The RSRP is comparable to the CPICH RSCP measurement in WCDMA. This measurement of the signal strength of an LTE cell helps to rank between the different cells as input for handover and cell reselection decisions. The RSRP is the average of the power of all resource elements which carry cell-specific reference signals over the entire bandwidth. It can therefore only be measured in the OFDM symbols carrying reference symbols.
The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision. RSRQ is the ratio between the RSRP and the Received Signal Strength Indicator (RSSI), and depending on the measurement bandwidth, means the number of resource blocks. RSSI is the total received wideband power including all interference and thermal noise. As RSRQ combines signal strength as well as interference level, this measurement value provides additional help for mobility decisions.
From field:
RSSI = -79 dBm RSRP = -93 dBm
Using RSRQ = #ofRB in BW * (RSRP/RSSI) we have for 10 Mhz BW:
RSRQ = 10*log(50) + (-93+79) = 17 dB + -14 = 3dB
Ref:
1. Parkland Wireless by Wei Xu
2. Wire Free Alliance posted by R Mahant
Kay Yes 
Hi,
The RSRP measurement bandwidth is determined by the eNode B,
it’s means the measurement BW maybe is not the whole BW(50RBs) everytime.
So how to know the certain BW when we want to calculate the RSSI ?
In my humble opinion, this posting is misleading and incorrect. RSRQ can NEVER be a positive value. 0 is the highest and it will be always negative. Closer value to 0 is better than larger negative numbers (-3 is better than -15). Kindly correct the posting. Thanks and Best Wishes!
Agree, the problem with the definition is that it assumes that RSRP is measured across all RBs that are used by UE but RSSI (interference) is only measured on one RB. Better to exclude 10*log(50) and define RSRQ as normalized RSRQ (per RB).
RSRQ(dB) = 10log(RSRP) dBm -10log(RSSI) dBm = -93 – (-79) = -14 dB