paper survey ——Underwater optical communication

本博文为本人阅读水下通信系列的笔记

 

 

 

 

Gaussian kernel-aided deep neural network equalizer utilized in underwater PAM8 visible light communication system

VLC has advantages that include high speed, the absence of electromagnetic radiation, safety for human eyes, and reliability of security communications

In LED-based VLC, because the wavelength of blue-green light is located in the transmission window of water (the absorption factor is small), blue/green LED-based VLC can achieve relatively long-distance and high-speed underwater communication at the same time. Therefore, LED-based underwater wireless communication may be a promising solution for future high-speed underwater communication networks.

However, the complex underwater environment includes turbulence, scattering, and diffusion, which leads to a highly nonlinear distortion channel.Hence, the existing linear equalization techniques, for example, recursive least squares (RLS), least mean squires (LMS), and scalar modified cascaded multi-modulus algorithm (S-MCMMA) , cannot effectively restore the nonlinear distortion signal. Thus, an effective scheme to eliminate the nonlinear deterioration is essential for practical underwater VLC systems.

Because of the capacity of DNN to model continuous models over any closed interval and the application of nonlinear active functions (tanh, sigmoid, rectified linear unit (ReLU),etc.), DNN can be applied as a solution for joint high-performance nonlinearity channel equalization and de-mapping, i.e., DNN can automatically find the relationship between input features and the classification tag through the training process. In this way, an artificial neural network equalizer has proved capable of restoring the received nonlinear deteriorated signals （信号恶化） in VLC systems.

As we know the LED-based visible light communication will suffer from nonlinear impairments due to the nonlinear transfer function of LED, the nonlinear amplification in the driving circuit and receiver.

Adopting neural networks is an effective method for nonlinear channel equalization. But the relatively long learning rate （极限学习） will limit its practical application in real-time communication system.

finding nonlinear mitigation（非线性缓解）scheme with comparable performance with DNN while with much less training rate is a great challenge in high-speed VLC system. In this paper we propose, for the first time, a Gaussian kernel aided DNN to do nonlinear mitigation。The Gaussian kernel is applied to pre-emphasis the training sequence thus can remarkably speed up the convergence process and reduce the training rate.

水下VLC信道模型

where a is the absorption coefficient and b is the scattering coefficient.

However, this channel only takes optical pass loss into concern. The real underwater VLC channel including the frequency response of electronic devices (signal amplifier, preequalizer, LED driver, LED etc.) and an optical channel. Therefore, the channel model in our system is given by

In wireless communication, the transmitted signal can be express as

why the ANN Equalizer is needed？

The nonlinear distortion is mainly caused by the nonlinearity of LED.

 

 

Underwater wireless optical communication using an arrayed transmitter/receiver and optical superimposition-based PAM-4 signal

Laser diodes (LDs) with the merits of high modulation bandwidth and high power density have been extensively studied for high-speed and long-distance UWOC in recent years.However, their extremely small divergence angles and thus strong directionality have restrictive requirements for link alignment in realistic underwater environment.（同理，相对于点对点的通信，跟camera结合，可以大大提升接收面积）

Multi-level pulse amplitude modulation (PAM) also enjoys high spectral efficiency but features simpler structure, more flexible implementation and lower computational complexity

 

34.5 m underwater optical wireless communication with 2.70 Gbps data rate based on a green laser diode with NRZ-OOK modulation

High-speed long-distance underwater wireless communications are strongly demanded given the growing underwater activities including oceanography investigation, offshore oil exploration, and sea floor monitoring. Traditional underwater acoustic communication suffers from low data rate of tens of kbps over km range. Radio frequency (RF) communication distance is limited in a range less than 10 m. For underwater optical wireless communication (UOWC), optical signal attenuation in the blue-green spectrum region is relatively small

 

Security weaknesses of underwater wireless optical communication

we uncover that the level of security we have traditionally taken for granted on underwater wireless optical communication (UWOC) may not always be there.

In real scenario, due to the inherently nonzero divergence angle of light beam and the scattering effect of water on light, the light spot diffuses as the transmission distance increases. The gradually diffused light beam may provide eavesdroppers with opportunities to wiretap or modify the transmitting signals, implying that the security of UWOC may become a serious and knotty problem in practical applications.（在实际场景中，由于光束本身的非零发散角和水对光的散射效应，光斑随着传输距离的增加而扩散。逐渐扩散的光束可以为窃听者提供窃听或修改发射信号的机会，这意味着超宽带通信的安全性在实际应用中可能成为一个严重而棘手的问题。）

UWOC may suffer from serious security threat due to the scattering effect.

 

Underwater wireless optical communication system using a 16-QAM modulated 450-nm laser diode based on an FPGA

 

Large-coverage underwater visible light communication system based on blue LED employing equal gain combining with integrated PIN array reception

quadrature amplitude modulation-discrete multitone (QAM-DMT)

 

Performance Analysis of MC-CDMA based Underwater Wireless Sensor Network

We have also proposed Optical communication along with Acoustic communication technique to improve the performance of Underwater Wireless Sensor Network (UWSN) in the harsh underwater environment.

 

A Complete Model for Underwater Optical Wireless Communications System

 

Sea-Trial of Optical Ethernet Modems for Underwater Wireless Communications

Submarine exploration and monitoring （海底勘探与监测）is now being extended to greater regions and depths, thanks to the development of autonomous underwater vehicles (AUVs) , which allow for an increasing number of tasks.

这篇论文有比较多实验的细节。后面可以认真看看

 

 

 

中文调研

相比于陆上通信，无线电波在水下的通信性能受到严重限制；声波虽能进行远距离通信，但水声信道传输时延长，传输速率低。根据海水的透光窗口特性，光谱中 的 可 见 光 部 分 可 以 实 现 高 速 通 信 ， 因 此 近 年 来 水 下 可 见 光 通 信 （ Visible  Light  Communication,  VLC）技术受到了广泛地重视。

无线电通信在陆地上的发展应用已经非常成熟，但是在海水中，无线电波的衰减非常严重，且波长越短、频率越高，在海水中的衰减就越厉害，传输距离越短。低频长波无线电波水下实验可以达到６~８米的通信距离。３０~３００Ｈｚ的超低频电磁波对海水穿透能力可达１００多米，但需要很长的接收天线，不利于水下设备的小型化

水声通信技术的发展历史可以追溯到１９世纪末，三者之中其发展最成熟，在水下的应用也最广泛。与其它通信载体如光波和无线电波相比，水声通信最大的优势在于声波在水中的衰减小，传输距离远，可达几十千米。但水声通信也存在诸如带宽低、传输时延高、传输速率低等不足之处：声波在传输的过程中沿不同路径到达接收端，从而导致接收信号不德定水声通信的速率低，一般仅在几百ｋｂｐｓ的级别；由于水声通信的通信延迟大，其延迟时间可迗５０毫秒至１００毫秒，较难提供实时的信息传输；此外，水声通信的收发端设备体积和功耗一般较大。

与水下无线电通信和水声通信相比，水下无线光通信（Ｕｎｄｅｒｗａｔｅｒ Ｏｐｔｉｃａｌ ＷｉｒｅｌｅｓｓＣｏｍｍｕｎｉｃａｔｉｏｎ，ＵＯＷＣ）可以传输的距离较为适中，但光载波的带宽高，可以获得Ｇｂｐｓ级别的高通信速率。此外，相比于水声通信，无线光通信系统的延迟很小，能够保证系统通信的实时性，同时，光通信不会对海洋生物产生噪声干扰。水下无线光通信作为一种新型的通信技术，以其带宽高、容量大、抗干扰能力强、保密性好等优势，已成为世界大国竞相发展的重要通信技术，将在水下自航式机器人、深海遥控机器人、载人深潜器、海底原位观测站以及水下无线传感器网络等应用中发挥重要作用
 

水下光束能量衰减的比尔定律(Beer'Law)

光在水中传播与海水的光学特性密切相关。海水中包含许多悬浮颗粒、浮游植物以及有色溶解有机物等。海水的成分直接影响海水的固有光学特性，包括吸收和散射。光在水中的衰减主要由于水中粒子对光的吸收和散射所造成的。

 

空中—水下无线光通信

可用于低空无人机与水下平台间的数据传输
传统的空中－水下无线通信是采用射频通信的方式［一般采用频率在３~３０ｋＨｚ之间的无线电波，主要用于对潜艇的单向发射信号。射频通信在空中－水下无线通信中的优势主要在于，其在空水界面处的传输比声波或者光波更平滑，其跨界面的传输性能好；而且射频通信对水的扰动的容忍性更好。然而，由于射频波在水中的衰减非常大，水下的潜艇需要非常靠近水面伸出天线接收信号，才能实现和空中平台的通信，这样就失去了潜艇的隐蔽性。

而对于空中－水下的数据采集来说，采用甚低频射频通信的方式则大大削减了灵活性。而且甚低频通信发射机输出功率一般在十几千瓦至数兆瓦，需要庞大的天线设备，天线高度可达２００米以上。此外，其通信速率一般在数百ｂｐｓ以下，仅用于传输简单的指令信息，如低速电报；而若要传输如图片、音频、视频等的数据，必须达到１Ｍｂｐｓ以上的通信速率。因此采用射频实现空中－水下无线通信非常耗时耗力。

随着海洋研究、探测、开发和保护等活动的不断增多，大量的自容式传感器或传感网络布放于水下，如何高效地采集水下传感器节点数据成为亟待解决的问题。水下无线光通信具有带宽高、安全性高、灵活性好、低功粍和低延迟的优点尤其是蓝绿激光在水中的传输距离要比射频更远，可迗６００米的水下深度。大量研究表明，水下无线蓝绿光通信可以在中等的传输距离下获得高速的通信速率。因此，采用蓝绿光实现空中平台到水下平台间的无线通信，既可以避免水下潜器浮出水面进行通信，大大增加空中－水下无线通信的安全性和灵活性，又可以实现高速的通信速率，保证了通信的实时性。
 

图像传感器是由众多光电二极管阵列组成，具有天然的空间分集接收能力，有着很好的应用前景。

潜水员在水下可以使用带有解码软件的手机，直接通过无线光通信与其他潜水员或是水面上的人员进行语音或视频通话。而且在水下的探测、监测等任务中，摄像头是必不可少的传感器，如果能同时将摄像头作为通信的接收端，既可以节省水下设备造价，又可以缩小水下设备的体积，大大提高其灵活性。尤其适合应用在基于无人机的空中－水下无线光通信系统中，因为目前的无人机都带有高分辨率的摄像头，采用这种信号接收方式无需部署新的设备，非常高效方便