Fuente: Wipo "IOT" The invention discloses a NB-IoT UE comprising a radio modem for receiving and transmitting radio frequency signals from a base station, an application controller for controlling the NB-IoT UE, a sensor interface for controlling a sensor, a power management unit for regulating a battery, a reference clock oscillator controlled by an external reference clock crystal and a sleep clock oscillator for controlling sleep periods of the NB-IoT UE and a method performed by the NB-IoT UE. The objective to provide a possibility that UE modem operates precisely without the bill-of-material cost contribution of the external crystal will be solved by a quartz crystal-free sleep clock oscillator which is integrated on-chip of the NB-IoT UE and controls a duration of long inactivity periods of the NB-IoT UE, and the reference clock oscillator is configured to support a normal operation mode utilized during transmit and receive operation as well as a low-power mode, which is utilized to bridge short inactivity periods of the NB-IoT UE.