Mumbai university > Comp > SEM 8 > Mobile Communication

Marks: 10M

Year: Revised 2012

• CDMA reverse channel uses frequency band of 869-894 MHz,

• Bandwidth of each channel is 1.25 MHz. The reverse link is separated from forward link by 45 MHz.

• There is no spreading of the data symbols using orthogonal codes instead orthogonal codes are used for waveform encoding.

• 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.

• Modulation scheme used is OQPSK.

• 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.

• 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.