Mobile Computing COE 446 IS-95

1. Mobile Computing COE 446IS-95 Tarek Sheltami KFUPM CCSE COE www.ccse.kfupm.edu.sa/~tarek Principles of Wireless Networks K. Pahlavan and P. Krishnamurth

2. 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”)

3. 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)

4. 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

5. d0 = 1 1 1 1 1 1 1 d1 = -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 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 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 1 1 1 1 1 M Di = SZi,m.cm m=1 M d0 = 1 d1 = -1 CDMA Encode/Decode channel output Zi,m Zi,m= di.cm data bits sender slot 0 channel output slot 1 channel output code slot 1 slot 0 received input slot 0 channel output slot 1 channel output code receiver slot 1 slot 0

6. CDMA: two-sender interference

8. 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)

9. 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

11. IS-95 (Cont’d) Forward 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

12. 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

17. 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