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    CDMA

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    Multiple Access Methods

    FDMA (Frequency Division MultipleAccess )

    TDMA (Time Division Multiple Access )

    CDMA (Code Division Multiple Access )

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    FDMA

    Frequency Division Multiple Access(FDMA)

    An approach to sharing a channel by

    separating the simultaneous users in

    frequency

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    FDMAf

    c

    t

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    Analog Cellular Systems (FDMA Systems)

    AMPS (Analog Mobile Phone Service)

    TACS (Total Access Communication System)

    NMTS (Nordic Mobile Telephone System)

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    Time division multiple access (TDMA) Approach for allotting single-channel

    usage amongst many users, by

    dividing the channel into slots of timeduring which each user has access to

    the medium.

    TDMA

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    TDMA

    f

    t

    c

    2.17.1

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    TDMA/FDD

    f

    c

    2.18.1

    t

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    CDMA

    Code Division Multiple Access (CDMA) Spread spectrum technique using

    high-speed pseudo random (PN) codes

    to scramble data words and spreadspectral occupancy for added

    robustness.

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    CDMA

    f

    t

    c

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    CDMASpreading and De-Spreading

    The base band signal to be transmitted ismultiplied by the user specific unique code

    The code word is of the order of 64 bits( to

    enable generation of a large number of

    combinations)

    Thus, the communication demands a largerspectrum spread than a typical FDMA

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    Defining Spread Spectrum

    A complete definition to Spread Spectrum, is the one given

    by Haykins..

    Spread Spectrum is a means of transmission in which

    the data sequences occupy a minimum bandwidth necessary

    to send it on bandwidth which is excess to it.

    Spread Spectrum is accomplished before transmissionthrough the use of a code that is independent of data

    sequences .

    The same code is used at the receiver to despread thereceived signal so that the original data sequence may be

    recovered

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    Advantages of Spread Spectrum

    Multi path access: Number of users uses a

    common channel for communication.

    Random access possibilities: Users can start their

    transmission at any arbitrary time.

    Privacy due to unknown random codes: The

    applied codes are - in principle - unknown to ahostile user. This means that it is hardly possibleto detect the message of an other user.

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    Concept of Spread Spectrum

    When the information bearing signal and a PNsequence is multiplied at a multiplier we obtain the

    desired modulation. The question is how do we getthe increase spectrum? This is a simple FourierTransform property.

    Multiplication in time domain is convolution infrequency domain. Hence by multiplying a narrowband information signal and a wide-band codesequence, the multiplied signal will have the spectrumsimilar to the wide-band PN code sequence.

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    How Spread Spectrum gives securecommunication?

    The spread signal gets the characteristics ofthe PN code sequence. This signal appearsnoise like to a receiver that has no idea aboutthe spreading code.

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    Spread Spectrum Technique

    Encoding Encoding Channel DecodingModulator

    PseudoRandom

    Generator

    PseudoRandom

    Generator

    De-modulator

    Spreading

    Code

    Spreading

    Code

    InputData

    OutputData

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    DSSS: Transmitter

    Modulator(BPSK)

    Pseudo

    NoiseBitSource

    Spread SpectrumSignal

    Spreader

    DataInput

    (Binary)

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    0

    1

    1 1 1

    0 0 0

    1

    1 1 1 1 1 1 1

    0 0 0 0 0 0 0 0 0 00 00

    A

    B

    C

    A = Data Input

    B = PN Bit Stream

    C= A + B

    Direct Sequence Spread Spectrum : Transmission

    0 0

    1

    0

    0 1 0 1 0 1 0 0 0 1 0 0 0 1 0 1 0 1 0 1 0 1 0 0 0 1

    1 0 1 0 1 0 0 1

    1 0 1 0 0 1 1 1 1 0 0 0 1 0 1 0 0 1 0 1 1 0 1 1 0 1

    0

    0 0 0 00 00 0 00 000 0

    1 1 1 1 11 1 1 1 1 1 1 1

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    DSSS:Receiver

    Demodulator(BPSK)

    Pseudo

    NoiseBitSource

    Spread SpectrumSignal

    Despreader

    DataOutput(Binary)

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    0

    1

    1 1 1

    0 0 0

    1

    1 1 1 1 1 1 1

    0 0 0 0 0 0 0 0 0 00 00

    A

    B

    C

    C = Recd.Signal

    B = PN Bit Stream

    A = C + B

    Direct Sequence Spread Spectrum : Reception

    0 0

    1

    0

    0 1 0 1 0 1 0 0 0 1 0 0 0 1 0 1 0 1 0 1 0 1 0 0 0 1

    1 0 1 0 0 1 1 1 1 0 0 0 1 0 1 0 0 1 0 1 1 0 1 1 0 1

    0 0 0 00 00 0 00 000 0

    1 1 1 1 11 1 1 1 1 1 1 1

    1 0 1 0 1 0 0 1 0

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    Quadrature Phase Shift Keying(QPSK)

    A

    B

    C

    D

    E

    F

    Carrier

    Sin Wct

    Cos Wct

    Pi/2

    2 bitSerial toparallel

    converter

    Summer

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    For example, if two adjacent bits are represented

    by +1, -1.The output at D will be [+1][SinWct] and that at E

    will be[-1][SinWct+pi/2] and the summer outputat F will be

    [Sin(Wct)]+[Sin(Wct)+(pi/2)-(pi)]=Sin (Wct)+Sin[(Wct)-pi/2]=

    2 sin[(Wct)-(pi/4)]Cos(pi/4).

    i.e., the two adjacent bits 1, 0 represented by (1,-1) now combined and denoted as a phase shiftof the carrier by pi/4.

    The following table gives all the phase shift valuesfor a combination of two bits taken at a time.

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    -Pi/4 (as per example above)01

    -3Pi/400

    3Pi/410

    Pi/411Phase Shift of the CarrierCB

    I

    Q11 State01 State

    10 State00 State

    QPSK

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    Offset Quadrature Phase Shift Keying(OQPSK)

    A

    B

    C

    D

    E

    F

    Carrier

    Sin Wct

    Cos Wct

    Pi/2

    2 bitSerial toparallel

    converter

    Summer

    Delay

    OQPSK differs from the conventional QPSK in that prior to carriermultiplication, a delay of a half-bit interval (with respect to the I path) is

    placed in the Q path.

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    Basic Model and Concepts

    pj

    psTransmitterReceiver

    Jammer

    Wss

    Bit Rate = Rb

    Wss = Bandwidth of the spreadsignal

    Rb= Bit rate of base band signalPs = Signal powerPj = Jammer power

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    E b =Ps/Rb and Nj = Pj/Wss. E b/Nj= (Wss/Rb)/ (Pj/Ps), where Wss/Rb iscalled processing gain (PG)

    Capacity of CDMA system : The capacity of a CDMA system depends on

    E b/No Frequency ReuseVoice Activity Factor Sectorization.For M users, we have the basic capacity equation as

    M-1=[PG]/[E b/No], where PG is the processing gainWss/Rb

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    Considering voice activity factor (v), frequencyreuse advantage (f), and the sectorizationadvantage (s), the capacity equation gets

    modified as:

    M = [PG] * [E b/No]-1

    * (1/v)*f*s.

    Ex: If Rb=9600 bps, Rc=1.2288Mcps, v=40%,f=0.65 and s=2.55, find the capacity?

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    Features of CDMA

    1. High Capacity : More channels per carrier as against 8 in GSM. Better Traffic Handling capacity Use of vocoders increases capacity Simpler frequency planning / frequency re-use.

    2 Lower Transmit Power :

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    2. Lower Transmit Power :

    The reduction in required Eb/No means thatthe MS has to Transmit less power. Thismeans that the cost of the MS comes down and

    the battery life increases. This also results inminimum interference, which means moreusers can talk simultaneously. Hence, more

    capacity.3. Improved privacy :

    The PN sequence operation, wide bandsignalling and certain addressee specificprotection features provide very good security

    to the users.

    4 Better performance in Fading/

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    4. Better performance in Fading/interference prone environment :

    This is derived from the basic concept ofspread spectrum technique which separates

    signals based on specific PN sequences. Thesystem has inherent multipath diversityfeatures.

    5. Improved capabilities: Variable rate vocoders for different grades of

    service.

    Interfaces to ISDN /Wireless PBXS/PCN/Cellular. Local loop applications in the PSTN.

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    GSM Vs. CDMA(Frequency Reuse)

    GSM CDMA

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    Models of Spread Spectrum CDMA Systems

    There are basically 4 models of SS-CDMASystems:

    1. One User One path

    2. One User Many paths3. Many Users One path

    4. Many Users Many paths

    One user one path Model

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    One user one path Model

    1. The base band signal of User j is denoted by. Bj(t).

    2. The PN sequence used for spreading thespectrum is Cj(t).3. The transmitted spectrum for user j is ..

    Bj(t) X Cj(t).

    4. At the receiver, the signal is received after adelay Tj. i.e., Bj(t-Tj) X Cj(t-Tj).

    5. We generate the PN sequence locally, with an

    arbitrary delay : Cj(t-T).6. The receiver output is then described by :Bj(t-Tj) X Cj(t-Tj)X Cj(t-T).

    7 If the local delay T is equal to the propagation delay Tj

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    7. If the local delay T is equal to the propagation delay Tj,then the receiver output becomes:

    Bj(t-Tj) X Cj(t-Tj)X Cj(t-Tj)= Bj(t-Tj) i.e. at the receiveroutput we get back the base band delayed by Tj.

    8. If the delay T generated locally at the receiver is notequal to Tj, then the receiver output becomes :

    Bj(t-Tj) X Cj(t-Tj)X Cj(t-T)= Bj(t-Tj) Cj(t-Tq). Thismeans that the receive output is still a SPREAD data.

    9. Thus, all other signals arriving at the receiver appear as

    spread data, excepting the desired data which gotdecoded as Bj(t-Tj). The other signals appear asnoise.

    10. Here, we have taken that there is only one direct path

    between the transmitter and the receiver and theeffects of multipath are not considered.

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    One user Many path Model

    The signals arrive at the receiver through anumber of paths, generated by reflections /

    scattering from buildings, trees, mountains,water etc. Therefore, the input to the receiver always has

    2 components:

    a. A direct path component given by : Bj(t-Tj) XCj(t-Tj).b. A multi path component given by : Bj(t-T jm)X C

    j(t-T

    jm).

    Where Tjm is the multipath delay for the signal fromuser J.

    S h l ll d PN S (

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    Suppose, the locally generated PN Sequence (at

    the receiver) is delayed by Tj. Let us look at thereceiver now:

    Receiver input : Bj(t-T jm) X Cj(t-Tjm).

    Locally generated PN Code: Cj(t-Tj). Receiver output : Direct path component +

    Multipath component .

    = [ Bj(t-Tj) X Cj(t-Tj) + Bj(t-T jm) X Cj(t-Tjm)] .Cj(t-Tj).

    = Bj(t-Tj) + [ Bj(t-T jm) X Cj(t-Tjm) X Cj(t-Tj) ]

    The term inside the bracket is the interferencesignal appearing as a spread multipath signal.

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    The spread interference or the multipath

    signal has a (t-Tjm) component.If (t-Tjm) is less than one chip (one bit )

    duration of Cj, the multipath component cannot

    be resolved and the demodulated output isdistorted.

    However, if (t-Tjm) is greater than one chipperiod, then it could be resolved. This could bedone by introducing a second receiver which hasits local PN generator delayed by Tjm.

    i.e., the second receiver is synchronized with

    the multipath component. One of the mostpopular type of such receivers is RAKE receiver.

    M M lti th M d l

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    Many users Multi path ModelIn a real life situation, there would be many usersoperating at the same frequency, at the same time.Each User would be spreading the signal with a uniquePN Sequence.

    The signals sent by each user undergo multipathpropagation and hence would have direct path andmultipath components at the receiver. The desiredsignal is separated by the receiver by using a local PNsequence generator as explained in the above model.

    Also, if the time difference (t-Tj) is greater than onechip duration, then for each user, the multipathcomponent could be resolved by using RAKE receiverarrangement.

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    However, for each user, there would be additional

    interference signals in the form of signals fromother users operating at the same frequency.

    If there are 3 users p, q and r.

    Then for user p, the receiver will have : A direct path component given by Bp(t-Tp) Spread interference signals pertaining to pq and

    pr.Components pq and pr are interfering signals

    from users q and r received at p.

    In other words, pq and pr are the co channelinterference for the desired signals. i.e., Bp(t-Tp).

    R k R i

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    Rake Receiver

    The rake receiver is a techniquewhich uses several base bandcorrelators to individually process

    multi path signal components. Theoutputs from the different correlatorsare combined to achieve improved

    reliability and performance.

    RAKE RECEIVER

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    Arake receiver is a radio receiver designed tocounter the effects of multipath fading. It doesthis by using several "sub-receivers" each

    delayed slightly in order to tune in to theindividual multipath components.

    Each component is decoded independently, butat a later stage combined in order to make themost use of the different transmissioncharacteristics of each transmission path.

    This could very well result in higher signal-to-noise ratio (or Eb/N0) in a multipath

    environment.

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    The rake receiver is so named because of itsanalogous function to a garden rake, eachfinger collecting bit or symbol energy similarly

    to how tines on a rake collect leaves.

    Rake receivers are common in a wide variety ofCDMA and W-CDMA radio devices such asmobile phones and wireless LAN equipment.

    RAKE RECEIVER Schemetic is shown below.

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    CDMA ARCHITECTURE

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    CDMA IS-95A ARCHITECTURE

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    BSC

    WPT 0

    WPT1

    TAP

    WPT 2

    BTS

    TE1

    TE2

    MSC

    OS AUX IWF

    MSC

    DMH

    HLR

    ACEIR

    VLR

    Other

    VLR

    A

    O XL

    Ai

    P

    E

    F C B Mi

    D

    Um Abis

    I

    I

    PSPDN

    PSTN

    ISDN

    PLMN

    TE2

    TE2

    TE2

    TE1

    TA

    W R

    S

    R

    R

    DCE

    G

    D

    HMS

    TE2

    Sm

    Rm

    Rm

    WPT : Wireless Personal Terminal

    DCE : Data communication equipment

    TE : Terminal Equipment

    Base Station subsystem

    External network

    The reference model elements are :

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    Mobile Station :The MS terminates with radio path on the

    user side and enables the user to gain access

    to services from the network.It can be a stand alone device or other

    device like PC, fax machine etc.

    Base station Subsystem:

    Base station terminates the radio path and

    connects to the mobile switching center (MSC).The base station subsystem is divided in to BTSand BSC.

    BTS (Base Transceiver System):

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    BTS terminate the radio path on thenetwork side. It may be co-located with BSC orindependently placed.

    The BTS contains the RF components thatprovide the air interface for a particular cell.This is the part of the CDMA network, which

    communicates with the MS. The antenna isincluded as part of the CDMA.

    BTS includes Transceivers, base bandsections, digital signal processing cards, powercards, Transmission cards, Antenna and filter

    sections etc.

    BSC (Base station controller):

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    It is the control management for one or more BTS.BSC exchanges information between BTS and MSC.

    The BTSs and BSC may either be located at thesame cell site co-located, or located at different sites

    Remote. In reality most BTSs will be remote, asthere are many more BTSs than BSCs in a network.

    Another BSS configuration is the daisy chain. ABTS need not communicate directly with the BSC

    which controls it, it can be connected to the BSC via achain of BTSs.

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    Daisy chaining reduces the amount ofcabling required to set up a network as aBTS can be connected to its nearest BTS

    rather than all the way to the BSC. Problemsmay arise when chaining BTSs, due to the

    transmission delay through the chain.

    The length of the chain must, therefore,

    be kept sufficiently short to prevent theround trip speech delay becoming too long.

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    Network switching subsystem:M bil S it hi C t

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    Mobile Switching Center:

    It is an automatic system interfaces with theuser traffic from the wireless network or other

    wireless networks.Depending upon the function of MSC, it can

    be classified into different ways.

    1. Provides radio contact in a call2. Controls BTSs adjacent to location of a MS.

    3. Accepts a call or a hand off

    4. Directing an incoming a call towards MS.

    5 C tl idi i t ll

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    5. Currently providing service to a call.

    6. Providing only trunk connections7. Selected for Handoff

    8. Providing service to MS.

    DMH (Data Message Handler) :Uses to collect billing data.

    Home Location Register (HLR):

    It manages mobile subscribers information

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    It manages mobile subscriber s informationby maintaining all subscribers information.

    The information is like ESN, IMSI, UserProfiles, and current location etc.

    It can be co-located or placedindependently. One HLR servers multipleMSCs.

    Visitor Location Register (VLR):

    It is linked to one or more MSC. Itdynamically stores subscribers information(ESN, User profiles)

    These informations are obtained from HLR

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    These informations are obtained from HLR.When a roaming MS enters a new service area,MSC informs the associated VLR about the MS

    by querying the HLR after MS goes through aregistration procedure.

    Authentication Center :It manages the information associated with

    the individual subscriber. It can be located within an HLR, or MSC or located independently ofboth.

    Equipment Identity Register:

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    Equipment Identity Register:Provides information about the mobile

    station for record purpose.

    Inter working function :

    Enables MSC to communicate with othernetworks.

    External Networks:They can be PSTN, ISDN, PLMN, PSPDN.

    Operation subsystem (OSS) :

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    Operation subsystem (OSS) :Responsible for overall management. Some

    of the functions of OSS are

    Event/Alarm Management. Fault Management.

    Performance Management. Configuration Management. Security Management.

    Interfaces between various elements of the system

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    BS to MSC (A-interface) :This interface between the base station and

    the MSC, supports signalling and traffic (bothvoice and data).

    A-interface protocols have been defined

    using SS 7, ISDN BRI/PRI and frame relaytransport (TIA IS-634).

    BTS to BSC interface : Abis if the base station issegmented into a BTS and BSC, this internal interface

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    is defined. MSC to PSTN interface : Ai. This is an analog

    interface, using either DTMF or MF signaling.

    MSC to VLR : B-interface. This interface is in the TIA-IS-45 protocol specification.

    MSC to HLR : C-interface. This interface is in the TIA-IS-45 protocol specification.

    HLR to VLR : D-interface. It is the signaling interfacebetween HLR and VLR based on SS7.

    MSC to ISDN : Di interface. This interface is thedigital interface to the PSTN and is a T1 interface.

    MSC to MSC : E interface. This interface is thetraffic and signaling interface between wireless

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    g g

    networks. MSC to EIR : F interface.VLR to VLR : G interface. When communication

    is needed between VLR, this interface is used.

    HLR to AC : H interface.

    DMH to MSC : I interface. MSC to IWF : I interface, this interface isdefined by the interworking function.

    MSC to PLMN : Mi interface. It is for anotherwireless network. MSC to OS : O interface.

    MSC to PSPDN : Pi interface It is defined by

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    MSC to PSPDN : Pi interface. It is defined bythe packet network.

    BS to MS : Um interface. This is an air

    interface. MSC to AUX: X interface. This depends on the

    auxiliary equipment connected to MSC

    PSTN to DCE : W interface. This is the outsidethe scope of PCS and is defined within the

    PSTN system.

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    CDMA Subscriber UnitArchitecture

    Transmitter Section

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    Transmitter Section

    Speech

    SourceEncode

    ChannelEncode

    MultipleAccess

    Modulate Transmitter

    Receiver Section

    SourceDecode

    ChannelDecode

    MultipleAccess

    Demodulate Receiver

    ReconstructedSpeech

    Voice Coding:

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    Voice Coding:Variable Rate Coding :Codecs used in CDMA systems are CELP (Code Excited Linear Predictor) EVRC (Enhanced Variable Rate Codec)(20 msec of speech is taken at a time for voice

    coding).

    Vocoders are source dependent codingsystems. They analyse the voice signal at thetransmitter end, and transmit parametersderived from the analysis. The receiver then

    synthesizes the voice signals according to theparameters received.

    Channel coding : Channel coding maps the

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    Channel coding : Channel coding maps theinput data into coded symbols containing morenumber of bits than the original signal. Differenttypes of Channel coding are

    1. Block coding , as the name implies, code aninformation sequence one block at a time.

    2. Convolution coding has a memory property.

    3. Turbo Coding is the most recent codingsystem which is used in 3G systems. Theprinciple used is based on convolution codingbut, the method is used for high speed wirelesscommunication requirements.

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    After this coding, bit output from thevocoder is grouped in terms of theirprobability to errors and are appropriately

    treated with FEC codes. The output codedsymbols are then spread over several dataframes by the method of interleaving thedata along with the FEC codes. This isdone to minimize the loss of data during

    transmission.

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    CODES IN CDMA

    EX-OR FUNCTION

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    011

    101

    110

    000

    Y=A BBA

    A

    B

    Y

    EX-OR Function Truth Table

    EX-OR gate produces HIGH output, if odd numberof inputs or HIGH, in all other cases, the output will beLOW.

    PN SEQUENCE

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    PN SEQUENCE :

    A Pseudo Random Noise sequence is one in

    which the bits appear in a random manner witha specified length and the pattern is repeatedfor subsequent sequences.

    PN sequence is the best choice as it appearsas noise to all other users excepting the desired

    receiver.

    Requirements of a PN sequence:

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    Be easy to generate

    Have random properties Have long periods

    Be difficult to reconstruct from a shortsegment.

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    PN SEQUENCE GENERATION Example : Suppose we have 4 digit words.

    The natural sequence is from 0000 to 1111.Purely random sequence could be a series of15 word sets, with the combination of words ineach set being random

    The PN sequence is important because, thereceiver needs a replica of the transmittedsequence to de spread the signals.

    The PN sequence has a random set of wordswhich repeat after a specific sequence length.

    Example of a PN sequence :

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    Consider a 4 bit sequence.. 0001.

    EX-OR

    1 23

    4

    0 0 0 1

    The first 4 bit sequence is 0001.

    The next 4 bit sequence will be 1000, 1100, 1110, 1111, 0111, 1011,0101, 1010, 1101, 0110, 0011, 1001, 0100, 0010.

    The next 4 bit after this again starts with 0001.

    Example for PN Sequence generator is (SSRG)

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    SIMPLE SHIFT REGISTER GENERATOR.

    A generic form of a simple shift registergenerator is shown below.

    1 2 3 4 5 6 N

    Feed back logic-exclusive OR circuits

    Properties of PN sequences:

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    PN Sequences exhibit the following properties: The maximal length of the sequence is 2n-1,

    where n is the number of stages in the shift

    register. The number of 1s will be 2(n-1) and that of 0s

    will be 2(n-1) -1. i.e., the number of 1s will be

    one more than the number of 0s. If a maximal SRG sequence is added to a phase

    shift (time shift) of it, then the resultingsequence is another phase shift of the originalsequence. This is called the shift and addproperty of SSRGs.

    A 4 stage SSRG type PN sequence generator

    0 0 0 1

    0001

    1000

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    EX-OR

    1 2 3 410001100

    1110

    1111

    0111

    1011

    0101

    1010

    1101

    0110

    0011

    1001

    0100

    0010

    The output sequence is : 1000 1111 0101 100

    It is 15 bits Long.i.e., L= 2n-1

    There are 8 ones and 7 zeroes

    In any period, half the run of continuous 1s or 0s are of length1. One fourth

    run are of length 2, one eighth are of length 3, and so on.

    Shift and Add property : If the PN sequence is shifted in time, theresulting sequence is another shift of the original sequence itself if the shift isby one bit/chip, then the original pattern repeats after 2n-1 sequence

    Example :Code : 100011110101100 100011110101100 100011110101100

    Shift 1 000111101011001 000111101011001 000111101011001

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    Shift 1: 000111101011001 000111101011001 000111101011001..

    XOR : 100100011110101 100100011110101 100100011110101

    The original code is marked with white digits in the result ofXOR. By shifting the sequence successively by one bit, it canbe verified that the original sequence repeats after 15sequences (2n-1).

    Example 2:

    For the same tappings, change the initial seeding of theshift register or feedback points and obtain the outputsequence. What are your observations, on the new sequence?

    Alternate configuration of the 4 stage SSRG

    3

    0 0 0 1

    0001

    1000

    0100

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    EX-OR

    1 2 3 4 0010

    1001

    1100

    0110

    1011

    0101

    10101101

    1110

    1111

    0111

    0011

    ..

    0001

    Out put sequence : 1000 1001 1010 111.

    Auto Correlation

    Thi d ib th t t f lik b t

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    This describes the extent of likeness between arandom variable and its time shifted version, itcan be defined by a simplified formula:

    Auto Correlation =x(t) * x(t-T) dTFor PN sequences this could be written as :

    Auto correlation = 0T

    C J (t) * C J(t-T)dT.The Auto correlation function of the PN

    sequence has a positive value = 2n -1 for zero

    time shift instances and has a small negativevalue at other instances, when the shift in timeis equal to or more than one chip duration.

    Cross Correlation

    C C l ti d fi th lik b t

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    Cross Correlation defines the likeness betweentwo different random variables and could bedescribed by :

    Cross Correlation =x(t) * y(t-T) dT

    For a PN sequence, this could be re written as :

    Cross correlation = 0T C J (t) * C k(t-T)dT.The cross correlation function will have verysmall negative values.

    Auto correlation of PN sequences :Consider the original sequence

    h d h

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    1000 1111 0101 100. This is compared withreceive sequences with time shifts T0, T1, T2 etc.

    T0 means zero time shift or exactsynchronization between transmitted andreceive sequences.

    To calculate the auto correlation values, we

    give a mark +1 for every bit that matches withthe reference (original) sequence and -1 forevery bit mismatch.

    R f C d 100011110101100 A t l ti

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    Ref Code : 100011110101100 Auto correlationTS 0 : 100011110101100 15

    TS 1 : 000111101011001 -1TS 2 : 001111010110010 -1

    TS 3 : 011110101100100 -1 andso on.

    14

    15

    Auto

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    0 1 2 3 4 5 6 7 8 9

    -1

    1

    32

    4

    5

    6

    8

    7

    9

    10 11 12 13 14 0

    10

    11

    12

    13

    14

    Cor

    relat

    ion

    v

    alue

    t

    Cross correlation of PN sequences:

    If we change the feed back tappings for our 4

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    If we change the feed back tappings for our 4stage SSRG, we get an entirely different PNsequence. Let us take the feed back from the3rd and 4th stage outputs with the initial loadingas 0001, from left to right (as shown in the

    slide above). The new PN sequence is : 1000 1001 1010 111We take a time shifted code from the

    original sequence 1000 1111 0101 100 andcompare that with the new code generated.

    This would give us the cross correlation valuesbetween the 2 different PN sequences

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    between the 2 different PN sequences.

    It is also possible to have the comparison

    between the first code and a time shiftedversion of the second.

    It is observed that the cross correlation values

    vary from -5 to +7, depending upon the extentof similarity between the 2 sequences.

    The comparison between the 2 PN sequences isgiven below and the cross correlation functionis presented pictorially in the coming slide.

    Ref Code : 100010011010111 cross corelation

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    Ref Code : 100010011010111 cross corelationTS 0 : 100011110101100 -1

    TS 1 : 000111101011001 -1TS 2 : 001111010110010 -1

    TS 3 : 011110101100100 -5

    TS 4 : 111101011001000 -5TS 5 : 111010110010001 +1

    TS 6 : 110101100100011 -5

    TS 7 : 101011001000111 +2

    2nd code

    Ref Code : 100010011010111 cross corelationTS 08 : 010110010001111 +1

    TS 09 : 101100100011110 -1

    2nd code

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    TS 09 : 101100100011110 -1TS 10 : 011001000111101 -5

    TS 11 : 110010001111010 +1TS 12 : 100100011110101 +7

    TS 13 : 001000111101011 -1

    TS 14 : 010001111010110 -1-------------------------------------------------------

    TS 0 : 100011110101100 -1

    Note that the cross correlation values vary from-5 to +7.

    7

    cros

    s

    8

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    0 1 2 3 4 5 6 7 8 9

    -6

    -4-5

    -3

    -2

    -10

    1

    10 11 12 13 14 0

    2

    3

    4

    5

    6Cor

    relat

    ion

    v

    alue

    t1 2 3 4

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    PN long code : It is having 42 bit length (242)and generated at a rate of 1.2288 Mcps. The

    code sequence repeats after 41 2 days

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    code sequence repeats after 41.2 days.242=4.39X1012.

    Chip Duration = 1/(1.2288X106)

    = 0.8138 Sec.

    Hence code duration = 4.39X1012

    X 0.8138X10-6

    = 3.579 X 106 sec

    = (3.579X106)/(24X60X60)

    = 41.2 days.

    In forward link, long code is used to encryptuser ID, in reverse link long code is used (ESN

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    user ID, in reverse link long code is used (ESN Electronic serial number). The code mask isdone by using ESN.

    Both Base station and respective user have

    knowledge of PN sequence at any given instantof time as a specified private code mask isexchanged between them.

    PN short code :

    This sequence uses 15 bits and code is

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    This sequence uses 15 bits and code isgenerated at 1.2288 Mcps. The code repeatswith a period of 26.67 m sec.

    215 = 32.768X103

    Chip duration = 1/(1.2288X106

    ) = 0.8138 SecHence, the code duration

    = 32.768X103X 0.8138X10-6

    = 26.67 m sec.

    The code is used by Base station and isused for final spreading of signal in the

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    The code is used by Base station and isused for final spreading of signal in theforward channel and is also transmitted

    as a pilot sequence by Base Station.

    All the base station use the same short

    code by distinct off-set time foridentification.

    i.e., PN offsets are used for BTS

    identity.

    ORTHOGONAL CODES

    Two codes are said to be orthogonal, if theproduct (Ex-or) of the two codes produces

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    Two codes are said to be orthogonal, if theproduct (Ex-or) of the two codes producesequal number of 1s and 0s.

    Ex : if A= 111010 and B=011001, then XoringA and B gives the answer as 100011, whichhas three number of 1s and three number of

    0s.Conventionally, 1 is mapped as +1 and 0 as

    -1.

    Example for orthogonal codes areWALSHCODES.

    WALSH CODES

    Most commonly used orthogonal codes in

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    y gCDMA systems

    A set of length n consists of n rows ofnXn Walsh matrix. W1= (0).

    In general, W2n= [ Wn, Wn, Wn, Wn ],where n is the dimension of the matrix.The over score denotes the logical NOT ofthe bits in the matrix.

    The matrix has the property that every

    row is orthogonal to every other row andalso to the logical NOT of every other row

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    g yalso to the logical NOT of every other row.

    W2x2 = 0 0

    0 1

    W4x4 = 0 0 0 0

    0 1 0 1

    0 0 1 1

    0 1 1 0

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    HAND OFFS IN CDMA

    Handoff Process in CDMA

    What is hand off?

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    What is hand off?In cellular telecommunications, the term

    handoffrefers to the process of transferringan ongoing call or data session from onechannel connected to the core network toanother.

    In a cellular telephone network, handoff is thetransition for any given user of signal transmissionfrom one base station to a geographically adjacentbase station as the user moves around. In an idealcell la telephone net o k each end se 's telephone

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    cellular telephone network, each end user's telephoneset or modem (the subscriber's hardware) is always

    within range of a base station. The region covered byeach base station is known as its cell. The size andshape of each cell in a network depends on the natureof the terrain in the region, the number of base

    stations, and the transmit/receive range of each basestation. In theory, the cells in a network overlap; formuch of the time, a subscriber's hardware is within

    range of more than one base station. The networkmust decide, from moment to moment, which basestation will handle the signals to and from each andevery subscriber's hardware.

    Each time a mobile or portable cellular subscriberpasses from one cell into another, the network

    automatically switches coverage responsibility fromone base station to another. Each base-station

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    transition, as well as the switching process orsequence itself, is called handoff.

    In a properly functioning network, handoff occurssmoothly, without gaps in communications andwithout confusion about which base station should bedealing with the subscriber. Subscribers to a networkneed not do anything to make handoff take place, norshould they have to think about the process or about

    which base station is dealing with the signals at anygiven moment.

    verse

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    Base Station

    Base Station

    Control

    Equipment

    Network InterfacePSTN

    Forward

    Rev

    er

    Handoff

    Base Station

    Control

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    Base Station

    Base Station

    Cell

    Boundary

    Types of Hand off

    Soft Hand off Soft Soft Hand off

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    So t So t a d o

    Softer Hand off

    Hard Hand offAll types of soft hand offs are called as

    make before break connections, and these

    are done only between cells having the sameCDMA channel (frequency) assignments.

    Hard hand off is called as break beforemake connection and these are donebetween cells having different CDMA channelfrequencies.

    Soft Hand off :This is the condition when two cells in a

    mobile network are in simultaneouscommunication with the mobile receiver

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    communication with the mobile receiver.

    This situation will continue till one of thepilot signals from a particular base stationfalls below a chosen threshold level.

    From this, the mobile will be transferred tothe remaining base station.

    Soft Soft Hand off :The situation here is the same as that

    described above. However, the number of cellsinvolved in this case are 3 This number is

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    involved in this case are 3. This number isdictated by the provision of rake receiver in themobile. This has 3 fingers for the separatereception and processing of three stations at a

    given time.

    Softer Hand off :

    This hand off procedure is similar tothe soft hand off But comes into picture

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    the soft hand off. But, comes into picturewhen the mobile transition is from one

    sector to another in the same cell area.The mobile is served by two sectorssimultaneously till one drops off.

    Hard Hand off :N di f t iti

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    Needing frequency transition

    This is similar to GSM. This occurs onlywhen the mobile transit from one area toanother where the allocated / used channel

    frequency happens to be different.

    Pilot sets in Hand off Procedure

    Hand offs are done by measuring pilotsignal strengths from neighboring cells.

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    g g g g

    Measurement message sent to basestation when a pilot of adequate strength,

    not belonging to the mobiles currentassignment of traffic channels is detected.

    Hand off process is dependent on pilotsearches.

    Pilots can be grouped into 4 sets.

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    1. ACTIVE SET

    2. CANDIDATE SET

    3. NEIGHBOURING SET

    4. REMAINING SET

    1. Active Set :

    The pilots associated with the forward

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    traffic channels assigned to the mobile

    station.

    2. Candidate Set :

    These pilots which are NOT in thecurrent Active Set but have sufficient

    signal strength to qualify for hand-off.

    3. Neighbour Set :

    This contains a list of neighbouring pilotswhose signal strengths are strong enough to

    k th did t il t Thi li t i

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    make them candidate pilots. This list is

    entered through the data base. Even if apilot from an adjoining cell is very strong, themobile will not look at if it is NOT mentioned

    in the neighbor list.

    4. Remaining Set :

    The set of all pilots other than the threetypes mentioned above.

    ACTIVE SET

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    Remaining SetNeighbour Set

    Candidate Set

    Parameters used in Hand-Offs

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    The following Parameters are very important

    in Hand-off situations:1. T_ADDs :

    Suggested value : -13 dB(Range : -31.5 to 0dB).

    T_ADD is the Pilot Detection Threshold.

    2. T_COMPs :

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    Suggested Value : 2.5 dB

    ( Range : 0 to 7.5 dB )

    This is the ACTIVE SET Vs CANDIDATESET COMPARISION THRESHOLD.

    3. T_DROPs :

    Suggested Value : -15 dB

    ( Range : -31 5 to 0 dB)

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    ( Range : -31.5 to 0 dB)

    This is the PILOT DROP THRESHOLD.

    4. T_TDROPs :

    Suggested Value : 2 seconds

    ( Range : 0 to 15 seconds )

    This is the PILOT DROP TIMER VALUE.

    Neighbour Set Remaining Set

    Search for Pilot

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    P exceeds T_ADD

    1. Move the pilot to candidate set

    2. Send Measurement report to base station

    P exceeds an active pilotby T_Comp X0.5

    No YES

    Keep pilot in candidate set

    Send measurement report

    Receive H/O Direction message

    Move Pilot to Active set

    Send Hand off com letion Messa e

    yes

    No

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    POWER CONTROL IN CDMA

    1TP

    Ideal CDMA System

    1TP

    Near-Far Effect

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    MS1

    MS2

    d

    12

    1

    21

    ==

    =

    R

    R

    RR

    P

    P

    I

    C

    PP

    P

    f1.25 MHz

    1RP

    2RP

    2TP MS1

    MS2

    d

    ( )

    16

    116

    1

    2/

    1

    2

    1

    21

    142

    ==

    =

    R

    R

    RR

    RR

    P

    P

    I

    C

    PP

    Pd

    P

    P

    f1.25 MHz

    12 16 RR PP =

    2TP

    d/2

    What is Power Control?

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    Power control refers to the strategies

    or techniques required in order to adjust,correct and manage the power from theBS/MS in both directions (i.e. uplink and

    downlink) in an efficient manner.

    Power control is essential to the

    smooth operation of a CDMA system.Because all users share the same RF band

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    through the use of PN codes, each user

    looks like random noise to other users.

    The power of each individual user,therefore, must be carefully controlled sothat no one user is unnecessarily

    interfering with others who are sharingthe same band.

    To illustrate how power control is essentialin CDMA, we consider a single cell that

    has two users.

    User 2 is much closer to the base station

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    User 2 is much closer to the base station

    than user 1. If there is no power control,both users would transmit a fixed amountof power pt.

    However, because of the difference indistance, the received power from user 2or pr2 would be much larger than the

    received power from user1 or pr1.

    This would be a great disadvantage.

    Power control is implemented toovercome the near-far problem and toi th lit d d th

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    improve the quality and reduce the

    interference.

    Power control is where the transmitpower from each user is controlled suchthat the received power of each user at

    the base station is equal to one another.

    pr1 pr2

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    User 2User 1

    pt

    pt

    pr2

    A base station with two users, each user is transmitting to the base station a fixedamount of power pt

    pr1 pr2

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    User 2User 1

    pt1

    pt2

    A base station with two users, each user is transmitting to the base station withdifferent powers, depending on the distance from the base station. (power control)

    Solution for Near far problem One Problem that has to be immediately

    solved in power control is the initial mobiletransmit power. Before the mobileestablishes contact with the base station

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    establishes contact with the base station,

    the mobile cannot be power controlled bythe base station.

    when the mobile first attempts to

    access the base station, what power levelshould the mobile use to transmit? At thispoint, the base station has not yet made

    contact with the mobile user, and the basestation has no idea as to the location ofthe mobile user.

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    The second option is that the mobile canrequest access from the base station with al t it

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    low transmit power.

    such low power decreases the probabilitythat the base station will receive the mobiles

    access request.But, the advantage is that the mobile

    wont cause much interference to other

    users.

    ACCESS PROBES

    Access probes are a series oftransmissions of progressively higher power.The mobile transmits its first access probe at

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    The mobile transmits its first access probe at

    a relatively low power, then it will wait for aresponse back from the base station.

    If after a random time interval the mobiledoes not receive an acknowledgement fromthe base station, then the mobile transmits a

    second access probe at a slightly higherpower.

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    MobileTransmitPower

    Secondaccess Probecorrection

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    Initial

    transmitpower

    First accessProbecorrection

    correction

    TimeRandom Time intervals

    Fig : A series of access probes by the mobile to access the system.

    TYPES OF POWER CONTROL

    Basically, there are 2 methods of powercontrol available in cdma

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    1. Open Loop Power Control :This is purely a mobile unit function. It

    gives open estimation. This is done only

    during the initial stage as soon as themobile is turned ON.

    After the call is established and mobile

    moves around within a cell, the pathlosses between MS and BS will continueto change.

    As a result the received power at mobilewill change and open loop power control willcontinue to monitor mobile received power

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    continue to monitor mobile received power

    and adjusts the mobile transmit poweraccording to the following equation.

    Pt initial= - Pr -73 +NOM_PWR + INIT_PWR+sum of all access probes correction.

    2. closed loop power control :

    This involves both base station and themobile unit and gives a closed loop powercorrection

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    correction.

    BS continuously monitors Eb/No onreverse link. If Eb/No is very high, the BScommands the mobile to decrease thetransmit power of mobile.

    If Eb/No is low, i.e., below a threshold

    value, it commands the mobile to increasetransmit power of mobile.

    The mobile should use the power level itreceives from the base station to estimatehow much to initially transmit.

    In other words, if the mobile sees a

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    strong signal from the base station, then itassumes that the base station is nearby andthus transmits initially at a relatively lowlevel.

    If the mobile sees a weak signal from thebase station, then it assumes that the base

    station is far away and thus transmits initiallyat a relatively high level.

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    CHANNELS IN CDMA

    CDMA channels can be divided in to two

    types of links.1. Forward Link : It is from BTS to Mobile

    St ti

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    Station.

    Frequency Range : 869 894 MHz.

    2. Reverse Link : It is from Mobile Station toBTS.

    Frequency Range : 824 - 849 MHz.

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    Forward CDMA Channel Forward link consists of up to 64logical channels

    (code channels) The code channels are distinguished by a set of 64

    Walsh functions

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    Walsh functions

    Walsh function code number zero is always reservedas thepilot

    Short Code(with period 215, 27.667 ms at 1.2288 MHz) spreading the CDMA Forward Channel

    used in conjunction with the Long Codefor spreading theCDMA Reverse Channel

    Long Code(with period 242 - 1) spreading the CDMA Reverse Channel

    Long Code Maskserves as a reverse link address

    Logical Forward CDMA Channel

    Forward CDMA Channel

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    Pilot

    Chan

    Sync

    Chan

    W0 W32

    Paging

    Ch 1

    W1

    ...Up

    to W7

    Paging

    Ch 7

    W8

    ...Up

    to

    Traffic

    Ch N

    Traffic

    Ch 1

    Traffic DataMobile Power

    Control Sub-Channel

    ...

    W31

    Traffic

    Ch 24Up

    to

    ...

    W63

    Traffic

    Ch 55

    W33

    Traffic

    Ch 25

    CDMA Channel Structure (Forward Link)

    0- Pilot Channel

    Paging Channel

    1

    0

    1.2288 MHz

    W0

    W1

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    Paging Channel

    Traffic Channel

    Synchronisation Channel

    Traffic Channel

    Traffic Channel

    Traffic Channel

    Traffic Channel

    7

    8

    9

    31

    32

    33

    63

    W7

    W32

    W8

    W31

    W33

    W63

    Pilot Channel

    Unmodulated signal

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    Unique per sector/cell Signal level is 4~6 dB higher than traffic

    channel

    Perfect phase/time/signal strengthreference for MS

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    Walsh Function

    W0 @ 1 2288Mcps BBF A

    I- Pilot PN [email protected]

    PILOT CHANNEL

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    (All 0s)

    W0 @ 1.2288Mcps BBF

    BBF

    To QPSKModulator

    A

    B

    Q- Pilot PN [email protected]

    A

    C

    Sin W t

    QPSK MODULATOR

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    carrier

    Pi/2

    B

    D

    ESUMMER

    Sin Wc t

    Cos Wc t

    The PILOT Channel is identified by Walsh

    function 0 (W0). The Channel itself containsno base band information. The base band isstream of zeroes, that are spread by Walsh

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    function zero, which is also a sequence of allzeroes.

    The resulting sequence still all zeroes, is

    then spread are multiplied by a pair ofquadrature sequence.

    The pilot channel is effectively PN-Sequence

    itself.

    Both Walsh function zero and PN-Sequenceare running at a rate of 1.2288 Mcps.

    The pilot channel is transmitted continuouslyby Base Station Sector.

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    It provides the mobile with timing and phasereference. The mobile measurements of signal to noise

    ratio of pilot channel also gives an indicationof which is strongest serving sector of thatmobile.

    Pilot power is 4 to 6 dB higher than trafficchannel.

    BBF A

    I- Pilot PN Sequence

    @1.2288MCPS

    Syncchannel

    Walsh Function

    W32 @ 1.2288Mcps

    SYNC CHANNEL

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    BBF

    To QPSKModulator

    B

    Q- Pilot PN [email protected]

    Blockinterleaver

    SymbolRepetition

    ConvolutionalEncoderR=1/2

    channelbits

    1.2kbps 2.4kbps 4,8kbps 4,8kbps

    1.2288

    Mcps

    A

    C

    Sin Wc t

    QPSK MODULATOR

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    carrier

    Pi/2

    B

    D

    ESUMMER

    c

    Cos Wc t

    Sync Channel

    Sync channel operates at a fixed bit rateof 1200 bps. Used for the mobile to get timing and

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    long code references.

    It is not scrambled

    It does not carry the power control bits Uses the same pilot PN offset as the pilot

    channel

    Unlike the pilot channel, the sync channelcarries base band information.

    The information contained in sync channel

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    message that notifies the mobile ofimportant information about systemsynchronization and parameters.

    To convey pilot PN sequence offset, timeof day, and long code state to allowimmediate sync of MS to the network

    The sync data is passed through

    convolutional coder. Output of convolutional coder is 2400 bps

    and this is passed through code repetition

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    p g pcircuit to get 4800 bps.

    The output of repetition circuit is then

    block interleaved and subjected to directsequence spreading using Walsh code 32and then given to Quadrature spreading

    circuit.

    Sync ch super frame

    Sync ch Sync ch Sync ch

    Sync ch super frame

    Sync ch Sync ch Sync ch

    -------------

    ------

    80 msec, 96 bits

    SYNC CHANNEL FRAME STRUCTURE

    26.66 msec

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    frame frame frame

    S

    OM

    SOM

    SOM

    1 0 0

    frame frame frame

    S

    OM

    S

    OM

    S

    OM

    0 0 0

    ---

    Sync channel message capsule = 96 X Ns, Ns is the no of super frames needed.,

    Sync channel message Padding bits

    MSG_LENGTH Message Body CRC

    8 bits 2-1146 bits 30 bits

    SOM = Start Of Message

    + Padding bits if needed

    Sync channel is sent in super frames of 96bits in 80 msec duration.

    Each super frame has 3 frames of 26.66..

    SYNC CHANNEL FRAME STRUCTURE

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    Msec each.A number of super frames form a sync

    channel message capsule.

    A capsule has a message length indicator,Message body and CRC, Message length inoctets.

    It also has padding bits set to 0; thenumber of padding bits is to make the

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    total number of bits equal to an integermultiple of 96 bits.

    The beginning of a super frame has anSOM bit set to 1 for the first frame and setto 0 for other frames.

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    Sync Channel Message contains: system identification (SID) and network

    identification (NID)

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    PN sequence offset and long code state

    system time, leap seconds, offset fromUTC, etc.

    paging channel data rate (PRAT).

    Only the Sync Channel Messageis senton the sync channel. MS

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    obtains information from Sync ChannelMessage

    adjusts its timing to normal system timing

    begins monitoring its Paging Channel

    Paging Channel

    Data rate: 4800, 9600 bps One 9600 bps Paging Channel can support

    180 pages/sec.

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    Does not carry power control sub channels Use the same PN sequence offset as the

    pilot channel Up to 7 paging channels are possible, the

    first one taking Walsh code number W1

    Paging Channel conveys four major types ofmessages:

    overhead

    paging

    d

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    order channel assignment

    Configuration of the system is conveyed infour overhead messages: System Parameter Message

    Access Parameter Message

    Neighbor List Message

    CDMA Channel List Message

    Overhead Messages

    System Parameter Message: configuration of the Paging Channel

    i t ti t

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    registration parameters parameters to aid pilot acquisition

    Access Parameter Message configuration of the Access Channel

    control parameters used to stabilize the

    Access Channel

    Neighbor List Message time offset of the pilot basic neighbor configuration

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    CDMA Channel List Message

    CDMA frequency assignment that containPaging Channels

    Paging Channel Messages

    Page Message: contains pages to one or more mobile stations.

    Order Message:

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    a broad class of messages used to control aparticular MS.

    Channel Assignment Message: let BS to assign a MS to the traffic channel

    change Paging Channel Assignment

    direct the MS to use the analog FM system

    BBF A

    I- Pilot PN [email protected]

    Block

    interleaver

    Symbol

    Repetition

    ConvolutionalEncoderR=1/2

    Walsh FunctionW1-7 @ 1.2288Mcps

    PAGING CHANNEL

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    BBF B

    Q- Pilot PN [email protected]

    interleaverRepetitionR=1/2

    4.8kbps 9.6 kbps 19.2ksps

    1.2288Mcps

    Long codeGenerator

    Decimator64:1

    Long code Maskfor pagingchannel

    19.2ksps

    19.2ksps

    carrier

    A

    C

    Sin Wc t

    QPSK MODULATOR

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    carrier

    Pi/2

    B

    D

    ESUMMER

    Cos Wc t

    Like Sync channel, paging channel alsocarries base band information. But, unlike

    sync channel, the paging channel transmitsat a higher rate.

    It can transmit at either 4.8 or 9.6 kbps.

    PRAT field in the sync message inform the

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    PRAT field in the sync message inform themobile of data rate of paging channel. Oncethe mobile acquires timing & synchronization

    using sync channel, the mobile begins tomonitor paging channel.

    Although there can be up to 7 pagingchannels per sector, each mobile onlymonitors one paging channel.

    From the above figure, the base band

    information is error protected then if thedata rate is at 4.8kbps, the bits arerepeated once, after that interleaving will

    be taken place

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    be taken place. The data is first scrambled by a decimated

    long PN sequence, then it is spread by aspecific Walsh function and further spreadby short PN sequence assigned to serving

    sector.

    Max. Paging Channel Slot Cycle (80 msec)

    PCHslot 0

    PCHslot 1

    PCHslot 2

    PCHslot2047

    PCHslot 3 ----------------------------------------------

    192 bits

    20 msec

    PAGING CHANNEL FRAME STRUCTURE

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    PCHframe

    PCHframe

    PCHframe

    PCHframe

    PCHhalf

    frame

    PCHhalf

    frame

    PCHhalf

    frame

    PCHhalf

    frame

    PCHhalf

    frame

    PCHhalf

    frame

    PCHhalf

    frame

    PCHhalf

    frame

    SCI

    SCI

    SCI

    SCI

    SCI

    SCI

    SCI

    SCI

    20 msec

    96bits

    10msec

    SCI : Synchronised Capsule Indicator

    Paging Channel Frame Structure

    Synchronized paging channel message has length of47 x N or 95 x N bits

    Messagelength Data (2 to 1146 bits) CRC Padding bits

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    Sent in N paging channel slots: 1 slot (80 ms) = 8 paging channel half-frames 1 half-frame (10 ms, 48 or 96 bits) = 1-bit SCI + 47 (or 95)-

    bit data

    SCI = 1 : start of a paging channel message

    Messagelength(8 bits)

    Data (2 to 1146 bits) CRC(10bits)

    Padding bitsall 0s

    Message contains:

    system parameters

    access parameter (for access channel)

    channel assignment

    TMSI (temporary MS identification)

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    TMSI (temporary MS identification)assignment

    BBF A

    I- Pilot PN [email protected]

    Block

    interleaver

    Symbol

    Repetition

    ConvolutionalEncoder

    R=1/2

    Walsh Function@ 1.2288Mcps

    FORWARD TRAFFIC CHANNEL

    mux

    PCB80bps

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    BBF B

    Q- Pilot PN [email protected]

    1.2 kbps

    2.4 kbps

    4.8 kbps

    9.6 kbps

    2.4 kbps 19.2ksps

    1.2288Mcps

    Long codeGenerator

    Decimator64:1

    Long code MaskSoecific user ESN

    19.2ksps

    19.2ksps

    Decimator 24:1

    1.2288MCps

    The forward traffic channel at variable spreadsup to 9600 bps with one pilot channel, one

    sync channel and seven paging channels. We can have a maximum of 55 traffic channels.

    From the figure of forward traffic channel thei l d th h l ti l

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    From the figure of forward traffic channel, thesignals are passed through a convolutionalencoder R=1/2. for every input bit, we get 2

    bits at output. The symbol repeater after theconvolutional encoder repeats the symbol twicefor 4800 bps speed, 4 times for 2400 bps and 8

    times at 1200 bps.

    The block interleaver output is scrambledusing a decimator. The scrambling is done

    by modulo 2 addition (Ex-or) of interleavedoutput with binary value of long code chip.The resulting PN Sequence is same as long

    code as 1 2288 MHz clock rate where onlyth fi t t t f 64kb i d f

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    code as 1.2288 MHz clock rate where onlythe first output of every 64kbps is used forscrambling data.

    The scrambled data (19.2kbps) is thenpassed through multiplexers, whereas powercontrol bits are added. For scrambling

    power control bits, the output of 64:1decimator to produce a bit rate of 800 bps.

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    In forward traffic channel, there are 2types of rate sets.

    Rate set 1 supports 4 data rates. They9 6 4 8 2 4 d 1 2 kb

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    Rate set 1 supports 4 data rates. Theyare 9.6,4.8,2.4 and 1.2 kbps.

    Rate set 2 supports the data rates of 14.4,7.2, 3.6, 1.8 kbps.

    12 8172

    192 bits (20 m sec)

    9600 bps frame structure

    8 880

    96 bits (20 m sec)

    4800 bps frame structureF

    F

    T

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    840

    48 bits (20 m sec)

    2400 bps frame structure

    816

    24 bits (20 m sec)

    1200 bps frame structure

    F

    T

    TF : Frame quality indicator (CRC)

    T : Encoder Tail Bits

    Forward Link Channel Parameters

    Channel Sync Paging Traffic

    Data rate 1200 4800 9600 1200 2400 4800 9600 bps

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    Code repetition 2 2 1 8 4 2 1

    Modulationsymbol rate 4800 19,200 19,200 19,200 19,200 19,200 19,200 sps

    PN chips/modulation symbol 256 64 64 64 64 64 64

    PN chips/bit 1024 256 128 1024 512 256 128

    CDMA Channel Structure (Reverse Link)

    Access Channel

    2

    1

    32

    1.2288 MHz

    Access Channel

    Access Channel

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    Traffic Channel

    Traffic Channel

    Traffic Channel

    Traffic Channel

    32

    1

    2

    3

    62

    Access Channel

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    Access Channel

    Access Channel provides communicationsfrom MS to BS when MS is not using a

    Traffic Channel.

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    All Access Channel use 4800 bps mode

    Access Channel Message: call origination response to pages

    orders registrations

    Access Channels

    To access the system, respond the page,make call origination and process other

    messages between the MS and the BS

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    g

    4.8 kbps slotted random access channel

    MS is identified by orthogonal long code

    BBF

    BBF

    A

    B

    I- Pilot PN [email protected]

    Blockinterleaver

    Symbol

    Repetition

    ConvolutionalEncoder

    R=1/3

    4 8kbps 14 4 28.8ksps

    ACCESS CHANNEL

    28 8ksps

    D

    64-aryOrthogonalmodulator

    To QPSK

    Modulator

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    BBF B

    Q- Pilot PN [email protected]

    4.8kbps 14.4kbps

    8 8 sps

    1.2288Mcps

    Long codeGeneratorLong code Mask

    for pagingchannel

    28.8ksps D

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    The base band information is first error

    protected by an R=1/3 convolutionalencoder. The lower encoding rate makeserror protection more robust on the

    reverse link. The symbol repetition function repeats the

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    The symbol repetition function repeats thesymbol once, giving a code symbol rate of

    28.8ksps. The data is then interleaved. Following

    interleaving, the data is coded by a 64-ary

    orthogonal modulator.

    The orthogonal modulated date is then

    spread by the long PN sequence. The longPN sequence is running at 1.2288Mcps, andthe bandwidth of the data after spreading is1.2288 Mcps.

    The data is further scrambled in the in phase

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    The data is further scrambled in the in phaseand quadrature phase paths by the short PN

    sequences defined . Because the reverse link uses OQPSKmodulation, the data in the Q path is delayed

    by one half a PN Chip.

    BBF

    A

    B

    I- Pilot PN [email protected]

    Blockinterleaver

    Symbol

    Repetition

    ConvolutionalEncoder

    R=1/3

    Reverse Traffic Channel

    D

    64-aryOrthogonalmodulator

    To QPSK

    Modulator

    BBF

    Data BurstRandomizer

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    Q- Pilot PN [email protected] code

    GeneratorLong code Mask

    D1.2kbps

    2.4 kbps

    4.8kbps

    9.6kbps

    3.6kbps

    7.2 kbps

    14.4kbps

    28.8kbps

    28.8ksps 28.8ksps

    1.2288 MCps

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    CDMA CALL PROCESSING

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    To process a call, MS goes through thefollowing stages to get traffic channel.

    SYSTEM INITIALISATION STATE SYSTEM IDLE STATE

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    SYSTEM IDLE STATE

    SYSTEM ACCESS STATE

    TRAFFIC CHANNEL STATE

    Tune to CDMA Carrier

    Mobile acquires pilot channel and sync channel

    Mobile acquires paging channel and monitors for messages

    System initialization state

    Idle state

    CALL PROCESSING

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    Mobile sends messages on access channel, BS sendsmessages on paging channel.

    Speech communication on forward and reverse trafficchannel, Power control on forward link

    Access State

    Traffic channel state

    CALL PROCESSING STATE:

    In system initialization state the mobileacquires a pilot channel by searching the PN offsets and selects the strongest pilot signal.

    Then it acquires synchronization channel usingW32 and detecting timing offset of pilot

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    channel. Then mobile acquires the system

    configuration and timing information. Then the MS enters the system idle state where

    it monitors the paging channel. Now the mobile

    can receive necessary message to initiate orreceive a call.

    If a call is placed or received, MS enters thesystem access state exchanging necessary

    parameters. MS transmits its response onaccess channel and BS transmits on pagingchannel.

    On access, the MS enters traffic channel

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    state and communicate. It also transmits

    control messages replacing speech bysignaling.

    Power control messages are sent on forwardlink channel.

    SYSTEM INITIALIZATION STATE

    Power up andmobile enters

    initialization state

    System Pilot Sync channel Timing

    Enter idlestateAnalog

    initialization

    Begin analog

    mode

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    Systemdetermination sub state

    Pilotacquisitionsub state

    Sync channelacquisitionsub state

    Timingchange sub

    state

    CDMAsystem is

    selected

    Pilot channel isacquired

    Sync channelis acquired

    System timingis acquired

    System determination sub state :Mobile can select analog or CDMA, if the

    system selects CDMA, the mobile sets theCDMA channel parameters to channelnumber.

    Pilot channel acquisition sub state:Mobile acquires pilot channel of CDMAt d t t CDMA h l d t

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    system and tunes to CDMA channel and setsits code channel for the pilot channel andsearches for pilot channel within 20 msec, themobile should acquire pilot channel, elsemobile enters system determination sub stateindicating failure.

    Synchronization acquisition sub state :Mobile acquires the sync channel and

    obtains system configuration and timinginformation for CDMA system and sets itscode channel for sync channel if mobile does

    not receive a sync channel message within21 msec, mobile enters the syncd b h l

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    determination sub state with a protocol

    mismatch indication.If it receives within 21 msec, mobilestores system configuration and timing

    information.

    Timing change sub state :

    If a mobile synchronizes its long codetiming of system timing to those of CDMAsystem after receiving and processing syncmessages.

    After sync channel is acquired and synci i d it t th

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    message is received, it stores the

    following information like protocol revisionmessage, system identification, networkidentification, pilot PN offset, system time,long code at system time, paging channeldata rate etc..

    CDMA initialization

    Acquires primary pagingchannel (W1)

    monitors paging channel

    Idle State

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    monitors paging channel

    Authentication

    Idle hand off

    Page Response

    Mobile origination

    Receives an incoming call

    Registration

    Idle State :

    Mobile monitors paging channel.Mobile can receive messages from basestation to initiate or receive a call.

    It can also initiate a registrationprocess, message transmission. Afterentering idle state mobile sets its Walsh

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    entering idle state, mobile sets its Walsh

    code to the primary paging channel andsets its rate to the rate obtained from syncmessage.

    Paging channel is subdivided into 80msec slots are called paging channel slots.

    In slotted mode, the mobile monitors only

    certain assigned slots. When pagingchannel is not monitored, mobile can stopor reduce its processing activities to save

    battery power.

    In the non slotted mode paging and

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    In the non slotted mode, paging and

    control data can be received on any of thisslot. The mobile monitors all the slots oncontinuous basis.

    Idle Hand off :Idle hand off or change of paging channel

    occurs when a mobile has moved from

    coverage of one base station to coverage ofanother, during idle state. Hand off shouldoccur when it detects new pilot that is stronger

    than current pilot.Pilots are identified by short PN offsets.

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    o s a e de ed by s o o se sThese pilot sets are maintained by active,

    candidate, neighbor, remaining. If neighborset or remaining set is stronger than active setand candidate set, pilot hand off is performed

    in the non slotted mode. On receiving newmessage, from new paging channel, it resumesoperations.

    SYSTEM ACCESS STATE:

    It includes following sub states.

    1. Update overhead information

    2. Mobile State origination Attempt3. Page Response

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    3. Page Response

    4. Registration Access sub state5. Mobile Station Transmission

    Traffic channel sub state:Speech communication takes

    associated with control messages.There are 5 sub states in this.

    1. Traffic Channel initialization :In this state, mobile verifies it can

    receive forward traffic channel and starts

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    receive forward traffic channel and starts

    transmit on reverse link.2. Waiting for order sub state:

    In this, MS waits for an alert with

    information message.

    3. Waiting for MS answer sub state:

    In this state, the MS waits for user toanswer the call.

    4. Conversation sub state:Here, the MS exchange primary traffic

    data packets with base station

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    data packets with base station.

    5. Release Sub state:In this case, MS disconnects a call.

    CDMA REGISTRATION

    Registration is a process in which theMS tells the base station about thewhereabouts. It notifies base station of

    its location status, identification, slot cycleand other characteristics.

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    and other characteristics.

    Purpose of registration is to enable thebase station to page the mobile in case ofan incoming call.

    CDMA supports 9 types of registration.

    1. Power up registration : The mobile registers,when it powers up.

    2. Power down registration : The mobileregisters, when it powers down informing thesystem that it is no longer active.

    3. Time based registration : the mobile registersat regular intervals of time. Its use also alerts thesystem to automatically deregister mobile stations

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    system to automatically deregister mobile stations

    that did not perform a successful power downregistration.

    4. Distance based registration : Mobile performsa registration when distance between current

    base station and the base station which it hasprevious registered exceeds the threshold.

    5. Zone based registration : Mobile registers when itenters a new zone.

    6. Parameter changed registration : Mobile does a

    registration when it enters a new system or some ofits stored parameters change.

    7. Ordered registration : mobile registers when a

    base station asks for it.8. Implicit registration : When a MS sends asuccessful origination message or page response

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    successful origination message or page responsemessage, base station can ask a mobile of its location.

    9. Traffic channel registration : Base station tellsthe mobile, it is registered.

    Only power up, power down registrations

    are presently supported.

    CALL PROCESSING (Mobile Station to Base Station)

    Sends origination message Receives origination messageAccess Channel

    Receive Paging Channel Sets up Traffic Channel, startssending null traffic data

    Paging Channel

    Sets up reverse traffic chAcquires reverse traffic Ch

    Rev traffic ChStarts sending Traffic Ch

    preamble Fwd traffic ChReceives