Satellite Communications System Capacity Allocation Multiplexing Transponders Applications Maria Leonora Guico Tcom 126 Lecture 13
Capacity Allocation Strategies Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)
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
Frequency-Division Multiple Access Factors which limit the number of subchannels provided within a satellite channel via FDMA Thermal noise Intermodulation noise 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
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
Satellite Satellite payload are several repeaters, known as transponders mounted on platform called bus containing solar cells for power, telemetry and control systems, and small jets to slightly adjust orbit (station-keeping)
Transponder Types Signals are received at one freq, amplified and moved to another frequency for retransmission Transponder is basically a linear amplifier and frequency translator Usable with analog and digital signals) Transponder Block Diagram (C-band)
Regenerative Repeater Used for purely digital signals Demodulate signal to baseband, decode the digital information, then remodulate the signal on a carrier with a different frequency Allows the satellite repeater to regenerate the signal while removing accumulated noise and distortion
Frequency Allocation
Earth station Tel Data Video Baseband in FDM or PCM/TDM Modulator (FM, PSK or QAM) Mixer Generator Band pass filter (BPF) High Power Amplifier To (HPA) Satellite transponder From satellite transponder Up-Converter AN EARTH STATION TRANSMITTER Low noise Amplifier (LNA) Mixer Generator Band pass filter (BPF) Demodulator (FM, PSK or QAM Baseband out (FDM or PCM/TDM) Data Video Tel Down-Converter AN EARTH STATION RECEIVER
Earth station transmitter Tel Data Video Baseband in FDM or PCM/TDM Modulator (FM, PSK or QAM) Mixer Generator Band pass filter (BPF) High Power Amplifier (HPA) To Satellite transponder Up-Converter AN EARTH STATION TRANSMITTER - Intermediate freq (IF) modulator converts the input baseband signals to either an FM, a PSK or a QAM modulated intermediate frequency. - The up converter converts the IF to an appropriate RF carrier freq. - The High Power Amplifier (HPA) provides the adequate input sensitivity and output power to propagate the signal to the satellite transponder.
Earth station receiver From satellite transponder AN EARTH STATION RECEIVER Low noise Amplifier (LNA) Mixer Generator Band pass filter (BPF) Demodulator (FM, PSK or QAM Baseband out (FDM or PCM/TDM) Data Video Tel Down-Converter - LNA which is highly sensitive and low-noise device amplifiers the received signal. - The RF to IF down-converter is a mixer and bans pass filter combination, which converts the received RF signal to an intermediate frequency (IF)
Transmission Modes Beam switching output from a transponder can be switched to any one of several antennas (applicable to digital signals, can use TDM) Cross-links (operate at 58-62 GHz range) Transmit signals directly between satellites Allow signals to be transmitted to areas outside the coverage area of one satellite, w/o using an extra earth station in between
Application of satellite communication Some of the application s of satellite communications are: Digital audio broadcasting Television distribution Serving remote areas Point-to-multipoint communications Remote monitoring and control Vehicle tracking Mobile communications Maritime and air navigation Video teleconferencing
Applications of Geostationary Satellites Television and radio broadcasting Telephony Data transmission
Television and Radio Broadcasting Carry network program feeds Transmit pay-tv and other cable-only channels to CATV company head-ends Transmit remote news feeds Radio broadcasts are carried, usually as subcarriers on the same transponders Typically uses analog FM modulation Direct-to-home TV system transmits digital video ( 3 to 7.5 Mb/s data transmission rate)
Telephony via Satellite Not ideal because of time delay Used for remote areas where it s too costly to install cable or microwave relays Important for ships at sea and interoceanic communications where connection is via Inmarsat satellites Uses FDMA-FM and SCPC
Data via Satellite Large data users can lease a transponder or a part of a transponder for a point-to-point or point-to-multipoint link Data network with a central hub communicating with many remote sites Remote sites use very small aperture satellites (VSATs) which are low-cost installations with small antennas (1.2 to 2.4 m) and lowpower transmitters Hub uses relatively expensive earth station with large antenna (5 to 7 m) and powerful transmitter Typical applications are inventory control, point-of-sale, and banking
Mobile Telephone Systems Using Geostationary Satellites Global coverage achieved with only 3 GEO satellites INMARSAT (International Maritime Satellite Organization) Original mandate was to provide voice and data service to ships at sea; services have expanded to include land and aeronautical mobile communication Refer to http://www.alphatelecom.ru/inmarsat/engindex.htm for the different Inmarsat standards and corresponding services
INMARSAT Total of eleven GEO satellites controlled from the HQ in London via ground stations located around the globe. Each satellite has: hemispherical beam and spot beams Maximum EIRP of 48 dbw in spot beams Power and BW dynamically allocated L-band (1.5/1.6 GHz)
INMARSAT/2 Inmarsat mini-m (operate on land and coastal waters) has typical portable transceiver, including antenna the size of a laptop computer
MSAT (Mobile Satellite) Joint Canadian and US project that uses one GEO satellite to provide coverage for North and Central America, the Caribbean and Hawaii (via spot beam), and surrounding coastal waters About 10 times more powerful than those used by Inmarsat EIRP of at least 57.3 dbw in coverage area Mobile terminals use reasonably compact roof-mounted antenna; portable terminals about the size of notebook computer with lidmounted antenna Only one ground station with an 11 m dish antenna
LEO Satellites Preferred when satellites are used with portable or mobile equipment (lower transmitter power and antenna gain) Requires many satellites (40 to 70) and a complex network Main problems with LEOs are: 1. Position in space is not fixed wrt ground station 2. Tendency to disappear below the horizon use a constellation of satellites to address problem 3. Doppler effect (transmitted freqs are shifted higher as satellite approaches a point on the ground and lower as the satellite recedes) requires careful receiver design
LEO Satellites/2 Most complex and expensive wireless communication systems One system (Iridium) went bankrupt after going into service As of April 2001, two LEO wireless telephony systems (Iridium and Globalstar) were operating; both were in financial difficulty
Iridium 66 Iridium LEO satellites provide 100% Global Coverage except Hungary, Poland, N. Korea and N. Sri Lanka Satellites are cross-linked Uses digital modulation, FDMA/TDMA
Globalstar Globalstar constellation consists of 48 LEO satellites, with an additional four satellites in orbit as spares, and operates at an altitude of 876 miles (1414 km) in space CDMA; utilizes soft handoff techniques Designed to provide high quality satellite-based services to a broad range of users, including: Voice calling Short Messaging Service (SMS) Roaming Positioning Facsimile Data transmission http://www.alphatelecom.ru/globalstar/engindex.htm
Little LEOs ORBCOMM Operational in 1988; 35 LEO satellites as of Feb 2001; used for SMS, email, and vehicle tracking LEO One Designed to use 48 satellites for paging and SMS E-Sat Designed to use 6 satellites using CDMA for remote meter reading
MEOs More satellites are needed than for GEO (on the order of 6 to 20 for real time communication) but fewer than for LEO Delay and propagation loss lesser than for GEO Main advantage over LEOs is financial
ICO One MEO satellite launched in June 2001, referred to as "F2," which currently provides data gathering services for an agency of the U.S. government Next-generation mobile satellite services (MSS) operator Building a hybrid system to offer satellite and terrestrial wireless services throughout the US ICO s network is being designed to provide wireless voice, data, video, and/or Internet service throughout the United States on mobile and portable devices.
Ellipso Uses an interesting combination of elliptical and circular orbits. Designed to initially include 6 satellites, later increasing to 10, in a circular orbit about 8000 km above the equator for worldwide coverage and to be complemented by 8 active satellites (plus two spares) in inclined elliptical orbits ( 520-7800 km) CDMA technology; voice communication using portable and mobile terminals
Advantages/disadvantages of satellite system Advantages of a satellite system include: It can access to wide geographical area Wide bandwidth High reliability Distance sensitive cost Independent of terrestrial infrastructure Disadvantages of satellite system High initial cost It has propagation delay
System performance HPA Uplink Gt Po Pt Lf Earth station transmitter Gr Lp Transponder Gain, Gsat Gt Lp Downlink Gr Pr Pin LNA Earth station receiver HPA high power amplifier Po - HPA output power Lf - feeder loss Gt - transmit antenna gain Lp - path loss Gr - receive antenna gain LNA low noise amplifier Pt - total radiated power, Pt = Po - Lf EIRP - Effective Isotropic Radiated Power EIRP = Pt * Gt
Link-Power Budget Formula Link-power budget calculations take into account all the gains and losses from the transmitter, through the medium to the receiver in a telecommunication system. Also taken into the account are the attenuation of the transmitted signal due to propagation and the loss or gain due to the antenna. The decibel equation for the received power is: [P R ] = [EIRP] + [G R ] - [LOSSES] Where: [P R ] = received power in dbw [EIRP] = equivalent isotropic radiated power in dbw [G R ] = receiver antenna gain in db [LOSSES] = total link loss in db
Link-Power Budget Formula Variables Link-Power Budget Formula for the received power [P R ]: [P R ] = [EIRP] + [G R ] - [LOSSES] The equivalent isotropic radiated power [EIRP] is: [EIRP] = [P S ] + [G] dbw, where: [P S ] is the transmit power in dbw and [G] is the transmitting antenna gain in db. [G R ] is the receiver antenna gain in db. [LOSSES] = [FSL] + [RFL] + [AML] + [AA] + [PL], where: [FSL] = free-space spreading loss in db = P T /P R (in watts) [RFL] = receiver feeder loss in db [AML] = antenna misalignment loss in db [AA] = atmospheric absorption loss in db [PL] = polarisation mismatch loss in db The major source of loss in any ground-satellite link is the free-space spreading loss.