UE Expected Open Loop Transmit Power

UE Expected Open Loop Transmit Power

Last updated: January 15, 2009

The UE uses open loop power control to determine the transmit power of the first PRACH preamble and of the DPCH as it is first turned on. Once the DPCH is established its power is controlled solely by closed loop power control (see Uplink Closed Loop Power Control Parameters and UE Target Power ).

Open Loop Power Control

The test set calculates the expected transmit power of the first PRACH preamble and of the DPCH (at turn-on the DPCH consists only of the DPCCH) and displays them in the UE Expected Open Loop Transmit Power window on the Call Setup Screen .

Initial PRACH TX Power

The test set calculates the expected transmit power of the first PRACH preamble using the following formula (see 3GPP TS 25.331 8.5.7):

Initial PRACH TX Power = Primary CPICH TX power - CPICH RSCP + Uplink Interference + Constant value

Three parameters are directly user-settable: Primary CPICH TX Power , Uplink Interference , and Constant Value .

The fourth parameter, CPICH RSCP (Received Signal Code Power) , is the result of the UE reported measurement of the CPICH code channel power. You can change the test set's CPICH code channel power by varying Cell Power and/or CPICH Level (see Downlink Channel Levels and CALL:STATus:CPIChannel[:CELL[1]]:POWer[:AMPLitude]? ).

   
NOTE
Ensure that you do not set these parameters such that the UE would be required to transmit its PRACH at a power level equal to or greater than its maximum transmit power or the power specified by the Maximum Uplink Transmit Power Level setting. See Call Setup Fails Due to Transmission of Single PRACH Preamble .
   

GPIB command: CALL:UPLink:PRAChannel:POWer:INITial?

Initial DPCCH TX Power

Per 3GPP TS 25.331 s8.5.3, the UE determines its initial DPCCH transmit power using the following formula:

DPCCH_Initial_power = DPCCH_Power_offset - CPICH_RSCP

The UE determines "CPICH_RSCP" by measuring the test set's CPICH code channel power. "DPCCH_Power_offset" is signalled by the test set in the RRC Connection Setup and Radio Bearer Setup messages used to establish the connection. The test set signals a value for DPCCH_Power_offset that places the UE's initial transmission near the UE Target Power setting (because the test set's receiver is ranged based on the UE Target Power setting). Thus, to determine what DPCCH_Power_offset value to signal during call establishment, the test set considers the UE Target Power setting, as well as the Cell Power and CPICH Level settings (which determine the CPICH RSCP the UE will measure).

As mentioned above, the test set signals a value for DPCCH_Power_offset that places the UE's initial transmission near the UE Target Power setting. The UE's initial transmission consists only of the DPCCH. The UE Target Power setting is the level you wish for the UE's DPCCH+DPDCH transmission. Thus, the test set places the initial transmit power of the UE's DPCCH slightly lower than the UE Target Power setting so that when the DPDCH is turned on, the total UE power matches the UE Target Power setting. The initial DPCCH power can thus be determined by the following formula:

Initial DPCCH TX Power = UE Target Power + 10 log (Beta c ^2/(Beta c ^2 + Beta d ^2))

All three parameters are user settable, see UE Target Power and Uplink DPCH Bc/Bd Control .

As an example, when Cell Power = -75 dBm, CPICH Level = -3.3 dB, UE Target Power = -5 dBm, Betac = 8/15 and Betad = 15/15, the value signalled for DPCCH_Power_offset is -44 (-88 dB) so that the Initial DPCCH TX Power is near -11.55 dBm (note that the actual level achieved is within 2 dB due to the fact that the information element's range is specified in steps of 2).

GPIB command: CALL:UPLink:DPCChannel:POWer:INITial?

Related Topics

How Do I Change UE TX Power Levels?

How Do I Change Cell Parameters?

Uplink Closed Loop Power Control Parameters and UE Target Power