Channel estimation and error correction for UWOC system with vertical non-line-of-sight channel

Recently, underwater wireless optical communication (UWOC) system has placed more attention towards increasing data rate, high accuracy, higher bandwidth and providing highly secured transmission. Generally, light propagation in underwater medium is disturbed due to some degrading effects such as turbulence, scattering and absorption which will degrade the performance. Therefore, channel estimation is essential and need to adapt suitable correction techniques that compensate the errors due to those effects. In this work, channel estimation and error correction technique are proposed for the UWOC system with vertical non-line-of-sight channel based on MIMO-OFDM approach. Initially, input sequence error is eliminated by HVD Turbo coding method and error rectified signal is modulated with M-ary OAM-PPM modulation technique. Modulated signal is transmitted through Vertical NLOS channel. During transmission, the signal undergoes scattering, absorption and channel fading. In receiver side, channel characteristics are estimated by invariant embedding method. Finally, estimated signal is demodulated and decoded by same techniques. MATLAB environment is used to implement the proposed work. Resultant performances like channel capacity, BER, SNR, data rate, receiving power and MSE for different water types like pure sea water, ocean water, costal water and turbid water are compared and results are examined.

     

平坦衰落环境中多输入多输出系统衰落相关与信道容量研究

为分析模型物理参数和天线排列方式对多输入多输出系统信道容量的影响,提出了一种平坦衰落环境中信道容量的研究方法。该方法基于接收均匀圆阵和均匀线阵分别构建了蕴含模型物理参数的相关矩阵,并利用Wishart分布的性质推导了信道容量上下限。该方法回避了求取衰落相关矩阵特征值的概率密度函数,降低了运算量;可被推广到多天线-频分复用系统。仿真结果表明,天线间距较小时,采用均匀圆阵比均匀线阵的系统信道容量要高;天线间距增大到一定程度后,系统信道容量达到饱和。散射角越大,信道容量的增长速率越快且采用均匀线阵比均匀圆阵系统的信道容量高。接收信噪比较大时,平均信道容量上下限基本接近其实际值。     

3×3多天线系统衰落相关与信道容量研究

为分析天线间距和散射角等信道物理参数对多输入多输出系统信道容量的影响,提出了一种相关衰落环境中信道容量的分析方法。该方法基于接收均匀圆阵构建了蕴含信道物理参数的衰落相关矩阵。并由此详细推导了3×3多天线系统信道容量的闭式表达。结果表明,相关矩阵特征值的个数和大小决定了系统信道容量的大小。该方法回避了已有算法需求取相关衰落信道特征值概率密度函数的问题,降低了运算量,可以被推广到任意收发天线数的多输入多输出和多输入多输出-频分复用系统。仿真结果表明,天线间距增大,信道容量随之增大。但是当天线间距增大到衰落相关的第一个过零点时,信道容量达到最大值,再增大天线间距对信道容量影响很小。散射角越大,信道容量收敛到最大值速率越快。     

多输入多输出系统信道容量研究

该文研究接收端采用均圆阵天线阵放置空间有限的条件下,存在相关衰落时MIMO系统的信道容量。建立了衰落相关模型,分析了散射角大小及天线数对信道容量的影响,采用随机理论推导了MN的MIMO系统信道容量的闭式解。分析结果表明,MIMO系统信道容量主要由衰落相关矩阵的特征值决定。仿真结果表明,在接收端空间有限的情况下,天线数增大到一定程度时,信道容量达到饱和,再增加天线数,对信道容量的影响很小。     

Optimal Transmission with Imperfect Channel State Information at the Transmit Antenna Array

We study the optimal transmission strategy of a multiple-inputsingle-output wireless communication link. The receiver has perfectchannel state information while the transmitter hasonly long-term channel state information in terms of the channelcovariance matrix. It was recently shown that the optimal eigenvectors of the transmitcovariance matrix correspond with the eigenvalues of the channelcovariance matrix. However, the optimal eigenvalues are difficult tocompute. We study the properties of these optimal capacity achieving eigenvalues, and present a necessary and sufficient condition for theoptimal eigenvalues of the transmit covariance matrix. Furthermore, we develop a necessary and sufficient condition forachieving capacity when transmitting in all directions. We compare thecapacity gain of an optimal diversity system with a system which works with beamforming, and we derive an upperbound. We answer the main questions regarding the system design using the developed results. Additionally, we show inwhich way the multiplexing gain can be computed in case the channel covariancematrix is given. We compute the maximum number of required paralleldata streams, and we define a multiplexing function inorder to obtain a measure for the available multiplexinggain. Furthermore, we show that the capacity gain is small considering theadditional complexity at the receiver. We illustrate allresults by numerical simulations.     

Modeling of non-line-of-sight ultraviolet scattering channels for communication

A stochastic non-line-of-sight (NLOS) ultraviolet (UV) communication channel model is developed using a Monte Carlo simulation method based on photon tracing. The expected channel impulse response is obtained by computing photon arrival probabilities and associated propagation delay at the receiver. This method captures the multiple scattering effects of UV signal propagation in the atmosphere, and relaxes the assumptions of single scattering theory. The proposed model has a clear advantage in reliable prediction of NLOS path loss, as validated by outdoor experiments at small to medium elevation angles. A Gamma function is shown to agree well with the predicted impulse response, and this provides a simple means to determine the channel bandwidth. The developed model is employed to study the characteristics of NLOS UV scattering channels, including path loss and channel bandwidth, for a variety of scattering conditions, source wavelength, transmitter and receiver optical pointing geometries, and range.     

Environmental issues for MIMO capacity

Wireless communication using multiple-input multiple-output (MIMO) systems enables increased spectral efficiency for a given total transmit power. Increased capacity is achieved by introducing additional spatial channels that are exploited using space-time coding. In this paper, the environmental factors that affect MIMO capacity are surveyed. These factors include channel complexity, external interference, and channel estimation error. The maximum spectral efficiency of MIMO systems in which both transmitter and receiver know the channel (using channel estimate feedback) is compared with MIMO systems in which only the receiver knows the channel. Channel complexity is studied using both simple stochastic physical scattering and asymptotic large random matrix models. Both uncooperative (worst-case) and cooperative (amenable to multiuser detection) interference are considered. An analysis for capacity loss associated with channel estimation error at the transmitter is introduced.     

On the asymptotic capacity of stationary Gaussian fading channels

We consider a peak-power-limited single-antenna flat complex-Gaussian fading channel where the receiver and transmitter, while fully cognizant of the distribution of the fading process, have no knowledge of its realization. Upper and lower bounds on channel capacity are derived, with special emphasis on tightness in the high signal-to-noise ratio (SNR) regime. Necessary and sufficient conditions (in terms of the autocorrelation of the fading process) are derived for capacity to grow double-logarithmically in the SNR. For cases in which capacity increases logarithmically in the SNR, we provide an expression for the "pre-log", i.e., for the asymptotic ratio between channel capacity and the logarithm of the SNR. This ratio is given by the Lebesgue measure of the set of harmonics where the spectral density of the fading process is zero. We finally demonstrate that the asymptotic dependence of channel capacity on the SNR need not be limited to logarithmic or double-logarithmic behaviors. We exhibit power spectra for which capacity grows as a fractional power of the logarithm of the SNR     

Indoor broadband optical wireless communications: optical subsystems designs and their impact on channel characteristics

With proper system design, infrared multispot diffusing (MSD) configuration communications links promise several orders of magnitude higher bit rates than radio links. Essential to the communications system are the optical subsystems: transmitter and receiver optics. Preliminary experiments on fabrication of beamshaping optical elements for the transmitter and receiver optical front-end have been conducted. The impact of optical subsystems on channel characteristics is investigated, and the results undoubtedly prove the great potential of MSDC. Use of holographic optical elements at both transmitter and receiver increases the signal-to-noise ratio by at least 11 dB and at the same time significantly improves the power budget of the system by reducing path loss by more than 6 dB.     

Receiver designs and channel characterization for multi-spothigh-bit-rate wireless infrared communications

This paper addresses one of the most promising candidates for high-speed in-house wireless communications, namely, the multi-spot diffusing configuration (MSDC). Since it uses the optical infrared medium for data transmission, it has the inherent potential for achieving very high capacities. The channel characteristics in MSDC are simulated and the causes for channel distortion are analyzed. Then, conditions for creation of a virtually ideal channel are derived. It is shown that the 3-dB channel bandwidth can be extended up to beyond 2 GHz. This bandwidth comes at the cost of a poor power efficiency. In order to compensate for that, a novel receiver optical front-end design is proposed and its performance is analyzed. Taking advantage of the unique properties of the holographic optical elements, the conventional optical front-end, consisting of a concentrator and a filter, is replaced by a single holographic curved mirror. The utilization of such a holographic optical element improves the signal-to-shot noise ratio by up to 18.5 dB     

Finite time of carrier energy relaxation as a cause of optical-power limitation in semiconductor lasers

It is shown that the reason why the maximum attainable optical power in semiconductor lasers is limited is the finite time of carrier energy relaxation via scattering by nonequilibrium optical phonons in the quantum-well active region. The power and spectral characteristics of semiconductor lasers are studied experimentally at high excitation levels (up to 100 kA/cm²) in pulsed lasing mode (100 ns, 10 kHz). As the drive current increases, the maximum intensity of stimulated emission tends to a constant value (“saturates”), and the emitted power increases owing to extension of the spectrum to shorter wavelengths. The intensity saturation is due to limitation of the rate of stimulated recombination, caused by a finite time of the electron energy relaxation via scattering by polar optical phonons. It is found that the broadening of the stimulated emission spectrum is related to an increase in carrier concentration in the active region, which enhances the escape of electrons into the waveguide layers. As the drive current increases, the carrier concentration in the waveguide reaches its threshold value and there appears an effective channel of current leakage from the active region. The experiment shows that the appearance of a band of waveguide lasing correlates with a sharp drop in the differential quantum efficiency of a semiconductor laser.     

云的多次散射对激光雷达测量结果影响的研究

为了更加准确地获得大气消光系数和后向散射系数,利用半解析Monte Carlo方法对云的大气多次散射激光雷达回波信号进行了模拟计算。分析了激光雷达接收视场角及光学厚度对多次散射回波信号的影响及水云和卷云多次散射因子与光学厚度的关系。激光雷达测量数据的对比结果表明,多次散射对卷云和水云消光系数影响较明显,而对后向散射系数的影响可以忽略。     

Array gain and capacity for known random channels with multipleelement arrays at both ends

Two arrays with M and N elements are connected via a scattering medium giving uncorrelated antenna signals. The link array gain relative to the case of one element at each end is treated for the situation where the channels are known at the transmitter and receiver. It is shown that the maximum mean gain achieved through adaptive processing at both the transmitter and the receiver is less than the free space gain, and cannot be expressed as a product of separate gains. First, by finding the singular values of the transmission matrix, fundamental limitations concerning the maximum gain and the diversity orders are given, indicating that the gain is upper bounded by (√M+√N) ² and the diversity order is MN. Next an iterative technique for reciprocal channels which maximizes power at each stage transmitting back and forth is described. The capacity or spectral efficiency of the random channel is described, and it is indicated how the capacity is upper bounded by N parallel channels of gain M(N相似文献   

PMT阵列在水下MIMO无线光通信中的应用

水下无线光通信具有的高带宽、低时延等特点,已成为水下通信的可行选择。系统发送端光源由6只绿光发光二极管(LED)构成阵列,接收端由3只光电倍增管(PMT)构成阵列,形成了6×3的多输入多输出(MIMO)传输方式。在室内10 m水槽水下信道下,实现了1 Mbps的信息传输速率。通过MATLAB软件对接收平面光功率分布仿真,最大值为?35.8 dBm。此外,测试了PMT阳极输出电压波形,并推导出阴极电流波形。理论计算得出信噪比为19.4 dB,理论误码率约为1.1×10?5。所选PMT模块理论上最小接收功率可低至1.5×10?9 W,体现出极高的探测灵敏度。最后,通过蒙特卡洛(Monte Carlo)数字仿真说明,在信噪比25 dB可达到约35 bit·s-1·Hz-1的信道容量。     

超结构光纤光栅的正交波分复用系统实现方案及接收机串扰分析

解释了超结构光纤光栅(SSFBG)作为脉冲成型滤波器的特性,说明SSFBG能够产生规则的时域矩形光脉冲,在频域上功率谱密度表现为sinc函数的形式.当光波道的频率间隔为码元速率的整数倍时,相邻波长的功率谱零点位于信号波长的中心频率处,频谱正交交叠.研究了一种基于SSFBG的正交波分复用(OWDM)系统,发送端用SSFBG进行脉冲整形,接收端采用窄带滤波器.给出基于理想窄带滤波器和高斯窄带滤波器两种接收机的串扰噪声模型,并得出了仿真结果.结果显示,两种窄带滤波器均可抑制严重的信道串扰,当接收机窄带滤波器通带足够窄时,接收机串扰比信道串扰降低约10 dB;理想窄带滤波器优于高斯窄带滤波器2～5 dB.     

超声调制光信号的调制深度与组织光学特性关系

超声调制光学成像技术融合了光学成像和超声成像的优点,已实现对浑浊介质内隐藏异体的成像,是一种很有前景的医学成像技术。通过实验探究超声调制光信号与组织光学特性的关系、超声焦区内外组织的光学特性对调制信号的影响,发现超声对光信号的调制深度与超声焦区组织的光学性质呈简单的线性关系,调制深度随样品散射系数的增大而减小,随样品吸收系数的增大而增大。超声区信号调制深度基本不受超声调制区域外复杂组织的光学特性的影响。这为超声调制光学成像提供必要的图像重构依据。     

Adaptive turbo-coded modulation for flat-fading channels

We consider a turbo-coded system employed on a flat-fading channel where the transmitter and receiver adapt the encoder, decoder, modulation scheme, and transmit power to the state of the channel. Assuming instantaneous and error-free channel gain and phase knowledge at the transmitter and the receiver, we determine the optimal adaptation strategy that maximizes the throughput of this system, while achieving a given bit-error rate under an average power constraint. Our optimized adaptive modulation strategy is based on an extensive set of existing turbo-coded modulation schemes. We find that adapting both the turbo encoder (rate) and the transmit power can achieve performance within 3 dB of the fading channel capacity.     

Cooperative interference power allocation in physical layer security under imperfect time synchronization

In physical layer security,considering imperfect time synchronization,a power allocation scheme was proposed to maximize the system secrecy capacity.By analyzing the residual interference power at the authorized receiver caused by imperfect time synchronization,the optimal power distribution method was given,and the relationship between the optimal interference power and the relative channel quality was discussed.Theory and simulation analysis show that when the synchronization error is small,the synchronization error consumes extra power to reach the preset secrecy capacity,and the interference power increases with its increase.As the synchronization error increases,the secrecy capacity of the system gradually fails to reach the preset target,and may even drop to zero.     

On the Multiuser Capacity of WDM in a Nonlinear Optical Fiber: Coherent Communication

Previous results suggest that the crosstalk produced by the fiber nonlinearity in a WDM system imposes a severe limit to the capacity of optical fiber channels, since the interference power increases faster than the signal power, thereby limiting the maximum achievable signal-to-interference-plus-noise ratio (SINR). We study this system in the weakly nonlinear regime as a multiple-access channel, and show that by optimally using the information from all the channels for detection, the change in the capacity region due to the nonlinear effect is minimal. On the other hand, if the receiver uses the output of only one wavelength channel, the capacity is significantly reduced due to the nonlinearity, and saturates as the interference power becomes comparable to the noise, which is consistent with earlier results. The results hold in channels with or without memory. Every point in the capacity region can be achieved without knowledge of the nonlinearity parameters at the transmitters. The structures of optimal/suboptimal receivers are briefly discussed     

Optical Communications Between Underwater and Above Surface (Satellite) Terminals

A multiple scattering model is used and extended to characterize the channel between underwater and airborne (satellite) terminals at optical frequencies. The effects of the air/sea interface are also included with approximate solutions accurate for elevation angles above 45°. The results are presented in terms of a radiance function which is related to the transform of the spatial covariance function (mutual coherence function). The primary losses are shown to be a result of the water absorption coeffecient and not the extinction coefficient. The scattering losses can be isolated from the absorption losses and for certain cases, where the receiver is imbedded in the scattering medium, can be completely recovered. New components may be required to achieve this performance. The effects of ocean roughness are shown to have a minimal effect upon the subsurface reception while causing possible beam steering of subsurface transmission. Although substantial losses are experienced, duplex operation can be achieved at modest data rates.