1. Lecture Week 5 Satellite Communications Fibre Optic Communications
Computer Networks

2. Wireless Communications & Mobile Telephony
Last Lecture: 04
Wireless Communications & Mobile Telephony
Introduction to Wireless Communications.
Radio Wave Frequency Bands.
Modes of Propagation of Signals.
Mobile Radio Systems.

3. Satellite Communications
Introduction to Satellites,
Components of a Human-Made Satellite,
Launching a Satellite,
Orbital Altitudes,
Satellites in Orbit,
Satellite Systems, GSO, MEO and LEO Satellites,
Satellite Payload.

4. Satellite Communications
A satellite is an object that goes around, or orbits, a larger object, such as a planet.
While there are natural satellites, like the moon, hundreds of man- made satellites also orbit the Earth.
Communications via satellite is a natural outgrowth of modern technology and of the continuing demand for greater capacity and higher quality in communications.
Satellite communications are reliable, survivable, secure, and a cost effective method of telecommunications.
Today, there are more than 150 communications satellites in orbit, with over 100 in geosynchronous orbit.

5. Components of a Human-Made Satellite System
UPLINK
DOWNLINK

6. Launching a Satellite
A satellite must be launched at 27,500 kmph if its forward momentum is to balance the earth’s gravity thus causing it to circle the earth.
If launched at more than 38,000 kmph it will leave the gravitational pull of the Earth.
If launched at less that 27,000 kmph it will fall back to earth.

7. Frequency Spectrum for Satellite
The most commonly used satellite frequency carrier bands are:
C band (4-8 GHz),
Ku band (10-18 GHz),
Ka band (18-31 GHz).
FSS Uplinks
FSS Downlinks
Mobiles
1 GHz
10 GHz
20 GHz
30 GHz L S C Ku Ka X
The chart shows the “principal” bands used for commercial satellite communications. There are various other small allocations scattered around the spectrum, such as low speed messaging in the VHF and UHF bands. The Fixed Satellite Service (FSS) shown in the chart also include those for Broadcast Satellite Services (BSS).
Many of the FSS/BSS allocations are shared bands with terrestrial services and there is no guarantee that they may all be used in any particular country. Location and frequency licensing of an earth station has to be very carefully researched and co-ordinated with national licensing authorities and those in neighbouring countries, in accordance with the Radio Regulations.
While the C-band has been used since the 1960s, the Ku band came into use in about 1980, to satisfy the increasing demand for more spectrum. The use of the Ka band is new and as yet is fairly experimental for FSS / BSS.
The rain fade problem increases with increasing frequency, as the wavelength becomes smaller and thus the size of rain-drops / snow-flakes becomes significant w.r.t. a wavelength.
Rain fade counter-measures to maintain the quality of service, currently in use in the Ku bands have included the use of “Up Path Power Control”, large fade margins in the link power budget and, occasionally, a location diversity antenna. Fade Margins in the Ku band are typically in the range of dB, depending on the rainfall density in the region in question, and the quality of service required.
In the Ka-band, Fade Margins of 12 to 25 dB would be required. However, more efficient use of the satellite power for FSS would be achieved by using “Far End Operated Power Control”. Statistically, not all links are likely to fade at the same time, and therefore each point to point link would not require the full theoretical fade margin. As long as sufficient satellite power is kept in reserve to allow a relatively small number of links to increase power by the necessary margin, when required, then each link could operate with a small fade margin, thus allowing more transmission capacity through any satellite.
Mobiles operating in L and S bands are inherently immune to the rain-fade problem. This combined with the fact that there is a clear 9 to 12 dB path loss advantage, compared to the figures shown on the previous page, make the hand-held mobile earth station a very viable proposition, when used with a LEO or MEO type of satellite.
Rainfall
Fading
Negligible
Significant
Severe

8. Satellite Power Source
Many satellites are powered by rechargeable batteries. Solar panels recharge many satellite batteries.
Other satellites have fuel cells that convert chemical energy to electrical energy, while a few rely on nuclear energy.

9. Satellite System Consist of Space Segments and Ground Segments.
The Space Segment is composed of satellites, classified as:
Geostationary Orbit (GSO) satellites,
Non-geostationary Orbit (NGSO) satellites, including Medium Earth Orbit (MEO) and Low Earth Orbit (LEO) satellite, according to the orbit altitude above the Earth's surface.
A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around the Earth with an orbital period of one sidereal day (approximately 23 hours 56 minutes and 4 seconds), matching the Earth's sidereal rotation period.[1] The synchronization of rotation and orbital period means that, for an observer on the surface of the Earth, an object in geosynchronous orbit returns to the exactly same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky traces out a path, typically in the form of an analemma, whose precise characteristics depend on the orbit's inclination and eccentricity.
A special case of geosynchronous orbit is the geostationary orbit (popularly, the term "geosynchronous" may sometimes be used to mean, specifically, geostationary).[2] This is a circular geosynchronous orbit at zero inclination, that is, directly above the equator. A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to ground observers. Communications satellites are often given geostationary orbits, or close to geostationary, so that the satellite antennas that communicate with them do not have to move, but can be pointed permanently at the fixed location in the sky where the satellite appears.
A semisynchronous orbit has an orbital period of 0.5 sidereal days, i.e., 11 h 58 min. Relative to the Earth's surface it has twice this period, and hence appears to go around the Earth twice every day. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System.
A geostationary orbit (GEO) is a circular geosynchronous orbit in the plane of the Earth's equator with a radius of approximately 42,164 km (26,199 mi) (measured from the center of the Earth)

10. Orbital Altitudes GEO LEO MEO First Van Allen Belt
Second Van Allen Belt
We have already seen that a satellite placed in a circular equatorial orbit at an altitude of 35,786 km can be regarded as geostationary, if moving in the same rotation as the Earth’s rotation. The selection of other orbital altitudes is mainly determined by the requirements of:
Minimising the radio path loss (increases as a square of the frequency and distance)
Minimising the transmission time delay (Latency - linear relationship with distance).
Achieving some kind of relationship with the Earth’s rotation (optional).
Avoiding the Van Allen Radiation Belts.
The Van Allen Belts are layers of charged particles emitted by the Sun, which have become trapped by the Earth’s magnetic field. These particles are sometimes known as the Solar Wind. The first Van Allen Belt mainly consists of heavy particles such as Protons and Neutrons. The Second Van Allen Belt is mainly light particles I.e. electrons. It is generally accepted that a satellite moving quickly through the Van Allen Belts may suffer damage to some of the delicate external components, either physical damage or through electro-magnetic interference. These altitudes therefore tend to be avoided.
The following is a rough comparison of the 3 favourite orbital altitudes Geostationary Earth Orbit (GEO), Medium Earth Orbit (MEO) and Low Earth Orbit (LEO), based upon using a common frequency of 6 GHz. This may then be scaled by 6dB up or down for each doubling or halving of the frequency being used.
GEO - One way path loss 200 dB; One hop latency mSec; 3 satellites for global coverage (between ±70° latitude); No satellite switch-over required for 24 hour service.
MEO - One way path loss dB; One hop latency mSec; 10 satellites for global coverage; 4 switch-overs per 24 hours.
LEO - One way path loss dB: One hop latency mSec; Minimum of 48 satellites for equivalent of GEO coverage; 144 switch-overs per 24 hours.
99.9% of current satellite transmission capacity is in GEO, currently totalling around 250 GEO satellites.

11. Satellite Footprints
The footprint identifies the ground coverage of a satellites transmission.
Global
Hemispheric
Regional or Zone
Spot
Most satellites have multiple antenna arrays which can create a variety of “Footprints” on the Earth’s surface, within the possible coverage area. Thus use of multi-feed antennae or phased arrays enables to footprints to be shaped to required coverage area.
In GEO satellites, there may be a global, hemispheric, zone or spot beams. MEO satellites can create regional and spots beams, but LEO satellites can only make spot beams, due to the low altitude. The smaller the size of the beam, the better the performance of the satellite in terms of uplink sensitivity and downlink power, within the beam.
The required footprints are created for the uplinks as well as the downlinks, and the number of beams created depends upon the complexity of the antenna arrays.
An input and output switching matrix on board the satellite, enables different uplink and downlink and downlink footprints to be interconnected.
The use of multiple footprints enables the re-use of the frequency spectrum on any satellite, as long as they don’t overlap. Frequency re-use in overlapping footprints can be enabled by using orthogonal polarisation methods.

12. Comparing GSO, MEO and LEO Satellites
Parameter
GSO
MEO
LEO
Distance from Earth (km)
36,000
3,000 – 36,000
200 – 3,000
Antenna Size
Large
Medium
Small
Transmitter Power
Round Trip Propagation Delay (ms)
250 – 280
110 – 130
20 – 25

13. Comparing GSO, MEO and LEO Satellites
Parameter
GSO
MEO
LEO
Distance from Earth (km)
36,000
3,000 – 36,000
200 – 3,000
Antenna Size
Large
Medium
Small
Transmitter Power
Round Trip Propagation Delay (ms)
250 – 280
110 – 130
20 – 25

14. Summary Introduction to Satellites,
Components of a Human-Made Satellite,
Launching a Satellite,
Orbital Altitudes,
Satellites in Orbit,
Satellite Systems, GSO, MEO and LEO Satellites,
Satellite Payload.

15. Fibre Optic Communications
Introduction to Optical Communications,
Technological Developments,
System & Data Link Considerations,
System Components,
Optical Fibre Principle of Operation,
Types of Optical Fibre,
Optical Fibre Transmission Characteristics,
Wavelength Division Multiplexing.

16. Technological Developments
The invention of laser during the 1950s,
The invention of glass optical fibres during the 1960s,
Initially α = 1000 dB/km
1970s α = 20 dB/km
1980s α < 1 dB/km (for single mode glass optical fibres),
1990s α dB/km (for single mode glass optical fibres).
Where: α = attenuation
Initial installations in the late 1970s to early 1980s,
Transcontinental and Sub-sea communications,
In lakes and around continents under the sea.

17. Generic Optical Fibre Communication System
Light Sources used are:
Laser (Light Amplification by Stimulated Emission of Radiation),
LED (Light Emitting Diode).
Light Detectors:
With internal amplification - Avalanche Photodiodes (APD),
Without internal amplification.

18. The Structure of an Optical Fibre
Core made of glass or plastic,
Cladding made of glass or plastic,
Jacket made of plastic elastic material.
The refractive index of the core (n1) is higher than the refractive index of the cladding (n2) to ensure total internal reflection:
n1 > n2

19. Acceptance Angle & Numerical Aperture
Light is injected in the core of the fibre up to a maximum angle, known as the acceptance angle,
The acceptance angle in within a cone shaped zone, known as Numerical Aperture, NA.
Where: n0 = the refractive index of air (n0 ≈ 1),
n1 = the refractive index of the core,
n2 = the refractive index of the cladding.

20. Thank You