Once the data has been scrambled with the user specific long code, the closed loop power control data is then punctured into the data stream. Remember that the power control bits are sent every 1.25 milli-seconds – once in every power control group (a CDMA frame is 20 milli-seconds with each frame having 16 1.25 milli-second power control groups).
- Long Code is Decimated Down to 800 bps
- Decimated Long Code Controls the Puncture Location
- Power Control Bits Replace Voice Data
- Voice Data is Recovered by the Mobile’s Viterbi Decoder
In each 1.25 milli-second power control group there are 24 modulation symbols of data (the data stream at this point is 19.2 kbps so each of the 24 symbols has a period of 52.08 micro-seconds). The power control bits are placed somewhere in the first 16 modulation symbols in each power control group.
The exact location of the power control bits are determined by decimating the long code down to a rate of 800 bps and then using the data to point to one of the modulation symbol locations. For a 9.6 kbps voice channel, two modulation symbols are punctured allowing the power control data to be sent twice.
For 14.4 kbps voice channels, only a single modulation symbol is punctured with the power control bit.
Since the power control bits replace the encoded voice data, holes (missing data) are introduced into the data stream from the receiver’s point of view. These holes are accepted and the system uses the Viterbi decoder in the receiver to restore the data lost by puncturing. The recovery of the missing data uses some of the available processing gain in the system.
This results in a loss of capacity, but the loss has been accounted for in the system’s design. Another way to think of this is that slightly more power is required to maintain the link because of the missing data introduced by the power control puncturing.
The power control data is only sent once in the 14.4 case since the reduced processing gain results in higher power being transmitted from the base station to maintain an acceptable signal to noise ratio. The higher power results in a much lower symbol error rate and the need to send the power control data twice is eliminated.