Mobile Computing COE 446 IS-95
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04/21/23 1
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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