAPR Reduction of OFDM Systems

In this paper, we propose a new peak-to-average power ratio (PAPR) reduction scheme of orthogonal frequency division multiplexing (OFDM) system, called a modified selected mapping (SLM) scheme, which considerably reduces the computational complexity with keeping the similar PAPR reduction performance compared with the conventional SLM scheme. The proposed scheme is analytically and numerically evaluated for the OFDM system specified in the IEEE 802.16 standard. For the OFDM system with 2048 subcarriers, the proposed scheme with 4 binary phase sequences can reduce the complex multiplications by 63.5% with the similar PAPR reduction compared with the SLM scheme with 16 binary phase sequences.