RSRP, RSRQ and SINR (LTE signal quality)
Updated
RSRP (Reference Signal Received Power) is the power of the LTE reference signal received by a device, in dBm; RSRQ (Reference Signal Received Quality), in dB, relates it to the total received power and interference. Both are defined in 3GPP TS 36.214; SINR, the signal to interference plus noise ratio, completes the picture.
In practice
As an indicative field rule, an RSRP above -90 dBm is good, -90 to -105 dBm is usable and below -110 dBm a standard 4G device often struggles; LTE-M and NB-IoT can go lower. The AT command AT+CESQ returns these values on most modules.