Satellite and radio communication

8/10/2019 Satellite and radio communication

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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

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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.

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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

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Satellite and radio communication

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