Satellite Communications Chapter 9

MicroWave frequencies

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

Classification of Satellite Orbits Circular or elliptical orbit Circular with center at earth’s center Elliptical with one foci at earth’s center

Classification of Satellite Orbits 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

Classification of Satellite Orbits Altitude of satellites Geostationary orbit (GEO) Medium earth orbit (MEO) Low earth orbit (LEO)

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 Minimum elevation angle To obtain maximum satellite coverage, we would like to use an elevation angle of 0 , which would enable the satellite's coverage to extend to the optical horizon from the satellite in all directions Coverage angle - the measure of the portion of the earth's surface visible to the satellite

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=> attenuation by absorption or mutlipath Atmospheric attenuation is greater at low elevation angles => the signal traverses the atmosphere for longer distances the smaller the elevation angle

GEO Orbit circular orbit 35,863 km above the earth's surface rotates in the equatorial plane of the earth, rotate at exactly the same angular speed as the earth =>will remain above the same spot on the equator as the earth rotates

GEO Orbit Advantages of the GEO orbit Disadvantages of the GEO orbit No problem with frequency changes (no doppler shift) Tracking of the satellite is simplified by the earth stations High coverage area, roughly a fourth of the earth; three satellites in geostationary orbit separated by 120 degree cover most of the inhabited portions of the entire earth excluding only the areas near the north and south poles. Disadvantages of the GEO orbit Weak signal after traveling over 35,000 km Polar regions are poorly served by geostationary satellites Signal sending delay is substantial

GEO Orbit Speed of light = 3 10^8 m/s Propagation delay?? Round Trip Delay?

Doppler effect the change in frequency of a wave for an observer moving relative to its source. It is commonly heard when a vehicle sounding a siren approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession. (F*Vij)/c

LEO Satellite Characteristics Circular/slightly elliptical orbit under 2000 km Orbit period ranges from 1.5 to 2 hours Diameter of coverage is about 6000 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 Because the motion of the satellite relative to a fixed point on earth is high To provide broad coverage over 24 hours, many satellite are needed

LEO Categories Little LEOs Big LEOs Frequencies below 1 GHz 5MHz of bandwidth Data rates up to 10 kbps Aimed at paging, tracking, and low-rate messaging Orbcomm is an example of such satellite system, using over 30 satellites 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 Globalstar is an example, it uses CDMA, 48 operating satellite

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 12,000 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

Frequency Bands Available for Satellite Communications For any given frequency allocation for a service, there is an allocation of an uplink band and a downlink band, the uplink band always of higher frequency. The higher frequency suffers greater spreading, or free space loss, than its lower frequency counterpart. The earth station is capable of higher power, which helps to compensate for the poorer performance at higher frequency.

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

Satellite Footprint At microwave frequencies, which are used in satellite communications, highly directional antennas are used. Thus, the signal from a satellite is not isotropically broadcast but is aimed at a specific point on the earth, depending on which area of coverage is desired. The center point of that area will receive the highest radiated power, and the power drops off as you move away from the center point in any direction. This effect is typically displayed in a pattern known as a satellite footprint; an example is shown in Figure 9.6. The satellite footprint displays the effective radiated power of the antenna at each point, taking into account the signal power fed into the antenna and the directionality of the antenna.

Satellite Footprint

Satellite Network Configurations

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

Frequency-Division Multiplexing The overall capacity of a communications satellite is divided into a number of channels. 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

Frequency-Division Multiplexing

Frequency-Division Multiplexing Example of an FDM scheme, which is typical of GEO communications satellites; this particular allocation is used in the Galaxy satellites from PanAmSat. PanAmSat is the largest satellite operator in the world. It is a private corporation providing satellite communications capacity worldwide. The satellite uses C band frequencies (4-8GHz) Provides a 500-MHz bandwidth, which is broken up into 24 40-MHz channels. Each 40-MHz channel includes a 4-MHz guardband, so each channel is actually 36 MHz wide.

Frequency-Division Multiple Access Each satellite channel is divided into a number of smaller channels (sub-channels) The ability of multiple earth stations to access the same channel is referred to FDMA Frequency-division multiplexing (FDM) is also distinct from FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on the other hand, is an access method in the data link layer.

FDMA Factors which limit the number of subchannels provided within a satellite channel via FDMA Thermal noise Intermodulation noise Crosstalk

Noise Terminology Intermodulation noise – occurs if signals with different frequencies share the same medium Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies the mixing of signals at frequencies f1and f2 might produce energy at the frequency f1+f2. This derived signal could interfere with an intended signal at the frequency f1+f2. Crosstalk – unwanted coupling between signal paths experienced by anyone who, while using the telephone, has been able to hear another conversation

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 pre-assigned FDMA – multiple stations access the satellite by using different frequency bands Uses considerable bandwidth

FAMA-FDMA Figure 9.11 is a specific example of FAMA-FDMA, with seven earth stations sharing the 36-MHz uplink capacity; a similar downlink diagram can be drawn. Station A is assigned the 5-MHz bandwidth from 6237.5 to 6242.5 MHz, in which it can transmit 60 VF channels using FDM-FM. That is, FDM is used to carry the 60 channels, and FM is used to modulate the channels onto the carrier frequency of 6240 MHz. A has traffic for other stations as follows: 24 channels to B, 24 channels to D, and 12 channels to E. The remaining spectrum of the 36-MHz channel is divided among the other earth stations according to their traffic needs.

FAMA-FDMA Considerable bandwidth is used. For example, station A has 60 VF channels to transmit, which occupy only 240 kHz (one VF channel = 4 kHz). Yet the satellite bandwidth allocation is 5 MHz. This is due to the use of FM (rather than AM) to maintain signal over the long distance of the satellite link and to minimize satellite power requirements.

FAMA-FDMA The FAMA-FDMA scheme just described is not efficient. Typically, in the C band, each channel has a usable bandwidth of 36 MHz. One INTELSAT FDMA scheme divides this into 7 5-MHz blocks, each of which carries a group of 60 VF channels, for a total of 420 channels. When the bandwidth is divided into 14 2.5-MHz sub-channels, two groups of 48 VF channels can be carried in each channel for a total of 336 channels. It turns out to be more efficient to avoid groupings altogether and simply to divide the 36-MHz bandwidth into individual VF channels. This technique is known as single channel per carrier (SCPC). Single channel per carrier (SCPC) – bandwidth divided into individual VF channels Attractive for remote areas with few user stations near each site=> it provides direct end use service. Suffers from inefficiency of fixed assignment

DAMA-FDMA 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 Common Signaling Channel CSC

DAMA-FDMA Example: The first commercially available DAMA SCPC system was SPADE (single channel per carrier, pulse code modulation, multiple-access, demand-assignment equipment), currently available on INTELSAT satellites. Each subchannel carries a 64-kbps QPSK signal, which occupies 38 kHz, plus a 7-kHz guardband. Typically, the signal is used to carry PCM voice traffic. A total of 794 subchannels are available (Figure 9.12a). These subchannels are paired such that two channels 18.045 MHz apart are always used to form a full-duplex circuit (e.g., 3 and 404, 4 and 405, 399 and 800). In addition, there is a 160-kHz common-signaling channel (CSC) that carries a 128-kbps PSK signal.

DAMA-FDMA

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 Uplink

FAMA-TDMA Downlink

Reference burst : to identify the beginning of the frame for synchronization purposes

Appendix

Ways to Categorize Communications 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

Frequency-Division Multiplexing Vertical polarization is when the electric field is vertical.   When people talk about polarization they are referring to the electric field vector. References to 'vertical' and 'horizontal' are frequent since terrestrial antennas are normally vertical or horizontal relative to the ground. Example 1:   Vertical polarization is produced by a vertical dipole antenna.  At an instant the top is positive voltage and the bottom is negative voltage - so the electric field produced is vertical.