at 1. NPDCCH Format 0 occupies only one NCCE, whereas NPDCCH Format 1 occupies two NCCEs. As shown in Fig. 6.13, there are only a few subframes that can be allocated to carry NPDCCH or narrowband physical downlink shared channel (NPDSCH). To reduce UE complexity, all downlink channels use LTE TBCC. Furthermore, the maximum trans- port block size (TBS) of NPDSCH is limited to 680 bits. The NRS is used to provide phase reference for coherent demodulation of the downlink channels. The NRS resources are time and frequency multiplexed with information bearing symbols in subframes carrying NPBCH, NPDCCH, and NPDSCH, using eight resource elements per subframe per antenna port. The NPDCCH structure is depicted in Fig. 6.13. In this example, we show the map- ping for an in-band NB-IoT operation assuming a single antenna port in the LTE cell and two antenna ports in NB-IoT. The parameter lNPDCCHstart, the LTE control region size sig- naled by NB-SIB1 [4], indicates the OFDM start symbol, which helps avoid conflict with the LTE control channel in the in-band operation. For the guard-band and standalone opera- tion modes, the control region size is by default zero, which provides more resource ele- ments for the NPDCCH. In order for the UE to find the control information with reasonable amount of decoding complexity, NPDCCH is grouped into the following search spaces: Subframe Slot 0 Slot 1 NCCE1 NPDCCH format Number of NCCEs LTE CRS LTE PDCCH NPBCH NCCE0 NRS Port 0 NRS Port 1 NPDCCHstart Figure 6.13 NPDCCH structure in time and frequency [2,25]. 764 Chapter 6 Type-1/1A common search spaces used for paging Type-2/2A common search spaces used for random access UE-specific search space A UE is not required to simultaneously monitor an NPDCCH UE-specific search space and a Type-1 or Type-2 NPDCCH common search space. An NPDCCH search space NSK (L',R) at aggregation level (AL) L' E {1,2} and repetition level R E {1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, ,2048} is defined by a set of NPDCCH candidates where each candidate is 