RSSI represents the total received wide-band power measured over entire bandwidth by UE. RSSI is not reported to eNodeB. It is computed to be used for the calculation of RSRQ in LTE system. RSSI is also a power and its measurement unit is dBm same as RSRP.
RSSI=Serving Cell Power + Neighbour Co-Channel Cells Power + Thermal Noise
Under full Load condition the RSRP (dBm)= RSSI – 10*log (12*N)
|Bandwidth||1.4 MHz||3 MHz||5 MHz||10 MHz||15 MHz||20 MHz|
|No. of Resource Blocks (N)||6||15||25||50||75||100|
|Scaling Factor: 10*log(12*N)||18.57||22.55||24.77||27.78||29.54||30.72|
Using Above equation one can say that if RSRP is -70 dBm and the cell is fully loaded (20MHz) and there is not interference from other cell then RSSI measured by UE should be -70 dBm +30.72 = -39.28 dBm. Let’s say the UE is experiencing 10dBm power from a Neighbour cell operating at same frequency then the RSSI should be -39.28 dBm +10 dBm = -29 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 table, but UE start seeing more signal on the same frequency from other cell then RSSI value become even better.