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UNIT 5
MULTI-USER RADIOCOMMUNICATION
1
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Generations of Cellular Mobile Radiophones
1G Advanced Mobile Phone System (AMPS): 1980s, Frequency
Modulation (FM), Frequency Division Multiple Access (FDMA),handover between cells, limited roaming between networks
2G Global System for Mobile communications (GSM): 1990s, digital-
coding of voice, Time Division Multiple Access (TDMA),Subscriber Identity Module (SIM), data communications
3G
3G Partnership Project (3GPP), Universal MobileTelecommunications System (UMTS): 1998-, Wideband CodeDivision Multiple Access (WCDMA), use of GSM network model,global roaming; 2 Mbps data
4G All-IP-based, 100 Mbps data
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Generations of Cellular Systems Generations of cellular systems include:
AMPS 1stgeneration
GSM 2ndgeneration
W-CDMA 3rd generation
Cellular systems operate based on variousprotocols, and use RF (radio frequency)waves that propagate through the air fortransmission of information. These systemstypically use the 800-900 MHz or 1800-1900MHz frequency band of the radio spectrum.
But what is the radio spectrum?
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The Radio Spectrum
The Radio spectrum is composed of many frequencybands
Communication systems have the liberty to transmitsignals at various frequencies, and the FCC (FederalCommunications Commission) regulates whichfrequencies to use
Source:http://howstuffworks.lycoszone.com/radio-spectrum1.htm
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Radio Spectrum Bands
MF
AM Radio
Maritime
HF
Maritime
Amateur Radio
VHF
Television
FM radio
Aviation
UHF/SHF
Satellite
Television
Cell Phones
Microwave
EHF
Astronomy
Low Frequency High Frequency
~400-2400 MHz frequency range
FM radio stations between: 88-108 MHz
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Advanced Mobile PhoneService
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AMPS Components
Mobile Units
contains a modem that can switch between manyfrequencies
3 identification numbers: electronic serial number,system ID number, mobile ID number
Base Transceiver
full-duplex communication with the mobile Mobile Switching Center
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AMPS
Spectral allocation in North America
Two25-MHz bands are allocated to AMPS: 869-894MHz from the base station to the mobile unit, 824-849 MHzfrom the mobile unit to the base station
The bandwidth has been split into two 12.5 MHz in eachdirection for two operators to compete each other.
A 12.5 MHz channel allows 416 channels.
Spatial allocation
10-50 frequenciesare assigned to each cell Original cells are 6.5-13km in size. 1.5-km is the practical
minimum size. Too small size will have more frequencychange.
Transferring from one base transceiver to another is calledhandoff.
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AMPS FREQUENCY SPECTRUM
9
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AMPS Control channels
Used for exchanging control informationbetween mobile units and base stations.
21 control channels in A and 21 controlchannels in B
Used for call orgination,call termination
and to obtain system information.
10
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Frequency ReuseA Seven-Cell Cluster
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Channel Assignment Strategies
Channel assignment strategy fixed channel assignment
dynamic channel assignment
Fixed channel assignment
each cell is allocated a predetermined set of voice channel
any new call attempt can only be served by the unused channels
the call will be blockedif all channels in that cell are occupied
Dynamic channel assignment
channels are not allocated to cells permanently.
allocate channels based on request.
reduce the likelihood of blocking, increase capacity.
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Frequency Reuse
Each cellular base station is allocated a group of radiochannels within a small geographic area called a cell.
Neighboring cells are assigned different channel groups.
By limiting the coverage area to within the boundary of the
cell, the channel groups may be reused to cover differentcells.
Keep interference levels within tolerable limits.
Frequency reuse or frequency planning
seven groups of channel from A to G
footprint of a cell - actual radio
coverage
omni-directional antenna v.s.
directional antenna
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Handoff Strategies
When a mobile moves into a different cell while a
conversation is in progress, the MSC automatically transfersthe call to a new channel belonging to the new base station.
Handoff operation
identifying a new base station
re-allocating the voice and control channels with the newbase station.
Handoff Threshold
Minimum usable signal for acceptable voice quality (-
90dBm to -100dBm)
Handoff margin cannot be too large or too small
usableminimum,, rhandoffr PP
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Global Systemfor Mobile Communication
Developed to provide common 2nd-generationtechnology for Europe
GSM transmission is encrypted, using stream cipher A5for transmissions from subscriber to transceiver. A3 isused for authentication.
It uses subscriber identity module(SIM) in the form ofsmart card.
Supports both data and image services based on ISDNmodel, with rates up to 9.6 kbps
Spectral allocation: 25 MHz for base transmission(935960 MHz), 25 MHz for mobile transmission (890915 MHz)
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GSM Layout
Mobile Service Switching Center (MSSC)
HLR, VLR, AuC, EIR
HLR: home location register database VLR: visitor location register
AuC: authentication center EIR: equipment identity register database
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BASE STATION SYSTEM (BSS)
n BTS n BTS
BSC
BSC
BSC
MSC/VLR
BSS
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BTS & BSC
BTShouses the radiotransceivers ofthe cell and handles the radio-link
protocols with the mobile BSCmanages radio resources
(channel setup, handover) for one or
more BTSs
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MSC
MSCMobile Switching Center
The central component of the network
Like a telephony switch plus everythingfor a mobile subscriber: registration,authentication, handovers, call routing,
connection to fixed networks. Each switch handles dozens of cells
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Continue..
HLRdatabase of all users + currentlocation.
VLRdatabase of users + roamers insome geographic area. Caches the HLR
EIRdatabase of valid equipment
AuCDatabase of users secret keys
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More GSM
GSM comes in three flavors(frequencybands): 900, 1800, 1900 MHz. 900 is
the Orange flavour in Israel.Voice is digitized using Full-Rate coding.
20 ms sample => 260 bits . 13 Kbps
bitrate
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Sharing
GSM uses TDMA and FDMA to leteverybody talk.
FDMA: 25MHz freq. is divided into 124carrier frequencies. Each base stationgets few of those.
TDMA: Each carrier frequency is dividedinto bursts [0.577 ms]. 8 bursts are aframe.
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Multiple Access
Four ways to divide the spectrumamong active users
frequency-division multiplexing (FDM) time-division multiplexing (TDM)
code-division multiplexing (CDM)
space-division multiplexing (SDM)
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Choice of Access Methods
A random access scheme using FDM, TDM, SDM or CDM todynamically assign sub-channels to users is called randomaccess method, e.g. FDMA, TDMA, CDMA, SDMA.
FDM, used in 1st generation systems, wastes spectrum
Debate over TDMA vs CDMA for 2nd generation
TDMA advocates argue there is more successful experience withTDMA.
CDMA proponents argue that CDMA offers additional features aswell, such as increased range.
TDMA systems have achieved an early lead in actualimplementations
CDMA seems to be the access method of choice for third-generation systems
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Third Generation Systems IMT-2000 defined the 3rd-generation capacities:
voice quality, 144kbps data rate for high speed mobile, 384 kbps data ratefor low speed mobile, 2.048 Mbps office use, packet/circuit switching,Internet interface, more efficiency of spectrum use, more mobile equipmentsupport, flexible for new services and technologies.
Intended to provide high speed wireless communications formultimedia, data, and video
Personal communications services (PCSs) and personalcommunication networks (PCNs) are objectives for third-generation wireless.
Planned technology is digital using TDMA or CDMA to provide
efficient spectrum use and high capacity PCS handsets are designed to be low power, small and light
Future public land mobile telecommunications systems(FPLMTS) includes both terrestrial and satellite-based services
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Geostationary Satellites
Circular orbit 35,838 km above theearths surface
rotates in the equatorial plane of theearth at exactly the same angular speedas the earth
will remain above the same spot on theequator as the earth rotates.
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LEO and MEO Orbits
Alternatives to geostationary orbits
LEO: Low earth orbiting (320-1100 Km)
Stronger signals Propagation time is smaller
Coverage can be better localized
Needs more satellites (66 for Iridium system)
MEO: Medium earth orbiting (>10,000Km)
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Satellite Orbits
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Types of LEOs
Little LEOs: Intended to work atcommunication frequencies below1 GHz
using no more than 5 MHz of bandwidthand supporting data rates up to 10 kbps
Big LEOs: Work at frequencies above 1
GHz and supporting data rates up to afew megabits per second
I idi A 3rd G ti
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Iridium: A 3rdGenerationSatellite System
66 small LEOs
Services: voice, paging, wireless phone
Proposed in 1987
Put in service 1999
Named for the element iridium because 77 electrons match thenumber of satellites
Transmissions between satellites
$5 billion to implement
Motorola 9505 terminal for Iridium weighs about 13 oz. (370g)2.4 hour talk time, 24 hours standby time
Using L band (1600-1700 MHz) for ground communications and18-30 GHz between satellites