Wireless Cellular Networks
TDDC22
• Extensive evolution and fast deployment
• First-Generation Mobile Phones: analog voice
(e.g. AMPS, NMT)
Overview of Cellular Networks
Overview of Satellite Networks
• Second-Generation Mobile Phones: digital
voice and some data (e.g. GSM, IS-95)
• Third-Generation Mobile Phones: digital voice
and data (e.g. 3G)
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Principles of cellular networks [1]
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Principles of cellular networks [2]
• Mobile Terminals and Base Stations
• Frequency reuse: use the same
frequency spectrum in different set of
cells
• Communication area divided in hexagonal
cells
• Cell dimensions from hundreds of meters till
tens of kilometers (e.g. GSM: 100m to 35
Km)
• Cells that reuse the same frequency
must be distant enough for avoiding
interference
• Each cell served by a base station formed by
a transceiver and a control unit
• Transmission power control
• Each cell allocated a frequency band for
communication
• Migration of a mobile station from
one cell to another with continuance
of communication -> handoff
• Communication from MS to BS -> reverse
link
• Communication from BS to MS -> forward
link
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Cellular systems - general architecture
Methods for increase capacity in cellular
networks
• General cellular system:
• Adding new channels
– Mobile Station (MS)
– Base Station (BS)
– Mobile telecommunication switching office (MTSO)
• Frequency borrowing: congested cells use frequencies
taken from adjacent cells
• Cell splitting:
– due to initial network design
• Communication between mobile station and base station
use:
– high-used cells are divided in smaller cells and frequencies
are reallocated
• Cell sectoring:
– cells divided in sectors (e.g. 3, 6 sectors)
– each sector has allocated its own set of channels
– base stations use directional antennas for covering sectors
• Microcells and picocells: very small cells
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– Control channels: exchange control data for calls management
– Traffic channels: data or voice connections between users
– Dominant switching mode: circuit-switch
BS
MS
BS
BS
MS
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Cellular systems operation example [1]
(a) Mobile station initialization
(b) Mobile-originated
call
(c) Paging
(d) Call accepted
Cellular systems operation example [2]
(e) Ongoing call
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(f) Handoff
Other operations:
call blocking
call termination
call drop
calls to/from fixed and remote mobile subscriber
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Aspects of Cellular Networks [1]
Aspects of Cellular Networks [3] - Access
Methods
• Radiowave propagation:
• Frequency Division Multiple Access ->FDMA
– signal strength
– fading
– diverse propagation patterns
• Handoff: assigning a MS to a BS other than the current one
when MS move from one cell toward other cell
• Different handoff parameters: cell blocking probability, call
dropping, call completion, handoff success, handoff
blocking, ...
• Handoff strategies:
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relative signal strength
relative signal strength with threshold
relative signal strength with hysteresis
relative signal strength with hysteresis and threshold
prediction techniques
– two (frequency) channels assigned per user, one for forward and one for
reverse
– used in first generation cellular (e.g. AMPS)
• Time Division Multiple Access ->TDMA
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each physical channel divided in logical subchannels
two logical channels assigned for user, for forward and reverse links
transmission in repetitive sequence of frames divided in time slots
each time slot position forms a logical channel that is assigned to the
user
– used in second generation cellular (e.g. GSM)
• Code Division Multiple Access -> CDMA
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– direct-sequence spread spectrum transmission -> use a chipping code
for data
– two logical channels per user
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– used in second and third generation cellular
GSM Cellular Network
Second Generation Cellular Networks
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First, Second, Third Generation Cellular
Networks
• More in 2G than 1G:
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Satellite Communication
2G have digital traffic channels, 1G pure analog
encryption
error detection and correction
dynamic channel access -> users share dynamically channels
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• 3G capabilities:
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voice quality comparable with switched telephone network
up to 384 kbps data rate outdoor
support for up to 2.048 Mbps indoor
symmetrical/asymmetrical data transmission rates
support for circuit switched and packet switched data services
support for wide variety of equipment
efficient spectrum usage
Internet interface
flexibility
Communication between earth stations and satellites
Uplink-> earth station to satellite
Downlink-> satellite to earth station
Satellite categorization:
Coverage area: global, regional, national
– Service type: fixed service satellite (FSS), broadcast
service satellite (BSS), mobile service satellite (MSS)
– General usage: commercial, military, amateur, experimental
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Satellite Networks Configurations
Satellite Communication and Wireless
Terrestrial Communication
• Advantages of satellite communication:
• Point-to-point link
– extensive area of coverage
– relative slowly variant conditions for communication between
satellites
– transmission cost independent of distance
– support for broadcast, multicast and point-to-point
communication
– high bandwidth and high data rates
– high quality of transmission
• Drawbacks of satellite communication :
• Broadcast link
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expensive installation
transmission delay
needed terminals power
needed number of satellites for global coverage
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GEO and LEO
Satellite and Orbits
• GEO - geostationary orbit satellites
• LEO - low earth orbit satellites
• MEO - medium earth orbit satellites
• GEO characteristics:
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no problems with the frequency change due to satellite movement
simplified tracking of satellite
very large area coverage (e.g. 3 satellite for almost whole Earth)
weak signal and extensive delay (e.g. 0.5 s) due to long distance
polar region poorly served
• LEO characteristics:
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small coverage area-> big number of satellites
satellite visibility cca. 20 minutes
frequency changes due to satellite movement
significant atmospheric drag
small latency
high data rates, up to few Mbps for Big LEOs
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Issues in Satellite Communication
Frequency Bands for Satellite
Communication
• Problems:
– Distance between earth station antenna and satellite
antenna
– Downlink: terrestrial distance between earth antenna and
the “aim point” of the satellite
– Atmospheric attenuation: oxygen, water, higher
frequencies
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Capacity Allocation Strategies:
frequency division multiple access (FDMA)
time division multiple access (TDMA)
code division multiple access (CDMA)
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Satellite Communication Examples
IRIDIUM LEO Satellite System
TDMA Satellite Transmission
(a)
(b)
Frågor
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