cdma -utl traing
TRANSCRIPT
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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
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2.17.1
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TDMA/FDD
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2.18.1
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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
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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