Satellite Communication

Basics of Satellite Communication. How it works. Digital Communication by Satellite.

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What is Satelitte Communications?

Satellite communications is an extension of LOS microwave technology. The satellite must be within line-of-sight of each participating earth terminal. We are more concerned about noise in satellite communication links than we were with LOS microwave. In most cases, received signals will be of a much lower level.

Block diagram of a transponder of a typical communication satellite

On satellite systems operating below 10 GHz, very little link margin is required; there is no fading, as experienced with LOS microwave. The discussion here only deals with geostationary orbit (GEO) communication satellites.

Satellite communications presents another method of extending the digital network. These digital trunks may be used as any other digital trunks for telephony, data, the Internet, facsimile, and video. However, fiber optics has become a strong competitor of satellite communications. Only very small aperture terminal (VSAT) systems are showing any real growth in the GEO arena. A new type of communication satellite is being fielded.

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Satelitte Communications basics

Satellite communication is nothing more than LOS microwave using one (or two) satellites located at great distances from the terminal earth stations, as illustrated in Figure 2 Because of the distance involved, consider the slant range from the earth station to the satellite to be the same as the satellite altitude above the equator. This would be true if the antenna were pointing at zenith (08 elevation angle) to the satellite. Distance increases as the pointing angle to the satellite decreases (elevation angle).

Figure 2 - Distances involved in satellite communications

We thus are dealing with very long distances. The time required to traverse thesedistances - namely, earth station to satellite to another earth station - is on the order of 250ms. Round-trip delay will be 2 × 250 or 500 ms. These propagation times are much greaterthan those encountered on conventional terrestrial systems. So one major problem is propagationtime and resulting echo on telephone circuits. It influences certain data circuits indelay to reply for block or packet transmission systems and requires careful selection oftelephone signaling systems, or call-setup time may become excessive.

Naturally, there are far greater losses. For LOS microwave we encounter free-spacelosses possibly as high as 145 dB. In the case of a satellite with a range of 22,300mi operating on 4.2 GHz, the free-space loss is 196 dB and at 6 GHz, 199 dB. At 14GHz the loss is about 207 dB. This presents no insurmountable problem from earthto satellite, where comparatively high-power transmitters and very-high-gain antennasmay be used. On the contrary, from satellite to earth the link is power-limited for tworeasons: (1) in bands shared with terrestrial services such as the popular 4-GHz bandto ensure noninterference with those services, and (2) in the satellite itself, which canderive power only from solar cells. It takes a great number of solar cells to produce theRF power necessary; thus the downlink, from satellite to earth, is critical, and receivedsignal levels will be much lower than on comparative radiolinks, as low as −150 dBW.A third problem is crowding. The equatorial orbit is filling with geostationary satellites.Radio-frequency interference from one satellite system to another is increasing. Thisis particularly true for systems employing smaller antennas at earth stations with theirinherent wider beamwidths. It all boils down to a frequency congestion of emitters.

Frequency Bands: the most desirable

The most desirable frequency bands for commercial satellite communication are in thespectrum 1000–10,000 MHz.These bands are:

  • 3700–4200 MHz (satellite-to-earth or downlink);
  • 5925–6425 MHz (earth-to-satellite or uplink);
  • 7250–7750 MHz (downlink);
  • 7900–8400 MHz (uplink).

These bands are preferred by design engineers for the following primary reasons:

  • Less atmospheric absorption than higher frequencies;
  • Rainfall loss not a concern;
  • Less noise, both galactic and man-made;
  • Well-developed technology;
  • Less free-space loss compared with the higher frequencies.

Multiple Access to a Communication Satellite

Multiple access is defined as the ability of a number of earth stations to interconnecttheir respective communication links through a common satellite. Satellite access isclassified (1) by assignment, whether quasi-permanent or temporary, namely, (a) preassigned multiple access or (b) demand-assigned multiple access (DAMA); and (2)according to whether the assignment is in the frequency domain or the time domain,namely, (a) frequency-division multiple access (FDMA) or (b) time-division multipleaccess (TDMA). On comparatively heavy routes (≥10 erlangs), preassigned multipleaccess may become economical. Other factors, of course, must be considered, such aswhether the earth station is “INTELSAT” standard as well as the space-segment chargethat is levied for use of the satellite. In telephone terminology, “preassigned” meansdedicated circuits. DAMA is useful for low-traffic multipoint routes where it becomesinteresting from an economic standpoint. Also, an earth station may resort to DAMAas a remedy to overflow for its FDMA circuits.

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Digital Communication by Satellite

There are three methods to handle digital communication by satellite: (1) TDMA, (2)FDMA, and (3) over a VSAT network. Digital access by FDMA is handled in a similarfashion as with an analog FDM/ FM configuration. Several usersmay share a common transponder and the same backoff rules hold; in fact they are evenmore important when using a digital format because the IM products generated in thesatellite TWT high-power amplifier (HPA) can notably degrade error performance. Inthe link budget, once we calculate C/N0, we convert to Eb/N0 with thefollowing formula:

Eb/N0 = C/N0 - 10 log(bit rate).

The Eb/N0 value can now be applied to the typical curves found in Figure 9.10 to derivethe BER.

As mentioned previously, satellite communication is downlink limited because downlinkEIRP is strictly restricted. Still we want to receive sufficient power to meet errorperformance objectives. One way to achieve this goal is to use forward error correctionon the links where the lower Eb/N0 ratios will still meet error objectives. ThusINTELSAT requires coding on their digital accesses.

The occupied satellite bandwidth unit for IDR carriers is approximately equal to 0.6times the transmission rate. The transmission rate is defined as the coded symbol rate.To provide guardbands between adjacent carriers on the same transponder, the nominalsatellite bandwidth unit is 0.7 times the transmission rate.

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