High efficiency wireless optical links in high transmission speed wireless optical communication networks

Power link budgets are prepared for wireless optical communication systems to illustrate the optical losses that happen during transmission. This paper has presented optical wireless links, which offer ultra multi gigabit per second data rates and low system complexity. For ground space and/or terrestrial communication systems, these links suffer from atmospheric loss mainly due to fog, and scintillation. Optical wireless links provide high bandwidth solution to the last mile access bottleneck. However, an appreciable availability of the link is always a concern. Wireless optical links are highly weather dependent, and fog is the major attenuating factor reducing the link availability. Link margin, received signal power, transmission bit rate, bit rate distance product, signal‐to‐noise ratio, and BER are the major interesting design parameters in the current study. Copyright © 2013 John Wiley & Sons, Ltd.     

Transmission Characteristics Evaluation Under Bad Weather Conditions in Optical Wireless Links with Different Optical Transmission Windows

Optical wireless links offer gigabit per second data rates and low system complexity. For ground space and or terrestrial communication systems, these links suffer from atmospheric loss mainly due to fog, scintillation and precipitation. Optical Wireless link provides high bandwidth solution to the last mile access bottleneck. However, an appreciable availability of the link is always a concern. Wireless Optics links are highly weather dependent and fog is the major attenuating factor reducing the link availability. Optical wireless links offer gigabit per second data rates and low system complexity. For ground space and or terrestrial communication scenarios, these links suffer from atmospheric loss mainly due to fog, scintillation and precipitation signals and then to upgrade the transmission bit rate distance product for ultra long transmission links. This paper has presented the bad weather effects such as rain, fog, snow, and scattering losses on the transmission performance of wireless optical communication systems. It is taken into account the study of bit error rate, maximum signal to noise ratio, maximum transmission optical path lengths and maximum transmission bit rates under these bad operating conditions.     

大动态能量自适应蓝/绿激光探测方法

针对水下无线光通信链路中近距功率饱和与远距功率不足导致的光信号采集失效的难题,提出一种大动态能量自适应蓝/绿激光探测方法。该方法在水下无线光通信系统中加入由电控液晶光阀、可调增益光电倍增管及FPGA板卡组成的增益可控探测模块,该模块在链路功率和探测器增益两个维度对通信链路形成闭环调控,以保证光电倍增管始终工作在最优状态,从而实现水下远距离、高速、大动态范围无线光通信。     

高斯信道超宽带通信信号设计

超宽带无线通信是利用频带非常宽、中心频率相对较低的信号进行信息传输。由于它可以用较低的发射功率进行高速传输，因而成为室内短距离无线通信的最佳选择。通过分析信号的特性及其在高斯信道中传播的特点，对Euclidean矩和比特误码率进行具体的仿真分析，最后给出了二维超宽带通信信号的设计公式。     

Overview of networking protocols for underwater wireless communications

Underwater wireless communications can enable many scientific, environmental, commercial, safety, and military applications. Wireless signal transmission is also crucial to remotely control instruments in ocean observatories and to enable coordination of swarms of autonomous underwater vehicles and robots, which will play the role of mobile nodes in future ocean observation networks by virtue of their flexibility and reconfigurability. To make underwater applications viable, efficient communication protocols among underwater devices, which are based on acoustic wireless technology for distances over one hundred meters, must be enabled because of the high attenuation and scattering that affect radio and optical waves, respectively. The unique characteristics of an underwater acoustic channel -- such as very limited and distance-dependent bandwidth, high propagation delays, and timevarying multipath and fading -- require new, efficient and reliable communication protocols to network multiple devices, either static or mobile, potentially over multiple hops. In this article, we provide an overview of recent medium access control, routing, transport, and crosslayer networking protocols.     

海洋光学系统中的时空方法

海洋光学系统在海洋探索、开发和监测中起到了越来越重要的作用。水下无线光通信、水下激光雷达是两种迅速发展且有良好应用前景的海洋光学系统。水下无线光通信凭借高速率与低延迟的特点在中短距离应用中成为理想的通信选择;水下激光雷达在获取地理信息、目标探测等应用中也是常用的高精度、高效率的观测方法。然而,海水信道的复杂光学特性为海洋光学系统性能的进一步提升带来了挑战。在海水信道中,不仅吸收与散射作用较强,而且信道中可能有湍流、气泡等动态变化的干扰因素。为应对这些挑战,一方面可通过时间或空间方法提高信噪比;另一方面,时空信息转换的方法有利于提升系统的性能。文中对以上解决方案进行综述,并指出海洋光学系统的发展趋势。     

Laser diode intersatellite links: Communications subsystem

This paper analyzes the critical areas in optical communications systems, placing particular emphasis on the modulation/demodulation techniques used in the ISL communications subsystem design for the ESA/PSDE programme. A 4-PPM/QPM demodulator has been designed, and problems of implementing the system at high data rates have been addressed. Maximum likelihood demodulation techniques and slot and frame synchronization were used. An attempt was made to devise a way to achieve synchronization tracking without using unique words along data patterns. Finally, for the optical ISL the use of coding is suggested as a way to relax link budget requirements. Coding could reduce problems caused by pointing subsystem instabilities. The use of on-board soft decoding without system bandwidth expansion is also foreseen.     

水下光通信防恶性码卷积码设计

为了防止在水下光通信系统中激光器发送长"0"或长"1"信号序列,避免通信系统发生无穷多个比特错误,采用一种新的卷积码编码方案,进行了编码设计和Viterbi译码理论分析与误比特性能仿真验证,用VHDL语言实现了编码器现场可编程门阵列设计,得到了编码系统与未编码系统的误比特率和达到同样误码性能所需的信噪比。结果表明,这种编码能够降低水下无线光通信系统的误比特率,提高通信的可靠性,降低系统对信噪比的需求。     

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.

     

水下无线光自动对准系统的设计与实现

水下无线光通信(UOWC)具有保密性好、容量大、传输速度快等优点,可应用于水下信息传输、资源勘查等领域,通信链路的快速建立和持续稳定是水下无线光通信实际应用的基本条件。在平台扰动和海水信道杂质干扰的条件下,如何实现远距离的快速对准是水下无线光通信必须解决的问题。针对水下无线光通信过程中系统发射端和接收端之间的链路由于干扰问题引起底层平台不断移动而无法对准的问题,搭建了一个基于激光二极管(LD)的水下无线光自动对准系统,该系统具有自动对准控制的特点,即系统在底层平台移动的情况下,依然可以实现链路的对准。     

Monte Carlo–based channel estimation and performance evaluation for UWOC links under geometric losses

Underwater wireless optical communication has been studied and proposed as a potential technology for various underwater applications including monitoring and surveillance using wireless sensor networks. In this context, we present a statistical model using an exhaustive Monte Carlo approach for an underwater wireless optical communication link between sensor nodes in a data mulling environment. This work provides insights into the science of photon transport underwater, its utilization in a communication system, evaluation, and optimization of underwater communication. We characterize the temporal channel behavior taking into account multiple scattering of photons involving the constraints of detector aperture diameter, field‐of‐view (FOV), and link distance for an aligned point‐to‐point link. We analyze the designed model in terms of total received power and bit error rate for clear ocean waters. Our simulation results show that the detector aperture and FOV has an impact towards a successful communication in the distance from 5 to 20 m. In addition, results show that the FOV is an affecting factor only in the presence of larger aperture sizes of the detector.     

空间调制无线光通信的信号设计

建立了空间调制无线光通信的光强信道模型,并给出了空间信号必须满足的两个基本约束条件。通过定义于二维空间的正交基函数建立了矢量信号空间,并定义了三个衡量空间信号设计方案性能的指标,即误符号率,频带利用效率和功率效率等。参照传统时域信号设计方案,提出了三种空间调制信号具体设计方案,并从带宽效率和功率效率两性能上进行了比较,结果表明正交幅度调制(QAM)具有带宽效率高、功率效率低的特点,而且采用的信号点越多其带宽效率就越高,另外其空间信号图案还具有圆对称性,因此非常适合在空间带宽有限的远距离无线光通信中使用。     

Weather effects on hybrid FSO/RF communication link

Free Space Optics (FSO) or optical wireless systems provide high data rate solution for bandwidth hungry communication applications. Carrier class availability is a necessity for wide scale acceptability which is extremely difficult to achieve in the case of optical wireless links. FSO links are highly weather-dependent and different weather effects reduce the link availability. Employing a hybrid network consisting of an FSO link and a back up link in the GHz frequency range renders high availability besides providing comparable data rates. In this paper effects of fog, rain and snow on FSO/GHz hybrid network are studied so that GHz frequencies with best complementary behaviour can be selected as a back up link. As a prime conclusion of the article, it is suggested that free space optical links can be supplemented with 40 GHz RF links to achieve near carrier class availability.     

A high-speed transmission method for wireless personal communications

The next-generation wireless personal communications systems are expected to support a wide range of high-quality services that require high data rates. Communicating at high transmission rates over the harsh wireless environment, however, creates many difficult and challenging problems. In this paper, we describe a technique for the high-speed transmission of data in wireless personal communications which we denote as multicode modulation. In this technique, the high-rate bit transmitted data is serial to parallel converted into low-rate bit streams in a similar fashion to multicarrier or multitone modulation. However, in contrast to the multicarrier method, here we propose to modulate each low-rate bit stream using direct-sequence spread-spectrum on a single carrier. It is demonstrated that by selecting the processing gain properly the total required bandwidth will be of the same order as the original high-rate data stream; thereby, gaining the inherent benefit of multipath rejection without expanding the bandwidth of the original high-rate stream. To demonstrate the potential and merits of the proposed method as an alternative technique for high-speed transmission for wireless personal communications, various simulation results over a multipath Rayleigh fading link are presented.Supported in part by the Hong Kong Research Grant Council, Project No. HKUST562/94E and the Hong Kong Telecom Institute of Information Technology, Project No. HKTIIT94/95.EG03.     

Underwater Ad Hoc Wireless Communication for Video Delivery

Due to the high attenuation of the water at high frequencies, underwater communications in freshwater are not being investigated so much. Many current underwater communication systems are based on acoustic or optical techniques. The use of electromagnetic (EM) waves in this medium, even in short distances, increases the bandwidth, which definitely implies a great advantage for video delivery. Related literature shows that the speed propagation and absorption coefficient in freshwater are independent of the working frequency of the transmitted signals. No work shows any temperature dependence with the electromagnetic waves propagation. In this paper, we study the EM wave’s behavior when we vary the temperature at 2.4 GHz in underwater freshwater environments. We are going to study the signal behavior in this medium in order to deliver video images from the marine fish cages with the purpose of monitoring the fish activity. To carry out our study, we fix the water conditions and measure the maximum distance as a function of several network parameters such as the working frequency, data transfer rate, modulations and water temperature. Our results show that some combinations of temperature and working frequency generate better results than others. Finally, we will compare our results with the statements extracted from other works.     

孔径接收对强海洋湍流DPSK调制的水下光通信系统误码率影响研究

水下光通信（underwater wireless optical communication,UWOC）系统具有宽带宽、高保密等优点,适用于水下通信。然而,水下吸收、散射和湍流效应影响了光束传输,限制了系统在长距离方面的应用。本文探讨了差分相移键控（differential phase-shift-keying, DPSK）调制和孔径接收对在Gamma-Gamma强湍流信道中传输的UWOC系统误码率（bit error rate,BER）的影响。基于Whittaker函数,推导出了系统采用DPSK调制高斯光束在强湍流中传输的BER解析式。仿真观察了系统调制方式、孔径接收直径、传输距离以及四种海洋参数对BER性能的影响。结果表明,采用DPSK和较大尺寸的孔径直径接收可以有效地提高BER性能。同时,系统在均方温度耗散率和温度和盐度波动对海洋湍流贡献的比值较小、动力粘度较大,湍流动能耗散率较大或较小的短距离海洋湍流环境中可以获得较好的通信性能。      

双光源水下无线光通信系统的研究与实现

随着海洋探测研究的不断深入,水下无线通信技术已成为制约其发展的关键。针对水下高速、远距离无线通信的需求,设计了一种面向海洋商用的水下双向链路通信系统。系统在发射端采用激光LD及LED双发射源,提出了双发射光源的系统设计方案,并通过设计相应的驱动电路验证了方案的可行性;在接收端利用5 mm大面积APD及高灵敏度PMT双接收探测器接收光信号,适用于远距离、高速通信;系统的信息处理部分由FPGA完成,通过网络通讯方式与PC端进行信息交互;最后完成整个系统设计并进行了商用工程化。开展的水下模拟实验表明,系统在通信距离5 m、误码率10?7时,通信速率可达60 Mbps;在远距离通信60 m、误码率10?7时,通信速率达10 Mbps。     

Multiple-subcarrier modulation in optical wireless communications

This article overviews multiple-subcarrier modulation techniques in optical wireless communications. The basic principles and characteristics of MSM techniques in optical wireless communications are presented. MSM optical wireless systems are explained where some block codes that convert information bits to be transmitted to the symbol amplitudes of subcarriers are used to increase the minimum value of the MS electrical waveform. MSM optical communications systems using subcarrier signal point sequences (SSPS) that can improve the power efficiency of MSM systems are also explained. The performance of MSM optical communications systems is presented in the intensity modulation with direct detection (IM/DD) channel without dispersion and in atmospheric optical communications where the effects of scintillation exist.     

Channel and source considerations of a bit-rate reduction technique for a possible wireless communications system's performance enhancement

In wireless commercial and military communications systems, where bandwidth is at a premium, robust low-bit-rate speech coders are essential. They operate at fix bit rates and those bit rates cannot be altered without major modifications in the vocoder design. A novel approach to vocoders, in order to reduce the bit rate required to transmit speech signal, is proposed. While traditional low-bit-rate vocoders code original input speech, the proposed procedure operates on the time-scale modified signal. The proposed method offers any bit rate from 2400 b/s to downwards without modifying the principle vocoder structure, which is the new NATO standard, Stanag 4591, Mixed Excitation Linear Prediction (MELP) vocoder. We consider the application of transmitting MELP-encoded speech over noisy communication channels by applying different modulation techniques, after time-scale compression is applied. Three different time-scale modification algorithms have been evaluated and waveform similarity overlap and add (WSOLA) algorithm has been selected for time-scale modification purposes. Computer simulation results, both source and channel, are presented in terms of objective speech quality metrics and informal subjective listening tests. Design parameters such as codec complexity and delay are also investigated. Simulation results lead to a possible wireless communications system, whose performance might be enhanced by using the spared bits offered by the procedure.     

基于10 Gb/s传输链路的40 Gb/s光传输实验研究

基于中国自然科学基金网(NSFCNet)的400 km×10 Gb/s光传输链路实现了40 Gb/s光传输,没有出现误码率(BER)平台,说明在常规的中短距离10 Gb/s系统可以直接升级至40 Gb/s系统,而不需要升级传输链路。但是,由于相对10 Gb/s系统而言40 Gb/s系统的色散容限非常小,在升级时必须精确补偿原有链路的色散,在接收机前一般需要加可调色散补偿单元。同时,还分析了光纤注入功率对系统性能的影响,结果表明在设计这种由10 Gb/s向40 Gb/s升级的系统时,不仅要考虑信号带宽增加带来信噪比要求的提高,而且必须充分考虑光纤非线性的影响。