Access Technologies in Cellular Communication

8/3/2019 Access Technologies in Cellular Communication

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ACCESS TECHNOLOGIES INCELLULAR COMMUNICATION


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CONTENTS

Basics of cellular ofcellular technology

Cellular systemarchitecture

Cellular systemcomponents

Cellular accesstechnologies

FDMA TDMA

CDMA

TDMA versus CDMA


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Basics of Cellular Technology In a cellular network, cells are generally

organized in groups of seven to form a

cluster. Size of a cell depends on the density of

subscribers in an area

All base stations of each cell areconnected to a central point, called theMobile Switching Office (MSO


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Basics of Cellular Technology MSO is generally

connected to the PSTN(Public SwitchedTelephone Network)

Cellular technologyallows the hand-off ofsubscribers from one

cell to another as theytravel around


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Cellular System Architecture Cells

Is the basic geographicunit of a cellular system

Are base stationstransmitting over smallgeographic areas that arerepresented as hexagons.

Size varies depending onthe landscape.

Clusters is a group of cells. No channels are reused

within a cluster


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Cellular System Architecture Frequency Reuse

A way to reuse radiochannels to carry more

than one conversation ata time

Is based on assigning toeach cell a group of radiochannels used within asmall geographic area

Cells are assigned agroup of channels that iscompletely different fromneighbouring cells


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Cellular System Architecture Footprint

coverage area of cells iscalled the footprint.

footprint is limited by aboundary.

the same group ofchannels can be used indifferent cells that are far

enough away from eachother so that theirfrequencies do notinterfere.


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Cellular System Architecture Cell Splitting

is used to split a

single area intosmaller ones.

can be split into asmany areas as

necessary to provideacceptable servicelevels in heavy-trafficregions


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Cellular System Architecture Handoff

occurs when the mobiletelephone networkautomatically transfers a

call from radio channel toradio channel as a mobilecrosses adjacent cells

When the mobile unitmoves out of the coveragearea of a given cell site, thereception becomes weak

After requesting Handoffsystem switches the call toa stronger-frequencychannel in a new sitewithout interrupting the callor alerting the user


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Cellular System Components PSTN (Public Switched Telephone Network

is made up of local networks, the exchange areanetworks, and the long-haul network

interconnect telephones and other communicationdevices on a worldwide basis.

Mobile Telephone Switching Office (MTSO) MTSO is the central office for mobile switching

Houses the mobile switching centre (MSC), fieldmonitoring, and relay stations for switching callsfrom cell sites to wire line central offices (PSTN)


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Cellular System Components The Cell Site

used to refer to the physical location of

radio equipment that provides coveragewithin a cell.

Mobile Subscriber Units (MSUs)

consists of a control unit and a transceiverthat transmits and receives radiotransmissions to and from a cell site.


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Cellular Access Technologies

TDMA Time Division Multiple Access

FDMA Frequency Division Multiple Access

CDMA Code Division Multiple Access


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Frequency Division Multiple

Access (FDMA) Is an analogue transmission technique

used for mobile phone communications,

in which the frequency band allocatedto a network is divided into sub-bandsor channels.

multiple users can share the availableband without the risk of interferencebetween the simultaneous calls.


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Frequency Division Multiple

Access (FDMA) Working

FDMA splits the allocatedspectrum into many channels.

When a FDMA cell phoneestablishes a call, it reserves

the frequency channel for theentire duration of the call. Thevoice data is modulated intothis channels frequency band(using frequency modulation)and sent over the airwaves

At the receiver, the informationis recovered using a band-pass

filter. The phone then uses acommon digital control channelto acquire channels.


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Advantages of FDMA If channel is not in use, it sits idle

Channel bandwidth is relatively narrow (30kHz)

Simple algorithmically, and from a hardwarestandpoint

Fairly efficient when the number of stations issmall and the traffic is uniformly constant

Capacity increase can be obtained by reducing theinformation bit rate and using efficient digital code

No need for network timing

No restriction regarding the type of baseband or

type of modulation


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Disadvantages to using FDMA

The presence of guard bands

Requires right RF filtering to minimize

adjacent channel interference

Maximum bit rate per channel is fixed

Small inhibiting flexibility in bit ratecapability

Does not differ significantly from analogsystem


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Time Division Multiple Access(TDMA)

is digital transmission technology that allowsa number of users to access a single radio-

frequency (RF) channel without interferenceby allocating unique time slots to each userwithin each channel.

divides a single channel into six time slots,

with each signal using two slots, providing a3 to 1 gain in capacity over advanced mobile-phone service (AMPS).


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Time Division Multiple Access(TDMA) :-Working

TDMA relies upon the factthat the audio signal hasbeen digitized; that is,divided into a number of

milliseconds-long packets. It allocates a single

frequency channel for ashort time and then movesto another channel.

The digital samples from asingle transmitter occupydifferent time slots in severalbands at the same time


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Advantages of TDMA

Flexible bit rate

No frequency guard band required

No need for precise narrowband filters Easy for mobile or base stations to initiate

and execute hands off

Extended battery life

TDMA installations offer savings in basestation equipment, space and maintenance

The most cost-effective technology forupgrading a current analog system to digital


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Disadvantages to using TDMA

Requires network-wide timingsynchronization

Requires signal processing fro matchedfiltering and correlation detection

Demands high peak power on uplink in

transient mode

Multipath distortion


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Code Division Multiple Access(CDMA)

Is the latest, greatest cellulartechnology.

CDMA channels the packets of voiceand data over wireless radiofrequencies so a cellular user can hear

better, quicker and with more qualitythen ever before.


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CDMA, after digitizing data, spreads it outover the entire available bandwidth.

Multiple calls are overlaid on each other onthe channel, with each assigned a uniquesequence code.

Data is sent in small pieces over a number ofthe discrete frequencies available for use atany time in the specified range.

Code Division Multiple Access(CDMA)


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Code Division Multiple Access(CDMA):- Working

All of the users transmit in thesame wide-band chunk ofspectrum. Each user's signal isspread over the entirebandwidth by a unique

spreading code. At the receiver, that same

unique code is used to recoverthe signal. Because CDMAsystems need to put anaccurate time-stamp on eachpiece of a signal.

Between eight and 10 separatecalls can be carried in the samechannel space as one analogAMPS call

operates in both the 800-MHzand 1900-MHz frequency bands


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Advantages of CDMA

Many users of CDMA use the same frequency,TDD or FDD may be used

Multipath fading may be substantially reducedbecause of large signal bandwidth

No absolute limit on the number of users

Easy addition of more users Impossible for hackers to decipher the code

sent

Better signal quality

No sense of handoff when changing cells


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Disadvantages to using CDMA

As the number of users increases, theoverall quality of service decreases

Self-jamming

Near- Far- problem arises


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The Future of CDMA

CDMA has overcome most cynicism to dominate theworldwide wireless voice market

What about data services? Scheduling vs.

Interference Averaging CDMA appears to be an underdog for 4G, but still

may win Ongoing research on CDMA Increase capacity by joint decoding (multi-user

detection & interference cancellation) Applying CDMA to other applications: optical CDMA,

ad hoc networks, dense wireless LANs MultiCDMA: multiple antenna CDMA, multicarrier

CDMA, multicode CDMA


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BIBLOGRAPHY

www.iec.org/online/tutorials/tdma http://www.iec.org/online/tutorials/cell_comm/ http://people.deas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/cdma/cd

ma.html http://www.tsp.ece.mcgill.ca/Telecom/Docs/cdma.html http://cas.et.tudelft.nl/~glas/ssc/techn/ http://www.umtsworld.com/technology/cdmabasics.htm http://www.cellphonecarriers.com/compare-tdma-cdma.html http://www.cellphonecarriers.com/cdma-wireless-phone.html http://searchmobilecomputing.techtarget.com/sDefinition/0,,sid40_gci213959,00

.html http://en.wikipedia.org/wiki/TDMA http://electronics.howstuffworks.com/cell-phone.htm/printable www.cellphonecarriers.com/analog-fdma.html http://people.deas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/lecture_7.

html
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Mobile phone

Mobile phone is a device used by a subscriber forwireless communication with a cellular mobiletelephone network. The subscriber will usually be

able to communicate with other mobile phones withinthe same or different mobile networks, and is alsoable to connect with phones in the traditional wiredtelephone network. Many mobile phones also haveconnection ports to enable them to interface with

other equipment, for example to link up to acomputer, which makes it possible to download filessuch as text, games or multimedia


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Frequency Band

is a term used in telecommunications torefer to a range of frequencies

authorized for specific purposes. Eachband will be a continuous spectrum offrequencies, with upper and lowerlimits, and international use of these

bands is regulated by the InternationalTelecommunication Union (ITU) toprevent interference.

Documents

Access Technologies in Cellular Communication