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Overview of GSM and CDMA

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    An Overview on

    G.S.M.&

    C.D.M.A.

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    What is Tele-Communication ?

    Tele means DISTANCE and Communication

    means INFORMATION TRANSFER So transfer of information between two ormore entities which may be far apart is called

    as Tele-Communication

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    Types of Communication tech.

    Wireline

    Wireless

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    Communication Path

    Wireline Physical layout of wires or optical fiber cables between

    entities . A physical contact is essential between the

    peer entities for communication Wireless

    NO Physical connectivity is required . Communicationis done through radio links ( Electro Magnetic Waves )

    and repeaters ( CELL SITES) are placed in betweenpeer entities for efficient communication

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    What does one expects

    from Wireless ?

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    Lower cost

    Roaming

    Better service and coverage

    NO dropped calls Enhanced Privacy

    NO Speech Clipping

    NO Echo

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    Wireless It is type of communication technology

    where electromagnetic waves carry thesignal (voice and data) on whole or part

    of communication path. Wireless telecommunication involves

    converting an audio signal into a

    Frequency (RF) signal and broadcastingit using radiating devices calledantennas.

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    MSC

    ARCHITECTURE

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    COMPONENTS OF WIRELESS

    Main Switching Center (MSC) Cell Site

    Cells

    Hand set

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    Main Switching Center ( MSC)

    The center where the wireless switch issituated is called of main switching

    center. All the call are first come to mscthen they are then routed to their

    respective handsets through cell sites

    locating the shortest path to therespective handset.

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    CELL SITE

    A Cell site contains a BTS(Base trans-receiver system ) which manages, sends,

    and receives traffic from the mobiles in itsgeographical area to a cellular telephoneswitch. It also employs a tower which has

    antennas, and provides a microwave linkto the distant cellular switch called MSC

    and Handset

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    Base Trans-Receiver System(BTS)

    The Base Station Subsystem controls the radio

    link with the Mobile Station It is physicalsystem which is installed with cell site. It hasReceiver , Transmitter and Power back up. Itreceives signal and add strength to the

    signals thus repeating the signal to its originalform and transmitting it again to next entitywhich may be cell site or hand set

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    CELL

    CELL is the geographical area covered by

    a cell site . All the receivers ( handsets)

    in this particular area respond to thiscell site i.e. they receives and transmits

    signal to a particular cell only

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    Techniques Used in Wireless

    Global System of Mobile Communication(GSM)

    Wireless in Local Loop (WLL)

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    G.S.M.GSM is an international standard ensuring that the

    same mobile telephony system is used in asmany countries as possible all over the world.

    This makes it possible to use the same mobilephone in different locations worldwide.

    GSM employs TDMA + FDMA Technique

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    FEATURES

    Already deployed as a worldwide

    standard

    National/International roaming

    Voice quality comparable to wire line

    SIM Facility

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    FDMA

    In Frequency Division Multiple Access, thefrequency band is divided in slots. Each

    user gets one frequency slot assignedthat is used at will.

    It could be compared to AM or FM

    broadcasting radio where each stationhas a frequency assigned.

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    TDMA

    In Time Division Multiple Access, the frequencyband is not partitioned but users are allowed to

    use it only in predefined intervals of time, oneat a time.

    Each caller is assigned a specific time slot fortransmission

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    CELL STRUCTURE AND FREQUENCY

    ALLOCATION

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    A frequency (channel) can be usedagain within an FDMA or TDMA network,

    but cells using the same frequencymust be separated by an appropriate

    distance. Adjacent cells must beassigned a different set of frequencies.For example, a cell using frequency A

    must not be adjacent to another cellusing frequency A.

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    How GSM works ?

    It works on the principle of

    FDMA + TDMA

    This means the bandwidth allotted isfirstly divided accordingly cell i.e. eachcell region works on different frequencyallotted to it and each cell frequency is

    different from the adjacent cells.

    This is FDMA

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    So each cell works at a particularfrequency and now this frequency is

    divided in time slots ie each user isgiven a specified time in which data of

    that respective user is transferred .

    This is TDMA

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    Wireless in Local Loop

    WLL is also a standard evolved andused in USA,It is emerging because

    of its advantages over GSM.

    WLL uses CDMA as communication

    technique

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    CDMA

    CODE DIVISION MULTIPLE ACCESS

    Code Division means the voice analog

    signal is converted into itscorresponding digital signal but this

    digital signal is accordingly a uniquecode assigned to that user.

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    Multiple Accessmeans the same frequencyrange i.e. bandwidth is used by all the users

    But there is no interference between the usersbecause each user is is talking to its

    respective counterpart in a unique codeassigned to it which is different from the

    other users

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    CDMA is a ..

    modulation and access system that

    employs signature codes (rather thantime slots or frequencybands) to

    arrange simultaneous and continuousaccess to a network by multiple users.

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    SPREAD SPECTRUMTECHNIQUE

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    CDMA works on the principle SPREADSPECTRUM Technique, which means

    that it spreads the informationcontained in a particular signal of

    interest over a much greater bandwidth

    than the original signal i.e. large rangeof frequencies are used for informationtransfer

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    Due to Spread Spectrum Technique

    Capacity increses of 8-10 times that of gsmsystem

    Improved call quality,with better and more

    consistent sound . Simplified system planning through the use of

    same frequency in every sector of cell

    Enhanced privacy

    Improved coverage

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    Cell Structure & Frequency

    allocation of CDMA

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    Each CELL-SITE in a CDMA networkcan use all available frequencies.

    Adjacent Cells can transmit at thesame frequency because users are

    separated by Code Channels

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    DIFFERENCES

    between

    GSM & CDMA

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    GSM Works on

    TDMA +FDMA

    Different cell works ondifferent frequencies

    Due to change in frequencyfrom one cell to anothercomplex hand off

    Due to Complex procedureCall interference is morelikely

    WLL Works on CDMA with

    Spread SpectrumTechnique

    Universal frequency use

    Soft Handoff tech leads tolower call drops

    Rake receiver technologyleads low call interferenceand .

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    Hands-Off

    Handoff is the process of

    transferring a call from one cell to

    another. This is necessary tocontinue the call as the phone

    travels.

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    Hands-Off in GSM

    TDMA and FDMA systems use a hard handoff

    when the mobile is moving from one cell siteto another. These technologies do not allow

    for any type of make-before-break handoff. Ahard handoff can increase the likelihood of a

    dropped call. A hard handoff requires themobile to break the connection with the oldBTS prior to making the connection with the

    new one. Hard handoffs are also calledBreak-Before-Make

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    Hard Hands-Off

    Break before Make

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    Soft Hands-OffMake before Break

    CDMA

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    Call Interference leads to

    Disturbance ,Low Speech,Echo

    Signals sent over the air can take a direct

    path to the receiver, or they can bounce offobjects and then travel to the receiver. Thesedifferent paths, called multi-paths, can resultin the receiver getting several versions of the

    same signal but at slightly different times.Multi-paths can cause a loss of signal through

    cancellation in other technologies

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    GSM

    When a signal travels to a handset through

    different paths ,in GSM handset ,it will pickthe first signal reaching to it, no matter how

    weak that signal is. Thus leading to CallDisturbance, Low Speech and some times

    Echo

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    Rake Receiver in CDMA

    CDMA's rake receiver is multiple receivers inone. The rake receiver identifies the three

    strongest multi-path signals and combinesthem to produce one very strong signal. Therake receiver therefore uses multi path to

    reduce the power the transmitter must send.

    Both the MOBILE and the CELL SITES use rakereceivers

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    Power Adjustments

    If all mobiles transmitted at the same powerlevel, the base station would receive

    unnecessarily strong signals from mobilesnearby and extremely weak signals frommobiles that are far away. This would reduce

    the capacity of the system.

    This problem is called the NEAR-FAR problem

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    Power Adjustment in GSMIf all mobiles transmitted at the same power

    level, the base station would receive

    unnecessarily strong signals from mobilesnearby and extremely weak signals from

    mobiles that are far away. Thiswould reduce

    the capacity of the system. This problem iscalled the near-far problem

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    Power Adjustment in CDMA

    Power control is a CDMA feature that enablesmobiles to adjust the power at which they

    transmit. This ensures that the base stationreceives all signals at the appropriate power.The CDMA network independently controlsthe power at which each mobile transmits.

    Both forward and reverse links use powercontrol techniques.

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    ADVANTAGES

    CDMA capacity is ten to twentytimes that of analog systems, and

    it's up to four times that of TDMA. CDMA's universal frequency reuse

    CDMA users are separated by code

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    CDMA's soft handoff leads to low

    or no call drops The Rake Receiver leads to better

    voice quality

    NO INTERFERENCE

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    Power control keeps power at anoptimal level.

    Power control helps the networkdynamically expand the coveragearea.

    CDMA's wide band signal reducesfading.

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    CDMA technology has numerous

    advantages including:

    Coverage

    Capacity

    Clarity

    Cost

    Compatibility

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    Coding provide the ability to covermore users for the same amount of

    available power used in other systems.

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    THANK YOU


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