Joint clip and quantization effects characterization in OFDM receivers

An accurate method to analyze the joint effect of clipping, quantization and thermal noise in the analog-to-digital conversion in high-speed orthogonal frequency division multiplexing (OFDM) receivers is presented. The model provides information about the spectral properties of the distortion noise, which allows the analytical characterization of the signal-to-noise distortion ratio at the input of the demodulator. Oversampling, filtering, uniform, and optimal nonuniform quantization effects are taken into account. The optimal automatic gain control working point is evaluated for each parameter configuration. The differences from the results obtained by the classical pseudo-quantization noise model are highlighted showing that, due to the spectral characteristics, the gain introduced by oversampling is generally a few decibels lower than what foreseen by the classical theory. Moreover, the impact of these effects on the OFDM receiver performance degradation is evaluated and the validity of simplified approximations is discussed.     

Distortion of OFDM Signals on Radio-Over-Fiber Links Integrated With an RF Amplifier and Active/Passive Electroabsorption Modulators

The integration of orthogonal frequency division multiplexing (OFDM) and radio-over-fiber (RoF) techniques have made cost-effective and high-data-rate mobile wireless Internet networks possible, such as wireless broadband networks. This paper describes the distortion effects of OFDM signals fed via an RF amplifier integrated with an RoF link employing active and passive electroabsorption modulators (EAM) for chiefly broadband in-building network applications. First, peak-to-average power ratio was investigated for RoF links. Second, the adjacent channel power ratio, which estimates the degree of spectral re- growth due to the in-band and out-of-band interference resulting from distortion effects from nonlinear amplification, error vector magnitude, and system distortion effects, was also observed for the proposed system. In this study, we considered a combined Volterra-series and impulse response-based analytical model for WiBro systems and compared it to the observed measurements. We analyzed the different nonlinear distortion effects for OFDM signals driven via an RF amplifier that was integrated with an RoF link employing a distributed feedback laser as a transmitter and an active and passive InP EAM as a receiver. The results show significant agreement between the suggested analytical model and the measurement case. This study is unique in that it examines the distortion effects of RoF links with active and passive EAMs as an access point for OFDM-based wireless access networks.     

Reduction of PAPR of OFDM Signals Using Nonlinear Companding Transform

In this paper, we propose and evaluate a novel nonlinear companding transform (NCT) scheme for peak-to-average power ratio (PAPR) reduction in orthogonal frequency division multiplexing (OFDM) systems. The key idea of the proposed scheme is to transform the original Gaussian-distributed OFDM signals into a specific statistics form, whose target probability density function is defined by a piecewise function with an inflexion point. By properly choosing the transform parameters, this scheme can enable more flexibility and freedom in the companding form so that a favorable tradeoff between PAPR reduction and bit error rate (BER) performance can be achieved. Moreover, compared to existing NCT techniques, this scheme dramatically decreases the impact of companding distortion on the BER performance to reach a given PAPR level. The analytical expressions regarding the achievable transform gain in PAPR, complementary cumulative density function, attenuation coefficient, and selection criteria of transform parameters are derived. Simulation results justify the significance and accuracy of the analytical expressions presented.     

Analytical evaluation technique of self-phase-modulation effect onthe performance of cascaded optical amplifier systems

We propose an approximate analytical evaluation technique of self-phase-modulation (SPM) in cascaded optical amplifier systems. The effect of SPM on optical pulse distortion is represented by the increase in the α-parameter of the transmitter. The waveform distortion is approximated by assuming that the pulse is transmitted over a dispersive fiber without nonlinearity. This technique greatly reduces the calculation time, and is especially useful in designing cascaded amplifier systems, evaluating theoretical maximum transmission distance     

Co-channel interference cancellation for space-time coded OFDM systems

Space-time coded orthogonal frequency division multiplexing (OFDM) is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. Such combination benefits from the diversity gain within the multiple-input-multiple-output ST coded system and the matured OFDM modulation for wideband wireless transmission. However, STC transmit diversity impairs the system's interference suppression ability due to the use of multiple transmitters at each mobile. We propose an effective co-channel interference (CCI) cancellation method that employs angle diversity based on -steering beamforming or minimum variance distortion response beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the system's interference suppression ability without significant bit-error rate performance degradation. Furthermore, it is demonstrated that the proposed method can significantly combat the delay spread detrimental effects over multipath fading channels without the use of interleaving.     

Effects of nonlinear distortion and imperfect parameters estimation on the performance of OFDM-based DTTB systems

As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency-division multiplexing (OFDM) is extensively used in modern digital television terrestrial broadcasting (DTTB) systems, such as the cyclic prefix OFDM (CP-OFDM) based DVB-T DTTB systems and the time domain synchronous OFDM (TDS-OFDM) based DMB-T DTTB systems, to support high performance bandwidth-efficient multimedia services. OFDM-based DTTB systems are very sensitive to the nonlinear distortion at the transmitter and the imperfect parameters estimation at the receiver. In this paper, the combined effects of nonlinear distortion and imperfect parameters estimation on the performance of the OFDM-based DTTB systems are investigated, a closed form of symbol error probability of the OFDM-based DTTB systems with nonlinear distortion and imperfect parameters estimation is derived. Theoretical results show closely matching with those obtained by simulations for both the CP-OFDM based DVB-T DTTB systems and the TDS-OFDM based DMB-T DTTB systems.     

Effects of Power Amplifier Distortion and Carrier Frequency Offset on the Performance of WLAN System

Due to the efficiency of mitigation multipath delay spread, orthogonal frequency division multiplexing (OFDM) is extensively used in the wireless local area network (WLAN) domain, such as the IEEE 802.11a standard defined by the IEEE 802.11 standardization group and the HIPERLAN/2 defined by the European Telecommunication Standards Institute Project on Broadband Radio Access Networks. OFDM based WLAN system is very sensitive to the power amplifier distortion and the carrier frequency offset. The performance of the OFDM based WLAN system in the presence of the power amplifier distortion and the carrier frequency offset under practical fading channel is researched and analyzed in this paper. A closed form of bit error rate (BER) is derived for the OFDM based WLAN system in the presence of the power amplifier distortion and the carrier frequency offset under practical fading channel. The effects of the power amplifier distortion and the carrier frequency offset on the OFDM based WLAN system performance are comparatively studied by the theoretical method and by the simulation method under practical multipath fading channels. Studies show that the theoretical and simulated results match well.     

Performance analysis and optimization of OFDM receiver with blanking nonlinearity in impulsive noise environment

A simple method of improving orthogonal frequency division multiplexing (OFDM) receiver performance in an impulsive noise environment is to precede a conventional OFDM demodulator with blanking nonlinearity. This method is widely used in practice since it is efficient and very simple to implement. However, performance analysis of this scheme has not yet appeared. In this paper, we study performance of the OFDM receiver with blanking nonlinearity in the presence of impulsive noise. Closed form analytical expressions for the signal-to-noise ratio (SNR) at the output of blanking nonlinearity and the optimal blanking threshold that maximizes SNR are derived. Simulation results are provided that show good agreement with theory if the number of OFDM subcarriers is sufficiently large.     

The effect of four-wave mixing in fibers on opticalfrequency-division multiplexed systems

The limiting effects of four-wave mixing on optical frequency-division multiplexing (OFDM) systems are described. The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various system parameters. An analysis of FWM in both undirectional and bidirectional transmission systems is included. The receiver sensitivity degradation from FWM crosstalk is measured in a 16-channel coherent system     

Performance Comparison of Uncoded OFDM Systems with Trellis Shaping over Nonlinear Channels

Orthogonal frequency-division multiplexing (OFDM) can achieve remarkable performance in terms of spectral efficiency, but its power amplifier efficiency becomes poor because of its high peak-to-average power ratio (PAPR) nature of transmitted signals. Trellis shaping is an effective method to reduce peak and average power without signal distortion at the cost of transmitter complexity and its performance depends on the metric used for the trellis decoder. Several metrics, defined in either time or frequency domain, have been proposed in the literature. In many practical OFDM systems, however, slight nonlinearity is tolerable as long as the resulting distortion meets the system requirements such as bit error rate (BER) and spurious level of transmitted signals. In this paper, we make a thorough performance comparison of trellis-shaped uncoded OFDM systems with different metrics in terms of complexity, achievable PAPR, and resulting BER as well as power spectrum in a practical channel where the signal envelope is distorted by a power amplifier (PA). Our study shows that the metric designed in frequency domain is mostly favorable in such a scenario.     

一种基于LS/SVD的基带预失真算法

自适应基带预失真技术是用于补偿高功率放大器非线性失真的一种有效技术。早期对预失真的研究大多局限于无记忆非线性,但对于OFDM等宽带应用,放大器的记忆效应明显。介绍了一种基于最小二乘奇异值分解(LS/SVD)的自适应算法来实现有记忆非线性功放的自适应基带预失真。仿真结果表明,该方案用于正交频分复用(OFDM)系统中能有效抑制带外频谱扩散,减小带内失真,实现有记忆非线性功率放大器的自适应预失真。     

Theoretical analysis and performance of OFDM signals in nonlinear fading channels

This paper presents an analytical framework to calculate the average symbol-error rate (SER) of uncoded orthogonal frequency-division multiplexing (OFDM) systems in realistic scenarios impaired by transmitter nonlinearity and frequency-selective fading channels. The results are applicable to cyclically extended OFDM signals characterized by a high number of carriers, which can be modeled as complex Gaussian processes. To avoid intercarrier interference, we also assume that the symbol duration is shorter than the channel coherence time. We derive analytical SER results in Rayleigh and Rice frequency-selective fading channels, for both the nonlinear amplification and the ideal predistortion case. Simulations results demonstrate the validity of the analytical results.     

Comparison of FWM- and XPM-induced crosstalk using the Volterra series transfer function method

New analytical tools to calculate the variance due to cross-phase modulation (XPM) and four-wave mixing (FWM) induced intensity distortion are derived based on the Volterra series transfer function method. The analysis for both the XPM and FWM effects is based on the same system configuration with a continuous-wave (CW) probe channel plus modulated pump channels, which makes possible a fair comparison between the two nonlinear effects. Effective ways to reduce the XPM- and FWM-induced intensity distortion are given. The new results on the variance of the nonlinearity-induced intensity fluctuation also make it possible to study both synchronous wavelength-division multiplexing (WDM) systems with fixed channel delays and asynchronous WDM systems with random channel delays. The new analytical results provide accurate and efficient ways for system parameter optimization to reduce these two nonlinear effects.     

一种改进的电力线通信系统脉冲噪声抑制方法

为了抑制脉冲噪声对电力线正交频分复用（OFDM）通信系统的影响,最常用的方法之一是在接收端OFDM解调器之前前置一个置零非线性单元,即传统置零法。然而,由于引入了非线性失真,其性能并不理想。针对传统置零法引起的非线性失真问题,提出了一种基于迭代消除非线性失真的改进置零法。首先,对接收到的时域OFDM信号进行脉冲噪声检测和置零处理;然后,在频域利用已检测的符号来重构时域置零处理引入的非线性失真,并通过迭代提高重构的准确性;最后,从频域接收信号中减去重构的非线性失真。仿真结果表明,所提改进算法与传统置零法相比,有非常大的性能提升,增强了电力线OFDM通信系统对脉冲噪声的抵抗能力。     

一种正交频分复用系统的非线性补偿技术

针对正交频分复用(OFDM)系统在功率放大器(PA)非线性较强时的性能问题,基于一种无线设备非线性与无线信道的联合估计技术,提出了一种基于训练序列的OFDM非线性信道估计与补偿技术.首先基于最小二乘(LS)算法进行发射机非线性与无线信道单位脉冲响应的联合估计,然后依次进行无线信道与发射机非线性的补偿.仿真结果显示,提出方法可逼近不考虑PA非线性时OFDM无线通信系统的完美均衡理论解析值.     

Adaptive Predistorters for Linearization of High-Power Amplifiers in OFDM Wireless Communications

Orthogonal frequency division multiplexing (OFDM) has several desirable attributes which make it a prime candidate for a number of emerging wireless communication standards. However, one of the major problems posed by OFDM is its high peak-to-average power ratio (PAPR), which seriously limits the power efficiency of the high-power amplifier (HPA) because of the nonlinear distortion resulting from the high PAPR. We provide a new mixed computational/analytical approach for adaptive compensation of this nonlinear distortion for cases in which the HPA is a traveling wave tube amplifier (TWTA) and solid state power amplifier (SSPA). TWTAs are used in wireless communication systems when high transmission power is required as in the case of the digital satellite channel, and SSPAs are generally used in mobile communication systems. Compared to previous predistorter techniques based on LUT (look-up table) or adaptive schemes, our approach relies on the analytical inversion of Saleh's TWTA model and Rapp's SSPA model in combination with a nonlinear parameter estimation algorithm. This leads to a sparse and yet accurate representation of the predistorter, with the capability of tracking efficiently any rapidly time-varying behavior of the HPA. Computer simulations results illustrate and validate the approach presented.     

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.     

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     

WCDMA系统中基于查找表的预失真技术的研究

在无线通信系统中,高功率放大器因其非线性,导致AM／PM效应使得微分相位、微分增益和互调失真变坏。高质量的通信系统设计应尽可能减小功率放大器的AM／PM效应。因此,针对宽带码分多址（WCDMA）功放非线性失真问题,采用一种基于查找表（LUT）的自适应预失真方法,改善功放的非线性失真。仿真表明．该方法能有效补偿放大器产生的AM—AM、AM-PM失真,并将功放的邻道功率泄漏比（ACPR）改善到30dB左右。