Subject : Mobile Communication

Topic : CDMA

Difficulty : Medium

The figure above shows the processing step for transmission in reverse CDMA channel.

A brief description of the various operations involved is given below:

Convolution Encoder:

• The data or digitised speech is encoded by a convolution encoder with rate 1/3,thus tripling the effective data rate to a maximum of 28.8 kbps.using this coding technique forward error correction provided. stronger rate 1/3 convolution code is used to compensate for the absence of a pilot channel(which generally consist of coherent detection )

Symbol Repetition:

• For lower rates the encoder outputs bits (called code symbols) are replicated to give the 28.8 kbpsrate

Interleaver:

• Communication over a radio channel are characterised by deep fades that can cause a large number of consecutive errors.

• Most coding schemes perform better on random data errors than on blocks of errors.

• bit interleaving data no two adjacent bits are transmitted near to each other , and the data errors are randomised

Orthogonal modulation using walsh codes[64-ary orthogonal modulator]:

• The data coming out of the block interleaver are grouped in units of 6 bits.

• Each 6 bit units serves as an index to select a row of the 6464 walsh matrix (26 =64),and that row is substituted for the inut.thus the data rate is expanded by a factor 64/6 to 307.2 kbps[28.864/6]i.e. walsh chips are transmitted at rate 307.2 kbps.

Data Burst Randomiser:

• The data from the 64-ary modulator is fed into the data burst randomiser.

• The data burst randomiser take advantage of the voice activity on the reverse link.

• This is used to reduce reverse link power during a quite period of speech by pseudorandom masking out redundant symbols produced by symbol repetition.

• It generates a masking pattern of 1s and 0s that randomly masks the redundant data generated by the code repetition process. It thus ensures that every repeated code symbol is transmitted exactly once.

Direct sequence spreading

• The long code PN sequence unique to the mobile is EX-ORed with the output of the randomiser to produce 1.2288 Mbps final data streams.

Quudrature modulation

The reverse link uses offset quadrature phase shift keying [OQPSK] modulation technique.

OQPSK involves the following operation:

• The digital bit streams at 1.2288 Mbps is separated into two bit streams called I and Q (In phase and quadrature)

• The I streams is Ex-Ored with I channel pilot PN sequence and Q streams is Ex-Ored with Q channel pilot PN sequence which is delayed by half a chip with respect to the I channel pilot PN sequence. This delay is used for improved spectral shaping and synchronisation

• Both streams are 1.2288 Mbps quadrature PN binary signals of 15 bits. The resulting quadrature signals at the output are then weighted and transmitted