PAR reduction in OFDM via active constellation extension

The high peak-to-average power ratio (PAR) in orthogonal frequency division multiplexing (OFDM) modulation systems can significantly reduce power efficiency and performance. Methods exist which alter or introduce new signal constellations to combat large signal peaks. We present a new PAR-reduction method that dynamically extends outer constellation points in active (data-carrying) channels, within margin-preserving constraints, in order to minimize the peak magnitude. This scheme simultaneously decreases the bit error rate slightly while substantially reducing the peak magnitude of an OFDM transmit block. Furthermore, there is no loss in data rate and, unlike other methods, no side information is required. PAR reduction for an approximated analog signal is considered, and about a 4.6 dB reduction at a 10/sup -5/ symbol-clip probability is obtained for 256-channel QPSK OFDM. The results show great promise for use in commercial systems.     

An orthogonal projection-based approach for PAR reduction in OFDM

High peak-to-average power ratio (PAR) of the transmitted signal is one of the limitations to using orthogonal frequency-division multiplexing (OFDM) systems. This article presents a novel orthogonal projection-based partial transmit sequences approach that achieves significant PAR reduction at low redundancy. The proposed method is applicable to OFDM systems with a large number of subchannels. This study also for the first time provides geometrical interpretation to combining the well-known partial transmit sequences scheme     

Low Complexity Partial Transmit Sequence with Complex Gain Memory Predistortion in OFDM Systems

In this paper the peak to average power ratio (PAPR) reduction and digital predistortion effects in orthogonal frequency division multiplexing (OFDM) systems are investigated. By applying a predistortion technique called complex gain memory predistortion (CGMP), power amplifier works at higher power efficiency. The proposed enhanced partial transmit sequence scheme is applied for PAPR reduction and integration with CGMP technique results in increasing in OFDM system efficiency and prolonged battery life. Simulation and results are examined with actual power amplifier and OFDM signal with quadrature phase shift keying (QPSK) modulation.     

A low complexity multicarrier PAR reduction approach based on subgradient optimization

We introduce a novel subgradient optimization-based framework for iterative peak-to-average power ratio (PAR) reduction for multicarrier systems, such as wireless orthogonal frequency division multiplexing (OFDM) and wireline discrete multitone (DMT) very high-speed digital subscriber line (DMT-VDSL) systems. The proposed approach uses reserved or unused tones to minimize the peak magnitude of the DMT symbol vector where these tone values are iteratively updated through a subgradient search. The algorithms obtained through this framework have very simple update rules, and therefore, low computational complexities in general. Since the approach is based on the direct update of some frequency domain parameters, the power spectral density (PSD) level constraints that exist in the communications standards can easily be incorporated into the algorithms. This feature also enables simple compensation for the effects of transmit filter on PAR. Furthermore, we can locate the Active Set PAR reduction method for real baseband signals as a special case of the subgradient approach and provide its natural extension to handle complex baseband DMT signals. In addition to the peak level cost function, we also introduce the $K$-peak energy cost function which is also used to develop effective subgradient algorithms as illustrated by the simulation examples.     

基于数字电视发射的功放线性化方案分析

根据数字电视OFDM信号对电视发射机功率放大器线性的要求,分析了功率回退法在数字电视地面传输具体环境下的应用条件,提出一种数字自适应基带线性化预校正方案,并进行了计算机仿真,对模拟电视发射机功率放大器适应OFDM信号的传输进行了探讨.     

A New PAPR Reduction Technique in OFDM Systems Using Linear Predictive Coding

High peak-to-average power ratio (PAPR) has always been as a major problem in orthogonal frequency division multiplexing (OFDM) that leads to a severe nonlinear distortion in practical hardware implementations of high power amplifier. In this article, a new PAPR reduction method is proposed based on linear predictive coding (LPC). This method proposes the use of signal whitening property of LPC as a preprocessing step in OFDM systems. Error filtering of the proposed method removes the predictable content of stationary stochastic processes which can reduce the autocorrelation of input data sequences and is shown to be very effective solution for PAPR problem in OFDM transmissions. It is shown that the proposed method can achieve a significant reduction in PAPR without degrading the power spectral level, error performance or computational complexity of the systems. It is also shown that the proposed method is independent of modulation schemes and can be applied to any number of subcarriers under both additive white gaussian noise and wireless Rayleigh fading channel.     

A Low Complexity Partition Dummy Sequence Insertion PAPR Reduction Method for the OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) technique has been widely adopted in many wireless communication systems because it can divide a wideband channel into several narrowband ones to avoid the frequency selective fading. However, one major drawback of the OFDM signal is the high peak-to-average power ratio (PAPR) problem. The high PAPR results in the in-band distortion and out-of-band radiation when the OFDM signal is fed into a nonlinear power amplifier. In this paper, we propose a Low Complexity Partition Dummy Sequence Insertion method to reduce the PAPR and computational complexity of the Partition Dummy Sequence method. Simulation results of the PAPR, Bit Error Rate and complexity reduction for the proposed method are evaluated to demonstrate its capabilities.     

Low-Power Analog Integrated Circuits for Wireless ECG Acquisition Systems

This paper presents low-power analog ICs for wireless ECG acquisition systems. Considering the power-efficient communication in the body sensor network, the required low-power analog ICs are developed for a healthcare system through miniaturization and system integration. To acquire the ECG signal, a low-power analog front-end system, including an ECG signal acquisition board, an on-chip low-pass filter, and an on-chip successive-approximation analog-to-digital converter for portable ECG detection devices is presented. A quadrature CMOS voltage-controlled oscillator and a 2.4 GHz direct-conversion transmitter with a power amplifier and upconversion mixer are also developed to transmit the ECG signal through wireless communication. In the receiver, a 2.4 GHz fully integrated CMOS RF front end with a low-noise amplifier, differential power splitter, and quadrature mixer based on current-reused folded architecture is proposed. The circuits have been implemented to meet the specifications of the IEEE 802.15.4 2.4 GHz standard. The low-power ICs of the wireless ECG acquisition systems have been fabricated using a 0.18 μm Taiwan Semiconductor Manufacturing Company (TSMC) CMOS standard process. The measured results on the human body reveal that ECG signals can be acquired effectively by the proposed low-power analog front-end ICs.     

Improved phase factor computation for the PAR reduction of an OFDMsignal using PTS

The peak-to-average power ratio (PAR) of an orthogonal frequency-division multiplexing (OFDM) signal can be substantially larger than that of a single carrier system. Partial transmit sequence (PTS) combining can improve the PAR statistics of an OFDM signal. As PTS requires an exhaustive search over all combinations of allowed phase factors, the search complexity increases exponentially with the number of subblocks. In this letter, we present a new algorithm for computing the phase factors that achieves better performance than the exhaustive search approach     

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.     

An overview of peak-to-average power ratio reduction techniques for multicarrier transmission

High peak-to-average power ratio of the transmit signal is a major drawback of multicarrier transmission such as OFDM or DMT. This article describes some of the important PAPR reduction techniques for multicarrier transmission including amplitude clipping and filtering, coding, partial transmit sequence, selected mapping, interleaving, tone reservation, tone injection, and active constellation extension. Also, we make some remarks on the criteria for PAPR reduction technique selection and briefly address the problem of PAPR reduction in OFDMA and MIMO-OFDM.     

On the design of multidimensional signal sets for OFDM systems

An orthogonal frequency division multiplexing (OFDM) system operating over a wireless communication channel effectively forms a number of parallel frequency-nonselective fading channels, thereby obviating the need for complex equalization and thus greatly simplifying equalization/decoding. However, the OFDM system also exhibits two weaknesses relative to its single-carrier counterparts: (1) the diversity achieved by the OFDM system can be less than a single-carrier system employing the same error control code in a signaling environment rich in diversity and (2) the baseband transmitted signal can exhibit significant amplitude fluctuation over time, thereby precluding efficient transmit amplifier operation. In this paper, nonstandard multidimensional signal sets matched to the OFDM framework are prescribed that address both of these issues. The proposed signal sets are chosen to maximize the diversity achieved by an uncoded system under a constraint to control the peak-to-mean envelope power ratio (PMEPR) of the baseband transmitted waveform. The cost of employing the proposed signal sets is an increase in decoding complexity, as essentially a small amount of controlled equalization has been added to the receiver; thus, the resulting system can be viewed as a hybrid between an OFDM system and a standard single-carrier system. Numerical results are presented which suggest that: (1) the system can provide an attractive alternative to a standard OFDM system in terms of required average transmitted SNR versus receiver complexity and (2) the system yields a modest reduction in PMEPR versus a standard OFDM system     

高效率基站Doherty功放的研究

随着无线通信系统的发展,人们对数据和信息的需求在不断的增加。功率放大器作为通信系统中最重要的模块之一,功放的性能对整个系统性能的影响至关重要。面对高速增长的移动数据业务和频谱资源短缺的威胁,高峰均比（PAR）的调制方式不断出现,如OFDM调制方式,这就对功放的线性度提出了较高的要求。为了保证信号的线性度,一般采用功率回退的方法来实现。以NXP公司的140W晶体管为模型,在ADS仿真软件中设计对称Doherty仿真电路。设计完成的功放电路能够在6dB功率范围内保持高效率工作。     

Performance analysis of peak power and band-limited OFDM system with linear scaling

The performance of the orthogonal frequency-division-multiplexing (OFDM) system over the strictly peak power and band-limited channel is analyzed in terms of the required input back-off, bit-error rate (BER), and channel capacity on the assumption that the power amplifier is perfectly linearized. The peak-power limitation is implemented by linearly scaling the band-limited OFDM signal such that the maximum peak power of each OFDM symbol is always below the saturation level of the amplifier. The theoretical performance analysis requires the knowledge of the distribution of the peak power normalized by the symbol-wise (local) average power, referred to as symbol-wise peak-to-average power ratio (PAR) in the paper, and we also develop a method to numerically calculate its statistical distribution. The analysis of BER performance suggests that the linear scaling causes practically negligible degradation. Furthermore, the benefit of additional application of simple PAR reduction schemes, such as symbol selection and deliberate clipping and filtering, is also examined.     

Turbo-Coded OFDM Transmission Over a Nonlinear Channel

In this paper, deliberate level clipping and turbo coding are combined to achieve an orthogonal frequency division multiplexing (OFDM) transmission system with a low peak-to-average power ratio (PAR) and a good performance. Using the linear approximation technique based on the minimum mean square error (MMSE) criterion, we first modify the metric computation for the turbo decoding in order to consider the distortion effects of the nonlinearity, caused by the Cartesian clipper. Also, this paper introduces a modified turbo decoder which simultaneously performs the data estimation and signal reconstruction. In other words, the turbo decoder iteratively recovers the clipped signal by using the estimated data, and then improves the data estimation by using the newly recovered signal. Numerical results are presented showing an improvement in the performance of the OFDM transmission system over the nonlinear channel, an increase in the efficiency of the high power amplifier (HPA), and/or an expansion of the transmitter coverage area.     

Minimizing the Peak-to-Average Power Ratio of OFDM Signals Using Convex Optimization

The main disadvantage of orthogonal frequency-division multiplexing (OFDM) is the high time-domain peak-to-average power ratio (PAR) that limits transmitter power efficiency. Assuming no changes in receiver structure, the transmitter can only reduce PAR by distorting the data carriers or by adding power to the free carriers. This paper shows that the OFDM signal with globally minimum PAR, subject to constraints on the allowed constellation error and the free carrier power, can be efficiently computed using convex optimization. Simulation results are presented for the 802.11a/g wireless local-area network standard. The globally minimum PAR, subject to the constellation error constraint, ranges from 0.7 dB for 6-Mbps binary phase-shift keying to 4.1 dB for 54-Mbps 64-QAM. Tradeoff analysis shows that the free carrier power can be drastically reduced by backing off from this globally minimum PAR by less than 1 dB. A customized interior-point method (IPM) is derived for solving the OFDM optimization problem. The IPM reaches the desired tradeoff between PAR and free carrier power within two iterations, where the main computational complexity per iteration is four fast Fourier transforms plus the solution of a linear system of equations. The customized IPM is about 100 times faster than a general-purpose optimization algorithm.     

On the PAR reduction of OFDM signals using multiple signal representation

Multiple signal representation (MSR) techniques have been used to reduce the high peak-to-average power ratios (PAR) of orthogonal frequency division multiplexing (OFDM) signals. These includes partial transmit sequences (PTS), selected mapping (SLM), selective scrambling and interleaving. All MSR techniques often improve the PAR statistics and are iterative in nature. The PAR reduction obtainable depends on the number of iterations performed, which also increases the complexity of the OFDM transmitter. However, a means to estimate the achievable PAR reduction for a given number of iterations has not been reported in the literature so far. This paper derives a lower bound on the achievable PAR when a MSR technique with a given complexity is used. Our analytical results show a clear asymptotic behavior of the PAR as the number of iterations is increased. Simulation results justify the significance and accuracy of the PAR bound derived.     

Combination of PAPR reduction and linearization for the OFDM communication system

Orthogonal frequency division multiplexing (OFDM) has been widely used in many kinds of communication systems. However, OFDM signal has serious problem of high peak‐to‐average‐power ratio (PAPR) due to so many sub‐carriers. So, OFDM signal has very wide dynamic range. Therefore, the bit error rate (BER) performance may be degraded because of the nonlinear devices like the high power amplifier (HPA). Even if the linearization and large back‐off are used to compensate for the HPA nonlinearity, the power efficiency of the HPA is still very low since the PAPR is very high. Therefore, the PAPR reduction of the OFDM signal before the linearization would be more reasonable to improve the power efficiency and nonlinear compensation at the same time. In this paper, we propose a new combined method of SPW (sub‐block phase weighting) for PAPR reduction and linearization technique for the improvement of the power efficiency and for the nonlinear compensation of HPA. An updated SPW method is proposed to use a novel weighting factor multiplication of the complementary sequence characteristic and PAPR threshold technique. From the simulation results, it can be confirmed that BER performance is significantly improved and out‐of‐band spectrum radiations are much mitigated. Power efficiency of HPA can be enhanced since we can set small IBO (input back‐off) due to the PAPR reduction. The proposed system shows about 3 and 1 dB performance improvement than the LCP (linearized constant peak‐power)‐OFDM and LCP‐OFDM plus SPW at BER = 10⁻⁴. Copyright © 2010 John Wiley & Sons, Ltd.     

Practical determination of impulse sample power boosting factor in impulse postfix OFDM systems

An impulse postfix OFDM (IP-OFDM) system, which performs channel estimation in time-domain by exploiting the IP instead of pilot tones, has recently been proposed by Chang et al. Although the proposed system can achieve the enhanced bit error rate (BER) performance compared to that of conventional OFDM systems, there is an important peak-to-average power ratio (PAPR) issue of using the IP that needs to be resolved. In this letter, we propose decision criteria for determining the power boosting factor (PBF) of IP in practical IP-OFDM systems with a high power amplifier (HPA). The analysis is performed with two criteria, the complementary cumulative distribution function of signal PAPR and the input back-off to determine the PBF of IP. The best PBF can be the one which gives an equal power to the IP and the rest of the OFDM symbol. From the analytic results, however, the PBF maximizing the power of the IP out of the HPA turns out to be the best one due to the non-linearities of the HPA. Simulation results demonstrate the effectiveness of the proposed decision criteria for IP-OFDM systems.     

一种降低OFDM系统PAPR的TR方法及其FPGA实现

针对正交频分复用(OFDM)系统的高峰均功率比问题,介绍了一种改进的载波保留(TR)方法,该算法应用时域核函数构造时域核函数矩阵并进行缩放参数优化,迭代过程复杂度低且性能良好.设计了一种简化的OFDM发射机方案以验证该算法在实际系统中的有效性,给出了在FPGA上实现该方案的整体架构及关键模块.硬件测试结果表明该OFDM发射机的峰均比得到了有效抑制,验证了该TR算法的正确性,证明其有较好的应用前景.