RSRQ is defined as (N x RSRP)/RSSI in 3GPP 36.214, where N is the number of RBs over the measurement bandwidth.
RSRQ = N × RSRP/(EUTRA carrier RSSI),
As you see from the definition of RSSI, RSSI contains all sorts of power including power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc. Therefore, (N x RSRP)/RSSI indicates "What is the portion of pure RS power over the whole E-UTRA power recived by the UE".
As you see, this is not the direct measurement, it is a kind of derived value from RSRP and RSSI. By deviding RSRP by RSSI, it could give some information about interference as well in addition to the strength of the wanted signal.
Since this is the ratio of two different power value, the unit of RSRQ is dB and the value would be always negative (because RSSI value will aways be larger than N x 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 use the real RSRQ value. It sends a non-negative value ranging from 0 to 34 and each of these values are mapped to a specific range of real RSRQ value as shown in the following table from 36.133.
< 36.133 Table 9.1.7-1 : RSRQ measurement report mapping >
Just to save a little bit of time for readers, I expanded the table to show all the possible RSRQ value and mapping absolute power.
 3GPP - 36.214 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer - Measurements
 3GPP - 36.133 Evolved Universal Terrestrial Radio Access (E-UTRA);Requirements for support of radio resource management