CDMA takes advantage of quiet times during speech to raise capacity. A variable rate vocoder is used; for the original vocoder the channel is a 9,600 bps when the user is talking. When the user pauses, or is listening, the data rate drops to only 1,200 bps (CDMA Variable Rate Speech Coder).
Data rates of 2,400 and 4,800 bps are also used, though not as often as the other two. The CDG 14.4 kbps vocoder is similar with the four channel rates running at 14,400, 7,200, 3,600, and 1,800 bps.
The data rate is based on speech activity and a decision as to the appropriate rate is made every 20 msec. Normal telephone speech has approximately a 40% activity factor.
The mobile station lowers its data rate by turning off its transmitter when the vocoder is operating at less than 9,600 bps. Thus CDMA mobiles also operate in a TDMA mode (pulsing) when the vocoder determines that the transmission rate required for a given frame is less than full rate. At 1,200 bps, the duty cycle is only one-eighth of the full data rate (CDMA Variable Rate Speech Coder).
The choice of time for this duty cycling is stochastic based on a pseudo random algorithm. This has the affect of randomizing the transmission times of each mobile. When averaged over many users, the average transmitted power is lowered.
Lowering the transmit power at the mobile reduces the level of interference for all other users. This increases the capacity of CDMA by nearly a factor of two (CDMA Variable Rate Speech Coder).
HOW WORKS ?
- DSP Analyzes 20 Millisecond Blocks of Speech for Activity
- Selects Encoding Rate Based on Activity:
- High Activity Full Data Rate Encoding (9600 bps)
- Some Activity Half Data Rate Encoding (4800 bps)
- Low Activity Quarter Data Rate Encoding (2400 bps)
- No Activity 1/8 Data Rate Encoding (1200 bps)
- How Does This Improve Capacity?
- Mobile Transmits in Bursts of 1.25 ms
- System Capacity Increases by 1/Voice Activity Factor