The Open Loop Power Control require in CDMA due to following reason.
Assumes Loss is Similar on Forward paths and Reverse Paths
Receive Power + Transmit Power = -73
All Powers in dBm
Example: For a Received Power of -85 dBm
Transmit Power = (-73) – (- 85)
Transmit Power = +12 dBm
Provides an Estimate of Reverse TX Power for Given Propagation Conditions
Open loop power control is based on the similarity of the loss in the forward path to the loss in the reverse path (forward refers to the base-to-mobile link, while reverse refers to the mobile-to-base link).
Open loop control sets the sum of transmit power and receive power to a constant, nominally -73, if both reverse and forward powers are in dBm. A reduction in signal level at the receive antenna will result in an increase in signal power from the transmitter.
For example, assume the Forward received power from the base station is -85 dBm. This is the total energy received in the 1.23 MHz receiver bandwidth. It includes the composite signal from the serving base station as well as from other nearby base stations on the same frequency.
The open loop transmit power setting for a received power of -85 dBm would be +12 dBm. Thus open loop power control adjusts the transmit power of the phone to match the propagation conditions that the phone is experiencing at any given time.
By the TIA/EIA-98 standard specification, the open loop power control slew rate is limited to roughly match the slew rate of closed loop power control directed by the base station. This eliminates the possibility of open loop power control suddenly transmitting excessive power in response to a receiver signal level dropout.
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