Mobile Computing COE 446 IS-95

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Mobile Computing COE 446

IS-95Tarek Sheltami

KFUPMCCSECOE

www.ccse.kfupm.edu.sa/~tarek

Principles of Wireless Networks

K. Pahlavan and P. Krishnamurth

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Code Division Multiple Access (CDMA)

used in several wireless broadcast channels (cellular, satellite, etc) standards

unique “code” assigned to each user; i.e., code set partitioning

all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data

encoded signal = (original data) X (chipping sequence)

decoding: inner-product of encoded signal and chipping sequence

allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

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CDMA.. Let the data rate of the ith user by Ri(t) Let the code assigned to the ith user by Ci(t)

i.e. all the codes are orthogonal Each user uses its code to spread its signal – the signal

transmitted by the ith user is Si(t) = Ri(t)Ci(t) The signal received BS is the sum of all transmitted

signals (ignore multi-path copies for the time being),

∑ Si(t) = ∑ Ri(t)Ci(t)

i j

0, i jC (t) X C (t)

1, i j

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CDMA..

Demodulation (De-spreading): Receiver dedicates a path structure per user – multiplies the received signal with the kth user code

Ck(t) X ∑ Si(t) = Ck(t) X ∑ Ri(t)Ci(t)

= Rk(t)

i.e. only the kth signal is retrieved from the kth receiver path

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CDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M

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CDMA: two-sender interference

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IS-95 The data message handler (DMH) collects

billing information The interworking, function (IWF) that allows

an MSC to connect to other networks The auxiliary (AUX) equipment that can

connect to an MT The authentication center (AuC) in GSM is

shown as the AC and the operation and maintenance center

(OMC) is shown as the operation system (OS)

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IS-95 (Cont’d) CDMA occupies the same frequency

spectrum as AMPS and IS-136 Each carrier occupies 1.25 MHz,

whereas in AMPS and IS-136 was 30 KHz IS-95 forward channel consists of 4

types of logical channels Pilot Ch Synchronization Ch Paging Ch Traffic Ch

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IS-95 (Cont’d)Forward ChannelForward Channel Each carrier contains pilot, synchronization, up to seven

paging and number of traffic channels Channels are spread from one another using different

spreading codes Modulation scheme is QPSK Any information contained in the symbol form is modulated

by Walsh codes Each Walsh code identifies one of the 64 forward channels After the channel symbol spread using orthogonal codes,

they are scrambled in the in-phase and quadrature phase line by Pseudorandom Noise (PN)-spreading codes

PN-codes are not orthogonal

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IS-95 (Cont’d) PN-codes are not random, they are

deterministic, periodic sequences with the following properties: PN are generated by combining the

outputs of feedback shift registers The sequence are repeated every 2n-1 The initial contents of the stages and

the feedback determine the successive contents of the stages

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IS-95 (Cont’d) The orthogonal codes are used to

isolate the transmissions between different channels within a cell

The PN spreading codes are used to separate the transmissions between different cells

The PN sequences are used to differentiate between several BS

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