RSRP is the linear average of reference singal power (in Watts) accorss the specified bandwidth (in number of REs). This is the most important item UE has to measure for cell selection, reselection and handover. You can think of this as the one similar to CPICH RSCP in WCDMA.

Three questions come up every time you read an RSRP value in a log. What exactly did the UE measure? Which resource elements did it use? And how did a power in dBm become the integer in the MeasurementReport? I'll take them in that order.

• How does 36.214 define RSRP ?
• Which resource elements does the UE measure ?
• How does the UE report RSRP ?
• Reference

How does 36.214 define RSRP ?

The word average in RSRP is easy to misread, so let's start from the exact text. RSRP is a power per resource element, not the total power of the carrier. That one choice decides how an RSRP value compares with RSSI and with the transmit power of the eNB.

The formal definition of RSRP in 3GPP 36.214 is as follows :

Reference signal received power (RSRP), is defined as the linear average over the power contributions (in [W]) of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth. For RSRP determination the cell-specific reference signals R0 according TS 36.211 shall be used. If the UE can reliably detect that R1 is available it may use R1 in addition to R0 to determine RSRP.

Note : R0 is the Cell Specific Reference Signal for Antenna Port 0 and R1 is the Cell Specific Reference signal for Antenna Port 1 (Refer to 36.211 Figure 6.10.1.2-1. Mapping of downlink reference signals (normal cyclic prefix))

The current 36.214, v19.0.0 clause 5.1.1, keeps this sentence and adds several rules around it. Four of them matter when you compare RSRP values from different UEs or test setups.

The first is the reference point. RSRP is measured at the antenna connector of the UE, so the antenna gain is outside the number. The second is receiver diversity. If the UE uses more than one receive branch, the reported value shall not be lower than the RSRP of any individual branch. The third is the measurement set. The number of resource elements and the measurement period are left to the UE implementation, as long as the UE meets the accuracy requirements of 36.133. The fourth is the energy itself. The power per resource element comes from the useful part of the symbol, and the CP is excluded.

The clause also covers two newer cases. When higher layers configure measurements based on discovery signals, the UE measures RSRP in the configured discovery signal occasions. When higher layers configure measurements based on the resynchronization signal, RSS, the UE measures RSRP on the RSS instead. RSRP is applicable in RRC_IDLE and RRC_CONNECTED, for both intra-frequency and inter-frequency measurements.

• RSRP is a power per resource element : it is the linear average in W over the CRS resource elements, not the power of the whole carrier.
• R0 is mandatory and R1 is optional : the UE may add R1 only if it can reliably detect it.
• The reference point is the UE antenna connector : antenna gain is not part of RSRP.
• With receiver diversity, the reported RSRP is at least the best branch : a combined value may not fall below any single branch.
• The UE chooses how many resource elements to use : 36.133 accuracy requirements are the only limit.

Which resource elements does the UE measure ?

Following is an example of one downlink radio frame. The red part is the resource elements in which reference signal is being transmitted. RSRP is the linear average of all the red part power.

In the resource grid below, time runs from left to right over one radio frame, and frequency runs from top to bottom. Each small square is one resource element. The red squares are the cell-specific reference signals, and they repeat in a regular diagonal pattern across the whole bandwidth and the whole frame. The coloured columns near the left edge and in the middle are the synchronization signals and PBCH in subframes 0 and 5.

Figure 1. One downlink radio frame. The red resource elements are the only ones that RSRP averages, so data load in the white resource elements does not change RSRP.

• The red resource elements are spread evenly in frequency, so the UE can average them over any measurement bandwidth it chooses.
• They also appear in every subframe, so the UE can measure RSRP at any time without a schedule from the network.
• The white resource elements carry PDSCH or nothing at all. Their power is part of RSSI but not part of RSRP.

Since this measures only the reference power, we can say this is the strength of the wanted signal. But it does not gives any information about signal quality. RSRP gives us the signal strenth of the desired signal, not the quality of  the signal. For quality of the signal information another parameter called 'RSRQ' is used in some case.

Because RSRP is a per resource element power, it lines up directly with the power that the eNB transmits per resource element. The eNB broadcasts that CRS power as referenceSignalPower. 36.213 clause 5.1.1.1 then defines the downlink path loss that the UE uses for uplink power control as referenceSignalPower minus the higher layer filtered RSRP. For example, if referenceSignalPower is 15 dBm and the filtered RSRP is -95 dBm, the UE estimates a path loss of 110 dB.

Cell selection uses RSRP in the same direct way. In 36.304, Srxlev = Qrxlevmeas - (Qrxlevmin + Qrxlevminoffset) - Pcompensation - Qoffsettemp, and Qrxlevmeas is the measured RSRP of the cell. So the RSRP value decides whether a cell passes the S criterion at all, before any comparison between cells starts.

• RSRP reads only the CRS resource elements : PDSCH load in the other resource elements does not change it.
• RSRP tells you signal strength, not signal quality : RSRQ adds the interference and noise side.
• Path loss is referenceSignalPower minus filtered RSRP : both values are per resource element, so they can be subtracted directly.
• The S criterion starts from RSRP : Qrxlevmeas in Srxlev is the measured RSRP.

How does the UE report RSRP ?

UE usually measures RSRP or RSRQ based on the direction (RRC message) from the network and report the value. When it report this value, it does not use the real RSRP value. It sends a non-negative value ranging from 0 to 97 and each of these values are mapped to a specific range of real RSRP value as shown in the following table from 36.133.

Figure 2. 36.133 Table 9.1.4-1 in its original form. Each reported value covers a 1 dB step, and the two end values are open ranges.

• RSRP_00 means RSRP < -140 dBm, so every weaker value is reported as 0.
• RSRP_01 to RSRP_96 are 1 dB wide. A reported value n covers -141 + n <= RSRP < -140 + n dBm.
• RSRP_97 means -44 dBm <= RSRP, so every stronger value is reported as 97.

The current 36.133, v19.5.0 clause 9.1.4, extends the table at the low end. The reporting range of RSRP is now -156 dBm to -44 dBm with 1 dB resolution. The new reported values RSRP_-17 to RSRP_-01 cover the range below -140 dBm, with RSRP_-17 meaning RSRP < -156 dBm and RSRP_-01 meaning -141 <= RSRP < -140 dBm. RSRP_00 and RSRP_01 to RSRP_97 keep the meaning shown in Figure 2.

Let's see how the extension fits into the RRC message without changing the old field. 36.331 keeps the original type at 0 to 97 and adds a second type for the negative values.

Following is based on 36.331 v19.3.0 (Release 19)

RSRP-Range ::=                     INTEGER(0..97)

RSRP-Range-v1360 ::=               INTEGER(-17..-1)

• A field of type RSRP-Range-v1360 is signalled only if the corresponding original field of type RSRP-Range is set to 0.
• A UE that supports CE Mode B, when upper layers do not restrict CE Mode B, reports the RSRP-Range-v1360 value if the measured RSRP is less than -140 dBm.
• MeasResults carries it in fields such as measResultPCell-v1360, and each neighbour cell result carries it as rsrpResultNCell-v1360.

So a network that does not know the extension still reads 0, which means below -140 dBm, exactly as before. A network that knows it reads the extra field and gets the finer value.

For practical calculation example, refer to RSRQ Calculator (YouTube)

Just to relieve a little of headache to figure out the mapping between the reported RSRP and measured value in dBm, I just made precalculated table as shown below.

The table covers the original range from 00 to 97 only. For the extended values of 36.133 v19.5.0, the same rule continues below 00. A reported value of -n, from -1 to -16, covers -141 - n <= RSRP < -140 - n dBm, and -17 covers everything below -156 dBm.

• The UE reports an integer, not a dBm value : 36.133 Table 9.1.4-1 maps each integer to a 1 dB range.
• A reported value n from 1 to 96 means -141 + n <= RSRP < -140 + n dBm : 0 and 97 are the open ends of the original range.
• The current range reaches -156 dBm : RSRP_-17 to RSRP_-01 extend it for UEs in enhanced coverage.
• The extension is backward compatible : RSRP-Range-v1360 is sent only when the original field is 0.

Reference

[1] 3GPP - 36.214 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer - Measurements  

[2] 3GPP - 36.133  Evolved Universal Terrestrial Radio Access (E-UTRA);Requirements for support of radio resource management

[3] For practical calculation example, refer to RSRQ Calculator (YouTube)

[4]  RSRP and RSRQ Measurement in LTE  

[5] 3GPP - 36.214 v19.0.0, clause 5.1.1 Reference Signal Received Power - the current text read for this page

[6] 3GPP - 36.133 v19.5.0, clause 9.1.4 RSRP Measurement Report Mapping

[7] 3GPP - 36.331 v19.3.0, RSRP-Range information element

[8] 3GPP - 36.213 v19.4.0, clause 5.1.1.1 UE behaviour, for the path loss estimate

[9] 3GPP - 36.304 v19.2.0, clause 5.2.3.2 Cell Selection Criterion