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Chapter 7 Wireless Widearea Networks (Part Three in textbook)

Chapter 7 Wireless Widearea Networks (Part Three in textbook). Outline. 7.1 Satellite Communications 7.2 Cellular Wireless Networks 7.3 Cordless Systems and Wireless Local Loop. 7.1 Satellite Communications. 卫星通信的基本概念. 什么叫卫星通信 什么叫静止卫星和静止卫星通信系统 卫星覆盖区 实现全球通信的条件 衡量卫星通信系统性能的主要技术指标

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Chapter 7 Wireless Widearea Networks (Part Three in textbook)

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  1. Chapter 7 Wireless Widearea Networks(Part Three in textbook)

  2. Outline • 7.1 Satellite Communications • 7.2 Cellular Wireless Networks • 7.3 Cordless Systems and Wireless Local Loop

  3. 7.1 Satellite Communications

  4. 卫星通信的基本概念 • 什么叫卫星通信 • 什么叫静止卫星和静止卫星通信系统 • 卫星覆盖区 • 实现全球通信的条件 • 衡量卫星通信系统性能的主要技术指标 • 卫星通信的特点

  5. 卫星通信——利用人造地球卫星作为中继站转发无线电信号, 在两个或多个地球站之间进行的通信。

  6. 什么叫静止卫星？ • 一、卫星的种类 • 二、什么叫静止卫星？

  7. 卫星的种类——1 • 按卫星的结构划分 • 无源卫星 • 有源卫星 卫星内部是否含有有源器件

  8. 卫星的种类——2 • 按卫星的轨道划分 • 1）按卫星轨道的形状划分 • 圆形轨道卫星 • 椭圆形轨道卫星 • 2）按卫星距地球表面的高度划分 • 低高度：H<5,000Km ; h< 4小时 • 中高度：5,000Km< H <20,000Km; 4小时 < h < 12 小时 • 高高度：H> 20,000Km ; h > 12 小时 • 注：H：表示高度，h：旋转一圈所需时间

  9. 圆形轨道卫星

  10. 椭圆形轨道卫星

  11. 卫星的种类——2续 • 3）按卫星轨道平面与地球赤道平面的夹角 • θ = 0° ，赤道轨道卫星 ； • θ = 90° ，极轨道卫星; • 0°< θ < 90°,倾斜轨道卫星 注： θ表示夹角

  12. 卫星轨道平面与地球赤道平面的夹角示意图 • θ = 0° ，赤道轨道卫星 地球赤道 赤道轨道

  13. 卫星轨道平面与地球赤道平面的夹角示意图 • θ =90°极轨道卫星; 地球赤道 卫星轨道

  14. 卫星轨道平面与地球赤道平面的夹角示意图 • 0°< θ < 90°,倾斜轨道卫星 地球赤道 倾斜轨道

  15. 卫星轨道平面与地球赤道平面的夹角示意图 θ=90° 极轨道 0<θ<90°倾斜轨道 θ=0赤 道轨道

  16. 卫星的种类——2续 • 按卫星的轨道划分（续） • 按相对于地面观察点的位置划分 • 运动轨道卫星 • 同步轨道卫星 • 静止轨道卫星

  17. 什么叫静止卫星？ 卫星在地球赤道上空，距地面 35,786公里的圆形轨道上绕地球旋转，卫星轨道平面与地球赤道平面的夹角为 0°，其绕地球旋转一周的时间和地球自转一周所需时间相同为24小时，并且其围绕地球旋转的方向和地球自转的方向相同，不论在地球的什么地方观察卫星，卫星始终是相对静止不动的我们把这种卫星称为静止卫星。

  18. 什么叫同步卫星？ 同步卫星：其公转与轨道中央星自转的周期与方向均相同的卫星。 静止卫星是同步卫星的一个特例。

  19. 静止卫星 35786KM AB间弧长18100KM B 地球 卫星覆盖区 一、定义： 卫星在地球表面的投影 二、静止卫星覆盖区： 覆盖面积超过地球表面总面积的三分之一 A

  20. P1 A B P C P2 P3 实现全球通信的条件 在静止卫星轨道上等间距地放置三颗静止卫星,在卫星覆盖区的重叠部分建立转发站 ，则经过一次跳变，即通过一颗卫星或经过二次跳变和转发站便可实现二个地球站间的通信联络 ，并可基本实现全球通信 GMDSS原理与操作

  21. 衡量卫星通信系统性能的主要技术指标 • 有效全向辐射功率（EIRP） • 用来衡量收发器发射分系统性能的 • EIRP越大，则表明发射分系统的性能越好。 • 符号: EIRP • 单位： dBW 或 dBm； 1dBW = 30 dBm • C 船站EIRP: 12 dBW < EIRP<16 dBW

  22. 衡量卫星通信系统性能的主要技术指标 • 增益噪声温度比（G/T） • 增益噪声温度比又叫优值比 • 常用来衡量接收分系统性能的好坏 • 增益噪声温度比越大，则表明接收分系统的性能越好，接收微弱信号的能力越强。 • 符号: G/T • 单位是 db/°k C 船站 G/T >= -23db/°k

  23. 卫星通信的特点 • 优点 : • 覆盖面积大、通信距离远、灵活机动并可基本实现全球通信。 • 频带宽、通信容量大。 • 抗干扰能力强，通信质量高。 • 卫星通信系统是实时、全天候通信系统。 • 功效高。

  24. 缺点： • 技术难度大，投资多，费用高。 • 卫星通信有较大的信号延迟和回声干扰。 返回

  25. Satellite-Related Terms • Earth Stations – antenna systems on or near earth • Uplink – transmission from an earth station to a satellite • Downlink – transmission from a satellite to an earth station • Transponder – electronics in the satellite that convert uplink signals to downlink signals

  26. Ways to CategorizeCommunications Satellites • Coverage area • Global, regional, national • Service type • Fixed service satellite (FSS) • Broadcast service satellite (BSS) • Mobile service satellite (MSS) • General usage • Commercial, military, amateur, experimental

  27. Classification of Satellite Orbits • Circular or elliptical orbit • Circular with center at earth’s center • Elliptical with one foci at earth’s center • Orbit around earth in different planes • Equatorial orbit above earth’s equator • Polar orbit passes over both poles • Other orbits referred to as inclined orbits • Altitude of satellites • Geostationary orbit (GEO) • Medium earth orbit (MEO) • Low earth orbit (LEO)

  28. 寻星所需设备 • 卫星天线、高频头（馈源一体化）、卫星接收机、电视机、指南针、量角器以及连接线若干。 • 馈源的主要功能是将天线收集的信号聚集送给高频头（LNB）。

  29. 寻星所需参数 • 对于固定式天线系统，需要根据天线所在地的经纬度及所要接收卫星的经度计算出天线的方位角和仰角，并以此角度调整天线使其对准相应的卫星。 对于极化的卫星信号，还需调整高频头的极化角，圆极化信号则不必。

  30. Geometry Terms • Elevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite从接收点仰望卫星的视线与水平线构成的夹角就是仰角。 • Minimum elevation angle • Coverage angle - the measure of the portion of the earth's surface visible to the satellite

  31. Elevation angle

  32. Minimum Elevation Angle • Reasons affecting minimum elevation angle of earth station’s antenna (>0o) • Buildings, trees, and other terrestrial objects block the line of sight • Atmospheric attenuation is greater at low elevation angles • Electrical noise generated by the earth's heat near its surface adversely affects reception

  33. 方位角(azimuth) • 从接收点到卫星的视线在接收点的水平面上有一条正投影线，从接收点的正北方向开始，顺时针方向至这条正投影线的角度就是方位角。

  34. GEO Orbit • Advantages of the the GEO orbit • No problem with frequency changes • Tracking of the satellite is simplified • High coverage area • Disadvantages of the GEO orbit • Weak signal after traveling over 35,000 km • Polar regions are poorly served • Signal sending delay is substantial

  35. LEO Satellite Characteristics • Circular/slightly elliptical orbit under 2000 km • Orbit period ranges from 1.5 to 2 hours • Diameter of coverage is about 8000 km • Round-trip signal propagation delay less than 20 ms • Maximum satellite visible time up to 20 min • System must cope with large Doppler shifts • Atmospheric drag results in orbital deterioration

  36. LEO Categories • Little LEOs • Frequencies below 1 GHz • 5MHz of bandwidth • Data rates up to 10 kbps • Aimed at paging, tracking, and low-rate messaging • Big LEOs • Frequencies above 1 GHz • Support data rates up to a few megabits per sec • Offer same services as little LEOs in addition to voice and positioning services

  37. MEO Satellite Characteristics • Circular orbit at an altitude in the range of 5000 to 12,000 km • Orbit period of 6 hours • Diameter of coverage is 10,000 to 15,000 km • Round trip signal propagation delay less than 50 ms • Maximum satellite visible time is a few hours

  38. Frequency Bands Available for Satellite Communications

  39. Satellite Link Performance Factors • Distance between earth station antenna and satellite antenna • For downlink, terrestrial distance between earth station antenna and “aim point” of satellite • Displayed as a satellite footprint (Figure 9.6) • Atmospheric attenuation • Affected by oxygen, water, angle of elevation, and higher frequencies

  40. Satellite Footprint

  41. Satellite Network Configurations

  42. Capacity Allocation Strategies • Frequency division multiple access (FDMA) • Time division multiple access (TDMA) • Code division multiple access (CDMA)

  43. Frequency-Division Multiplexing • Alternative uses of channels in point-to-point configuration • 1200 voice-frequency (VF) voice channels • One 50-Mbps data stream • 16 channels of 1.544 Mbps each • 400 channels of 64 kbps each • 600 channels of 40 kbps each • One analog video signal • Six to nine digital video signals

  44. Frequency-Division Multiple Access • Factors which limit the number of subchannels provided within a satellite channel via FDMA • Thermal noise • Intermodulation noise • Crosstalk

  45. Forms of FDMA • Fixed-assignment multiple access (FAMA) • The assignment of capacity is distributed in a fixed manner among multiple stations • Demand may fluctuate • Results in the significant underuse of capacity • Demand-assignment multiple access (DAMA) • Capacity assignment is changed as needed to respond optimally to demand changes among the multiple stations

  46. FAMA-FDMA • FAMA – logical links between stations are preassigned • FAMA – multiple stations access the satellite by using different frequency bands • Uses considerable bandwidth

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