航院毕设英文原文.docx

1、航院毕设英文原文 毕业设计(论文)英文翻译原文院 （系）： 桂林航天工业学院 专 业： 计算机通信工程 学生姓名： 杨 康 学 号： 030111301585 指导教师： 孙山林 职 称： 教授 年 月 日Application of adaptive modulation for rain attenuation Abstract Rain attenuation is a dominant source of satellite link outages in higher frequencies satellite communication systems. The intensity

2、 and distribution of rainfall greatly affects transmission quality and limits systems availability. In order to overcome such difficulties, in the article, it use adaptive modulation and make SNR as the judgement threshold, this method furthest improves the throughput in satellite communication. It

3、has Iridium system in the back and OPNET as tool, veritably simulate the application of adaptive modulation under the environment of rainfall. Comprehensive theoretical analysis and dedicated simulations show that the simulation results and the theoretical results are matched perfectly. Key words：ad

4、aptive modulation; rain attenuation; OPNET; link budget; coverage1 INTRODUCTION Satellite communication links are affected by rain fall, and solar outage, among others. These affects must be taken into accounts in satellite link design and operation. This article describes the effects and techniques

5、 to quantify and alleviate the effect.The rapid development in communication systems has forced system designer to explore higher and higher frequencies. Rain is a dominant source of attenuation at higher frequencies. The intensity and distribution of rainfall greatly affects transmission quality an

6、d limits systems availability. Rain effects on microwave systems are more critical in tropical and equatorial zones, where rainfall is higher than in temperate zones. In temperate regions the rain effects becomes significant above 10 GHz, while in tropical climates in general and in equatorial clima

7、te particularly, since the rain drops are larger than in temperate climates, the incidence of rainfall on radio links becomes important for frequencies as low as about.In order to overcome such difficulties, in the article, it use adaptive modulation and make SNR as the judgement threshold, this met

8、hod can furthest improve the throughput in satellite communication. It has Iridium system in the back and OPNET as tool, veritably simulate the application of adaptive modulation under the environment of rainfall.2 THEORETICAL ANALYSIS OF ADAPTIVE MODULATION2.1 Basic theory of adaptive modulation In

9、 the traditional modulation mode, in order to ensure the BER performance of the system, it always selects the appropriate modulation on the basis of the worst situation of channel. This method can ensure the reliability of channel during the communication. As the worst time of the transmission is ve

10、ry short, so it will cause huge waste. In order to furthest improve the throughput of the information transmission, in this article, we will use adaptive modulation. The basic theory of adaptive modulation is change the modulation mode in line with the situation of the channel. In the system of adap

11、tive modulation, the users always choose the higher modulation mode in ideal channel, and choose the lower modulation mode in not ideal channel, this method not only can ensure the reliability of channel during the communication, but also canurthest improve the throughput. In the article we use the

12、fixed threshold method as the modulation mode in the system of adaptive modulation. It first sets a reliable BER threshold based on the traffic characteristics in the fixed threshold method, and calculate the SNR value for the appointed BER, then compare every threshold value and choose the correspo

13、nding modulation mode. 2.2 Calculation of judgement threshold The design of adaptive modulation is closely related to the calculation of judgement threshold. In the article we use the criterion of threshold as following: maintain a certain BER and the throughput is variable. The selection of modulat

14、ion mode exerts an essential effect on the system of satellite communication. In the article , we use BPSK_1/2FEC, BPSK., 8PSK, 64PSK_2/5FEC, 64PSK five modulation modes. Usually, SNR is directly related to channel state, in order to analyze easily, it use theapproximate expression between SNR and c

15、hannel, the calculation of BRE for M-ary is given: (1)Fig 1 The curve of BER and SNR under the condition of adaptive modulation The transmission efficiency of the modulation mode above is given: (2)BPSK_1/2FEC(M=2) BPSK(M=2) 8PSK(M=8) 64PSK_2/5FEC(M=64) 64PSK(M=64) Under condition that BER is in the

16、 range of 10-6 to 10-5, it always chooses he adaptive modulation mode of high effciency.According to the formula above, we can deduce the SNR threshold value of adaptive modulation: when SNR is less than 9dB, it only can choose BPSK_1/2FEC modulation mode, as there is 50% redundancy, so the throughp

17、ut will reduce by a half; When SNR is between 9dB and 13dB, the modulation of both BPSK_1/2FEC and BPSK satisfy the need of BER, but there is no reduction in BPSK modulation, so the throughput is higher; When SNR is between 13dB and 23dB, as SNR is higher, so it can choose the better modulation mode

18、, the throughput of 8PSK, 64PSK_2/5FEC and 64PSK is high, but only 8PSK satisfies the need of BER, so it chooses 8PSK, and the throughput is three times higher than the original; When SNR is between 23dB and 28dB, the throughput of both 8PSK and 64PSK_2/5FEC satisfy the need, in order to get the hig

19、her throughput, it chooses 64PSK_2/5FEC, and there is 40% redundancy; When SNR is more than 28dB, all kinds of modulation mode satisfy the need, but the throughput of 64PSK is the highest, so we choose 64PSK.According to the analysis above, we can deduce the selection scheme of adaptive modulation m

20、ode as following:Table1 Scheme of adaptive modulationRange of SNR（dB）Modulation modeSNR 9dBBPSK-1/2FEC9dBSNR13dBBPSK13dBSNR23dB8PSK23dBSNR28dB64PSK3. RAIN ATTENUATION MODELING3.1. Rain area distributionAccording to the ITU standard, the whole world can be divided into A, B, CQ fifteen rain areas, th

21、e figures in the following are the rain area distribution maps of Europe, Asia, and America. The relationship between rainfall intensity and time is shown in table 2 Fig 2 Rain area distribution maps of Europe, Asia, and AmericaTable 2 The relationship between rainfall intensity and time according t

22、o ITU(1)For , the slant path length is (3) For the following formula is used (4) (2)Calculate the horizontal projection LG (5) (3)Obtain the specific attenuation (6) (4)Calculate the horizontal reduction factor, for 0.01% of the time. (7)(5)Calculate the vertical adjustment factor, for the 0.01% of

23、the time For, else If else x=0 (8) (6)The effective path length is (9) (7)The predicated attenuation exceeded for 0.01% of an average year is (10) The rain will not only cause attenuation of signal power, but also will cause increase in noise power. In the clear sky, the temperature is (11) Aclear i

24、s attenuation in the clear sky. ,In the rain sky, the temperature is (12) The raise in the temperature caused by the rain is (13)Where the TLAN is the temperature of the LAN.3.2. CoverageFig 3 The coverage area of the LEO satelliteThe path loss is given by (14) Where d is the distance between the ea

25、rth station and Iridium satellite, and maximum and minimum visibility distances dmax, and dmin is given by (15)Combining the last two equations yields visibility distance d criterion, we have （16）Where rmax is maximum visibility radius and is the maximum earth central angle. Fig 4 The distance betwe

26、en the earth station and Iridium satellite The curves in Figure 4 vary between 780km and 2500km in approximately 2 hour period. It implies that a satellite can be visible again from the same plane in about 2 hours. In each period, there are several peaks and valleys whose values are different, corre

27、sponding to the different satellites coverage. Each handoff happens at different earth-satellite distances and the coverage lasts about 10 minutes. It is because that the orbit plane is shifting all the time, the coverage of each satellite is not simply repeating but slight different.4. OPNET SIMULA

28、TION4.1 Network layer modeling Fig 5 Iridium Network modelIn the simulation, the network is made up of sixty-six Iridium satellites, two earth stations and other mobile terminations, the orbit of Iridium satellites is created by STK and the network topology structure is. star. In the process of simu

29、lation, Iridium satellite nodes fly around the earth along the orbit of Iridium, and the two earth stations communicate with each other through sixty-six Iridium satellites. As the coverage of Iridium is limited, so the earth stations will communicate with each other through several satellites.4.2.

30、Node modeling Node is often considered as equipment or resource, is made up of hardware and software which can provide corresponding handling capacity. In OPNET, there is three kinds of node: the first is fixed node, such as router, switch, workstation, server and so on.; the second is mobile node,

31、such as mobile Station, vehicle and so on; and the third is satellite node which represents satellite. The attribute of every node is different from each other, for example, mobile node can use three-dimensional or two- dimentional mobile orbit, and the satellite can use satellite orbit. Fig 6 Earth station and Iridium subnet node modelFor the wireless channel, the message transmitting procedure between can be described by a series of stages. The fourteen stages in the OPNET can vividly simulate the process of sending and receiving message. 5. CONCLUSIONS SIMULATIONS AND ANALYSISTh