Wireless Local Loops and
Satellites
•Wireless Local loops
•FSO
•Satellites
•Deep Space communications
Copyright: A. Umar
Amjad
Umar
Wireless MAN (Local Loop)
• Wired technologies responding to need for
reliable, high-speed access by residential,
business, and government subscribers
– ISDN, xDSL, cable modems
• Increasing interest shown in competing wireless
technologies for subscriber access
• Wireless local loop (WLL)
– Narrowband – offers a replacement for existing
telephony services
– Broadband – provides high-speed two-way voice and
data service
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WLL Configuration
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Advantages of WLL over Wired
Approach
• Cost – wireless systems are less expensive due to
cost of cable installation that’s avoided
• Installation time – WLL systems can be installed
in a small fraction of the time required for a new
wired system
• Selective installation – radio units installed for
subscribers who want service at a given time
– With a wired system, cable is laid out in anticipation of
serving every subscriber in a given area
Copyright: A. Umar
Propagation Considerations for WLL
• Most high-speed WLL schemes use millimeter
wave frequencies (10 GHz to about 300 GHz)
– There are wide unused frequency bands available above
25 GHz
– At these high frequencies, wide channel bandwidths
can be used, providing high data rates
– Small size transceivers and adaptive antenna arrays can
be used
• Undesirable characteristics of millimeter WF
– Free space loss increases with the square of the
frequency; losses are much higher in millimeter WF
– Above 10 GHz, attenuation effects due to rainfall and
atmospheric or gaseous absorption are large
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– Multipath losses canCopyright:
be quite
high due to vegetation
Wireless Local Loops
Telephone
InterExchange
Switch
LAN
PBX, TV
Toll
Connecting
Trunks
Wireless
Local Loop
Offerings
(MMDS, LMDS)
Intertoll
Trunks
Telephone
Wired
Local
Loop
Local
Control
Office
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Toll
Connecting
Trunks
InterExchange
Switch
MMDS and LMDS
• Multichannel multipoint distribution service (MMDS)
–
–
–
–
–
Older standard for 2.15 GHZ to 2.68 GHZ
Also referred to as wireless cable (competes with cable TV)
Used mainly by residential subscribers and small businesses
Single MMDS channel can offer 27 Mbps over 50 km
Individual subscribers at 300 kbps to 3 Mbps
• Local multipoint distribution service (LMDS)
– Newer standard for 30 GHZ (US), 40 GHZ (Europe)
– Appeals to larger companies with greater bandwidth demands
– Can deliver upto 37 Mbps within 2 to 4 km
– point-to-multipoint communication system for
digital two-way voice, data, Internet, and video
Copyright: A. Umar
Tradeoffs between MMDS and LMDS
• MMDS
– MMDS signals have larger wavelengths and can travel
farther without losing significant power
– Equipment at lower frequencies is less expensive
– MMDS signals don't get blocked as easily by objects
and are less susceptible to rain absorption
– It has been proposed to assign a new frequency
band dedicated to digital MMDS
services, but this is impractical
• LMDS
– Relatively high data rates
– Capable of providing video, telephony, and data
– Relatively low cost in comparison with cable
alternatives
Copyright: A. Umar
802.16 Standards Development
• Standards for LMDS air interface and functions
– Use wireless links with microwave or millimeter wave
radios
– Use licensed spectrum
– Are metropolitan in scale
– Provide public network service to fee-paying customers
– Use point-to-multipoint architecture with stationary
rooftop or tower-mounted antennas
– Provide efficient transport of heterogeneous traffic
supporting quality of service (QoS)
– Use wireless links with microwave or millimeter wave
radios
– Are capable of broadband
(>2 Mbps)
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Umar
IEE802.16 Refernce Architecture
SNI
(STS Network
Interface)
Subscriber
Network
BNI
(BTS Network
Interface)
Air
Interface
STS
BTS
Core
Network
Repeater
(Optional)
Subscriber Network = (LAN, PBX, IP-based network)
Core Network = PSTN. Internet
BTS = Base transceiver station
STS = Subscriber transceiver station
Copyright: A. Umar
802.16.1: 10GHz-66GHZ
802.16.2: Coexistence
802.16.3: 2-11 GHZ
Protocol Architecture
• Physical and transmission layer functions:
– Encoding/decoding of signals
– Preamble generation/removal
– Bit transmission/reception
• Medium access control layer functions:
– On transmission, assemble data into a frame with address and error
detection fields
– On reception, disassemble frame, and perform address recognition
and error detection
– Govern access to the wireless transmission medium
• Convergence layer functions:
– Encapsulate PDU framing of upper layers into native 802.16
MAC/PHY frames
– Map upper layer’s addresses into 802.16 addresses
– Translate upper layer QoS parameters into native 802.16 MAC
format
– Adapt time dependencies of upper layer traffic into equivalent
MAC service
Copyright: A. Umar
Free-Space Optics (FSO)
• FSO uses lasers to transmit data, but instead of
enclosing the data stream in a fiber optic cable, the
data is transmitted through the air.
• FSO systems can support data rates between 1.25G
bit/sec to 150G bit/sec (theoretically) with link lengths
that can vary from more than 600 feet up to about a
mile.
• Common FSO networks support around 2.5 Gbps of
data, voice and video communications between 1000
to 2000 feet.
• FSO transceivers can be located on a rooftop, on a
corner of a building or indoors behind a window to
support the last mile.
• Highly secure line of sight communications in the last
mile
Copyright: A. Umar
Useful Web Sites for WLLs
– Broadband wireless exchange magazine
(http://www.bbwexchange.com/)
– IEEE 802.16 Working Group on Fixed
Broadband Wireless Standards
(http://grouper.ieee.org/groups/802/16/index.ht
ml
– Broadband Wireless Association (http://www.broadbandwireless.org/)
Copyright: A. Umar
Satellite Communications
Copyright: A. Umar
Satellite Systems
•
•
•
•
•
Basically a repeater in the sky - first launched in 1962
Deliver around 50 Mbps, (3-30 GHz, 300 - 500 MHz BW)
Earth Stations – antenna systems on or near earth
Uplink – transmission from an earth station to a satellite
Downlink – transmission from a satellite to an earth station
(different from uplink, typically faster, can be broad )
• Transponder – electronics in the satellite that convert/amplify
uplink signals to downlink
signals (16 to 20 per satellite, each
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with 36-50 MHz BW)
Ways to Categorize
Communications Satellites
• Coverage area
– Global, regional, national
– Transponder beams can cover wide (10,000km) to narrow
(250km)
– Each antenna aims at a transponder, sends a few frames,
then aims at another area (dwell time)
• Service type
– Fixed service satellite (FSS) - point to point
– Broadcast service satellite (BSS) - for homes
– Mobile service satellite (MSS) - senders/receivers mobile
• General usage
– Commercial, military, amateur, experimental
Copyright: A. Umar
Classification of Satellite Orbits
• Altitude of satellites
– Geostationary orbit (GEO) - around 35,000 Killometers
– Medium earth orbit (MEO) - 5,000 to 12,000 km
– Low earth orbit (LEO) - 500km to 1500 km (<2000 km)
• Circular or elliptical orbit
– Circular with center at earth’s center
– Elliptical with one foci at earth’s center
• Orbit around earth in different planes
– Equatorial orbit above earth’s equator
– Polar orbit passes over both poles
– Other orbits referred to as inclined orbits
• VSAT (Very Small Aperture Terminals) - 1 meter
antenna, 19.2 Kbps uplink, 512 kbps downlink
Umar coordinate users
– Use a hub to reinforceCopyright:
signalA. and
Examples of Satellite Systems
 Teledesic, funded by Microsoft and McCaw Cellular,
 a $9 billion low-earth orbit (LEO) satellite network project.
 A typical Teledesic user will operate at 64 Mbps downlink
and 2 Mbps uplink.
 Iridium, initiated by Motorola, is another LEO project
 Goal: cover every spot on earth plus 50 miles above
 Customers have been luke warm (high cost and technical
problems).
• Western Union satellites used by PBS
– TV programs broadcasted through satellites
• Many others (GE, Hughes, AT&T, IBM)
• Orbital arcs controlled by FCC and other agencies
– FCC formed in 1934. Satellites controlled by
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Communications Satellite Act of 1962
Classification of Satellites
Elliptical
Equatorial
Polar
Circular
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Examples
• Weather satellites - Polar with cameras
• GPS - 2 dozen
• Satellite constellations -- “LAN in the sky
to provide universal coverage
• Space junk
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Geometry Terms
A
Earth
• Elevation angle (A) - the angle from the horizontal to the
point on the center of the main beam of the antenna when the
antenna is pointed directly at the satellite
• Coverage angle - the measure of the portion of the earth's
surface visible to the satellite
• Minimum Elevation Angle: Reasons affecting minimum
elevation angle of earth station’s antenna (>0o)
– Buildings, trees, and other terrestrial objects block the line
of sight
– Atmospheric attenuation is greater at low elevation angles
– Electrical noise generated by the earth's heat near its
surface adversely affects reception
Copyright: A. Umar
GEO Orbit and GEO Satellites
• 35,000 km above the earth (Clark’s belt)
• Only 180 to avoid interference - placed 2 degree (out
of 360 degree eguatorial plane)
• Satellite appears stationary (rotates with earth)
• Advantages of the GEO orbit
–
–
–
–
No problem with frequency changes
Tracking of the satellite is simplified
High coverage area, high delay (240 to 270 ms)
One satellite can cover 120 degrees (3 can cover the earth)
• Disadvantages of the GEO orbit
– Weak signal after traveling over 35,000 km
– Polar regions are poorly served
– Signal sending delay is substantial
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LEO Satellite Characteristics
• Very few until 1990
• Iridium - Motorola filed an application with FCC in
1990 for 77 LEOs (element 77 is Iridium)
• Basic idea: many LEOs, when one out of view, next
one takes over (a great deal of activity)
• Circular/slightly elliptical orbit under 2000 km
• Orbit period ranges from 1.5 to 2 hours
• Diameter of coverage is about 8000 km
• Round-trip signal propagation delay less than 20 ms
• Maximum satellite visible time up to 20 min
• System must cope with large Doppler shifts
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Umar
• Atmospheric drag results
inA.orbital
deterioration
LEO Categories
• Little LEOs
–
–
–
–
Frequencies below 1 GHz
5MHz of bandwidth
Data rates up to 10 kbps
Aimed at paging, tracking, and low-rate messaging
• Big LEOs
– Frequencies above 1 GHz
– Support data rates up to a few megabits per sec
– Offer same services as little LEOs in addition to voice
and positioning services
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MEO Satellite Characteristics
• Circular orbit at an altitude in the range of 5000 to
12,000 km
• Orbit period of 6 hours
• Diameter of coverage is 10,000 to 15,000 km
• Round trip signal propagation delay less than 50
ms
• Maximum satellite visible time is a few hours
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Frequency Bands Available for
Satellite Communications
Ban
d
L
Frequenc
y Range
1 to 2 GHz
Total
Bandwidt
h
1 GHz
S
2 to 4
GHz
4 to 8 GHz
2 GHz
8 to 12.5
GHz
12.5 to 18
GHz
4.5 GHz
C
X
Ku
4 GHz
5.5 GHz
Typical
Applications
Mobile satellite
service (MSS)
NASA, deep sea
research, MSS
Fixed satellite
service (FSS)
Mobile satellite
service (MSS)
Mobile satellite
service (MSS)
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C and Ku mostCopyright:
frequently
used
Satellite Link Performance
Factors
• Distance between earth station antenna and
satellite antenna
• For downlink, terrestrial distance between earth
station antenna and “aim point” of satellite
– Displayed as a satellite footprint (Figure 9.6)
• Atmospheric attenuation
– Affected by oxygen, water, angle of elevation, and
higher frequencies
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VSATs
Server
User Site
Hub
User A.
Site
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Umar
Satellite versus Fiber
• Satellites were favored before 1984
• Breakup of AT&T resulted in development of fiber and
broadband communications
• Fiber has far more BW than many satellites
– BW of one fiber link 2,000 MHZ
– BW of one satellite (300 to 500 MHz )
• But fiber cannot reach every home (last mile)
• Niches for satellite:
– user needs 50 Mbps (use T3 or satellite)
– message needs to be broadcasted (tough with fiber)
– cannot lay cables due to hostile environment
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Capacity Allocation Strategies
• Typically a GEO will have 500MHz BW
• Subdivided into channels (e.g., 40 MHz)
• Special issues in multiple user allocation:
– Long delays (270 ms) creates several problems
– Cannot do polling, collision sense, etc
– Variants of Alloha
• Commonly used
– Frequency division multiple access (FDMA) -- now
– Time division multiple access (TDMA) -- future
– Code division multiple access (CDMA)
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Frequency-Division Multiplexing
• Alternative uses of channels in point-to-point
configuration
–
–
–
–
–
–
–
1200 voice-frequency (VF) voice channels
One 50-Mbps data stream
16 channels of 1.544 Mbps each
400 channels of 64 kbps each
600 channels of 40 kbps each
One analog video signal
Six to nine digital video signals
• Factors which limit the number of subchannels
provided within a satellite channel via FDMA
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– Thermal noise, Intermodulation
Forms of FDMA
• Fixed-assignment multiple access (FAMA)
– The assignment of capacity is distributed in a fixed
manner among multiple stations
– Demand may fluctuate
– Results in the significant underuse of capacity
• Demand-assignment multiple access (DAMA)
– Capacity assignment is changed as needed to respond
optimally to demand changes among the multiple
stations
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Use of TDMA Techniques
TDMA is being used more often because
• Cost of digital components continues to drop
• Advantages of digital components
– Use of error correction
Combined FAMA-TDMA Operation more popular
• Increased efficiency of TDM
– Lack of intermodulation noise
• Advantages of digital components
– Use of error correction
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FAMA-TDMA
Time
slot1
Time
slot3
To S2
To S3
To S1
To S3
To S1
To S2
Time
slot2
Time
slot1
Station S1
To S2
To S3
Station
S2
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A. Umar
Station S3
FAMA-TDMA
To Station2
Transmission
from satellite1
To Station 3
To Station 1
To Station 2
To Station1
To Station 3
To Station2
To Station 3
Station S1
Station
S2
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A. Umar
Transmission
from satellite3
Transmission
from satellite2
Transmission
from satellite1
Station S3
Deep Space Networking
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Interplanetary Internet
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Additional Info
• Inglis, A., and Lutner, A., “Satellite
Technology: An Introduction”, Focal Press,
1997
• Elbert, B., “Introduction to Satellite
Communications”, Artech, 1999
• http://www.thetech.org/exhibits_events/onli
ne/satellite/
• Lloyd Wood’s Web Site
(http://www.ee.surrey.ac.uk/Personal/L.Woo
d/constellations/
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