*More detail in next lecture: A sidereal day is the time between consecutive crossings of any particular longitude on the earth with reference to inertial space (or it’s own axis); I.e., in practice, with reference to any star other than the sun. This corresponds to a 360 degree rotation.
Transponders are microwave repeaters carried by communications satellites. Transparent transponders can handle any signal whose format can fit in the transponder bandwidth. No signal processing occurs other than that of heterodyning (frequency changing) the uplink frequency bands to those of the downlinks. Such a satellite communications system is referred to as a bent-pipe system. Connectivity among earth stations is reduced when multiple narrow beams are used. Hence, the evolution proceeded from the transparent transponder to transponders that can perform signal switching and format processing.
Breakeven Distance: As the cost of Satellite Circuit is independent of distance on the Earth between the two ends, whilst the cost of a terrestrial circuit is approximately directly proportional to that distance, the concept of a "breakeven" distance where the costs are equal has been used to determine where services should be routed via satellite. This breakeven distance varies according to the size of the route, growth rate, and any special networking requirements.
1 for N Diversity: Where there is negligible likelihood of route failure, there is no need for route diversity protection and the type of protection used is known as "1 for N". In point to point radio systems it is (typically 7 : 1) throughout the world. If a worker section down a route fails, the traffic is switched to a stand-by section. After repair of the worker, traffic is returned to it after a suitable period of time. This period of time is that necessary for a stability test, to check that the fault has been genuinely cleared. Traffic loss due to section failure can typically be reduced by several hundred times by the use of "1-for-N" protection.
FSS: Stands for Fixed Satellite Services. Satellite communications in the FSS frequency band were initially developed in order to provide transmission links between the public switched telephone networks (PSTNs) of different countries, first intercontinental and then regional (e.g. the Intelsat and Eutelsat systems respectively);
Multipl access and satellite Communication
Mobile Communications Diversity, Multiple Access Techniques 1
What is Diversity?• Idea: Send the same information over several “uncorrelated” forms – Not all repetitions will be lost in a fade• Types of diversity – Time diversity – repeat information in time spaced so as to not simultaneously have fading • Error control coding! – Frequency diversity – repeat information in frequency channels that are spaced apart • Frequency hopping spread spectrum, OFDM – Space diversity – use multiple antennas spaced sufficiently apart so that the signals arriving at these antennas are not correlated • Usually deployed in all base stations but harder at the mobile 2
Interleaving• Problem: – Errors in wireless channels occur in bursts due to fast fades – Error correction codes designed to combat random errors in the code words• Hamming codes can detect 2 and correct one bits error in a block of 7 bits• If 5 out of 7 bits are in error in a codeword, it is not possible to correct 5 errors• Idea: – Use block interleaving – Spread the errors into five codewords, so that each codeword “sees” only one error – Possible to correct each of the errors 4
Block Interleaving• Codewords are arranged on below the other• Bits are transmitted vertically• Burst of errors affect the serially transmitted vertical bits• Errors can be corrected• Delay at the receiver as several codewords have to be received before the voice packet is reconstructed• Receiver needs buffer to store arriving data 5
Frequency Hopping• Traditionally: transmitter/receiver pair communicate on fixed frequency channel• Frequency Hopping Idea: – Since noise, fading and interference change somewhat with frequency band used – move from band to band – Time spent on a single frequency is termed as Dwell Time• The centre frequency of the modulated signal is moved randomly among different frequencies• For FHSS, the spectrum is spread over a band that is 100 times larger than original traditional radios 6
Frequency Hopping (cont)• Two types: – Slow Hopping • Dwell time long enough to transmit several bits in a row (timeslot) – Fast Hopping • Dwell time on the order of a bit or fraction of a bit (primarily for military systems)• Transmitter and receiver must know hopping pattern/ algorithm before communications. – Cyclic pattern – best for low number of frequencies and combating Fast Fading : •Example with four frequencies: f4, f2, f1, f3, f4, f2, f3, …. – Random pattern – best for large number of frequencies, combating co-channel interference• Example with six frequencies: f1, f3, f2, f1, f6, f5, f4, f2, f6, …• Use random number generator with same seed and both ends 8
Frequency Hopping (cont)• Slow frequency hopping used in GSM• Fast hopping in WLANS• Provides frequency diversity• By hopping mobile less likely to suffer consecutive deep fades 9
Direct Sequence Spread Spectrum• Similar to FHSS• DSSS: Two stage modulation technique• Transmitter – First stage: the information bit is spread (mapped) into smaller pulses referred to as CHIPS – Second stage: the spreading signal is transmitted over a digital modulator• Receiver – Transmitted bits are first demodulated and then passed through a correlator • A correlator indicates the strength and direction of a linear relationship between two random variables 10
DSSS (cont)• Multipath fading is reduced by direct sequence signal spreading and better noise immunity• DS also allows lower power operation – harder to detect and jam• Spreading code spreads signal across a wider frequency band – As Bandwidth is inversely proportional to the duration of symbol – Spread is in direct proportion to number of chip bits W used – Processing gain G = W/R; W = chips per sec, R = information bit rate per sec• – Processing gain is a measure of the improvement in SNR gained by using the additional bandwidth from spreading (18-23 dB in cellular systems) 11
DSSS Modulation• The original DataStream is “chipped” up into a pattern of pulses of smaller duration• Good correlation properties• Good cross-correlation properties with other patterns• Each pattern is called a spread spectrum code 12
DSSS (cont)• Example: IEEE 802.11 Wi-Fi Wireless LAN standard• Uses DSSS with 11 bit chipping code – To transmit a “0”, you send [1 1 1 -1 -1 -1 1 -1 -1 1 -1] – To transmit a “1” you send [-1 -1 -1 1 1 1 -1 1 1 -1 1]• Processing gain – The duration of a chip is usually represented by Tc – The duration of the bit is T – The ratio T/Tc = R is called the – “processing gain” of the DSSS system – –For 802.11 R = 11 14
Multiple Access and Mode• Mode – Simplex – one way communication (e.g., broadcast AM) – Duplex – two way communication – TDD – time division duplex – users take turns on the channel – FDD – frequency division duplex – users get two channels – one for each direction of communication • For example one channel for uplink (mobile to base station) another channel for downlink (base station to mobile)• Multiple Access determines how users in a cell share the frequency spectrum assigned to the cell: FDMA,TDMA, CDMA• Wireless systems often use a combination of schemes; GSM – FDD/FDMA/TDMA 17
Multiple Access Techniques• FDMA (frequency division multiple access) – separate spectrum into non-overlapping frequency bands – assign a certain frequency to a transmission channel between a sender and a receiver – different users share use of the medium by transmitting on non- overlapping frequency bands at the same time• TDMA (time division multiple access): – assign a fixed frequency to a transmission channel between a sender and a receiver for a certain amount of time (users share a frequency channel in time slices)• CDMA (code division multiple access): – assign a user a unique code for transmission between sender and receiver, users transmit on the same frequency at the same time 18
FDMA• FDMA is simplest and oldest method• Bandwidth F is divided into T non-overlapping frequency channels – Guard bands minimize interference between channels – Each station is assigned a different frequency• Can be inefficient if more than T stations want to transmit• Receiver requires high quality filters for adjacent channel rejection• Used in First Generation Cellular (NMT) 19
FDD/FDMA General Scheme, example AMPS (B Block) 21
TDMA• Users share same frequency band in non-overlapping time intervals, eg, by round robin• Receiver filters are just windows instead of bandpass filters (as in FDMA)• Guard time can be as small as the synchronization of the network permits – All users must be synchronized with base station to within a fraction of guard time – Guard time of 30-50 microseconds common in TDMA• Used in GSM, NA-TDMA, (PDC) Pacific Digital Cellular 22
CDMA• Code Division Multiple Access – Narrowband message signal is multiplied by very large bandwidth spreading signal using direct sequence spread spectrum – All users can use same carrier frequency and may transmit simultaneously – Each user has own unique access spreading codeword which is approximately orthogonal to other users codewords – Receiver performs time correlation operation to detect only specific codeword, other users codewords appear as noise due to decorrelation 24
CDMA (cont)• Advantages – No timing coordination unlike TDMA – CDMA uses spread spectrum, resistant to interference (multipath fading) – No hard limit on number of users – Large Capacity Increase• Disadvantages – Implementation complexity of spread spectrum – Power control is essential for practical operation• Used in IS-95, 3G standards (UMTS, cdma 2000) 29
Overview• Satellite technology has progressed tremendously over the last 50 years since Arthur C. Clarke first proposed its idea in 1945 in his article in Wireless World.• Today, satellite systems can provide a variety of services including broadband communications, audio/video distribution networks, maritime navigation, worldwide customer service and support as well as military command and control.• Satellite systems are also expected to play an important role in the emerging 4G global infrastructure providing the wide area coverage necessary for the realization of the “Optimally Connected Anywhere, Anytime” vision that drives the growth of modern telecom industry.
Intelsat• INTELSAT is the original "Inter-governmental Satellite organization". It once owned and operated most of the Worlds satellites used for international communications, and still maintains a substantial fleet of satellites.• INTELSAT is moving towards "privatization", with increasing competition from commercial operators (e.g. Panamsat, Loral Skynet, etc.).• INTELSAT Timeline:• Interim organization formed in 1964 by 11 countries• Permanent structure formed in 1973• Commercial "spin-off", New Skies Satellites in 1998• Full "privatization" by April 2001• INTELSAT has 143 members and signatories .
Eutelsat• Permanent General Secretariat opened September 1978• Intergovernmental Conference adopted definitive statutes with 26 members on 14 May 1982• Definitive organization entered into force on 1 September 1985• General Secretariat -> Executive Organ• Executive Council -> EUTELSAT Board of Signatories• Secretary General -> Director General• Current DG is Giuliano Berretta• Currently almost 50 members• Moving towards "privatization"• Limited company owning and controlling of all assets and activities• Also a "residual" intergovernmental organization which will ensure that basic principles of pan-European coverage, universal service, non-discrimination and fair competition are observed by the company
Communication Satellite• A Communication Satellite can be looked upon as a large microwave repeater• It contains several transponders which listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals.
Satellite MissionsSource: Union of Concerned Scientists [www.ucsusa.org]
Satellite Microwave Transmission• Satellites can relay signals over a long distance• Geostationary Satellites – Remain above the equator at a height of about 22300 miles (geosynchronous orbits) – Travel around the earth in exactly the same time, the earth takes to rotate
Space Segment• Satellite Launching Phase• Transfer Orbit Phase• Deployment• Operation – TT&C - Tracking Telemetry and Command Station – SSC - Satellite Control Center, a.k.a.: • OCC - Operations Control Center • SCF - Satellite Control Facility• Retirement Phase
Ground Segment• Collection of facilities, Users and Applications• Earth Station = Satellite Communication Station
Satellite Uplink and Downlink• Downlink – The link from a satellite down to one or more ground stations or receivers• Uplink – The link from a ground station up to a satellite.• Some companies sell uplink and downlink services to – television stations, corporations, and to other telecommunication carriers. – A company can specialize in providing uplinks, downlinks, or both.
Satellite Communication When using a satellite for long distance communications, the satellite acts as a repeater. An earth station transmits the signal up to the satellite (uplink), which in turn retransmits it to the receiving earth station (downlink). Different frequencies are used for uplink/downlink.Source: Cryptome [Cryptome.org]
Satellite Transmission Links• Earth stations Communicate by sending signals to the satellite on an uplink• The satellite then repeats those signals on a downlink• The broadcast nature of downlink makes it attractive for services such as the distribution of TV programs
Direct to User ServicesOne way Service (Broadcasting) Two way Service (Communication)
Satellite Signals• Used to transmit signals and data over long distances – Weather forecasting – Television broadcasting – Internet communication – Global Positioning Systems
Satellite Transmission Bands Frequency Band Downlink Uplink C 3,700-4,200 MHz 5,925-6,425 MHz Ku 11.7-12.2 GHz 14.0-14.5 GHz Ka 17.7-21.2 GHz 27.5-31.0 GHzThe C band is the most frequently used. The Ka and Ku bands are reserved exclusively forsatellite communication but are subject to rain attenuation
Types of Satellite Orbits• Based on the inclination, i, over the equatorial plane: – Equatorial Orbits above Earth’s equator (i=0°) – Polar Orbits pass over both poles (i=90°) – Other orbits called inclined orbits (0°<i<90°)• Based on Eccentricity – Circular with centre at the earth’s centre – Elliptical with one foci at earth’s centre
Types of Satellite based Networks• Based on the Satellite Altitude – GEO – Geostationary Orbits • 36000 Km = 22300 Miles, equatorial, High latency – MEO – Medium Earth Orbits • High bandwidth, High power, High latency – LEO – Low Earth Orbits • Low power, Low latency, More Satellites, Small Footprint – VSAT • Very Small Aperture Satellites – Private WANs
Satellite Orbits Geosynchronous Orbit (GEO): 36,000 km above Earth, includes commercial and military communications satellites, satellites providing early warning of ballistic missile launch. Medium Earth Orbit (MEO): from 5000 to 15000 km, they include navigation satellites (GPS, Galileo, Glonass). Low Earth Orbit (LEO): from 500 to 1000 km above Earth, includes military intelligence satellites, weather satellites.Source: Federation of American Scientists [www.fas.org]
GEO - Geostationary Orbit• In the equatorial plane• Orbital Period = 23 h 56 m 4.091 s = 1 sidereal day*• Satellite appears to be stationary over any point on equator: – Earth Rotates at same speed as Satellite – Radius of Orbit r = Orbital Height + Radius of Earth – Avg. Radius of Earth = 6378.14 Km• 3 Satellites can cover the earth (120° apart)
NGSO - Non Geostationary Orbits• Orbit should avoid Van Allen radiation belts: – Region of charged particles that can cause damage to satellite – Occur at • ~2000-4000 km and • ~13000-25000 km
LEO - Low Earth Orbits• Circular or inclined orbit with < 1400 km altitude – Satellite travels across sky from horizon to horizon in 5 - 15 minutes => needs handoff – Earth stations must track satellite or have Omni directional antennas – Large constellation of satellites is needed for continuous communication (66 satellites needed to cover earth) – Requires complex architecture – Requires tracking at ground
HEO - Highly Elliptical Orbits• HEOs (i = 63.4°) are suitable to provide coverage at high latitudes (including North Pole in the northern hemisphere)• Depending on selected orbit (e.g. Molniya, Tundra, etc.) two or three satellites are sufficient for continuous time coverage of the service area.• All traffic must be periodically transferred from the “setting” satellite to the “rising” satellite (Satellite Handover)
Satellite OrbitsSource: Union of Concerned Scientists [www.ucsusa.org]
Advantages of Satellite Communication• Can reach over large geographical area• Flexible (if transparent transponders)• Easy to install new circuits• Circuit costs independent of distance• Broadcast possibilities• Temporary applications (restoration)• Niche applications• Mobile applications (especially "fill-in")• Terrestrial network "by-pass"• Provision of service to remote or underdeveloped areas• User has control over own network• 1-for-N multipoint standby possibilities
Disadvantages of Satellite Communication• Large up front capital costs (space segment and launch)• Terrestrial break even distance expanding (now approx. size of Europe)• Interference and propagation delay• Congestion of frequencies and orbits
When to use Satellites• When the unique features of satellite communications make it attractive• When the costs are lower than terrestrial routing• When it is the only solution• Examples: – Communications to ships and aircraft (especially safety communications) – TV services - contribution links, direct to cable head, direct to home – Data services - private networks – Overload traffic – Delaying terrestrial investments – 1 for N diversity – Special events
When to use Terrestrial• PSTN - satellite is becoming increasingly uneconomic for most trunk telephony routes• but, there are still good reasons to use satellites for telephony such as: thin routes, diversity, very long distance traffic and remote locations.• Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likely to dominate (e.g. GSM, etc.)• but, satellite can provide fill-in as terrestrial networks are implemented, also provide similar services in rural areas and underdeveloped countries
Frequency Bands Allocated to the FSS• Frequency bands are allocated to different services at World Radio- communication Conferences (WRCs).• Allocations are set out in Article S5 of the ITU Radio Regulations.• It is important to note that (with a few exceptions) bands are generally allocated to more than one radio services.• CONSTRAINTS – Bands have traditionally been divided into “commercial" and "government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilize "government" bands.
Earth’s atmosphere Source: All about GPS [www.kowoma.de]
Atmospheric Losses• Different types of atmospheric losses can disturb radio wave transmission in satellite systems: – Atmospheric absorption – Atmospheric attenuation – Traveling ionospheric disturbances
Atmospheric Absorption • Energy absorption by atmospheric gases, which varies with the frequency of the radio waves. • Two absorption peaks are observed (for 90º elevation angle): – 22.3 GHz from resonance absorption in water vapour (H2O) – 60 GHz from resonance absorption in oxygen (O2) • For other elevation angles: – [AA] = [AA]90 cosec θSource: Satellite Communications, Dennis Roddy, McGraw-Hill
Atmospheric Attenuation• Rain is the main cause of atmospheric attenuation (hail, ice and snow have little effect on attenuation because of their low water content).• Total attenuation from rain can be determined by: – A = αL [dB] – where α [dB/km] is called the specific attenuation, and can be calculated from specific attenuation coefficients in tabular form that can be found in a number of publications – where L [km] is the effective path length of the signal through the rain; note that this differs from the geometric path length due to fluctuations in the rain density.
Traveling Ionospheric Disturbances• Traveling ionospheric disturbances are clouds of electrons in the ionosphere that provoke radio signal fluctuations which can only be determined on a statistical basis.• The disturbances of major concern are: – Scintillation; – Polarisation rotation.• Scintillations are variations in the amplitude, phase, polarisation, or angle of arrival of radio waves, caused by irregularities in the ionosphere which change over time.• The main effect of scintillations is fading of the signal.
What is Polarisation?• Polarisation is the property of electromagnetic waves that describes the direction of the transverse electric field.• Since electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to each other.• it is necessary to adopt a convention to determine the polarisation of the signal.• Conventionally, the magnetic field is ignored and the plane of the electric field is used.
Types of Polarisation • Linear Polarisation (horizontal or vertical): – the two orthogonal components of the electric field are in phase; – The direction of the line in the plane depends on the relative amplitudes of the two components. • Circular Polarisation: – The two components are exactly 90º out of phase and have exactly the same amplitude. • Elliptical Polarisation: – All other cases.Linear Polarisation Circular Polarisation Elliptical Polarisation
Satellite Communications• Alternating vertical and horizontal polarisation is widely used on satellite communications• This reduces interference between programs on the same frequency band transmitted from adjacent satellites (One uses vertical, the next horizontal, and so on)• Allows for reduced angular separation between the satellites. Information Resources for Telecommunication Professionals [www.mlesat.com]
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