CDMA stands for Code Division Multiple Access.
It uses code to convert between analog voice signals and digital signals. It also uses codes to separate voice and control data into data streams called channels.
Steps of CDMA signal generation:
1. analog to digital conversion
– uses pulse code modulation
– voice compression uses vocoder
– variable rate vocoder, Four rates: Full(talking fast), 1/2,1/4 and 1/8(silent).
– 8 kbps vocoder, EVRC
3. encoding and interleaving
– built into BTS and phones
– purpose is to build redundancy into the signal
– convolutional encoding,, uses repetition code, each bit repeated three times, called symbols and the decoder uses a majority logic rule.
– burst error, caused by fading and interference.
– interleaving is method of reducing effect of burst error and recovering lost bits.
4. channelizing the signals
– Walsh codes channelize users on the forward link (BTS to mobile).
– Pseudorandom Noise (PN) codes channelize users on the reverse link (mobile to BTS), 4.4 trillion PN codes available
5. conversion of the digital signal to a Radio Frequency(RF) signal
Receiving end signal to voice recovering process:
1. Conversion from RF signal to digital signal
2. Despreading the signal
3. Deinterleaving and decoding
4. voice decompression
5. Digital to analog voice recovery
Foward Link channel:
– Pilot channel: BTS constantly transmits the pilot channel, mobile uses the pilot signal to acquire the system, it then uses the pilot signnal to monitor and adjust the power needed in order to transmit back to the BTS. Near-Far problem in CDMA.
– Sync channel: BTS provides the mobile with the system time and the identification number of the cell site. mobile ignores the sync channel after synchronization.
– Paging channel: uses upto 7 paging channels; sends commands and pages to the mobile; also sends commands and traffic channel assignment during call set-up; ignores paging channel after traffic channel is established.
– forward link traffic channel: 55 and 61 forward traffic channels to send both voice and overhead control data during a call.
Reverse Link Channel:
– Access Channel: is used when not assigned to a traffic channel; register with the network, originate calls, respond to pages and commands from the BTS, transmit overhead messages to the BTS.
– traffic channel: is used only when there is a call, transmits voice data to BTS, also overhead messages during the call.
Call Processing stages:
Initialization ———-> Idle Mode———–>Access Mode————->Traffic mode