Free-Space Optical Communications

With recent successes of laboratory, inatmosphere, and space demonstrations of free-space optical communications, there is no doubt that the technology is ready for operational deployment. While these successes have shown that there are no laws of physics against such systems, their estimated system costs are still much too high for serious considerations. Two types of development can reduce the cost dramatically. The first is via the improvement of physical-link communication efficiency by an order of magnitude using photon-counting receivers for vacuum channels, system complexity, weight, and power for space systems can be greatly reduced. The second is through the use of coherent systems in links where clear-air turbulence impairs communication efficiency, and in multiple access applications where coherent processing can reduce the level of interference, significant reduction in system costs can be realized     

Free-Space Optical Communications for Next-generation Military Networks

There is a continuing need for increased capacity for military applications, especially in network-centric operational concepts that promote the use of information as fundamental for gaining superiority on the battlefield. As an example, the access to, and distribution of, sensor data is a major tenet of network-centric warfare and yet radio frequency (RF) links will struggle to provide the needed capacity. Free-space optical communications (FSOC) has the potential to meet these emerging military needs by offering dramatic increases in capacity. However, there are many technical challenges al multiple layers of the communications protocol stack. This article describes these challenges and discusses some mitigation approaches to provide a path to realizing this capability on the battlefield     

软信息辅助硬判决译码在自由空间光通信中的应用

由于在性能和复杂度之间的良好折中,软信息辅助硬判决译码近年来受到了光通信领域的高度关注,其中包括了软信息辅助比特标记(Soft-Aided Bit-Marking,SABM)算法.为了易于硬件实现,本文基于阶梯码(Staircase Code,SCC)提出了一种改进型SABM算法(improved SABM,iSABM),称为iSABM-SCC.iSABM-SCC利用信道软信息,通过两个可信度阈值将硬判决输出比特标记为三种可信度等级,用以辅助硬判决译码识别译码错误和扩展纠错能力,达到提升阶梯码性能的目的.在受大气湍流影响的自由空间光通信中的仿真表明,iSABM-SCC性能显著优于标准SCC和RS码.以强湍流信道为例,码率为0.75的iSABM-SCC在4-PAM调制下较标准SCC产生的性能增益可达4.37 dB,在8-PAM调制下较RS码产生的性能增益可达11.06 dB.     

基于零点消除的无线OFDM系统信道缩短均衡器

针对无线信道的有限冲激响应及其全零点特性,本文设计了一种基于零点消除的无线正交频分复用(OFDM)系统信道缩短均衡器,该均衡器利用零点消除的方式有效地缩短信道时延扩展长度,并采用反馈滤波器的形式来实现无限冲激响应滤波器,该方案不仅降低了信道缩短均衡器算法的复杂度,而且也大大简化了无线OFDM系统的设计与实现.仿真结果验证了上述方案和算法的有效性.     

BPSK Subcarrier Intensity Modulated Free-Space Optical Communications in Atmospheric Turbulence

Free-space optical communications (FSO) propagated over a clear atmosphere suffers from irradiance fluctuation caused by small but random atmospheric temperature fluctuations. This results in decreased signal-to-noise ratio (SNR) and consequently impaired performance. In this paper, the error performance of the FSO using a subcarrier intensity modulation (SIM) based on a binary phase shift keying (BPSK) scheme in a clear but turbulent atmosphere is presented. To evaluate the system error performance in turbulence regimes from weak to strong, the probability density function (pdf) of the received irradiance after traversing the atmosphere is modelled using the gamma-gamma distribution while the negative exponential distribution is used to model turbulence in the saturation region and beyond. The effect of turbulence induced irradiance fluctuation is mitigated using spatial diversity at the receiver. With reference to the single photodetector case, up to 12 dB gain in the electrical SNR is predicted with two direct detection PIN photodetectors in strong atmospheric turbulence.     

降雨和雾气天气下自由空间光通信性能研究

天气的影响会导致自由空间光通信系统性能恶化。利用Mie理论和两种雨滴谱分布模型,计算了不同降雨量下大气激光传输的衰减;根据Kim和AlNaboulsi模型计算了激光在不同能见度下的衰减。研究了降雨和雾气对自由空间光通信系统性能的影响,分析了不同降雨量时降雨和雾气天气条件下传输距离与误码率的关系以及发射功率与误码率的关系。结果表明,降雨量不同会影响通信距离;雾天衰减比雨天衰减更为严重,通信距离变短,为保证系统正常工作,可以适当加大发射功率。     

Implementation of a Viterbi Processor for a Digital Communications System with a Time-Dispersive Channel

This paper describes the theory, design, and testing of a Viterbi processor for a digital communication system with intersymbol interference over fading time-dispersive channels. The requirement is to implement the Viterbi algorithm for a channel memory of 9 baud at a data rate of 2400 bits/s. The processor is partitioned into three subprocessors corresponding to the correlation, state metric evaluation, and state decision-making operations. For prototype evaluation, each subprocessor is being implemented as a separate chip using4-5 mum CMOS technology. The architecture, circuit design, and subsystem characterization of the correlator chip are described in some detail. The chip is required to evaluate 1024 state transition metrics in each baud interval (about 400 ns) using a pipeline architecture. Simulation and initial test results verify the correct operation of the chip with an adequate-speed safety margin. The theory of operation and architecture of the state metric chip are described. With off-chip memory for state metric storage, the state transition metrics from the correlator chip are used to determine the winning (optimal) path in the Viterbi trellis and to calculate the corresponding 16-bit state metric for each baud interval. Implementation of the third chip which is required to make a state decision regarding the bit sequence sent is presently being investigated.     

Blind, Adaptive Channel Shortening Equalizer Algorithm Which Can Provide Shortened Channel State Information (BACS-SI)

Channel shortening equalization plays an important role in multicarrier modulation (MCM) systems. In this paper, we propose a blind channel shortening equalizer structure named blind, adaptive channel shortening equalizer which can provide the shortened channel state information (BACS-SI). The algorithm depends on the minimization of a cost function defined as the sum-squared difference of the autocorrelations of the shortened channel impulse response (CIR) and a target impulse response. The surface is proven to be multimodal; however, minima are shown to be related to each other in a certain way. A two-phase approach is proposed. In the first phase, the cost function is minimized by a stochastic gradient descent algorithm in order to find an arbitrary minimum. In the second phase using the relation between minima, genetic algorithms are employed to find the best minimum according to a fitness function. The algorithm can both successfully shorten the channel and also explicitly provides shortened CIR which is a necessary information for the proper operation of a MCM receiver, in contrast to many other algorithms proposed in the literature which cannot directly provide this information.      

空间光通信RS-INTERLEAVING信道编码研究

分析了空间激光通信的两种信道数学模型--自由空间光通信信道和大气激光通信信道,针对信道时变突发的特点,引入信道编码技术和交织技术,交织对发送信号进行扰码以克服大气信道相关性,Reed-Solomon(RS)线性分组码克服信道的突发错误,并进行一定程度的纠错.计算机仿真结果表明,RS编码和交织技术非常适合空间光通信信道,能大大降低系统的误码率.     

应用于10 Gbit/s光通信及背板传输的自适应均衡器设计

描述了一种既可用于背板传输也可用于光纤通信的高速串行收发器前端均衡器的设计。为适应光信号在传播中的色散效应,使用前馈均衡器(FFE)加判决反馈均衡器(DFE)的组合,取代了背板通信中常用的连续时间线性均衡器(CTLE)和DFE的组合。设计使用3 pre-tap、3 post-tap和1个main tap的抽头组合方式,兼顾pre-cursor和post-cursor的信号失真,有效补偿范围为15 dB。补偿系数采用完全自适应算法调整,对FFE采用模拟MSE算法调整,DFE引擎采用1/16速率数字sign-sign最小均方差(LMS)算法实现。芯片使用UMC 28 nm工艺流片,输入信号频率为10 Gbit/s。     

GMSK解调系统中维特比均衡器的设计与优化

通信系统中维特比均衡器常用来克服频率选择性失真,但其计算的复杂度和占用的硬件资源一直是其应用时需要解决的关键问题。介绍了采用维特比均衡方法的GMSK解调系统,对其软判决的算法进行了分析,并采用ASIC设计方法,对其输入存储量,幸存路径计算复杂度,输出译码复杂度等进行了优化,减少了硬件实现的代价。     

BER Performance of Free-Space Optical Transmission with Spatial Diversity

Free space optical (FSO) communications is a cost-effective and high bandwidth access technique, which has been receiving growing attention with recent commercialization successes. A major impairment in FSO links is the turbulence- induced fading which severely degrades the link performance. To mitigate turbulence-induced fading and, therefore, to improve the error rate performance, spatial diversity can be used over FSO links which involves the deployment of multiple laser transmitters/receivers. In this paper, we investigate the bit error rate (BER) performance of FSO links with spatial diversity over log- normal atmospheric turbulence fading channels, assuming both independent and correlated channels among transmitter/receiver apertures. Our analytical derivations build upon an approximation to the sum of correlated log-normal random variables. The derived BER expressions quantify the effect of spatial diversity and possible spatial correlations in a log-normal channel.     

An Economical RC Active Equalizer with Applications in Communications

A simple active equalizer network employing two operational amplifiers (OA's), five resistors, and a pair of capacitors is presented. The same network can be used for both amplitude and delay equalizations. Experimental results show that the circuit can be conveniently used for amplitude equalization in voice communications, and for delay equalization in private line data circuits.     

A Novel Trellis Coded Overlapping Amplitude and Pulse Position Modulation Scheme for Gamma-Gamma Channel Free-Space Optical Communication

A novel trellis coded-4×8 overlapping amplitude and pulse position modulation(TC-4×8AOPPM) scheme is proposed to enhance bit error rate(BER) performance of free-space optical communication(FSO) system. In addition, an uncoded AOPPM referential scheme is also designed. The schemes manage to decrease BER by designing gamma-gamma(GG) channel applicable decoding and demodulation methods. Simulation results of 8, 16 and 64-state TC-4×8AOPPM show 2.5-3.3 dB SNR gain against traditional TC-4×8AOPPM scheme respectively. Thus significant BER performance improvement is achieved and the reliability of the FSO system is also enhanced.     

Recent Results on the Use of Concatenated Reed-Solomon/Viterbi Channel Coding and Data Compression for Space Communications

The development of sophisticated adaptive source coding algorithms together with inherent error sensitivity problems fostered the need for efficient space communication at very low bit error probilbilities(leq 10^{-6}). This led to the specification and implementation of a concatenated coding system using an interleaved Reed-Solomon code as the outer code and a Viterbi-decoded convolutionai code as the inner code. This paper presents the experimental results of this channel coding system under an emulatedS-band uplink andX-band downlink two-way space communication channel, where both uplink and downlink have strong carrier power. Test results verify that at a bit error probability of 10^-6or less, this concatenated coding system does provide a coding gain of 2.5 dB or more over the viterbi-decoded convolutional-only coding system. These tests also show that a desirable interleaving depth for the Reed-Solomon outer code is 8 or more. The imptict of this "virtually" error-free space communication link on the transmission of images is discussed and examples of Simulation results are given.     

Joint Optimization of Receive Filter and Equalizer

It is shown that a suitable adaptive receiver for digital communication over noisy dispersive unknown channels consists of an adaptive prefilter followed by an adaptive equalizer. The adaptive receiver is optimized by a minimization of the overall receiver mean-square error. The resultant adaptive receiver requires a dummy equalizer to supply a control signal for adjusting the tap gains of the prefilter.     

一种用於空间光通信系统的FSOP及应用

提出一种可用于空间光通信系统的通信协议(FSOP)。该协议在光路的物理连接之上建立一种可进行数据传输的逻辑链路,该链路具有数据实时备份、出错即时恢复等机制以保证传输数据的安全与可靠。本文提供的通信协议可以弥补空间光通信系统的通信质量易受外部环境影响而容易出错的不足,使光通信系统真正具有和光纤可相比较的传输稳定性和可靠性。同时可用於具有移动平台之光通信系统上。     

与偏振相关和无关的4×4自由空间光开关

设计了一种由相位型空间光调制器(PSLM)、偏振光分束器(PBS)、反射镜、半波片(HWP)和四分之一波片(QWP)构成的2×2光开关。该光开关所用器件少,具有结构紧凑规整、功能的实现与信号光的偏振态无关以及可以完成双向交换等特点。在此基础上通过2×2光开关的级联,设计了一种与偏振无关的4×4光开关的实验模块,根据其路由控制表对该实验模块功能的实现进行了分析。为与之作比较,还提出了一种与偏振相关的4×4光开关,该光开关利用对信号光偏振态的控制进行路由选择,以实现所需的交换和排序。     

Coded Modulation Faster-than-Nyquist Transmission with Precoder and Channel Shortening Optimization

Faster-than-Nyquist(FTN)signaling can improve the spectrum efficiency(SE)of the transmission system.In this paper,we propose a coded modulation FTN(CM-FTN)transmission scheme with precoder and channel shortening(CS)optimization to improve bit error rate(BER)performance and reduce the complexity of FTN equalizer.In our proposal,the information rate(IR)or spectral efficiency(SE)is employed and verified as a better performance metric for CM-FTN than the minimum Euclidian distance(MED).The precoder of CM-FTN is optimized for maximizing the IR criterion using the bare-bones particle swarm optimization(BB-PSO)algorithm.Further,a three-carrier CM-FTN system model is used to capture the broadening effect of precoder.Also targeting for the IR maximization,the inter-symbol interference(ISI)length for CS is optimized to reduce the receiver complexity without performance loss.Simulation results demonstrate that our method has a 0.6dB precoding gain compared with the nonprecoding scheme and a maximum of 87.5%of the complexity of FTN equalizer is reduced without BER loss.