Multiple Access and Satellite Communication

7/28/2019 Multiple Access and Satellite Communication

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Mobile Communications

Diversity,

Multiple Access Techniques

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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 tonot simultaneously have fading Error control coding! Frequency diversity repeat information in frequencychannels 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 notcorrelated Usually deployed in all base stations but harder at the mobile


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Performance Degradation and Diversity


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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 seesonly one error Possible to correct each of the errors


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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 receivedbefore the voice packet is

reconstructed Receiver needs buffer to store

arriving data


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Frequency Hopping

Traditionally: transmitter/receiver pair communicate on fixed frequencychannel

Frequency Hopping Idea:

Since noise, fading and interference change somewhat with frequencyband 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 amongdifferent frequencies

For FHSS, the spectrum is spread over a band that is 100 times larger thanoriginal traditional radios


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Frequency Hopping Concept


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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 FastFading :

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


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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


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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


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DSSS (cont)

Multipath fading is reduced by direct sequence signal spreading andbetter 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 persec

Processing gain is a measure of the improvement in SNR gained by usingthe additional bandwidth from spreading (18-23 dB in cellular systems)


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DSSS Modulation The original DataStream is

chipped up into a pattern of pulses of smaller duration

Good correlation properties

Good cross-correlationproperties with other patterns

Each pattern is called a spreadspectrum code


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DSSS Mod/Demod


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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 usuallyrepresented 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


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Output Without Spreading


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Output With Spreading


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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


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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 senderand 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 senderand a receiver for a certain amount of time (users share a frequencychannel in time slices)

CDMA (code division multiple access): assign a user a unique code for transmission between sender andreceiver, users transmit on the same frequency at the same time


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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)


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Frequency Division Multiple Access


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FDD/FDMA General Scheme, example AMPS (B Block)


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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 afraction of guard time

Guard time of 30-50 microseconds common in TDMA Used in GSM, NA-TDMA, (PDC) Pacific Digital Cellular


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Time Division Multiple Access


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CDMA Code Division Multiple Access

Narrowband message signal is multiplied by very largebandwidthspreading signal using direct sequence spread spectrum

All users can use same carrier frequency and may transmitsimultaneously

Each user has own unique access spreading codeword whichis approximately orthogonal to other users codewords

Receiver performs time correlation operation to detect onlyspecific codeword, other users codewords appear as noisedue to decorrelation


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Code Division Multiple Access


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simple example illustrating CDMA


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Simple CDMA Transmitter


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Simple CDMA Receiver


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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)


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Satellite Communication


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Overview Satellite technology has progressed tremendously over the

last 50 years since Arthur C. Clarke first proposed its idea in1945 in his article in Wireless World.

Today, satellite systems can provide a variety of servicesincluding broadband communications, audio/videodistribution networks, maritime navigation, worldwidecustomer service and support as well as military commandand control.

Satellite systems are also expected to play an important rolein the emerging 4G global infrastructure providing the wide

area coverage necessary for the realization of the OptimallyConnected Anywhere, Anytime vision that drives the growthof modern telecom industry.


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Intelsat INTELSAT is the original "Inter-governmental Satellite organization". It

once owned and operated most of the World's satellites used forinternational communications, and still maintains a substantial fleet of satellites.

INTELSAT is moving towards "privatization", with increasing competitionfrom 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 .


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Intelsat Structure


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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 areobserved by the company


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Eutelsat Structure


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Communication Satellite

A Communication Satellite can be looked uponas a large microwave repeater

It contains several transponders which listens

to some portion of spectrum, amplifies theincoming signal and broadcasts it in anotherfrequency to avoid interference with incomingsignals.


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Motivation to use Satellites


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Source: Union of Concerned Scientists [www.ucsusa.org]

Satellite Missions


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Satellite Microwave Transmission

Satellites can relay signals over a long distance Geostationary Satellites

Remain above the equator at a height of about22300 miles (geosynchronous orbits)

Travel around the earth in exactly the same time,the earth takes to rotate


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Satellite System Elements


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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


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Ground Segment Collection of facilities, Users and Applications

Earth Station = Satellite Communication Station(Fixed or Mobile)


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Satellite Uplink and Downlink Downlink

The link from a satellite down to one or more groundstations 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.


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Satellite Uplink and Downlink


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Source: Cryptome [Cryptome.org]

When using a satellite for longdistance communications, the satelliteacts as a repeater.An earth station transmits the signalup to the satellite (uplink), which inturn retransmits it to the receivingearth station (downlink).

Different frequencies are used foruplink/downlink.

Satellite Communication


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Satellite Transmission Links

Earth stations Communicate by sendingsignals to the satellite on an uplink

The satellite then repeats those signals on adownlink

The broadcast nature of downlink makes itattractive for services such as the distributionof TV programs


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Direct to User Services

One way Service (Broadcasting) Two way Service (Communication)


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Satellite Signals

Used to transmit signals and data over longdistances

Weather forecasting Television broadcasting Internet communication Global Positioning Systems


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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 GHz The C band is the most frequently used. The Ka and Ku bands are reserved exclusively forsatellite communication but are subject to rain attenuation


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Types of Satellite Orbits

Based on the inclination, i, over the equatorial plane: Equatorial Orbits above Earths equator (i=0 ) Polar Orbits pass over both poles (i=90)

Other orbits called inclined orbits (0


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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


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Source: Federation of American Scientists [www.fas.org]

Satellite Orbits

Geosynchronous Orbit (GEO):36,000 km above Earth, includescommercial and militarycommunications satellites, satellitesproviding early warning of ballistic

missile launch.Medium Earth Orbit (MEO): from5000 to 15000 km, they includenavigation satellites (GPS, Galileo,Glonass).Low Earth Orbit (LEO): from 500 to

1000 km above Earth, includesmilitary intelligence satellites,weather satellites.


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Satellite Orbits


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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)


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NGSO - Non Geostationary Orbits

Orbit should avoid VanAllen radiation belts:

Region of charged particlesthat can cause damage tosatellite

Occur at ~2000-4000 km and ~13000-25000 km


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LEO - Low Earth Orbits Circular or inclined orbit with < 1400 km altitude

Satellite travels across sky from horizon to horizon in 5 - 15minutes => 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


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HEO - Highly Elliptical Orbits HEOs (i = 63.4) are suitable to provide

coverage at high latitudes (including NorthPole in the northern hemisphere)

Depending on selected orbit (e.g. Molniya,

Tundra, etc.) two or three satellites aresufficient for continuous time coverage of the service area.

All traffic must be periodically transferredfrom the setting satellite to the rising

satellite (Satellite Handover)


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Source: Union of Concerned Scientists [www.ucsusa.org]

Satellite Orbits


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Why Satellites remain in Orbits

Ad t g f S t llit


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Advantages of SatelliteCommunication

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

Di d t g f S t llit


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Disadvantages of SatelliteCommunication

Large up front capital costs (space segmentand launch)

Terrestrial break even distance expanding(now approx. size of Europe)

Interference and propagation delay Congestion of frequencies and orbits


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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


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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 distancetraffic and remote locations.

Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likelyto dominate (e.g. GSM, etc.)

but, satellite can provide fill-in as terrestrial networks areimplemented, also provide similar services in rural areas

and underdeveloped countries


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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 RadioRegulations and is becoming less clear-cut as "commercial" operators move toutilize "government" bands.


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Earths atmosphere

Source: All about GPS [www.kowoma.de]


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Atmospheric Losses

Different types of atmospheric losses can disturb radiowave transmission in satellite systems:

Atmospheric absorption Atmospheric attenuation Traveling ionospheric disturbances


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Atmospheric Absorption

Energy absorption by atmospheric gases, whichvaries with the frequency of the radio waves. Two absorption peaks are observed (for 90

elevation angle): 22.3 GHz from resonance absorption in water

vapour (H 2O) 60 GHz from resonance absorption in oxygen (O 2)

For other elevation angles: [AA] = [AA]90 cosec

Source: Satellite Communications , Dennis Roddy, McGraw-Hill


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Atmospheric Attenuation

Rain is the main cause of atmospheric attenuation (hail, ice andsnow have little effect on attenuation because of their lowwater content).

Total attenuation from rain can be determined by:

A = L [dB] where [dB/km] is called the specific attenuation, and can be

calculated from specific attenuation coefficients in tabular form that canbe 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 fluctuationsin the rain density.


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Traveling Ionospheric Disturbances Traveling ionospheric disturbances are clouds of electrons

in the ionosphere that provoke radio signal fluctuationswhich 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 byirregularities in the ionosphere which change over time.

The main effect of scintillations is fading of the signal.


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What is Polarisation?

Polarisation is the property of electromagnetic waves thatdescribes the direction of the transverse electric field.

Since electromagnetic waves consist of an electric and amagnetic field vibrating at right angles to each other.

it is necessary to adopt a convention to determine thepolarisation of the signal.

Conventionally, the magnetic field is ignored and the planeof the electric field is used.


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Types of Polarisation

Linear Polarisation (horizontal orvertical): 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 twocomponents.

Circular Polarisation: The two components are exactly

90 out of phase and haveexactly the same amplitude.

Elliptical Polarisation: All other cases.

Linear Polarisation Circular Polarisation Elliptical Polarisation


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Satellite Communications

Alternating vertical and horizontalpolarisation is widely used onsatellite communications

This reduces interference betweenprograms on the same frequencyband transmitted from adjacentsatellites (One uses vertical, the nexthorizontal, and so on)

Allows for reduced angularseparation between the satellites.

Information Resources for Telecommunication Professionals[www.mlesat.com]

Multiple Access and Satellite Communication

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