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

Performance Degradation and Diversity

3

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 Concept

7

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

Output Without Spreading

15

Output With Spreading

16

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

Frequency Division Multiple Access

20

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

Time Division Multiple Access

23

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

Code Division Multiple Access

25

simple example illustrating CDMA

26

Simple CDMA Transmitter

27

Simple CDMA Receiver

28

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 World's 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 Missions

Source: 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 Services

One 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 GHz

The C band is the most frequently used. The Ka and Ku bands are reserved exclusively for
satellite 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 Orbits

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

Why Satellites remain in Orbits

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 θ

Source: 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]

Multipl access and satellite Communication

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