ss-rsrp is the linear average over the power contributions of the resource elements received over secondary synchronization signals. These secondary signals are specific to each cell are transmitted using source elements.
ss-rsrp is used to compare strengths of the signals from individual cells in 5g networks. It is used for cell selection, cell reselection, power control, mobility procedures and beam management procedures.
The measurement time resources are confined within SS/PBCH block measurement time configuration (SMTC) window duration.
ss-rsrp is defined in 3GPP TS 38.215 and 3GPP TS38.133.
For ss-rsrp determination, demodulation reference signals (DMRS) for PBCH with an option of CSI-RS (Channel state information reference signals) is used. ss-rsrp is measured by the linear averaging over the power contributions of the resource elements that carry corresponding reference signals taking into account of power scaling of reference signals.
The DMRS and SS signals are transmitted with equal power so results can be averaged directly. CSI-RS may have different transmit power.
ss-rsrp is average power received from single resource element allocated to SSS. The power is measured in mWatts, power is an energy received during useful part of symbol only , cyclic part not included.
Measurement is performed at UE antenna connector. Assumption is UE has single antenna element per receive path instead of antenna array.
Measurement is performed based upon combined signal strength from all antenna elements related to the single receive path. Assumption is UE has an antenna array for each receive path.
ss-rsrp layer 1 measurements useful for switching between the beams, and reporting to L3.
3GPP TS 38.133 specifies about ss-rsrp reporting and measurements.
As per 3GPP 5G NR specs, min allowed RSRP value is -156 dBm and max allowed is -31 dBm.
ss-rsrp is filtered at L1 to remove the impact of noise. L1 measurements are specific to beam level, as each ss-rsrp is measured on each SS/PBCH block.
ss-rsrp is filtered at L3 to remove fast fading and to reduce short term variations in measurement results. L3 measured rsrp value can be either beam level or cell level.
L1 ss-rsrp range is -140 dBm to -44 dBm, while L3 ss-rsrp range is between -156 dBm and -31 dBm. Both these values signaled/represented using 7 bits.
Reason for measuring ss-rsrp using only SSS as SSB carries other info PSS PBCH:
Some answers from internet:
- NR supports bandwidths upto 400MHz and scanning this entire range will drain UE power.
The center 20 RB carry SSS which also carries reference signals. So UE does not have to scan entire bandwidth.
-The Primary Synchronisation Signal (PSS) may be transmitted using an EPRE which is equal to the SSS EPRE, or it may be transmitted with an EPRE which is 3 dB greater than the SSS EPRE. There is scope to increase the transmit power of the PSS because there are unused Resource Elements both above and below the PSS (within the set of20 Resource Blocks used by an SS/PBCH Block).
Increasing the EPRE by 3 dB means that the total power within each symbol of an SS/PBCH Block remains approximately constant. The UE is not provided with information regarding the transmit power of the PSS so the UE must deduce the value of the transmit power offset. Having knowledge of the offset is useful for UE which are capable of Interference Cancellation ( IC). Knowledge of the offset simplifies the generation of the signal to be subtracted from the wanted signal. In addition, having knowledge of the offset is useful for the UE receiver Automatic Gain Control (AGC) which can be tuned according to the power or each symbol belonging to the SS/PBCH Block.
Practical Example of RSRP:
Let’s consider a single antenna system and say the Reference Signal Transmitted power is about 12 dBm. A UE is located somewhere in the cell experiencing a path loss of 100 dB. Then the RSRP measured by UE should be 12–100 = -88 dBm , When we compare -88dBm value in above table it is corresponding to RSRP Integer value 52 so UE can report RSRP=52 in the measurement report.
Normally, when anyone see the UE logs measurement report ,it contained the RSRP integer value, the quick way to find our absolute RSRP value in dBm is simply subtract the integer value from -140 .Let’s take an example Integer RSRP value is 52 then corresponding RSRP dBm = -140 + 52 = -88 dBm.