Abstract: A method of optimizing a monitoring time window of NB-IoT physical downlink control channel (NPDCCH) for one of i) narrow-band Internet-of-things (NB-IoT) for non-terrestrial network (NTN) including a satellite and a base station, or ii) 5th Generation New Radio NTN includes: receiving, by a user equipment (UE), a specified signal Kmac transmitted by the base station, wherein the specified signal Kmac enables the UE to calculate a signal transmission round-trip time (RTT) between the UE and the base station via the satellite; and adjusting, based on the specified signal Kmac, NPDCCH monitoring time window when the UE does not need to monitor a downlink (DL) signal transmitted by the base station in response to an uplink (UL) transmission. The signal RTT is equal to the sum of UE's TTA (timing offset between the start of a received downlink subframe and a transmitted uplink subframe) and Kmac.