Earth Station

Earth stations come in varying sizes and configurations to satisfy the requirements Telephone or telecommunication gateway, allowing remote user terminals or other gateways to gain access to public or private terrestrial networks including the PSTN and the Internet; Broadcasting uplink to originate video programming, audio (radio) programming, and data and other noninteractive forms of information;

Earth station design depends on the following parameters i) Transmitter power ii) Choice of frequency iii) Gain of antenna iv) Antenna efficiency v) Antenna pointing accuracy vi) Noise temperature vii) Local conditions such as wind, weather etc, viii) Polarization ix) Propagation losses

most essential and sensitive aspect is the RF terminal (RFT), which provides the critical direct interface to the space link. RFT radiates in the assigned frequency band, under control or direction of the satellite or ground segment operator Connection to baseband equipment is at a standard intermediate frequency (IF), to simplify connecting different types of transmission equipment

IF center frequency itself is determined by the RF bandwidth of the carrier on the link to the satellite. The actual RF frequency is established by the up converter and down converter, for transmit and receive

Typical IFs include: · 70 MHz, supporting a usable bandwidth of 36 MHz (the typical transponder bandwidth at C-band) up to a maximum of 54 MHz (one of a number of bandwidths used at Ku-band); · 140 MHz, supporting wider IF bandwidths, such as 110 MHz (the upper end of what has been used on some Ku-band satellites);

Transmitter chain consist : multiplexer, the modulator, the upconvertor, a high power amplifier, and the antenna.

Multiplexer combines the individual channels onto a single data stream. The information can be encrypted and encoded with a forward error correction code.

Why FEC? satellite links, the propagation delay is very long compared to the transmission time of a single frame. The result is a very inefficient system if we have error detection code.

It is desirable to be able to correct errors without requiring retransmission. Using the bits that were transmitted.

Forward Error Correction Code Forward Error Correction (FEC) Simplex connection, error correction codes The receiver tries to correct some errors The two main categories of FEC codes are block codes and convolutional codes. Block codes work on fixed-size blocks (packets) of bits or symbols of predetermined size. Convolutional codes work on bit or symbol streams of arbitrary length

Modulation Modulator modulates the baseband signal containing the desired information onto an intermediate frequency (IF) carrier, usually at 70 MHz. A common form of modulation used in digital satellite communication is M-ary phase shift keying

In this technique, the carrier can assume one of M phase states,each of which represents a symbol. In binary phase shift keying (BPSK), there are two phase states, 0° and 180°, representing a binary one or zero. In quaternary phase shift keying (QPSK), there are four phase states that represent the four symbols 11, 01, 00, and 10.

Upconverter The upconverter changes the carrier to the radio frequency (RF) signals used to transmit the signal,such as C-band (6 GHz) or Ku-band (14 GHz).

It accepts the modulated IF carrier from modulator and translates its IF freq. ω0 to the uplink RF freq. ωu in the uplink freq.spectrum of the satellite by mixing ω0 with a LO freq.ωl Accomplished by Single –Conversion Process Double –Conversion Process

& use the BPF at the o/p to select the upper sideband

HPA The high power amplifier (HPA) amplifies the modulated RF signals from the output of the upconvertors to the required power at the input terminals of the antenna. can be either a klystron, A traveling wave tube (TWT), or a solid state power amplifier (SSPA).

The bandwidth of a klystron is fairly narrow and is the same as the bandwidth of a transponder, or about 40 MHz at 6 GHz and 80 MHz at 14 GHz TWT is a broadband device with a bandwidth of about 500 MHz, or about the full bandwidth of a 24 transponder satellite comprising 12 transponders at each polarization..

TWT is more flexible,since it can put the same carrier into all 12 transponders. However, since it is a nonlinear device, it must be backed off to operate in the linear region when multiple carriers are present An SSPA is very efficient and thus does not produce much heat. A typical SSPA power is 2 or 3 watts, but can be as high as 80 or 100 watts. At Ku-band the HPA must be located near the antenna to minimize losses

RECEIVER CHAIN The LNA is mounted on the antenna itself to minimize waveguide loss. The LNA must have a high gain but contribute very little noise Instead of an LNA, a low noise block downconverter (LNB) may be used. LNB both amplifies the signal and downconverts the frequency to again to minimize losses

TYPES Depending Upon The Size: •    Large earth station •    Small earth stations •    Micro earth station: •    Very small aperture terminals (VSAT) •    Large earth station

Depending Upon Types Of Services : Fixed Mobile Maritime Aeronautical Terrestrial (trucks etc) Broadcasting

Depending Upon The Application •    Transmit and receive •    Receive only (DBS) •    Transmit only (DCS)

Standard A earth station: G/T = 40.9 db/k at C-band Dish diameter  = 30 m Antenna gain = 6.4 db Standard B earth station : G/T = 31.7 db/k at C-band Dish diameter  = 31 m Antenna gain = 51.5 db Standard C earth station: G/T = 39 db/k at C-band (at 14/11 GHz) Dish diameter  = 19 m (at 14/11 GHz) Antenna gain = 65 db (at 14/11 GHz)

Small earth stations #)Antenna size less than 60 wavelength in diameter. #)Diameter of antenna = (say) 5m at 4GHz = 1.6 at 11 GHz Gain = below 44db #)Type of parabola = Front feed parabola #)Fixed pointing: no need of expensive auto-track system. #)Application: TURO (TV receive only) systems, load mobile services, maritime services, Direct broadcast satellite (DBS).

Micro earth station: #)Size: 2 to 6 feet antenna #)Very inexpensive #)DBS application

Very small aperture terminals (VSAT) #)Low cost earth station Very small aperture terminals (VSAT) #)Low cost earth station. Low speed, packet data Networks applications. #)Antenna size: 1.2 to 2.5m dia #)G/T <15 db/k #)LNA is uncooled GaAs FET of about 100k #)HPA: 1 to 5w power also GaAsFET Low cost solution for route data communication between control host computer and a large no. of remote sites.

Generally employs star topology Generally employs star topology. VSAT employs packet transmission with switching at the master station. Such networks are ideally suited for non-voice applications like computer and other forms of data communication. These employ spread spectrum multiple access (SSMA) techniques. The data transmission rates are generally limited to 19.2 bits at 4 GHz.

Applications of VSATs are •    Data broadcasting services and •    Two-way data services •    News dissemination including text and graphic information •    Transmission of charts and documents. #)It is believed that in future, with spread spectrum technique with 20/30 GHz band, the size of the earth may come down o 12 cm, making earth stations truly portable. #)VSAT find applications in aircrafts ships and rural areas and mobile communications.

Mobile and Transportable earth stations: #)The small earth stations are portable and the antenna must be capable of relating in all directions. #)Application is the communication for shipping through IMMARSAT (International Maritime Satellite Organization) satellites. #)IMMARSAT: Earth station characteristics are Antenna diameter: 12 m Gain:23 db Beam width: 100 Capacity : single duplex telephony or telegraphy. Frequency: 1640/1540 GHz

TURO systems #)It has only the downlink section consists of dish (reflector), feed –horn , polarizer, LNA, down converter and TV/Video monitor. #)The dish can be spherical or parabolic, generally the later. #)The receiver part of TURO consists of down converter,  IF amplifier, demodulator and video/audio processor. #)Once the video and audio are properly processed, they may be received on the monitor.

outdoor unit receiving antenna feeds directly to a low-noise amplifier/converter combination parabolic reflector is generally used with offset feed