CE-OFDM调制中继选择AF系统的误码性能

恒包络（CE）正交频分复用（OFDM）能够开发OFDM的优点,并能消除OFDM峰均功率比高、对功率放大器非线性失真敏感的缺点。反馈时延等因素会恶化采用OFDM调制的中继选择放大转发（AF）协作系统的误码性能。最小均方误差（MMSE）维纳信道预测器能够减轻反馈时延对无线系统误码性能的恶化。为此,提出了一种能够克服OFDM缺点和反馈时延影响的、采用CE-OFDM调制和MMSE信道预测的中继选择AF协作系统方案,并推导其在瑞利块衰落信道上矩形正交幅度调制下的平均误比特率（ABER）下界表达式。数值计算和仿真结果表明：当调制系数较小时,上述系统的ABER下界具有较高的准确性,可用于CE-OFDM调制和MMSE信道预测的中继选择AF协作系统的设计。     

采用TAS/RAS的CE-OFDM系统性能分析

研究平坦信道上采用收发天线选择的恒包络正交频分复用系统的误码性能。采用矩生成函数方法,推导矩形M进制正交幅度调制的恒包络正交频分复用系统的平均误符号率下界的解析表达式。仿真表明在大信道衰落参数、小相位调制系数和高信噪比时下界与平均误比特率非常吻合。数值计算和仿真结果均阐明收发天线数目、信道衰落参数与相位调制系数对分集接收恒包络正交频分复用系统误码性能影响。     

基于Saleh模型的DRSS信道建模与仿真

对DRSS信道URE链路进行建模仿真,采用Saleh模型描述DRSS信道URE链路中HPA的非线性特性。通过仿真8PSK、方型16QAM、星型16QAM调制信号并经过DRSS URE非线性信道后的非线性ISI特性、频率特性、星座图和误码特性,分析了信道非线性特性对通信系统性能的影响。     

功放非线性对QAM解调误码率的影响

QAM调制由于高频谱效率得到了广泛应用,但由于QAM信号的功率峰均比较高,功率放大器的非线性会导致解调性能的下降。针对功放非线性对QAM解调性能的影响,传统方法采用仿真实验的方法来获得,而文中推导了高斯信道下功率放大器的非线性对QAM解调误码率影响的闭合表达式,可求出非线性功放在不同静态工作点对应的误码率。仿真结果验证了理论分析的正确性。     

基于相似变换的CEFQPSK的交叉调制模型设计与分析

针对传统FQPSK调制方式存在包络起伏及中心点不连续问题,在对FQPSK进行改进的基础上,提出一种波形集完全对称的恒定包络FQPSK调制方案。通过类比I/Q调制与连续相位调制过程,确定相似变换参量;通过等效替换,导出恒定包络FQPSK信号表示成连续相位调制的形式。对改进后恒包络FQPSK模型进行了仿真分析,仿真结果表明新方法继承了FQPSK调制信号良好的性能,实现简单,为非线性信道下数据传输处理提供了有价值的技术参考。     

基于相似变换的CEFQPSK交叉调制模型设计与分析

针对传统FQPSK调制方式存在包络起伏及中心点不连续问题,在对FQPSK进行改进的基础上,提出一种波形集完全对称的恒定包络FQPSK调制方案。通过类比I/Q调制与连续相位调制过程,确定相似变换参量;通过等效替换,导出恒定包络FQPSK信号表示成连续相位调制的形式。对改进后恒包络FQPSK模型进行了仿真分析,仿真结果表明新方法继承了FQPSK调制信号良好的性能,实现简单,为非线性信道下数据传输处理提供了有价值的技术参考。     

一种改进的变步长常数模算法

常数模算法是一种最为常用的盲均衡算法,普遍应用于恒包络信号和非恒包络信号的均衡,但存在收敛速度慢和剩余误差大的缺点。在深入研究常数模算法的基础上,该文提出一种改进的变步长算法,对其性能进行了理论分析,并对二值序列以及正交幅度调制信号进行了仿真。理论分析和仿真结果表明:该算法具有收敛速度快、剩余误差小的特点。     

数字微波系统中信道非线性对系统性能的影响

本文论述了数字微波系统中信道非线性对系统误码性能及频谱性能的影响,并以8PSK系统为例,利用计算机模拟计算了在非线性作用下,系统误码性能和频谱性能的恶化情况。     

AWGN信道中非恒包络信号SNR估计算法

本文提出一种利用信号自相关矩阵的奇异值分解(SVD)对加性高斯白噪声(AWGN)信道下非恒包络信号信噪比的盲估计算法,并作了详细理论分析.通过对该算法进行计算仿真,结果表明:该算法性能稳定,对MPSK、MQAM信号在实际信噪比为1~20dB时,估计误差小于0.5dB.     

非线性功放对数字通信影响的仿真分析

利用半解析的方式,构建了一种简化的PSK（相移键控）通信系统的仿真模型,该模型与蒙特卡罗仿真模型相比具有更高的运算效率。在功放工作于饱和条件下,利用该模型,分别仿真了BPSK（二进制相移键控）、QPSK（正交相移键控）信号通过饱和功放后的频谱以及误码特性,阐述了成型滤波器对PSK信号的影响,分析比较了OQPSK（交错正交相移键控）信号在非线性信道中的特点。为通信信号在雷达通道中进行传输提供了借鉴和一种分析方法。     

Binary Power Amplifier with 2‐Bit Sigma‐Delta Modulation Method for EER Transmitter

A novel power amplifier for a polar transmitter is proposed to achieve better spectral performance for a wideband envelope signal. In the proposed scheme, 2‐bit sigma‐delta (ΣΔ) modulation of the envelope signal is introduced, and the power amplifier configuration is modified in a binary form to accommodate the 2‐bit digitized envelope signals. The 2‐bit ΣΔ modulator lowers the noise of the envelope signal by fine quantization and thus enhances the spectral property of the RF signal. The Ptolemy simulation results of the proposed structure show that the spectral noise is reduced by 10 dB in a full transmit band of the EDGE system. The dynamic range is also enhanced. Since the performance is improved without increasing the over‐sampling ratio, this technique is best suited for wireless communication with high data rates.     

Nonlinear modeling and analysis of a Doherty power amplifier driven by non-constant envelope signals

A circuit-level CAD method based on circuit envelope (CE) simulation is presented to analyze the nonlinear behavior of Doherty power amplifiers (DPAs) in LDMOS technology, when driven by non-constant envelope signals. In this paper, a general nonlinear model has been extracted for an RF LDMOS power transistor. Then, a complete CE analysis is applied to a designed DPA using the extracted model at 3.5?GHz. To solve the CE equation at specific envelope samples, analytic expressions are developed for the Jacobian matrix and an effective method for quick calculation is suggested. Employing this nonlinear analysis gives insight for optimum design of DPAs and provides an accurate tool to precisely model the nonlinearity of the amplifier in order to predict its distortion effects in modern communication systems.     

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.     

非线性功放条件下的恒包络OFDM系统性能

正交频分复用技术具有较高的频谱效率和好的抗频率选择性衰落性能,被广泛应用于无线通信系统中。但是OFDM信号具有较高的峰均比,当系统采用非线性功放时,OFDM系统需要较大的输入功率回退以避免信号失真。为了研究非线性功放条件下的系统性能,分析了恒包络OFDM系统的实现方式,并以误码率为指标,在加性高斯白噪声信道和多径衰落信道场景下对比了恒包络OFDM系统与采用传统迭代限幅滤波PAPR抑制技术的OFDM系统的性能。     

Characterizing the effects of nonlinear amplifiers on linearmodulation for digital portable radio communications

The design and performance of linear modulation techniques and RF power amplifiers are discussed. Various types of power amplifier are used in the comparison, using simulation, of out-of-band radiation and power efficiency. The designs focus on high-frequency silicon bipolar power amplifiers suitable for low-power portable radio applications. Two different 4-QAM (4-level quadrature amplitude modulation) schemes are compared. Power efficiency as high as 40% can be achieved using a dynamically biased amplifier operating near saturation. The effects of nonlinear distortions from power amplifiers on adjacent channel interference as well as bit and block error rate performance are also investigated     

Differential modulation techniques for a 34 MBit/s radio channel using orthogonal frequency division multiplexing

The orthogonal frequency division multiplexing (OFDM) technique has been proposed for terrestrial digital transmission systems due to its high spectral efficiency, its robustness in different multipath propagation environments and the ability of avoiding intersymbol interference (ISI). Our studies consider a radio channel bandwidth of 8 MHz and a data rate of 34 Mbit/s.In the case of the OFDM transmission system a coherent 64-QAM requires a channel estimation process and a channel equalization in frequency-selective interference situations [4]. The equalization process can be realized by a multiplier bank at the FFT output in the receiver, a so-called frequency-domain equalizer. Alternatively, a multilevel differential modulation technique, the so-called differential amplitude and phase shift keying (64-DAPSK) considering the phase and simultaneously the amplitude for differential modulation, is proposed and presented in this paper. Differential modulation/demodulation techniques do not require any explicit knowledge about the radio channel properties in the differential channel equalization. It is therefore not necessary to implement a frequency-domain equalizer in an OFDM/64-DAPSK receiver, which reduces the computation complexity. The performance of both modulation techniques has been analysed in the uncoded and coded case referring to Gaussian and frequency-selective Rayleigh fading channels. Simulation results are presented in this paper.The OFDM signal has a non-constant envelope with large instantaneous power spikes possible primarily resulting in an overdriving of the high power amplifier (HPA) at the transmitter. This leads to nonlinear distortion causing intermodulation noise and spectral spreading. Both effects can be limited by introducing an appropriate input backoff (IBO). In this paper the performance of OFDM signals in the presence of nonlinearities is analysed quantitatively.     

Ultrafast integrable and reconfigurable XNOR, AND, NOR, and NOT photonic logic gate

A novel, simple, compact, and integrable scheme of reconfigurable and ultrafast photonic logic gate is demonstrated, based on a single semiconductor optical amplifier (SOA) and able to process ultrafast signals. XNOR function has been optically implemented exploiting four-wave mixing and cross-gain modulation in an SOA. The same scheme can be easily reconfigured to obtain AND, NOR, and NOT logic gates. Performances in terms of bit error rate for 20-ps return-to-zero signals at 10 Gb/s show a power penalty limited to 0.5 dB for all logic gates but the AND, which experiences regeneration (-2-dB power penalty) due to nonlinear SOA noise compression.     

A Polar Modulator Using Self-Oscillating Amplifiers and an Injection-Locked Upconversion Mixer

A polar modulator for use with non-constant envelope signals is presented. Techniques for efficient behavior of both the amplitude (AM) and phase (PM) path are introduced. The amplitude modulation technique is based on asynchronous pulse-width modulation of a phase-modulated RF signal. The digital signal generated in this way is suited for amplification using a nonlinear, efficient type of PA, while preserving the AM modulation, since this information is present in the signal. The efficiency of the switching amplifier is less dependent of the output power, compared to class B operation. Using a self-oscillating, asynchronous type of pulse-width modulator, the spurs are concentrated in narrow bands well separated from the signal band. These switching spurs are filtered out by the filters already present in the transmitter. The feasibility of this RF-pulse-width-modulation is proven by measurements with amplitude modulated signals on a prototype in 0.18 mum CMOS. Unmodulated, the circuit is able to deliver a peak output power of 8.26 dBm with a drain efficiency of 35%. In the phase-modulation path of the polar modulator, injection locking of an oscillator is used as a single-stage high-gain amplifier. Avoiding multiple stages and related power loss improves the overall power efficiency.     

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.     

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