基于非理想信道状态信息对DCO-OFDM-IM系统建模与分析

针对传统DCO-OFDM-IM系统进行信道估计时没有考虑信道误差影响,导致获得非理想信道状态信息（CSI）而造成信道估计有效性降低等问题,利用最小均方误差信道估计（MMSE）算法并基于非理想信道状态信息建立了DCO-OFDM-IM系统模型,推导了其理论误码率闭合表达式。同时考虑到DCO-OFDM-IM系统中采用最大似然检测算法（ML）当调制阶数增大时存在检测复杂度高,系统传输速率低等问题,而利用对数似然检测算法（LLR）降低检测复杂度,最后采用蒙特卡罗方法进行系统仿真性能的仿真对比。结果表明：当调制阶数和索引组合相同,误码率为10^-3时,LLR检测算法比ML所需信噪比平均改善约2 dB;在能量比为1∶8时,采用MMSE估计比LS信道估计算法系统性能改善约4 dB;当系统信道估计误差越小时,越可能获得理想CSI,提升了信道估计有效性。     

基于LWT的可见光OFDM-IM系统研究

针对索引调制正交频分复用(OFDM-IM)技术峰均比过高,对相位噪声和载波频偏敏感,系统复杂度高等问题,提出基于小波提升变换(LWT)的可见光OFDM-IM系统。首先选择正交小波基作为子载波,通过对信号的分裂、预测和更新,将信号分为高频信号和低频信号,再结合可见光信道构成LWT-OFDM-IM系统,最后通过理论分析和蒙特卡罗方法对系统的可靠性、峰均比特性和最佳小波分解层数进行了仿真验证,结果表明：当子载波N=256,子块中子载波数量L=4,激活子载波数量k=2,系统误码率为10^-4时,LWT-OFDM-IM比FFT-OFDM-IM改善约8 dB,相较于DWT-OFDM-IM提高约4 dB。当系统互补累计分布函数(CCDF)为10^-1数量级时,LWT-OFDM-IM的峰均比较FFT-OFDM-IM改善约2.3 dB。随着小波层数的递增,系统的误码性能越好,当小波层数为3层时,大约在10 dB附近,误码率可到10^-5量级。     

改善误码性能的索引调制正交频分复用混合映射方案

为进一步提升索引调制正交频分复用(OFDM-IM)系统在高信噪比(SNR)条件下的误比特率(BER),该文提出一种混合映射方案。该方案利用高信噪比时索引比特比数据比特错误率更低的特性,混合使用两种不同位数的索引比特分配策略,提高了索引比特的占比。相应地,将子载波激活模式(SAPs)分为超级SAPs和普通SAPs,相比于普通SAPs,超级SAPs对应的索引比特和数据比特分别增加1 bit和减少1 bit。将超级SAP对应的数据比特采用奇偶校验的方式补齐,然后映射为数据符号,提高了系统的分集度,增大了数据符号间的最小欧氏距离。仿真结果表明,相比于以往的映射方案,混合映射方案在误比特率为10–4时可取得1～3 dB的性能增益,有效改善索引调制OFDM系统的误码性能。     

基于可变长纠错码和掺杂调制的联合信源信道编码调制方法

针对无线通信中存在的频谱资源有限、信道衰落和多径效应等问题，该文将可变长纠错(VLEC)编码和掺杂调制相结合，提出一种新的联合信源信道编码调制方法。该方法利用外信息转移(EXIT)图分析系统的迭代译码特性，优化设计可变长纠错编码和掺杂调制的参数。主要包括：(1)设计有更大自由距离的可变长码，使其具有纠错能力；(2)设计优化掺杂调制的掺杂和映射规则的方法，使掺杂调制EXIT曲线与可变长纠错编码EXIT曲线匹配，降低迭代译码收敛所需的信噪比(SNR)。仿真结果表明，在AWGN信道和瑞利衰落信道下，该联合信源信道编码调制方法与分离的信源信道编码调制方相比，迭代收敛所需的信噪比减小了1 dB以上，相比其他的联合信源信道编码调制方法，也有更好的误码率性能。在误码率为10–4时，该方法距离AWGN信道和瑞利衰落信道的香农限分别为0.7 dB和1.0 dB。     

基于深度学习的水下无线光通信信噪比改善研究与实现

复杂的水下环境造成了水下光信号的质量下降,为提升水下无线光通信系统信号信噪比,结合深度神经网络提出了一种信噪比改善方法,该方法通过信号频谱有效抑制了信号噪声.实验结果表明:针对1m传输距离的水下无线光通信16阶正交振幅调制-正交频分复用(16QAM-0FDM)信号,该方法可以实现约17 dB的信噪比提升同时误码率降低至...     

基于小波的变换域通信系统抗干扰性能研究

变换域通信系统通过信道估计在变换域设计信号波形避开干扰,它没有采用载波调制,而是用一个类似噪声的基函数进行信息调制。对高斯白噪声信道下的取不同门限值的基于小波的变换域通信系统与DSSS的抗干扰性能作了比较分析,仿真结果表明基于小波的变换域通信系统相比于DSSS在单音、多音、线性调频、10%部分带宽干扰下误码率平均改进约10dB,在70%部分带宽干扰下误码率平均改进约5dB,并对基于小波的变换域通信系统门限值的选取给出了建议。     

数字调相通信系统的性能

本文分析数字调相系统在高斯噪声下的性能,并确定无衰落情况下所需信噪比与分立的相数及误码率的关系,既考虑到由本机产生参考载波的相干检波又考虑到相位比较检波。计算表明,多相调制较之多电平调幅在带宽换取信噪比方面更为有效。同时发现除二进制调制外,相位比较检波的性能较相干检波约低3 dB。而二进制调相在误码率约不超过0.001时,性能下降小于1dB。     

实现20比特/秒/赫兹的无线传输数字调制算法——VMSK/2

本文介绍了一种新的具有极高频带利用率的数字基带调制算法─1/2甚小频移键控(VMSK/2),VMSK/2可以在不损失信噪比的前提下极大地压缩信号传输所需的频带.将VMSK/2与模拟单边带抑制载波(SSB-SC)调制方式相结合,利用现有的硬件技术,可在射频(RF)传输系统中获得近20比特/秒/赫兹(bits/s/Hz)的频带利用率.本文对VMSK/2结合SSB-SC的调制方式进行了理论分析,并给出了仿真结果.数值实验表明在保证1.0×10^-6误码率(BER)的前提下,为实现20bits/s/Hz的频带利用率,VMSK/2结合SSB-SC无线传输系统所需的信噪比(SNR)为13分贝(dB).     

联合限幅和μ律压扩抑制可见光通信系统的峰均功率比北大核心CSCD

直流偏置光正交频分复用(DC-biased optical orthogonal frequency division multiplexing,DCO-OFDM)可见光通信(visible light communication,VLC)系统具有较高的峰均功率比(peak-to-average power ratio,PAPR)。为解决此问题,提出一种联合限幅和归一化μ律压扩算法抑制DCO-OFDM系统的PAPR。首先,该联合算法利用限幅操作对时域信号中的大峰值信号进行削波来降低信号的幅值;其次,对限幅后的信号再进行压扩变换。通过这种联合算法,DCO-OFDM系统的PAPR可以显著降低。实验仿真结果表明,当采用16QAM调制方式,互补累计函数的值为10^(-3)时,相比较于联合限幅和传统μ律压扩算法,此联合算法的PAPR下降了约2.206 dB。     

基于多载波的相干波分复用太比特传输实验研究

基于循环频移器(RFS)的多载波产生原理,得到了间隔为25GHz,载噪比为25dB的20个子载波的多载波输出,将波特率为16Gbaud(即64Gbit/s)的归零码十六进制正交幅度调制(RZ-16QAM)信号加载到这些子载波上,并且进行偏分复用(PDM)生成PDM-RZ-16QAM信号,实验实现了2.56Tbit/s PDM-RZ-16QAM信号792km标准单模光纤(SSMF)的传输和相干接收。实验测得的背靠背相干光波分复用(CO-WDM)系统在误码率为1×10-3的情况下,光信噪比需求比理论值(FEC)增加了4.1dB。传输后,最佳入射光功率处的误码率为4.5×10-3,小于前向纠错的软判决门限(2.4×10-2),此时对应的CO-WDM PDM-RZ-16QAM中心信道信号恢复后x和y偏振方向的星座图清晰规则,说明传输性能很好,并且系统的频谱效率达到了5.1bit/(s·Hz)。     

Line-of-sight PCM performance at 15.3 GHz

The effects of certain atmospheric conditions upon pulse-code modulation (PCM) data transmission at a carrier frequency of 15.3 GHz are discussed in this paper. Using an amplitude-shift keying (ASK) modulation to transmit 1300000 bit/s across a 9.5 mi line-of-sight path, the performance characteristics of the system, relating to conditions in the channel, were measured and are presented in this study. Theoretical curves of probability of error versus predetection signal-to-noise ratio (SNR), are compared to measured error probabilities; these are obtained at different received signal levels, and under various clear weather conditions ranging from humid to dry. Above 15 dB predetection SNR, the system operates more efficiently than theoretically predicted for ASK modulation over a Rayleigh fading channel. The detected envelope probability distributions are computed for different values of SNR. Results show that drier weather, which causes amplitude fluctuations of a shallow nature, results in greater error probabilities than those for more humid conditions; the term shallow fading is used to describe short term (5-10 min) received signal envelope variations of less than 1 dB. For example, error probabilities for a predetection SNR of 14 dB, ranged from 0.005 to 0.078, from humid to dry conditions respectively. These values fall between the theoretical values of 0.0005 for the nonfading case, and 0.0740 for Rayleigh fading.     

基于LDPC-OFDM编码调制在无源光网络的研究

将低密度奇偶校验码与正交频分复用相结合的编码调制方式引入到无源光网络中,并通过Optisystem和Matlab进行联合仿真,研究了基于LDPC-OFDM编码调制的PON系统的下行传输性能。结果显示,由LDPC-OFDM编码调制的信号,在无源光网络中可以实现10 Gbit·s-1的下行传输。在误码率均为10-4时,基于LDPC-OFDM编码调制的系统传输性能较未编码传输光有4.2 dB的提升。当信噪比＞2.6 dB时,Gallager构造法较Mackay构造法和BIBD构造法有更好的误码率性能,以及当信噪比＞4.6 dB时,LLR-BP及其改进译码算法能使系统性能得到改善。     

CDF and MGF based analysis over Rayleigh TWDP shadowed fading channel for indoor communication

Recently, the two-wave with diffuse power (TWDP) distribution has been extensively used to model the shadowing in multipath-faded/shadowed indoor environment. In this article, novel expressions for joint moments, mean, second moment, variance and cumulative distribution function for Rayleigh TWDP shadowed fading model are derived. By using the derived expression of mean and variance, the expression for amount of fading is obtained. Also, the outage probability, moment generating function and average bit error rate (ABER) for various modulation schemes namely binary phase shift keying (BPSK), binary frequency shift keying (BFSK), minimum shift keying (MSK), differentially coherent phase shift keying (DCPSK) and non-coherent frequency shift keying (NCFSK) are calculated. The derived expressions for cumulative distribution function, outage probability and ABER are presented in analytical format and have been numerically evaluated. Moreover, the numerical results of ABER using MSK and DPSK modulation schemes is compare with results of Rayleigh Gamma composite fading model. The study shows that better outage probability (0.01) is observed at 40dB average signal to noise ratio (SNR) with 5dB lowest threshold SNR, however, at higher threshold SNR (>5dB) with fixed average SNR (40dB), poor outage probability performance are obtained. Further, at higher shadowing (10dB), for fixed average SNR (15dB), minimum error probability (10^?4) is observe, while at lower shadowing (less than 10dB), higher error probability (greater than 10^?4) is observed that represents poor BER performance.     

Block adaptive subcarrier allocation for wireless multiuser OFDM system

In this paper, a blockwise adaptive subcarrier allocation algorithm for multiuser Orthogonal Frequency Division Multiplexing (OFDM) system is introduced. Assuming the knowledge of channel information for all users, the algorithm minimizes the total transmit power while satisfying the total power and users‘ rate constraints. The result of simulation shows that the proposed algorithm reduces the average bit Signal-to-Noise Ratio (SNR) by approximately 4 dB compared with OFDM-Frequency Division Multiple Access (OFDM-FDMA), and supports more users in a multiuser Rayleigh fading channel. As assigning subcarriers in block, the computational complexity of the algorithm is much lower than that of the derivation of “water-filling” algorithm.     

Efficient coding techniques for ADO-OFDM in IM/DD systems

Optical wireless communication (OWC) is an age-long technology, which is based on optical data transmission through free space, and it can be implemented in both indoor and outdoor applications. Asymmetrically clipped DC-biased optical orthogonal frequency division multiplexing (ADO-OFDM) is a modulation scheme that can be utilized in indoor OWC systems. It is based on transmitting DC-biased optical OFDM (DCO-OFDM) for even-frequency subcarriers and asymmetrically clipped optical OFDM (ACO-OFDM) for odd-frequency subcarriers. The ADO-OFDM exhibits better optical power performance than those of the conventional ACO-OFDM and DCO-OFDM. The main disadvantage of ADO-OFDM is the clipping noise, which mainly affects the even subcarriers. So, in this paper, convolutional and turbo coding techniques are investigated to improve the bit error rate performance of the ADO-OFDM over Additive white Gaussian noise and diffuse channels. Simulation results show that the proposed coded ADO-OFDM using convolutional and turbo coding techniques achieves significantly lower BERs compared to that of the uncoded ADO-OFDM. In addition, turbo-coded ADO-OFDM gives the best BER performance.     

可见光通信光叠加正交频分复用技术研究

针对可见光通信(VLC)正交频分复用(OFDM)系统 发光二极管(LED)动态范围窄、非线性和限幅失真严重等问题,利用光 源具有多个灯芯的特点,将OFDM频带划分为多个子带,每个子带内数据通过傅里叶反变换(IFFT)产生一路独立信号输入LED 灯芯,所有LED灯芯发出的光信号在空间上叠加形成光叠加OFDM(OSM-OFDM) 信号。在相同直流偏置情况下, OSM-OFDM系统每一路信号的调制深度较低,能够有效地避免LED非线性和限幅影响,特别是 在输入信号功率较大时,系统 误码率(BER)降低了至少1个数量级。根据有效信噪比(SNR) 最大原则能够得到OSM-OFDM系统最优直流偏置,而且在系统设计时可以根据 系统有效信噪比公式选择合适的LED灯芯数。实验表明,OSM-OFDM系统BER 性能要比直流偏置光OFDM(DCO-OFDM)系统提升了1个数量 级。     

水下复合信道对GMSK无线光通信系统性能的影响

The absorption and scattering of light in seawater channel cause signal attenuation, and the turbulence of seawater causes signal amplitude fluctuation, both of which will reduce the bit error rate (BER) performance of underwater wireless optical communication (UWOC) system. The effects of the two channel characteristics on the signal performance were considered comprehensively, and a method was proposed to equate the transmission distance and turbulence probability density function to the system signal-to-noise ratio (SNR) and turbulence noise, and then the signal attenuation and turbulence noise were combined into the signal waveform to establish the underwater composite channel signal transmission model. According to the experimental system parameters, the signal transmission waveforms of Gaussian minimum frequency shift keying (GMSK) modulation under composite channel were simulated, and the one-bit difference demodulation algorithm was used to compare the demodulated waveforms with the original waveform, and the influence relationships of composite channel on the system BER performance was analyzed. The simulation experiment results show that, compared with on-off keying modulation (OOK), pulse position modulation (PPM), GMSK system can obtain the SNR gain of 3.3 dB, 4.8 dB respectively only in the attenuation channel with seawater attenuation coefficient of 0.151 m?1. Under the composite channel, GMSK modulation performance is superior to OOK modulation and PPM modulation. When the water attenuation coefficient is 0.151 m?1, and turbulence intensity variance is smaller than 0.16, GMSK modulation system has no error rate limit, the system BER is decided by signal attenuation and turbulence noise and Gaussian noise together, GMSK modulation achieves SNR gain of 4.35 dB compared with PPM modulation. Furthermore, turbulence intensity variance is greater than 0.16, system BER arrives limit, which value is determined by the turbulence intensity, and the limit value of BER increases nonlinearly with the increase of turbulence intensity.     

Adaptive Modulation and Multiple Access for the OFDM Transmission Technique

For the transmission of flexible andhigh data rates in future mobile communicationsystems, the multicarrier transmission technique(Orthogonal Frequency Division Multiplexing, OFDM) canbe regarded as a possible alternative tosingle-carrier transmission that is used in today'ssystems. One of the advantages of OFDM is theflexibility concerning modulation and multiple accesstechniques. Since in OFDM the total bandwidth isdivided into a large number of subcarriers, it can beflexibly shared among all the users. In addition, themodulation scheme can be individually chosen for eachsubcarrier, thus granting the possibility of anadaptation to the radio channel characteristics. Inthis paper, the combination of two different multipleaccess schemes, OFDM-FDMA and OFDM-TDMA, and anappropriate adaptive modulation technique isconsidered for the downlink of a communication system.Different degrees of adaptivity are analysed andcompared to show the benefit of an ``intelligent'multiple access and modulation strategy. For thegiven parameters and assumptions, OFDM-TDMA withadaptive modulation yields an improvement of 6 dB insignal-to-noise ratio (SNR) at a bit error rate (BER)of 10^-2. When OFDM-FDMA with adaptive subcarrierallocation is applied, the SNR gain amounts to 12.5 dB(BER of 10^-2).     

A Multidigit Adaptive Delta Modulation (ADM) System

A multidigit adaptive delta modulation (ADM) system has been proposed where the error signal, between the input and the approximated signal produced by ADM coder, is coded in an auxiliary encoder. The error in the auxiliary coder is processed by another ADM and so on. The bit rate of each of these coders isf_{r}/Nwhere fris the overall transmission rate andNis the number of coders used. The bit streams are interleaved for transmission and at the receiver they are separated and decoded, and these signals are added and filtered. It is shown that for a given transmission rate, each coder operates at a basic sampling rate of frBsuch thatN_{opt} = f_{r}/f_{rB}gives the optimum number of coders to be used for maximum signal-to-noise ratio (SNR). A bound is derived for the maximum SNR of such a system and is compared with the bounds derived for other predictive coders. The experimental results of a two-digit ADM are presented. An average SNR of 30 dB is obtained with a dynamic range of 32 dB at fr= 32 kbits/s for band-limited noise signals. The SNR increases with the sampling rate at 15 dB/octave, as against 9 dB for a single-digit ADM. The frequency response is good and the variation of SNR with the message frequency of the delta coding system has been improved. The effect of channel errors has also been studied and the performance of the system is found satisfactory.     

Trellis code‐aided high‐rate M‐QAM space‐time labeling diversity using a unitary expansion

In this paper, we present a high‐rate M‐ary quadrature amplitude modulation (M‐QAM) space‐time labeling diversity (STLD) system that retains the robust error performance of the conventional STLD system. The high‐rate STLD is realised by expanding the conventional STLD via a unitary matrix transformation. Robust error performance of the high‐rate STLD is achieved by incorporating trellis coding into the mapping of additional bits to high‐rate codes. The comparison of spectral efficiency between the proposed trellis code‐aided high‐rate STLD (TC‐STLD) and the conventional STLD shows that TC‐STLD with 16‐QAM and 64‐QAM respectively achieves a 12.5% and 8.3% increase in spectral efficiency for each additional bit sent with the transmitted high‐rate codeword. Moreover, we derive an analytical bound to predict the average bit error probability performance of TC‐STLD over Rayleigh frequency‐flat fading channels. The analytical results are verified by Monte Carlo simulation results, which show that the derived analytical bounds closely predict the average bit error probability performance at high signal‐to‐noise ratios (SNR). Simulation results also show that TC‐STLD with 1 additional bit achieves an insignificant SNR gain of approximately 0.05 dB over the conventional STLD, while TC‐STLD with 2 additional bits achieves an SNR gain of approximately 0.12 dB.