In 5G new  radio (NR) system, reference signal   measurement are used to perform cell selection, cell reselection and handover etc.  Reference Signal (RS) measurements are further classified into three measurement parameters as given below: 

•        Reference Signal Received Power (RSRP)
•        Received Signal Strength Indicator (RSSI)
•        Reference Signal Received Quality (RSRQ)

 These measurements (i.e. RSRP, RSRQ, RSSI) are included in the RRC Information Elements (IEs) carries. 

Received Signal Strength Indicator (RSSI):

RSSI is defined as  the total received wide-band power measured over entire bandwidth by UE. RSSI is not reported to eNodeB. The RSSI is used in the calculation of RSRQ in LTE system. RSSI is also a power and its measurement unit is dBm

 RSSI = Serving Cell Power + Neighbour Co-Channel Cells Power + Thermal Noise

 Under full Load condition the  RSRP (dBm)= RSSI – 10*log (12*N)

where N  represents number of resource block. 

By using above equation RSSI can be calculated as:

If RSRP is -60 dBm and the cell is fully loaded (20MHz) and the UE is experiencing 10dBm power from a Neighbour cell operating at same frequency RSSI measured -60 dBm + 30.72 dBm +10 dBm = -19.98 dBm.

 So from the above calculation it can be concluded that if UE is seeing only one cell then there is relation between RSRP and can be directly scaled by the factor for particular bandwidth given in the table1, but UE start seeing more signal on the same frequency from other cell then RSSI value become even better.

 Reference Signal Received Power(RSRP):

RSRP is the linear average of reference signal power (in Watts) measured over a specified bandwidth (in number of REs). It is very similar to CPICH RSCP in WCDMA. It is applicable while UE in different RRC states i.e. RRC_IDLE intra-frequency, RRC_IDLE inter-frequency, RRC_CONNECTED intra-frequency, RRC_CONNECTED inter-frequency.

  Practical Example of RSRP:

 Let’s consider a single atnenna 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 then Table 2 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 45 then corresponding RSRP dBm = -140 + 45 = -95 dBm.

 RSRQ:

RSRQ is th Reference Signal Received Quality and it can be defined using following equation:

 RSRQ= (N* RSRP)/RSSI

As per the definition of RSSI, It is wide band power including signal power from serving cell, co-channel neighbour cell, interference from other cell interference and noise, in similar word one can say that RSRQ define the purity of Reference Signal (RS) across the system bandwidth. RSRQ is a calculated value from RSRP and RSSI is measure of signal and interference. As RSRQ is a ration of two signal powers with same  same unit i.e. dBm so RSRQ uses dB as a measurement unit.

 Similar to RSRP , UE reported an integer value to eNodeB and its range is from 0 to 34 (Table 3). 3GPP has provided a table shown below, by using this table the integer value can be translated to a range of RSRP value in dB.

 The best value of RSRP is about -3 dB and worst value can be -19.5dB. 

So, when there is no traffic, and assuming only the reference symbols are transmitted (there are 2 of them within the same symbol of a resource block) from a single Tx antenna then the RSSI is generated by only the 2 reference symbols so the result becomes:

 RSRQ = N / 2N = -3 dB for 1Tx

RSRQ = -6 dB for 2Tx

If all resource elements are active and are transmitted with equal power, then

 RSRQ = N / 12N = -10.8 dB for 1Tx

RSRQ = N / 20N = -13 dB for 2Tx

(because RSRP is measured over 1 resource element and RSSI per resource block is measured over 12 resource elements).

 Reference: TS 36.133

                  TS 36.304