Understanding 3GPP LTE/NR RRC parameter Q-RxLevMin it’s impact on coverage

Introduction:

The Q-RxLevMin parameter is set by the network in the SIB1 broadcast to the UE in LTE, or in the RRC Reconfiguration in NR-NSA. It sets the threshold value or lower RSRP limit that the device should remain attached to the eNB/gNB cell.

In urban situations of high cell density, having Q-RxLevMin stretched too thin can result in poor experience, where the UE may not be able to respond to the network since commercial UE Tx power is limited to 23 dBm (decibels per milliwatt) in Power Class 3, or 26 dBm in Power Class 3. 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 still be able to transmit back if the signal to noise ratio is decent and if UE Tx output power is strong enough. T-Mobile US has Q-RxLevMin for NR-SA n71 set to -140 dBm RSRP in rural areas, but lowers it to -118 dBm RSRP in urban areas, and is a good illustration of how this is implemented on a commercial network. Networks try to find a balance between coverage, performance, and impact on required UE Tx power levels.

The minimum allowed RSRP in the 3GPP standard for LTE is -140 dBm, and -156 dBm in NR. In the signaling, Q-RxLevMin is given according to the following formula: RSRP = (2 * integer). Therefore, in LTE a Q-RxLevMin value -70 is equal to -140 dBm RSRP, or 0x46 in Hex. In NR a Q-RxLevMin value -75 is equal to -156 dBm RSRP, or 0x4E in Hex. Qualcomm’s modem will not allow values lower than -150 in NR, so we’ll set Q-RxLevMin to 0x4B in hex instead of 0x4E.

3GPP compliant devices must respect this value, however it is still possible to override this.

Devices rendered non-compliant via custom Q-RxLevMin value may experience issues in situations where the UE Tx power results in the UE not being heard by the network. This may result in call completion failures. Proceed at your own risk. This is for use in lab testing only.

How to bypass Q-RxLevMin on LTE/NR networks:

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 setting the custom Q-RxLevMin threshold value

For LTE: navigate to /nv/item_files/modem/lte/rrc/efs/

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

List of NV item files:

q_rxlevmin_custom_v2 set to 0x46

For NR: navigate to /nv/item_files/modem/nr5g/RRC/

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

List of NV item files:

q_rxlevmin_custom set to 0x4B

rxlevmin_threshold set to 0x4B

Download from here