How Traffic Channel Forward Link Modifications in CDMA
In an effort to provide CDMA with even greater voice quality, the CDG (CDMA Development Group) has proposed and implemented a new vocoder. This new vocoder uses and improved, higher data rate of approximately 13 kbps to digitized voice signals. After adding bits used to support the traffic channel, the final traffic channel data rate with
This new vocoder is 14.4 kbps. To accommodate this new vocoder with the least impact to the existing 9.6 kbps traffic channel structure, the CDG simply modified the convolutional encoder rate from a one-half rate to a three-quarter rate encoder.
Thus, the output from the convolutional encoder is still the same 19.2 kbps used in the original CDMA system. No other changes are required in the coding structure which simplifies the implementation of this new voice quality mode.
- Replaces 8 kbps Vocoder with a 13 kbps Vocoder (both Variable Rate)
What Effects ?
- Provides Toll Quality Speech
- Uses a 3/4 Rate Encoder
- Reduces Processing Gain 1.76 dB
- Results in Reduced Capacity or Smaller Cell Sizes
Figure has shown that the improvements of the 14.4 vocoder result in voice quality that is the equivalent of good land-line long distance telephony! Obviously, this level of voice quality will be a distinct marketing advantage for CDMA in the highly competitive cellular and PCS markets. However, the voice quality improvement does not come for free.
By reducing the level of error correction provided in the convolutional encoder, the overall processing gain is reduced. In this case the overall processing gain is lowered from 21.07 dB to 19.31 dB. The result of lower processing gain is that something must give: either the capacity is reduced or the cell sizes must be reduced. The capacity loss for this reduction in processing gain is 1/3 .
Both of these choices have negative effects for operators: the cost to support the same number of uses will increase due to the need to install more cell sites. CDMA network operators will have to balance the benefits of this new vocoder against the costs of implementing it. One possible solution is the EVRC (Enhanced Variable Rate Coder). This new 8 kbps coder promises to produce voice quality equal to the 13 kbps coder without losing processing gain.
The TIA committee is in the process of standardizing the 13 kbps coder and is working on selecting the EVRC vocoder design.