Why Closed Loop Power Control in CDMA
Closed loop power control is used to allow the power from the mobile unit to deviate from the nominal as set by open loop control. This is done with a form of delta modulator. The base station monitors the power received from each mobile station in reverse link and commands the mobile to either raise power or lower power by a fixed step of 1 dB. This process is repeated 800 times per second, or every 1.25 msec.
The Reverse power control data sent to the mobile from the base station is added to the data stream by replacing the encoded voice data in Forward direction. This processes in called “puncturing”, since the power control data is written into the data stream by over writing the encoded voice data. The power control data occupies 103.6 micro-seconds of each 1.25 milli-second of data transmitted by the base station.
Because the mobile’s power is controlled to be no more than is needed to maintain the link at the base station, a CDMA mobile typically transmits much less power than an analog phone.
The base station monitors the received signal quality 800 times per second and directs the mobile to raise or lower its power until the received signal quality is just adequate ( BTS Send Each times command to increase or Decrease Power in Reverse Direction). This operating point varies with propagation conditions, the number of users, and the density and loading of the surrounding cells.
Analog cellular phones need to transmit enough power to maintain a link even in the presence of a fade. Most of the time, analog phones transmit excess power. CDMA radios are controlled in real time and kept at a power level to just maintain a quality transmission based on the changing RF environment.
This has the benefit of longer battery life and smaller, lower cost amplifier design. If recent health concerns over cellular phone radiation are determined to have some basis in fact, CDMA will be preferred because of its much lower RF output power.
What Closed Power Control Criteria ?
- Directed by Base Station
- Updated Every 1.25 msec
- Commands Mobile to Change TX Power in +/- 1 dB Step Size
- Fine Tunes Open Loop Power Estimate
- Power Control Bits are “Punctured” over the Encoded Voice Data
- Puncture Period is Two 19.2 kbps
- Symbol Periods = 104.2 usec