基于数字信号处理的高灵敏度水下光通信发收机设计与评估 下载: 846次
In recent years, there has been a strong demand for high-speed and long-range underwater wireless communication technologies owing to an increase in underwater activities, e. g., marine surveys, offshore oil exploration, submarine monitoring, and a series of new underwater monitoring and communication technologies such as an unmanned underwater vehicle(UUV), which has recently been rapidly developed. The traditional underwater acoustic communication frequency is generally between 10 Hz and 1 MHz, with low data transmission speed and high communication delay, which has gradually failed to meet the requirements of underwater activities. With the development of visible-light communication, underwater wireless optical communication (UWOC) has attracted increasing attention. UWOC has higher transmission bandwidth, faster data rate, lower link delay, higher security and lower cost than hydroacoustic communication, making it an attractive and viable alternative. Although underwater optical communication with blue and green light can minimise the transmission attenuation effect, in water, photons inevitably interact with water molecules and other particulate matter and suffer severe absorption and scattering effects, thereby weakening the transmission of optical signals and limiting the communication distance and quality. Therefore, it is essential to improve the sensitivity of underwater optical communication receivers.
The communication system designed in this study uses the on-off keying (OOK) modulation method, which occupies a small bandwidth and has a high transmission rate per code element. It is the preferred optical modulation technique for existing mature underwater laser communication systems. However, the traditional OOK underwater optical communication transmitter and transceiver can cause huge attenuation and power jitter in the transmitted signal owing to the underwater channel as well as pulse spreading and other signal distortion phenomena, which bring a great challenge to the system’s BER capability. Herein, we design a high-sensitivity underwater optical communication transceiver using hardware circuitry and digital signal processing through field-programmable gate array digital devices. Additionally, we improve the signal-to-noise ratio (SNR) of underwater communication using a series of digital algorithms, such as source coding, adaptive judgement threshold, and FIR filtering. Furthermore, we test the BER performance of the transceiver under different water quality conditions to verify the overall BER performance of the system.
We conducted internal and external field experiments using three different types of water. The experimental diagram of the indoor pool of the underwater communication system is shown (Fig. 8). The transceiver communication rate is 5 Mbps, modulation format is OOK, transmitting light source is a band blue light-emitting diode with 470 nm wavelength and the power is 1.2 W, the pass-light aperture is 75 mm; the underwater communication system’s external field test diagram is shown (Fig.9). The wireless optical communication terminal transmitter and receiver are placed in the lake water, 2-m deep from the lake surface. An indoor pool test with the water quality attenuation coefficient of 0.17 m-1 belongs to class I of water quality with the pseudo-random code sequence rate of 5 Mbps, when the error rate is 10-6. The communication distance is 20 m, and the system sensitivity reaches -38.28 dBm. The outdoor pool test with the water quality attenuation coefficient of 0.47 m-1 belongs to class II of water quality with the pseudo-random code sequence rate of 5 Mbps, when the error rate is 10-5. The communication distance is 10 m, and the system sensitivity reaches -38.46 dBm. At 2 m depth of Qiandao lake shore water, the test water quality attenuation coefficient is 1.33 m-1, which belongs to class III of water quality with pseudo-random code sequence rate of 5 Mbps. Then, the BER is 10-6 and the communication distance is 4.5 m. The SNR considerably improves after digital filtering. The reception sensitivity of the system reaches -37.52 dBm (Table 2).
This study describes the underwater OOK channel model and analyses the correspondence between BER and SNR of underwater OOK modulation methods in different water quality conditions. To cope with the impact of an underwater channel on the optical signal transmission, an underwater optical communication transmitter-transceiver based on hardware circuits and field-programmable logic gate devices is designed. The digital signal processing modules such as FIR filtering (to improve the SNR of the system), adaptive judgement threshold and sliding mean filtering are designed to improve the communication BER performance. The communication performance of the underwater communication transmitter and transceiver is verified under different water quality conditions. The experimental results show that the terminal can achieve a sensitivity of -38 dm at a transmission rate of 5 Mbps and BER of 10-6. The transmission distance can reach 20, 10 and 4.5 m in class I, class II and class III waters, respectively. In the class III water test, the communication distance of 5 m and BER of 10-5 can meet the demand of voice transmission. The distortion-free image and SD video transmission function can be realised at the communication distance of 4.5 m and BER of 10-6, verifying the feasibility of underwater optical communication based on digital signal processing.
1 引言
近年来,随着水下活动的增加,例如海洋调查、海洋石油勘探、潜艇监测以及最近几年快速发展的无人水下潜航器(UUV)等一系列新型的水下监测通信技术[1-4],对高速长距离的水下无线通信技术一直有着很强的需求。传统的水下声波通信频率一般在10 Hz~1 MHz之间,数据传输速度较低,而且具有较高的通信延迟,逐渐无法满足水下活动的需求。随着可见光通信的发展,水下无线光通信(UWOC)越来越受到人们的关注。水下无线光通信相比于水声通信具有更高的传输带宽、更快的数据速率和更低的链路延时,而且安全性高、成本低,这些优点使其成为一种具有吸引力的可行性替代方案。
不同波段的光在水中传输时的损耗有很大差异。研究证明在海水中光谱存在一个低损耗窗口,波长在470~525 nm范围内的蓝绿光波段相较于其他波段具有较低的损耗,可以最有效的降低水下信道对光信号带来的吸收和衰减,蓝绿波段光线在水下信道传输的损耗仅为其他波段的1%,这证明了水下光通信的可行性[5-6]。2005年麻省理工学院的Vasilescu等[7]实现了水下光通信网络,该网络由静止终端与动态终端组成,两者之间通信速率可以达到512 kbps。2010年,美国北卡罗莱纳州立大学的Simpson等[8],设计了一种应用于水下传感器网络节点的低成本无线光通信系统,该系统对以前发射系统的数字信号处理方式(包括编码技术)进行了改进,并在实验室长为3~5 m的水槽中实现了传输速率5 Mbps的无线光通信实验。2014年,哈尔滨工业大学的姚灿[9]设计了基于OOK调制的水下实时光通信系统,在实验室水槽以及近海岸浑浊水域中进行实验,结果表明,该系统在串口速率为9600 bps时,传输距离可达27 m,误码率为10-4。2017年4月,美国克莱姆大学的Baghdady等[10]利用轨道角动量空间复用技术,采用OOK-NRZ的调制格式,采用445 nm波段尾光纤半导体激光器作为发射光源,在衰减系数为0.4128 m-1、通信距离为2.96 m的水下信道中,实现了通信速率3 Gbps、误码率为2.03×10-4的水下通信。
虽然用蓝光和绿光进行水下光通信可以最大限度地减少透射衰减效应,但是在水中,光子不可避免地与水分子和其他颗粒物质相互作用,存在严重的吸收和散射效应,削弱了光信号的传输,限制了通信距离与质量,因此提高水下光通信接收机的灵敏度具有重要意义。
本文设计的通信系统采用了开关键控(OOK)的调制方式,该调制方式占用带宽小、单位码元传信率高,是现有成熟的水下激光通信系统的首选光调制技术[11]。但是传统的OOK水下光通信发收机,由于水下信道的影响会对传输信号造成巨大的衰减、功率抖动[12],并且出现脉冲展宽等信号畸变现象[13],使系统的误码能力面临很大的考验。本文设计了一种利用硬件电路以及现场可编程门阵列(FPGA)数字器件进行数字信号处理的高灵敏度水下光通信发收机,通过信源编码、自适应判决门限以及有限长单位冲激响应滤波器滤波(FIR)等一系列数字算法提高水下通信的信噪比,并且在不同水质条件中对发收机误码性能进行测试,验证系统整体的误码性能。
2 水下信道模型
2.1 水下光OOK信道
对于UWOC来说,海水信道是一个衰落信道,对水下光信号传输产生影响的主要是水分子和其他颗粒物质对光的吸收和散射,这导致了信号的衰减。当入射光通过海水信道时,接收到的光信号功率Pt可以表示为
在复杂的水质环境中,吸收和散射吸收呈现出较大的区别。一般将海水水质划分为三大类:Ⅰ类水质为清澈的海水,Ⅱ类水质为沿岸较为浑浊的海水,Ⅲ类水质为港口浑浊的海水。
表 1. 不同水质条件参数
Table 1. Parameters of different water quality conditions
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2.2 通信误码率分析
建立接收机信噪比与误码率方程,在等概率分布的情况下,误码率Pe可以表示为
考虑到高斯加性噪声对信道的影响,给出了误码率方程如(8)式
依据
图 1. 不同水质条件下OOK调制信噪比与误码率的关系
Fig. 1. Relationship between SNR and slot error rate of OOK modulation under different water quality conditions
由
3 数字接收终端设计
3.1 水下数字接收终端总体架构
水下数字接收机的结构设计如
APD采用的是HAMAMATSU的C12702,接收靶面直径为3 mm,温度控制系数参数为2.2 V/℃,温度敏感二极管D1N914安装在APD附近,温度灵敏度系数为-2.1 mV/℃。二极管的温度读数可以反馈到高压转换模块,该模块通过调整反向偏置电压来维持APD在更稳定的增益水平。APD的输出由一个初级跨阻放大器和一个低噪声场效应晶体管组成。在温度反馈控制下,APD的灵敏度在450 nm波段可以达到0.9×104 V/W。为了实现高灵敏度数字接收机的设计要求,使用了12位ADC和具有5 V范围的电路,对模拟信号的数字量化分辨率为2.4 mV。采样后的数字信号在FPGA逻辑单元中进行处理。数字信号处理主要包括三个模块:首先,信号进入有限元脉冲响应(FIR)滤波模块,此模块对信号的信噪比起重要作用;其次,进入自适应判决门限模块进行判决;最后,对判决后的信号采用滑动均值滤波滤除信号中的“毛刺”,最终还原出原始基带信号。
信号进入FIR滤波模块,该模块对信号的信噪比改善起较大作用,数字滤波器可以避免模拟滤波器无法克服的电压漂移和温度漂移等问题,设计滤波器参数也可以有效地滤除接收机电路中带来的低频闪烁噪声以及高频噪声。FIR滤波器其单位取样响应h(n)是一个N点长的有限长序列,0≤n≤N-1。滤波器的输出y(n)可表示为输入序列x(n)与单位取样响应h(n)的线性卷积[15-17]
由于水下OOK信道的严重衰减,在实际链路中由于距离的变化会产生较大的发射光功率的变化,固定门限不能适应信道环境的动态变化,因此加入自适应判决门限来解决此问题。将滤波后的信号分别送入自适应判决门限模块与信号延迟模块。自适应判决门限VT设置为[18]
判决门限仿真如
图 4. 解调基带波形自适应门限仿真
Fig. 4. Adaptive threshold simulation of demodulation baseband waveform
通过门限对滤波后的信号进行判决,在信号边沿处易产生毛刺,为还原的基带信号中带来误码。通过加入滑动均值滤波模块消除毛刺影响,滑动均值滤波实际上具有保留低频分量、滤除高频分量的特性。输入信号yi与输出信号xi的关系为
滑动均值滤波器包括两个基本组成部分:积分部分和梳状部分,滤波器结构简单,没有乘法器,只有加法器、积分器和寄存器,在FPGA中可以以非常低的资源消耗实现高速滤波,滤波器结构如
滤波器的传递函数为
滑动均值滤波模块仿真如
4 实验与结果
通信系统总体框图如
水下通信系统室内泳池实验图如
图 8. 水下通信系统室内泳池试验图。(a)接收端;(b)发射端;(c)室内泳池信道
Fig. 8. Experiment of underwater communication system in indoor pool. (a) Receiver; (b) transmitter; (c) indoor pool channel
表 2. 不同水质环境下实验结果
Table 2. Experimental results under different water quality environments
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水下通信系统外场试验图如
图 9. 水下通信系统外场试验图。(a)千岛湖湖水信道;(b)室外泳池信道
Fig. 9. Experiment of underwater communication system in real water environment. (a) Qiandao lake water channel; (b) outdoor pool channel
通过
根据
5 结论
描述了水下OOK信道模型,并分析了水下OOK调制方式在不同水质中的误码率与信噪比的对应关系。为了应对水下信道对光信号传输的影响,设计了一种基于硬件电路与现场可编程逻辑门器件的水下光通信发收机。通过设计FIR滤波提高系统信噪比、自适应判决门限和滑动均值滤波等数字信号处理模块来提高通信误码性能。在不同的水质条件下对水下通信发收机进行了通信性能验证,实验结果表明该终端可实现在5 Mbps传输速率、误码率为10-6条件时下达到-38 dBm的灵敏度,在Ⅰ类水质中传输距离达到20 m,Ⅱ类水质中传输距离10 m,Ⅲ类水质中传输距离可以达到4.5 m。在Ⅲ类水域试验中,通信距离5 m,误码率10-5条件下可以满足语音传输的需求;在通信距离4.5 m,误码率10-6条件下可实现无失真的图像和标清视频传输功能,验证了基于数字信号处理的水下光通信可行性。
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