Subject : Mobile Communication

Topic : CDMA

Difficulty : Medium

The forward IS95 channel consists of several code channels:

• The pibt channel - It is always transmitted by the BS on each active forward CDMA channel. It is unmodulated spread sequence signal (i.e. it does not contain spreading by the short walsh Hadamard code). The pilot tone is a PN-sequence which is used for synchronization.

• A sync channel - Operating at a fixed rate of 1200 bits/sec.

• Upto 7 paging channels

• a number of forward traffic channels

Each of these code channels is orthogonally spread by appropriate Walsh function and then spread by a quadrature pair of PN sequences at a fixed chip rate of 1.2288 Mcps / sec. Multiple forward CDMA channels may be used within a base station in frequency division multiplexed manner.

All channel except the pilot channels are convolutionally at rate 1/2. This encoder generates two code symbols for each user data code symbol. By means of EXOR-ing of selected taps of the time delayed i/p data sequence.

Every channel is EXOR-ed with a Walsh function at a chip rate 1.2288 Mcps/s. This provides orthogonal code division multiplexing. The length of Walsh function is 64. So 64 orthogonal codes exists.

After Walsh CDMA spreading, every signal is modulated by a quadrature sequence of length $ 2' = 32768 $. This sequence is called the pilot PN sequence. It is generated from.

$ Inphase \ P_x(x) = x^{15} + x^{13} + x^{9} + x^{8} + x^{7} + x^{5} + 1 $ and

$ Quadrature \ P_x(x) = x^{15} + x^{12} + x^{11} + x^{10} + x^{6} + x^{5} + x^{4} + x^{3} + 1 $

All the Base stations use the same PN sequences but with a different 'offset'. The offsets are by 64 chips, the total number of offsets = 32768/64 = 511 offsets.

The cells identified by short code PN offset 511 different ones are available, same as random no seed in random no. generator. Users are identified by Walsh code.

Rake receiver allows user to receive signal from multiple BS or multiple sectors simultaneously.

The MS processes the pilot channel to find the strongest signal. A search correlator sweeps through all possible frequency offsets to identify BSS in the area. The MS picks strongest pilot s/g, this s/g has PN-offset. The MS uses the PN-offset of this pilot to track the sync channel.