云層厚度對(duì)藍(lán)綠激光通信性能的影響分析

李松朗，毛忠陽(yáng)，劉傳輝，劉 敏

云層厚度對(duì)藍(lán)綠激光通信性能的影響分析

李松朗*，毛忠陽(yáng)，劉傳輝，劉 敏

海軍航空大學(xué)，山東 煙臺(tái) 264001

針對(duì)機(jī)載激光發(fā)射器位于云層上方或云層中央時(shí)，云層的存在會(huì)降低激光通信性能的問題，仿真分析了不同類型的云層對(duì)激光能量衰減、信噪比、最大碼元傳輸速率與誤碼率的影響。得到結(jié)論：云的存在主要造成激光能量衰減，影響最大傳輸速率與誤碼率，但對(duì)信噪比影響較小。鏈路余量大于18.9 dB的通信系統(tǒng)，鏈路上允許存在4 km的云層。云層對(duì)最大通信速率與誤碼率的影響主要是時(shí)間擴(kuò)展造成碼間串?dāng)_。卷云對(duì)通信性能幾乎無(wú)影響；積云對(duì)通信性能的影響較大；層云、層積云和積雨云對(duì)通信性能的影響更大，但三種云的差異很小，可不作區(qū)分；高層云和雨層云對(duì)通信性能影響最大，其中雨層云的影響比高層云更大。

無(wú)線光通信；大氣信道；光學(xué)厚度；云層厚度；藍(lán)綠激光通信

1 引 言

在機(jī)載藍(lán)綠激光通信中，如果激光發(fā)射器位于云層上方或中央，云層會(huì)使激光能量衰減，導(dǎo)致通信質(zhì)量下降，因此需要分析云層對(duì)藍(lán)綠激光通信的影響，為改善方法的研究提供理論依據(jù)。

云對(duì)激光的衰減主要是米氏散射。在現(xiàn)有云層對(duì)藍(lán)綠激光通信的分析中，Bucher和Mooradian通過(guò)實(shí)驗(yàn)給出了激光透過(guò)云之后的信號(hào)波形[1-2]；狄凌峰等通過(guò)實(shí)驗(yàn)計(jì)算了532 nm激光在近地大氣中的散射相位函數(shù)和氣溶膠分布[3]；Hess等通過(guò)理論公式計(jì)算了不同類型的云的衰減系數(shù)[4]；胡秀寒等仿真分析了云高度和云厚度對(duì)激光到達(dá)海面的能量、角度分布、空間分布和時(shí)間分布的影響[5]。在現(xiàn)有云層對(duì)紅外激光通信的分析中，Arnon等通過(guò)蒙特卡洛仿真給出了脈沖經(jīng)過(guò)云層后的雙Gamma擬合曲線和無(wú)碼間串?dāng)_時(shí)的最大傳輸速率[6-7]；劉建斌等計(jì)算了四種類型云的消光系數(shù)、單次散射返照率、不對(duì)稱因子和散射相函數(shù)[8]；陳純毅等計(jì)算了紅外波段激光通過(guò)云層后有碼間串?dāng)_的誤碼率[9]。

上述文獻(xiàn)在分析云層對(duì)通信性能的影響時(shí)僅在紅外波段分析了云層厚度對(duì)誤碼率和最大傳輸速率的影響，但實(shí)際通信系統(tǒng)中造成誤碼的因素除了云層外，還有湍流的影響，并且藍(lán)綠激光通信系統(tǒng)在傳輸距離和發(fā)散角與接收孔徑方面均比紅外通信系統(tǒng)大，因此不能直接參考紅外波段的研究結(jié)論；在藍(lán)綠波段僅從光學(xué)角度分析了能量的衰減與時(shí)間展寬，未對(duì)通信性能進(jìn)行分析，也未對(duì)云層進(jìn)行詳細(xì)分類。

本文詳細(xì)分析不同類型的云層及云層厚度對(duì)藍(lán)綠激光通信的影響，并比較云層和湍流對(duì)通信影響的差異，為后續(xù)減小云層對(duì)通信影響的技術(shù)研究提供理論依據(jù)。

2 云衰減模型

云由于所處高度不同，按其微觀粒子組成可分為水云、冰水混合云與冰云。積云、層云和層積云高度較低，屬于水云；雨層云、積雨云和高層云云層較厚，屬于冰水混合云；卷云高度較高，屬于冰云。其中積云與卷云一般在晴天出現(xiàn)，而其他類型的云一般在陰天出現(xiàn)，他們的衰減系數(shù)如表1[6,10-12]。

云的光學(xué)厚度與物理厚度存在如下關(guān)系：

根據(jù)Vande Hulst公式，激光經(jīng)過(guò)云層的能量透過(guò)率為[13]

3 云層厚度對(duì)鏈路能量衰減的影響

3.1 云層光學(xué)厚度與透過(guò)率

由于機(jī)載發(fā)射器距海平面最高為8 km，云的衰減系數(shù)最大為0.128 m-1，因此最大光學(xué)厚度為1024。仿真光學(xué)厚度0~1024的云層與相應(yīng)的激光透過(guò)率，仿真結(jié)果如圖1。

由圖1知，光學(xué)厚度小于200時(shí)，激光能量衰減比較劇烈，之后能量衰減漸緩；云層為最大光學(xué)厚度1024時(shí)，對(duì)光的衰減約為19 dB。因此對(duì)于鏈路余量大于20 dB的通信系統(tǒng)，云層的存在不會(huì)使通信中斷。

3.2 云層物理厚度與鏈路能量衰減

有云層時(shí)，通信系統(tǒng)的鏈路余量為

其中為無(wú)云時(shí)的鏈路余量。仿真不同類型的云及云層厚度對(duì)鏈路余量的影響，結(jié)果如圖2。

由圖2所示，卷云的透過(guò)率最大，云層厚度4 km時(shí)，激光通過(guò)-25 ℃卷云時(shí)能量減小了0.56 dB，-50 ℃卷云時(shí)能量減小了0.06 dB；層云、層積云和積雨云對(duì)能量衰減的趨勢(shì)基本一致，4 km的云層厚度使能量減小了11.5 dB~11.8 dB；其次是積云，4 km的云層使能量減小了12.5 dB；雨層云和高層云對(duì)能量的衰減最大，云層厚度4 km時(shí)，雨層云使激光能量減小了16.18 dB，高層云使激光能量減小了15.45 dB。

表1 不同類型云的衰減系數(shù)

圖1 光學(xué)厚度與透過(guò)率

值得注意的是，由于卷云與積云的水平面積小，呈現(xiàn)晴天，晴天云層下方氣溶膠對(duì)激光的衰減比陰天時(shí)的小。根據(jù)文獻(xiàn)[14]，中緯度532 nm激光從距地8 km處向下發(fā)射，陰天時(shí)大氣衰減比晴天多2.7 dB，在設(shè)計(jì)系統(tǒng)鏈路余量時(shí)需要考慮。因此，為使系統(tǒng)在鏈路上存在4 km的云層時(shí)也能正常接收信號(hào)，按能量衰減最為嚴(yán)重的雨層云計(jì)算，雨層云使激光能量減小了16.18 dB，由于出現(xiàn)雨層云時(shí)為陰天，陰天大氣衰減又比晴天時(shí)多2.7 dB，則整體鏈路余量應(yīng)大于18.88 dB。

4 云層厚度對(duì)信噪比的影響

不考慮云層厚度但考慮湍流強(qiáng)度時(shí)，通信系統(tǒng)的信噪比為[15]

考慮云層厚度與湍流強(qiáng)度時(shí)通信系統(tǒng)的信噪比為

在不考慮云層厚度和湍流強(qiáng)度的情況下，采用OOK調(diào)制時(shí)，信噪比為[16]

其中：為光電響應(yīng)度，T為激光發(fā)射功率，為電子電荷，為探測(cè)器濾波帶寬，atm為大氣衰減，geo為幾何衰減，其表達(dá)式為[15]

根據(jù)文獻(xiàn)[14]，中緯度532 nm激光從距地8 km處向下發(fā)射，晴天時(shí)大氣衰減atm為74.58％，陰天為40.06％。

由圖3與圖4所示，云層厚度對(duì)信噪比的影響均較小，其中卷云厚度對(duì)信噪比無(wú)明顯影響；積云次之；其次是層云、層積云和積雨云，不同湍流強(qiáng)度下信噪比均減小了不到0.03 dB；高層云和雨層云對(duì)信噪比的影響相對(duì)較大，云層厚度4 km時(shí)，高層云使中湍流信噪比減小了0.036 dB，弱湍流信噪比減小了0.061 dB；雨層云使中湍流信噪比減小了0.043 dB，弱湍流信噪比減小了0.073 dB。這也從側(cè)面反應(yīng)了隨著湍流強(qiáng)度減弱，云層厚度對(duì)信噪比的影響逐漸增大，但云層對(duì)信噪比的影響遠(yuǎn)不如湍流對(duì)信噪比的影響，湍流的存在使信噪比從166 dB~309 dB驟降到5 dB~7 dB，云層的存在只使信噪比下降了不到0.1 dB，說(shuō)明云層影響通信質(zhì)量的方式不是降低信噪比，文獻(xiàn)[8-9]指出，云層會(huì)使脈沖信號(hào)產(chǎn)生嚴(yán)重的時(shí)間展寬，從而造成高速率通信時(shí)的碼間串?dāng)_。

圖3 中湍流云層厚度對(duì)信噪比的影響

5 云層厚度對(duì)最大傳輸速率的影響

文獻(xiàn)[8-9]通過(guò)蒙特卡洛仿真得到了532 nm的激光脈沖通過(guò)五組不同光學(xué)厚度的薄云層時(shí)的波形，并擬合成雙Gamma函數(shù)：

式中：1、2、3和4為雙Gamma函數(shù)的常量系數(shù)，()為單位階躍函數(shù)，參數(shù)擬合結(jié)果如表2。

表2 雙Gamma函數(shù)的擬合結(jié)果

仿真曲線與擬合曲線如圖5。

最大碼元傳輸速率

圖4 弱湍流云層厚度對(duì)信噪比的影響

但云層的物理厚度較大時(shí)，上述公式不再適用，半功率點(diǎn)的時(shí)間擴(kuò)展公式為[17]

云層厚度對(duì)最大傳輸速率影響較大，相同物理厚度時(shí)，卷云的最大傳輸速率最高，其次是積云、層云、層積云和積雨云，傳輸速率最小的為雨層云和高層云。云層厚度4 km時(shí)，雨層云與高層云的最大傳輸速率小于104bps，積云、層云、層積云和積雨云略大于104bps，-25 ℃卷云為2′105bps，-50 ℃卷云為1.3′106bps。

圖5 脈沖響應(yīng)擬合曲線

圖6 脈沖能量探測(cè)比與最大碼元傳輸速率

6 云層厚度對(duì)誤碼率的影響

在通信速率小于最大傳輸速率時(shí)，由時(shí)間擴(kuò)展引起的碼間串?dāng)_并不會(huì)影響性能，誤碼率主要受湍流造成的光強(qiáng)閃爍和接收器噪聲影響；而當(dāng)通信速率超過(guò)最大傳輸速率，碼間串?dāng)_則成為主要因素，其波形示意圖如圖8。

根據(jù)文獻(xiàn)[18]，采用OOK調(diào)制，且發(fā)送“1”與“0”概率相等時(shí)，系統(tǒng)的平均誤碼率為

圖7 云層厚度與最大碼元傳輸速率

當(dāng)傳輸速率為5 Mbps時(shí)，云層的光學(xué)厚度不同時(shí)，信噪比與誤碼率之間的關(guān)系如圖9。

圖8 碼間串?dāng)_波形示意圖

圖9 信噪比與誤碼率

7 結(jié)束語(yǔ)

本文主要從能量衰減及時(shí)間擴(kuò)展角度研究不同類型的云及云層厚度對(duì)藍(lán)綠激光通信性能的影響。得到結(jié)論：云層主要影響鏈路余量、最大通信速率和誤碼率，對(duì)信噪比的影響很小。卷云對(duì)通信性能幾乎無(wú)影響；積云對(duì)通信性能的影響次之；其次是層云、層積云和積雨云，且三種云的差異很小，可不作區(qū)分；高層云和雨層云對(duì)通信性能影響較大，其中雨層云的影響更大。

從鏈路余量角度看，云層的影響占主導(dǎo)地位，一方面云層影響了天氣的陰晴，從而影響了云層下氣溶膠的衰減系數(shù)，導(dǎo)致鏈路余量下降；另一方面云層的厚度也直接導(dǎo)致了激光能量的衰減，且影響較大。但對(duì)鏈路余量大于20 dB的系統(tǒng)來(lái)說(shuō)，云的存在不會(huì)使通信中斷。

從信噪比角度看，湍流的影響仍然占主導(dǎo)地位，云層厚度只是使通信性能略微降低。中湍流時(shí)信噪比在5.63 dB~5.68 dB之間，弱湍流時(shí)信噪比在6.74 dB~6.82 dB之間，與無(wú)云時(shí)基本一致。

從最大通信速率角度看，脈沖通過(guò)云層時(shí)由于多徑效應(yīng)造成時(shí)間擴(kuò)展，當(dāng)通信速率過(guò)大時(shí)會(huì)造成碼間串?dāng)_，因此會(huì)限制最大通信速率。云層厚度4000 m時(shí)，雨層云與高層云的最大傳輸速率小于104bps，積云、層云、層積云和積雨云略大于104bps，-25 ℃卷云為2′105bps，-50 ℃卷云為1.3′106bps。

從誤碼率角度看，由于云層限制了最大通信速率，因此當(dāng)通信速率超過(guò)最大值時(shí)會(huì)造成碼間串?dāng)_，增加了誤碼率，造成通信質(zhì)量下降。改善方法有發(fā)送端速率自適應(yīng)和接收端信道均衡，有待于進(jìn)一步研究。

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Analysis of the effect of cloud thickness on the performance of blue-green laser communication

Li Songlang*, Mao Zhongyang, Liu Chuanhui, Liu Min

Naval Aviation University, Yantai, Shandong 264001, China

Schematic of laser passing through clouds

Overview:Using airborne wireless optical communication, if the laser transmitter located above or in the center of the cloud, the quality of communication on the receiving end will be severely degraded. Therefore, effective measures should be taken to improve communication performance. It is necessary to analyze the influence of clouds on the performance of wireless optical communication, which provides a theoretical basis for the rate adaptation and channel equalization design when there are clouds on the link.

In this paper, Vande Hulst formula is used to simulate the transmittance of blue-green laser passing through clouds, and calculate link margin and SNR. The monte carlo simulation method is used to simulate the waveform of the pulse passing through the thin clouds, and the waveform is fitted as a double gamma function. The relationship between the detection ratio of simulated pulse energy and the maximum symbol transmission rate is simulated. Stotts formula is used to calculate the time width of half power point, then estimate the maximum symbol transmission rate, and calculate the bit error rate in excess of maximum transmission rate.

It is concluded that cloud mainly causes laser energy attenuation, which affects maximum communication rate and bit error rate, but has little effect on SNR. Cirrus has little effect on communication performance, stratus, stratocumulus, and cumulonimbus have a great influence on the communication performance, but the differences between the three types of clouds are small and could be not be distinguished. Altostratus cloud and nimbostratus have great influence on communication performance, of which nimbostratus has greater influence than altostratus cloud.

From the perspective of laser energy attenuation, cloud plays a dominant role. For communication systems with link margin greater than 18.9 dB, 4 km cloud cover is allowed on the link. From the point of view of SNR, the influence of turbulence is still dominant, and the cloud thickness only reduces the SNR slightly, which is basically the same as that without cloud. From the perspective of maximum communication rate and bit error rate, the time expansion will be caused due to multi-path effect when the pulse passes through the cloud. The inter-symbol crosstalk will be caused when the communication rate is too high, so the communication rate will be limited. When the communication rate exceeds the maximum value, it will cause inter-code crosstalk, increase the bit error rate, and reduce the communication quality. The improved methods include the sending end rate adaptive and the receiving end channel equalization, which need to be further studied.

Citation: Li S L, Mao Z Y, Liu C H,Analysis of the effect of cloud thickness on the performance of blue-green laser communication[J]., 2020, 47(3): 190389

Analysis of the effect of cloud thickness on the performance of blue-green laser communication

Li Songlang*, Mao Zhongyang, Liu Chuanhui, Liu Min

Naval Aviation University, Yantai, Shandong 264001, China

When the airborne laser transmitter is located above or in the center of the cloud, the cloud will reduce the laser communication performance. In order to solve this problem, the effects of different types of clouds on laser energy attenuation, signal-to-noise ratio (SNR), maximum symbol transmission rate and bit error rate are simulated and analyzed. It is concluded that the cloud mainly causes laser energy attenuation, which affects maximum transmission rate and bit error rate, but has little effect on SNR. For communication systems with link margin greater than 18.9 dB, 4 km cloud cover is allowed on the link. The effect of cloud on the maximum communication rate and bit error rate is mainly caused by inter-symbol crosstalk caused by time extension. Cirrus has little effect on communication performance, cumulus has a great impact on communication performance, and stratus, stratocumulus, and cumulonimbus have a greater influence on the communication performance, but the differences between the three types of clouds are small and could be not be distinguished. Altostratus cloud and nimbostratus have greatest influence on communication performance, of which nimbostratus has greater influence than altostratus cloud.

optical wireless communication; atmospheric channel; optical thickness; cloud thickness; blue-green laser communication

TN929.12

A

10.12086/oee.2020.190389

: Li S L, Mao Z Y, Liu C H,. Analysis of the effect of cloud thickness on the performance of blue-green laser communication[J]., 2020,47(3): 190389

2019-07-08；

2019-10-21基金項(xiàng)目：國(guó)家自然科學(xué)基金資助項(xiàng)目(6170012154)；山東省“泰山學(xué)者”建設(shè)工程專項(xiàng)經(jīng)費(fèi)基金資助項(xiàng)目(ts20081130)

李松朗(1995-)，女，碩士研究生，主要從事藍(lán)綠激光通信的研究。E-mail：772591662@qq.com

李松朗，毛忠陽(yáng)，劉傳輝，等. 云層厚度對(duì)藍(lán)綠激光通信性能的影響分析[J]. 光電工程，2020，47(3): 190389

Supported by National Natural Science Foundation of China (6170012154) and Shandong Province "Taishan Scholars" Construction Project Special Funds (ts20081130)

* E-mail: 772591662@qq.com