(650536) Prerequisite: Digital Communications (610533) Instructor: Dr. Abdel-Rahman Al-Qawasmi

Transcription

1 Communications & Electronics Engineering Dept. Part 6 Satellite Communications Communication Networks (650536) Prerequisite: Digital Communications (610533) Instructor: Dr. Abdel-Rahman Al-Qawasmi Text Book Wireless Communications & Networking. William Stallings Published by: Pearson Education, 2002

2 Satellite Communications Satellite parameter and configurations: Earth Station: the antenna system on or near the earth. Uplink: transmission from an earth station to satellite. Downlink: Transmission from satellite to earth station. Transponder: the electronics in the satellite that takes an uplink signal and converts it to downlink signal. Categorizing communications satellites: 1. Coverage area: global, regional and national. 2. Service type: a) FSS- fixed service satellite b) BSS- Broadcast service satellite c) MSS- mobile service satellite. 3. General usage: Commercial, military. From textbook: the differences between satellite-based and terrestrial wireless communications Satellite orbits may be classified in number of ways: 1. Circular or elliptical 2. By planes; equatorial orbit, polar orbit. 3. By distance from earth: GEO, MEO and LEO

3 Distance d = Coverage angle Rsin( β ) sin( α ) Minimum elevation angle d 90 0 β R θ h R altitude The round trip transmission delay: 2 h 2( R h)sin( β ) t + c c(cosθ ) Earth radius =6370 km The coverage of satellite=2βr From textbook: the differences between GEO,MEO and LEO Frequency bands: Band Frequency range Total Bandwidth application L GHz 1 GHz MSS C GHz 4 GHz FSS Ku GHz 5.5 GHz FSS + BSS

4 Transmission Impairments The performance of a satellite link depends on three factors: 1. Distance between earth station antenna and satellite antenna. Pt 4πd Ldb = 10 log = 20log = 20log( λ) + 20 log( d ) db P λ r P,P t r- signal power at transmitting and receiving antennas Propagation distance between antennas Carrier wavelength d and λ are in the same unit Example: for GEO at distance 42,711 km: Ldb=-20log(λ) In the case of downlink, terrestrial distance between earth station antenna and the (aim point) of the satellite. 3. Atmospheric attenuation.

5 Capacity Allocation-Frequency Division All of the allocation strategies fall into one of three categories: es: 1. Frequency Division Multiple Access (FDMA) 2. Time Division Multiple Access (TDMA) 3. Code Division Multiple Access. (CDMA). Frequency Division Multiplexing: Over all capacity of communication satellites is divided into a number of channels. Each frequency assignment is used by two carriers with orthogonal polarization. Each channel could be used for one of number of alternative purposes: voice-frequency (VE) voice channels channels of 64 kbps each. 3. One analog video signals. Video signals combined with the audio signal = 6.8 MHz with carrier 6 GHz (FM). f=12.5. and by Carson's Rule B T = 2 F+2B=38.6 MHz.

6 Frequency Division Multiplexing

7 Frequency Division Multiple Access (FDMA) The ability of multiple earth stations to access the same channel is referred to FDMA Number of subchannels provided within a satellite channel via FDMA is limited by three factors: 1. Thermal noise 2. Intermodulation noise 3. Crosstalk. A frequency band can be used if antennas two polarized signals of the same frequency in orthogonal planes are employed. There are two forms of FDMA: 1. Fixed- assignment multiple access (FAMA) 2. Demand-assignment multiple access (DAMA). Fixed-assignment multiple access (FAMA-FDMA): The assignment of capacity within the over satellite channel is distributed in fixed manner among multiple stations. FAMA refers to fact that logical links between stations are preassigned. FDMA refers to the fact that multiple stations are accessing the satellite by using different frequencies. Although an earth station may transmit only one carrier up to the satellite (A with carrier 6.24 GHz with a bandwidth of 5 MHz), it must be able to receive at least one carrier for each location with which it wishes to communicate (A must receive three carriers, parts of transmission of B,D and E. The satellite performs no switching function. Although it is receiving portions of the 36 MHz channel from various sources, it simply accepts signals across the spectrum, translates them to the 4-GHz band and retransmission them. Considerable bandwidth used. A has 60 VF channels to transmit, which occupy only 240 khz (VF= 4khz)

8 FAMA-FDMA

9 DAMA-FDMA Single channel per carrier (SCPC): a technique that we use to divide the 36-bandwidth into individual VF channels (C-band). SCPC is attractive for remote areas where there are few user stations near each site. SCPC provides direct end-user services. SCPC is more bandwidth efficient than FDMA but it does suffer from the inefficiency of fixed assignment. To achieve more efficiency we use DAMA. With DAMA we can establish a full-duplex link between two earth stations, a pair of subchannels is dynamically assigned on demand. The first commercially available DAMA SCPC system was SPADE (single channel per carrier, pulse code modulation, multiple access, demand-assignment equipment), introduced in the early 1970s (INTELSAT). Demand assignment is performed in a distributed fashion, by the earth station, using 160-kHz common-signaling channel (CSC). Because only 49 stations can participate using CSC, most of the subchannels are not required for demand for demand assignment.

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11 Capacity Allocation Time Division TDM techniques are in increasingly widespread use because: 1. The continuing drop in in the cost of digital components. 2. The advantages of digital techniques, including the error correction. 3. The increased efficiency of TDM due to the lack of Intermodulation noise. TDMA Frame Frame Period Station 1 Station 1 Station 1.. N-1 N Carrier and bit time recovery Coordination data Guard Time Preamble Information Carrier and bit time recovery Unique word Station identification Network control request

12 Frame Format Frame begins with two reference bursts to define the beginning of the frame. Each reference burst begins with a carrier and bit timing recovery pattern, which is a unique pattern that allows all stations to synchronize to a master clock. The preamble contains control and timing information, plus the identification of the destination station. FAMA-TDMA: Individual earth stations take turns using the uplink channel and may put a burst of data in the assigned time slot. All stations must know not only which time slot to use for transmission, but which time slot to use for reception. The satellite also repeat the reference bursts and all stations synchronize the reception of the that burst. Comparison: Ordinary TDMA is more efficient than ordinary FDMA because the guard g times and control bits of TDMA utilize less capacity than the guard bands of FDMA. TDMA drops much more slowly as the number of time slots (channels) increases. The use of along frame time increase efficiency. An SCPC provides a constant capacity.

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14 SS-TDMA: The satellite can service a number of areas, each containing a number of earth stations. Communication among stations in different areas can be achieved if the satellite has the ability to switch time slots from one beam to another is known as satellite switched TDMA (SS-TDMA). Only one station can transmit at the same time within the same e area. The SS-TDMA is to some extent a DAMA system if the mode pattern can be changed by ground command.