The base station uses a slightly different scheme when the vocoder moves to lower rates. First, EIA/TIA-95-B CDMA base stations do not pulse their transmissions.

Rather, base stations repeat the same bit patterns as many times as needed to get back to the full rate of 9,600 bps. So, if the vocoder selects a frame to be half-rate, the data bits are sent twice to fill the entire frame.

The transmit power is then adjusted down by 3 dB, since repeating the data twice adds three dB more processing gain to the signal (21 dB + 3 dB = 24 dB for a half rate frame). Adjusting the gain down maintains the approximate same signal to noise ratio that existed for a full rate frame. Quarter and one-eighth rate frames repeat the data four and eight times to fill each frame, and are lowered in power by 6 and 9 dB respectively.

This allows more capacity on the forward link since frames operating below full rate are transmitted with lower power, which reduces the total interference.

 

Base Stations Do Not Pulse TX Channels.
How Does the Base Station Handle Variable Rate Vocoding?
  • Repeats Data Bits When Transmitting at Reduced Rates
  • Repeating Data Adds 3 dB Coding Gain
  • Lowers the TX Power 3dB for Each Lower Rate