How to check and control UE signal power ?

- UE TX power is based on path loss seen by the UE. UE computes the path loss based on:

Received Signal Strength
Network Transmitted Signal Strength, broadcasted by eNB in sib2 as "referenceSignalPower". Or, in case of NR, broadcasted by gNB in sib1 or in RRC signalling (NSA case) as "ss-PBCH-BlockPower".

If we artificially increase the referenceSignalPower or ss-PBCH-BlockPower (NR case), the UE will calculate a fake higher path loss and hence increase Tx power. If we decrease the referenceSignalPower/ss-PBCH-BlockPower, UE TX power will decrease.

To adjust the value of referenceSignalPower broadcasted in sib2 (LTE case):

the parameter manual_ref_signal_power must be set to true in the enb configuration file, under "cell_default" object.
referenceSignalPower can be set to the desired value in sib2 file

For NR, to adjust the value of ss-PBCH-BlockPower, add the following under “nr_cell_default” or for any NR cells under “nr_cell_list”

manual_ref_signal_power: true, ss_pbch_block_power: xx, /* tx_power */

Example to let a LTE UE transmits at max power:in enb.cfg config file, set manual_ref_signal_power: true,in included sib2_3.asn file, set referenceSignalPower 20

pdsch-ConfigCommon { referenceSignalPower 20, /* range: -60 .. 50 dBm */ p-b 1 }

The network also limits the UE TX power with the "p-Max" parameter broadcasted in sib1, its default value is set to 10dBm.

You can control this value in the enb configuration file (enb.cfg), according to the UE capabilities, for example:

p_max: 23, /* maximum power allowed for the UE (dBm) */

To have the UE continuously transmitting at max power you would need a stable UL traffic which fills the whole BW. To achieve that you can add the following line under the "cell_default" object.

force_full_bsr: true,

This will force the UE to always transmit in UL.

To verify the effect on UE UL power, in the enb screen, with "t" command, you can monitor the reported PHR (Power Headroom).PHR is the difference between MAX UE power (let's say 23 dBm) and current UE Tx power.

Table 9.1.8.4-1 of 3Gpp specification 36.133 defines the mapping between this difference in dB and reported value. For example, a reported PHR of 23 (0 dB difference) means that the UE is transmitting at 23 dBm.

NOTE : When you do UL max power testing, be cautious not to damage RF input port of SDR card. Refer to following RF input specification of SDR card and do proper measure (e.g, using external attenuator) to provent the damage.