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Satellite-Related Terms Earth Stations ( 地面站 )– 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 （转发器） – electronics in the
satellite that convert uplink signals to downlink signals
Ways to CategorizeCommunications Satellites Coverage area
Global, regional, national Service type
Fixed service satellite (FSS) Broadcast service satellite (BSS) Mobile service satellite (MSS)
指移动或便携电话使用的卫星通信网络 航空、陆地、海事
General usage Commercial, military, amateur, experimental
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 ( 赤道轨道 ) Polar orbit passes over both poles ( 极地轨道 ) Other orbits referred to as inclined orbits ( 倾斜轨道）
Altitude of satellites Geostationary orbit (GEO) ( 地球同步轨道 ) Medium earth orbit (MEO) ( 中地轨道 ) Low earth orbit (LEO) ( 低地轨道 )
Geometry Terms Elevation angle （仰角） - 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
Minimum elevation angle （最小仰角） Coverage angle （覆盖角） - the measure
of the portion of the earth's surface visible to the satellite
：仰角 ：覆盖角 H ：卫星距地高度
覆盖范围与仰角关系
卫星
地球
覆盖边缘
Minimum Elevation Angle（最小仰角） 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
GEO Orbit （地球同步轨道） Advantages of the the GEO orbit
No problem with frequency changes （多普勒效应） 因为卫星相对地球是固定的
Tracking of the satellite is simplified High coverage area
距地 35863 公里，卫星能和大约 1/4 地球通信 三个同步卫星可以覆盖除南北极附近区域的大部分有人居住区
Disadvantages of the GEO orbit Weak signal after traveling over 35,000 km Polar regions （极地和南北半球的高纬度地区） are poorly served Signal sending delay is substantial
同步卫星正下方两个位置之间的传播时延为 (235863/3000000.24 秒 对于非正下方的其他位置，时延会更大 对于采用（同步）卫星电话通信的双方，一个人说话到另一个人反应的
附加时延是 0.5 秒 另外一个特征：同步卫星在很大的范围内使用所分配到的频率
有益于电视广播业务 对于点到点通信来说则是一种浪费
LEO Satellite Characteristics Circular/slightly elliptical orbit under 2000 km Orbit period ( 轨道周期 ) ranges from 1.5 to 2 hours Diameter of coverage （瞬时地面覆盖直径——仰角 10
度） is about 6000 km Round-trip signal propagation delay ： several
milliseconds Maximum satellite visible time up to 20 min System must cope with large Doppler shifts Atmospheric drag ( 大气阻力） results in orbital
deterioration
LEO Satellite实际应用需要多个轨道平面，每个轨道上多颗卫星每次通信通常涉及到多颗卫星
LEO Categories Little LEOs
Frequencies below 1 GHz 5MHz of bandwidth Subscriber data rates up to 10 kbps Aimed at paging, tracking, and low-rate messaging E.g., Orbcomm 公司 :
1995 年发射了ＬＥＯ卫星２颗， 现拥有卫星３０颗 用户预约速率 2.4—4.8kbps. 可以跟踪汽车、火车车厢、重型设备及其他移动资产或油井、管道，也
可以用于联系世界范围的工作人员 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 全球星系统：现有４８颗星，８颗备用、轨道高度１４１３ km
全球星系统是和传统的话音承载网络相结合
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
Frequency Bands Available for Satellite Communications
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 从地球站到卫星的仰角
仰角越小，衰减越大
Satellite Network Configurations
Capacity Allocation Strategies Frequency division multiple access
(FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)
Frequency-Division Multiplexing 如当卫星采用Ｃ频段的 500MHz 频宽时，可以通过频
分的方式划成２４个 40MHz 的信道 , 其中保护带４MHz, 信道实际宽度 36MHz
Alternative uses of channels in point-to-point configuration (Fig. 9.8(a))
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
Frequency-Division Multiple Access
Factors which limit the number of subchannels provided within a satellite channel via FDMA Thermal noise Intermodulation noise
如果信号太弱，热噪声会淹没有用信号，而 如果信号太强，放大器的非线性则产生高互调噪声
Crosstalk
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
FAMA-FDMA FAMA – logical links between stations are
preassigned FAMA – multiple stations access the
satellite by using different frequency bands Uses considerable bandwidth
DAMA-FDMA Single channel per carrier (SCPC) – bandwidth
divided into individual VF channels Attractive for remote areas with few user stations near
each site Suffers from inefficiency of fixed assignment
DAMA – set of subchannels in a channel is treated as a pool of available links For full-duplex between two earth stations, a pair of
subchannels is dynamically assigned on demand Demand assignment performed in a distributed fashion
by earth station using CSC （公用信令信道）
Reasons for Increasing Use of TDM Techniques Cost of digital components continues to
drop Advantages of digital components
Use of error correction Increased efficiency of TDM
Lack of intermodulation noise
FAMA-TDMA Operation Transmission in the form of repetitive sequence of
frames Each frame is divided into a number of time slots Each slot is dedicated to a particular transmitter
Earth stations take turns using uplink channel Sends data in assigned time slot
Satellite repeats incoming transmissions Broadcast to all stations
Stations must know which slot to use for transmission and which to use for reception
FAMA-TDMA Uplinktim
e
FAMA-TDMA Downlinktim
e
tim
e
勘误 P.193, 图 9.1 左面， (3)…. 电噪声对吸收
起副作用 =〉… .电噪声对信号接收起副作用
P.196, 第 12行，传输时延 =〉 传播时延 P.204, 图 9.11(a) 中，“A 站发射的波长” =〉“A 站发射的频谱”
P.204, 第 6 行， 60 个信道 =〉 60 个 VF信道。这里 VF指音频 (Voice Frequency)
P.204, 倒数第一行，分别为发射到 B、 D 、E => 分别为 B、 D 、 E的发射的一部分
