A method performed by a user equipment, UE, the method comprising: detecting, by the UE in a narrowband-internet of things, NB-loT, system supporting time-division duplex, TDD, a narrowband physical downlink shared channel, NPDSCH, ending in subframe n; starting a transmission of acknowledgment/negative acknowledgment, ACK/NACK, after j 0 NB-IoT uplink, UL, slots following the end of subframe n+12, wherein, the j 0 is an offset value and NB-IoT UL slots are determined from a set of UL subframes configured for NB-IoT UL transmission; and determining a subcarrier spacing Δf, wherein the subcarrier spacing Δf is used for the transmission of the ACK/NACK; in response to determining that the subcarrier spacing Δf = 3.75kHz, setting the j 0 =0 in response to determining that a value in a ACK/NACK resource field in downlink control information, DCI, corresponding to one of {0, 1, 2, 3, 4, 5, 6, 7}; and setting the j 0 =4 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {8, 9, 10, 11, 12, 13, 14, 15}; in response to determining that the subcarrier spacing Δf = 15kHz, setting the j 0 =0 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {0, 1, 2, 3}; setting the j 0 =4 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {4, 5, 6, 7}; setting the j 0 =8 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {8, 9, 10, 11}; and setting the j 0 =10 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {12, 13, 14, 15}. detecting, by the UE in a narrowband-internet of things, NB-loT, system supporting time-division duplex, TDD, a narrowband physical downlink shared channel, NPDSCH, ending in subframe n; starting a transmission of acknowledgment/negative acknowledgment, ACK/NACK, after j 0 NB-IoT uplink, UL, slots following the end of subframe n+12, wherein, the j 0 is an offset value and NB-IoT UL slots are determined from a set of UL subframes configured for NB-IoT UL transmission; and determining a subcarrier spacing Δf, wherein the subcarrier spacing Δf is used for the transmission of the ACK/NACK; in response to determining that the subcarrier spacing Δf = 3.75kHz, setting the j 0 =0 in response to determining that a value in a ACK/NACK resource field in downlink control information, DCI, corresponding to one of {0, 1, 2, 3, 4, 5, 6, 7}; and setting the j 0 =4 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {8, 9, 10, 11, 12, 13, 14, 15}; setting the j 0 =0 in response to determining that a value in a ACK/NACK resource field in downlink control information, DCI, corresponding to one of {0, 1, 2, 3, 4, 5, 6, 7}; and setting the j 0 =4 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {8, 9, 10, 11, 12, 13, 14, 15}; in response to determining that the subcarrier spacing Δf = 15kHz, setting the j 0 =0 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {0, 1, 2, 3}; setting the j 0 =4 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {4, 5, 6, 7}; setting the j 0 =8 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {8, 9, 10, 11}; and setting the j 0 =10 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {12, 13, 14, 15}. setting the j 0 =0 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {0, 1, 2, 3}; setting the j 0 =4 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {4, 5, 6, 7}; setting the j 0 =8 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {8, 9, 10, 11}; and setting the j 0 =10 in response to determining that a value in a ACK/NACK resource field in DCI corresponding to one of {12, 13, 14, 15}.