Communication techniques and coding for atmospheric turbulence channels

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. In this paper, we describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of the fading, and the observation interval is shorter than the correlation time of the fading. We assume that the receiver has no knowledge of the instantaneous fading state. The techniques we consider are based on the statistical properties of fading, as functions of both temporal and spatial coordinates. Our approaches can be divided into two categories: temporal domain techniques and spatial domain techniques. In the spatial domain techniques, one must employ at least two receivers to collect the signal light at different positions or from different spatial angles. Spatial diversity reception with multiple receivers can be used to overcome turbulence-induced fading. When it is not possible to place the receivers sufficiently far apart, the fading at different receivers is correlated, reducing the diversity gain. We describe a ML detection technique to reduce the diversity gain penalty caused by such fading correlation. In the temporal domain techniques, one employs a single receiver. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to optimize performance. When the receiver also knows the temporal correlation of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. We describe two reduced-complexity implementations of the MLSD, which make use of a single-step Markov chain model for the fading correlation in conjunction with per-survivor processing. Next, we also investigate the performance of using error-control coding and pilot symbol-assisted detection schemes through atmospheric turbulence channels.     

Atmospheric channel effects on free-space laser communication

Free-space laser communication offers an attractive alternative for transferring high-bandwidth data when fiber optic cable is neither practical nor feasible. However, there are a variety of deleterious features of the atmospheric channel that may lead to serious signal fading, and even the complete loss of signal altogether. Physical obstructions—such as birds, insects, tree limbs, or other factors—can temporarily or permanently block the laser line-of-sight. Platform/building motion due to wind, differential heating and cooling, or ground motion over time can result in serious misalignment of fixed-position laser communication systems. But most importantly of all, absorption and scattering due to particulate matter in the atmosphere may significantly decrease the transmitted optical signal, while random atmospheric distortions due to optical turbulence can severely degrade the wave-front quality of a signal-carrying laser beam, causing intensity fading and random signal losses at the receiver.     

Adaptive detection technique for optical wireless communication over strong turbulence channels

Optical wireless communication (OWC) systems use the atmosphere as a propagation medium, so the atmospheric turbulence effects lead to fading related with signal intensity. The received signal of OWC over strong turbulence channels is assumed to be a mixture of K-distributed fading and Gaussian distributed thermal noise. Second-order spectral analysis is unable to separately estimate the mixed signal. In order to mitigate the fading induced by turbulence, the decision threshold-updating algorithm based on second and higher order cumulants is proposed and is able to operate in an unknown turbulence environment. The performance of the adaptive processing scheme has been evaluated by means of Monte Carlo simulations. Simulation results show the improvement of the bit error rate (BER) performance.     

The effect of turbulence on NLOS underwater wireless optical communication channels [Invited]

Conventional line-of-sight underwater wireless optical communication(UWOC) links suffer from huge signal fading in the presence of oceanic turbulence due to misalignment, which is caused by variations in the refractive index in the water. Non-line-of-sight(NLOS) communication, a novel underwater communication configuration,which has eased the requirements on the alignment, is supposed to enhance the robustness of the UWOC links in the presence of such turbulence. This Letter experimentally and statistically studies the impact of turbulence that arises from temperature gradient variations and the presence of different air bubble populations on NLOS optical channels. The results suggest that temperature gradient-induced turbulence causes negligible signal fading to the NLOS link. Furthermore, the presence of air bubbles with different populations and sizes can enhance the received signal power by seizing the scattering phenomena from an ultraviolet 377 nm laser diode.     

Outage performance analysis for relay-assisted UWOC systems over GGD weak turbulence with nonzero boresight pointing errors

In this paper, we mainly investigate the outage performance of serial and parallel relay-assisted underwater wireless optical communication (UWOC) systems based on a newly proposed aggregated underwater fading model. In order to overcome the deficiency of the traditional underwater weak turbulence models, that is, they could not accurately fit the measured data in the laboratory, the generalized gamma distribution (GGD) which has been verified by a series of experiments is chosen for the first time to characterize the weak oceanic turbulence. Then, we establish a new receiving signal model which has integrated the implicit path loss plus multipath propagation effect shown by fading free impulse response (FFIR), GGD weak oceanic turbulence, and nonzero boresight pointing errors. Next, we deduce the closed-form expression of the probability density function (PDF) of the hybrid fading considering GGD weak turbulence and nonzero boresight errors based on the new receiving signal model above through double-exponential Taylor expansion and higher transcendental Whittaker function. Finally, the analytical expressions of the outage probabilities for point-to-point (P2P) link, serial and parallel relay-assisted UWOC systems are further derived respectively under the proposed aggregated channel. Numerical simulations are also provided to validate the accuracy of the theoretical formulae derived above, and to show the effects of the key system parameters on the outage performance of relaying UWOC systems.     

基于重复编码的海上可见光通信大气信道建模

可见光通信作为一种新型无线通信技术,在海上舰船场景中的应用吸引了广泛的关注.海上可见光通信系统受多种因素的影响,包括海浪随机起伏和大气湍流,大气湍流将导致可见光信号的强度随机波动,降低可见光通信系统在大气中的链路质量.本文基于对数正态衰减分布,建立了采用重复编码的海上可见光通信的链路评估模型.在此基础上,根据Pierson-Moskowitz海谱,分析了海上风速、大气折射率结构常数、能见度、重复编码分集度以及接收器孔径对可见光通信系统平均误码率的影响.本文提出的海上大气链路评估模型可为海上可见光通信网络的搭建提供重要参考.     

基于高阶累计量的大气光通信自适应信号处理

针对强湍流信道下信号衰落的特点,分析了对数正态分布模型与K分布模型的适用范围.基于K分布模型建立大气光通信接收信号模型,并给出了自适应最优门限检测方法.采用四阶和六阶累计量对强湍流信道参量进行估计,采用二阶累计量对其它高斯噪音进行估计,得到K分布参量及高斯噪音统计量的预测值,实现自适应门限更新.基于Monte Calro算法进行仿真,给出了门限更新算法对通信系统误码率的影响,同时分析了信号采样率对估计参量偏差的影响.计算表明,在强湍流情况下,大气光通信系统的误码率性能得到极大的改善,优于基于MLSD检测的接收机.     

星地激光通信中分布式接收阵列的特性研究

星地激光通信中,发射机对准误差及大气湍流会引起接收信号衰落,对带前置光放大的阵列接收机,分析了分布式接收阵列的抗衰落性能。研究表明,分布式接收阵列对发射机对准误差引起的衰落具有一定抑制作用,阵列中各子接收孔径之间的距离可根据发射机对准误差及湍流的强弱进行优化设计。与传统式阵列相比,分布式阵列可以采用更窄的发射光束宽度,降低对发射功率的要求。分布式阵列的另一个重要特性还在于:当实际发射机对准误差标准差大于设计值时,通过调整阵列各子接收孔径之间的距离,可以在很大程度上降低发射机对准误差引起的功率损失。     

Air turbulence effects on performance of optical wireless communication with crosstalk in server backplane

Free space optical interconnections(FSOIs) are anticipated to become a prevalent technology for short-range high-speed communication. FSOIs use lasers in board-to-board and rack-to-rack communication to achieve improved performance in next generation servers and are expected to help meet the growing demand for massive amounts of inter-card data communication. An array of transmitters and receivers arranged to create an optical bus for inter-card and card-to-backplane communication could be the solution. However, both chip heating and cooling fans produce temperature gradients and hot air flow that results in air turbulence inside the server, which induces signal fading and, hence, influences the communication performance. In addition, the proximity between neighboring transmitters and receivers in the array leads to crosstalk in the received signal, which further contributes to signal degradation. In this Letter, the primary objective is to experimentally examine the off-axis crosstalk between links in the presence of turbulence inside a server chassis. The effects of geometrical and inter-chassis turbulence characteristics are investigated and first-and second-order statistics are derived.     

Performance of phase compensated coherent free space optical communications through non-Kolmogorov turbulence

The bit-error-rate (BER) performance of coherent free-space optical (FSO) links employing phase compensation techniques is investigated in weak non-Kolmogorov turbulence. Assuming that the amplitude fading and phase fluctuation follow lognormal model and Gaussian distribution respectively and using the expression of non-Kolmogorov turbulence in terms of Zernike polynomials, the signal-to-noise ratio (SNR) at the coherent receiver is analyzed and as a special case, a new closed-form expression using chi-square distribution is obtained. Thus, the influence of different compensation modes and normalized receiver diameter on BER performance is evaluated and an optimum normalized receiver diameter is suggested to achieve the minimum BER. Moreover, the impact of outer scale L₀ and the exponent value α in non-Kolmogorov spectrum is studied with the optimum diameter, which reveals that the BER has an obvious decrease with larger values of L₀ and α.     

激光雷达湍流大气探测

基于湍流对激光束的扩散和漂移效应,提出了一种采用激光雷达观测湍流强度的方法。湍流对光束产生扩散和飘移作用,使激光光束偏离激光雷达接收视场,造成后向散射光能量损失。对比常规散射回波信号与畸变回波信号的差别可以测量湍流强度。实验表明该方法与其他方法的测量结果具有一致性,证明了该方法的有效性。     

基于自适应滤波器的大气湍流噪声抑制

为了解决大气湍流效应所引起的光无线通信系统中的衰落问题,建立了与大气湍流噪声及其它加性噪声有关的信号模型,实现了最大输出信噪比可提高3-20dB的匹配滤波器。根据最大似然比准则,推导了强度调制/直接探测(IM/DD)的光无线通信系统的最佳判决门限。采用自适应滤波器实现了对探测门限中所含的信号和湍流的统计值的预测,该自适应门限探测技术对大气湍流噪声有很好的抑制作用,在强湍流情况下,可降低系统的误码率到10-5以下。     

大气激光通信中多光束传输性能分析和信道建模

多光束传输技术是克服大气激光通信中大气湍流效应的有效途径之一。首先从理论上分析了大气湍流对多光束大气激光通信系统性能的影响和多光束大气传输的光强起伏特性,然后利用统计分析的方法,建立了一个以传输距离z、光束数目n、发射孔径之间的距离St、接收孔径Dr等为参量的多光束大气传输信道模型。最后,结合相关文献提供的实验结果对该信道模型进行了实验验证和误码性能分析。结果表明,当S1≥√λz或Dr远大于大气湍流相干长度ρo时,随着n的增大,接收光强将趋于对数正态分布．降低了大气激光通信系统的误码率,从而验证了多光束传输对于克服大气湍流影响的有效性。     

大气弱湍流下星地光链路衰落特性影响因素分析

研究了弱湍流条件下星地光通信链路的衰落概率、平均衰落次数以及平均衰落时间3个重要参数的统计特性,并通过数值仿真分析了链路天顶角、光波长以及接收机海拔高度对链路衰落特性的影响。结果表明:减小天顶角和提高接收机探测灵敏度可以改善链路的衰落特性;1.55 m激光较0.85 m、1.06 m激光更适合于星地光通信;地面接收机应尽量建在2 000 m以上的高海拔地区。     

Scattering of a general partially coherent beam from a diffuse target in atmospheric turbulence

The second-order and fourth-order statistical moments of the speckle field from a diffuse target in atmospheric turbulence are studied which have great influence on the performance of lidar systems. By expanding a general rotationally symmetric beam as a sum of Gaussian–Schell model(GSM) beams, the mean intensity of the general beam propagating over a distance in an atmospheric turbulence is formulated. Expressions for the degree of coherence(DOC) and the normalized intensity variance of the scattered field of a general beam from a rough surface in turbulence are derived based on the extended Huygens–Fresnel principle. The general expressions reduce to the well-known forms for a GSM beam. Another example for the general beam used in this paper is the collimated flat-topped beam. The results of both kinds of beams show that the intensity profile on the target plane is a key factor affecting the statistical characteristics of the speckle field.A larger beam spot on the target plane induces a smaller coherence length and a smaller normalized intensity variance of the received field. As turbulence gets stronger, the coherence length becomes smaller, and the normalized intensity variance firstly increases and declines to unity finally.     

Analysis of interference cancellation techniques in OFDMA systems employing adaptive antenna arrays

We analyze the problem of interference cancellation in OFDMA cellular communication systems employing adaptive antenna arrays at the receiver and operating in the channels with frequency-selective fading and interference from the neighboring base stations. This problem is very important for modern broadband cellular systems operating with a low coefficient of multiple use of frequencies. Under such conditions of the network deployment, the system efficiency is restricted by the level of intrachannel interference from the neighboring base stations transmitting in the same frequency channel. In this work, we analyze different methods for measuring the interference characteristics and using them for spatial processing of received signals. Five different methods for signal processing in an adaptive antenna array of the receiver are discussed. Each approach (reception method) has its own interference immunity and complexity degree. Comparative analysis of the interference immunity of the considered receivers has been performed for different frequency-channel loads and different numbers of interfering stations. Recommendations for using the considered receivers are given.     

Lower bound on the capacity of distributed MIMO systems with spatially correlated Rician/Lognormal fading

This paper presents an analytic lower bound of ergodic capacity for distributed MIMO (D-MIMO) systems that experience not only Rician fading but also shadowing effects of Lognormal distributed. In particularly, we consider that the Rician fading channel is spatially correlated at both transmitter and receiver. In the communication environment corresponding to this setting, the angle spread at the transmitter and the angle spread at the receiver are both insufficient, and the non-fading components co-exists with the fading components. Such communication environment is very common. In the process of deriving the analytic lower bound, in order to avoid Hayakawa polynomials that cannot be analytically expressed, non-central quadratic forms are transformed to non-central Wishart matrices by use of inequality. The validity of the presented lower bound is verified by computer simulations. The simulation results show the influence of the number of radio ports at the transmitter, the number of antennas at the transmitter or receiver, correlation level (angle spreads) and Rician K-factor on the capacity of systems. In all cases, the presented bound remains tight across the entire signal-to-noise ratio (SNR) regime.     

Power coupling of Gaussian vortex beam propagation through an optical system with the Cassegrain-telescope receiver in turbulent atmosphere

Model of Gaussian vortex beam propagation through an optical system with the Cassegrain-telescope receiver in turbulent atmosphere is established. With this model, the analytical formulas of the average intensity distribution at the receiver plane are derived, and the influences of the optical topological charge, the propagation distance and the turbulence strength are numerically analyzed. These studies show that optical power at the receiver plane concentrates in an annular area, which is suitable for power coupling by the Cassegrain-telescope receiver; the optical topological charge of the vortex source need to be optimized to access the most power coupling. Under the H-V 5/7 turbulence model, power coupling efficiencies of the optical system with different parameters are calculated. Results show that in comparison with the Gaussian beams, Gaussian vortex beams have great advantages in power coupling of optical systems with the Cassegrain-telescope receivers in turbulent atmosphere, which can be a new attractive application of the vortex beams.     

Gallager Exponent Analysis of Coherent MIMO FSO Systems over Gamma-Gamma Turbulence Channels

This paper studies the Gallager’s exponent for coherent multiple-input multiple-output (MIMO) free space optical (FSO) communication systems over gamma–gamma turbulence channels. We assume that the perfect channel state information (CSI) is known at the receiver, while the transmitter has no CSI and equal power is allocated to all of the transmit apertures. Through the use of Hadamard inequality, the upper bound of the random coding exponent, the ergodic capacity and the expurgated exponent are derived over gamma–gamma fading channels. In the high signal-to-noise ratio (SNR) regime, simpler closed-form upper bound expressions are presented to obtain further insights into the effects of the system parameters. In particular, we found that the effects of small and large-scale fading are decoupled for the ergodic capacity upper bound in the high SNR regime. Finally, a detailed analysis of Gallager’s exponents for space-time block code (STBC) MIMO systems is discussed. Monte Carlo simulation results are provided to verify the tightness of the proposed bounds.     

Some limitations on optical communication reliability through Kolmogorov and non-Kolmogorov turbulence

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal, affecting link performance.Influence of Kolmogorov and non-Kolmogorov turbulence statistics on laser communication links are analyzed for different propagation scenarios, and effects of different turbulence spectrum models on optical communication links are presented. Statistical estimates of the communication parameters such as the probability of fade (miss) exceeding a threshold dB level, the mean number of fades, and BER are derived and examples provided. The presented quantitative data suggest that the non-Kolmogorov turbulence effects on lasercom channels are more severe in many situations and need to be taken into account in wireless optical communication. Non-Kolmogorov turbulence is especially important for elevations above the boundary layer as well as for even low elevation paths over water.