Subject : Mobile Communication

Topic : CDMA

Difficulty : Medium

1. CDMA reverse channel uses frequency band of 869-894 MHz,
2. Bandwidth of each channel is 1.25 MHz. The reverse link is separated from forward link by 45 MHz.
3. There is no spreading of the data symbols using orthogonal codes instead orthogonal codes are used for waveform encoding.
4. Spreading is carried out by 42 bit long PN codes. After spreading they are further scrambled in phase and Quadrature phase by PN spreading codes.
5. Modulation scheme used is OQPSK.

1. There are two types of reverse channels:

   A. Access channels:

   • It is a random access channel with each channel uniquely identified by their long codes
   • There are 32 access channels per reverse channel. It works at fixed data rate of 4800 bps.
   • It allows a mobile to communicate with the system for registration call origination.

   B. Reverse traffic channel:

   • It carries traffic from the mobile station to the base station.
   • It operates on variable data rates.
2. The reverse channel modulation process is as follows:

Convolution encoder and symbol repetition:

• The convolution encoder used is rate 1/3 and constraint length 9.
• The coder bits after convolution encoder are repeated before interleaving when the data rate is less than 9600 bps.
• After symbol repetition the symbol rate out of the coder is fixed at 28800 bps.

A. Block interleaver:

• Block interleaving is performed following convolution encoding and repetition.
• The block interleaver spans 20 ms and is an array with 32 rows and 18 columns.
• The code symbols are written into matrix by columns and read out by rows.

B. Orthogonal modulation:

• A 64-ary orthogonal modulation is for the reverse CDMA channel.
• One of 64 Walsh functions is transmitted for each group of six coded bits. Within a Walsh function 64 Walsh chips are transmitted at the rate of 307.2 kbps. 28.8 kbps * (64 Walsh chips)/ (6 coded bits) = 307.2 kbps

C. Variable data rate transmission:

• Variable rate data is sent over CDMA reverse channel.
• Data in each 20 ms frame is divided into 16 power control groups, each with 1.25ms. Some are gated-on some are gated-off.
• The data randomizer ensures that every repeated code symbol is transmitted exactly once.
• The data burst randomizer generates a masking pattern of 0’s and 1’s that randomly masks the redundant data generated by code repetition process.
• For the data rates is 9600 bps, 4800 bps, 2400 bps and 1200 bps transmission occurs on all 16, 8, 4, 2 power control groups respectively.

D. Direct sequence spreading:

• The reverse traffic channel is spread by the long code PN sequence which operates at the rate of 1.2288 Mcps.
• Each Walsh chip is spread by 4 long code PN chips.

E. Quadrature modulation:

• Prior to transmission, the reverse channel is spread by I and Q channel pilot PN sequences. Pilot sequences are used for synchronization.
• The reverse link modulation is OQPSK. The data spread by the Q pilot PN sequence is delayed by half a chip with respect to the data spread by I pilot sequence to improve spectral shaping and synchronization.