Understanding 3GPP LTE/NR RRC parameter p-max and it’s impact on coverage

Introduction:

There are two parameters that govern maximum UE Tx power levels. There is the p-max parameter in LTE/NR-SA, and p-NR-FR1 parameter in NR-NSA. In LTE/NR-SA the parameter is defined in the SIB1 broadcast to the UE. In NR-NSA it is defined in the the RRC Reconfiguration parameters at the time the network adds the component carrier. These parameters set the value or maximum Tx power limit that the device can transmit at.

In rural situations of low cell density, having p-max set too conservatively can result in poor coverage where the UE may not be able to respond to the network. Most commercial networks limit UE Tx power to 23 dBm (decibels per milliwatt) in LTE/NR-SA, otherwise known as Power Class 3, or 26 dBm combined power in NR-NSA. This can be exacerbated by the introduction of interference/noise (negative SINR), and proximity of other UEs. In rural areas with less cell density, despite low RSRP in the Rx, the UE may not be able to transmit back UE Tx output power is too weak. Networks try to find a balance between coverage, performance, and impact on required UE Tx power levels. Often times, the FCC certfication for a given device will allow slightly higher Tx power, as long as total ERP does not exceed the SAR limits. This results in some UEs not being able to reach their maximum allowed Tx and will never reach the peak SAR values if p-max is limited by the network.

3GPP compliant devices must respect this value, however it is still possible to override this in LTE/NR-NSA. Currently, there are no known NV items available to modify this parameter in NR-SA.

Devices with a p-max parameter override using this method will not transmit at higher Tx power levels than allowed by the device’s stock Tx power calibration values which is fixed and set by the regulatory body in which the UE is sold commercially. This may be lower, equal to, or slightly higher than p-max but never higher than allowed by the FCC per the device certification. Proceed at your own risk. This is for use in lab testing only.

How to adjust p-max by up to +2 dBm and up to the maximum Tx power allowed by the factory calibration

Step 1: Launch EFS explorer

Launch QPST with your device in diag mode. Then, open the EFS explorer.

Step 2: Upload the NV item files enabling the UE p-max adjustment

For LTE/EN-DC: navigate to /nv/item_files/modem/lte/ML1/

Right click and “Copy item file from PC”, do not drag or drop.

List of NV item files:

pmax_adjust set to 0x01

pmax_adjust_endc set to 0x01

For dual SIM, add the same files but suffixed with _Subscription01

Download from here