illustrated in Fig. 6.21. The scheduling commands are conveyed through DCI, which is carried by NPDCCH using aggregation-level (AL)-1 or AL-2 for transmission. With AL-1, two DCIs are multiplexed in one subframe, otherwise each sub- frame carries only one DCI (i.e., AL-2), resulting in a lower coding rate and improved COV- erage. Further coverage enhancement can be achieved through repetition. Each repetition occupies one subframe. DCI can be used for scheduling downlink data or uplink data. In the case of downlink data, the exact time offset between NPDCCH and the associated NPDSCH is indicated in the DCI. Since NB-IoT devices are expected to have reduced com- puting capability, the time offset between the end of NPDCCH and the beginning of the associated NPDSCH is at least 4 ms. In comparison, LTE PDCCH schedules PDSCH in the same subframe. After receiving NPDSCH, the UE needs to send HARQ ACK/NACK using NPUSCH format 2. The resources of NPUSCH carrying HARQ ACK/NACK are also sig- naled in DCI. Considering the limited computing resources in an NB-IoT device, the time offset between the end of NPDSCH and the start of the associated HARQ ACK/NACK is at least 12 ms. This offset is longer than that between NPDCCH and NPDSCH because the size of the transport block carried in NPDSCH can be as large as 680 bits, which is much 776 Chapter 6 NPDSCH ACK/NACK NPDCCH (AL = 2,Rep = 2) (4 resource units) NPUSCH ->4 ms >12 ms ACK/NACK NPDCCH NPUSCH (AL = 2,Rep = 2) NPDCCH >8 ms >3 ms NPDCCH NPDSCH NPUSCH NPDCCH (DCI for NPDSCH) (downlink data) (ACK/NACK) (next transmission) Downlink NPDCCH NPDSCH ->4 ms >12 ms >3 ms repetition repetition NPDCCH NPUSCH NPDCCH (DCI for NPDSCH) (uplink data) (next transmission) Uplink NPDCCH NPUSCH >8 ms >3 ms repetition repetition Figure 6.21 Illustration of the NB-loT HARQ operation [14]. larger than DCI that is only 23 bits long. The DCI for uplink scheduling grant needs to specify which subcarriers are allocated for the UE. The time offset between the end of NPDCCH and the beginning of the a