Open Loop Power Control in CDMA
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.