n parameters are fixed, there is no associated indication in the control channel. The SIB1-NB content may only be changed on each modification period, which has a length of 4096 radio frames; that is, every 40.96 seconds. This corresponds to four SFN periods, which is the reason for indication of two LSBs of the HFN in the MIB-NB. If such a modification occurs, it is Table 6.4: Resource allocation in NB-loT [2]. Subcarrier Spacing (kHz) Number of Tones Link Direction Resource Unit Size (ms) Uplink/Downlink Uplink Uplink Uplink Uplink Table 6.5: Uplink resource allocation in NB-loT [2,4]. Subcarrier Subcarriers Slots Duration of Number of Resource Uplink Data Type Spacing (kHz) per RU per RU RU (ms) Elements per RU Uplink data Uplink control information (ACK/NACK) Internet of Things 769 indicated in the NPDCCH using DCI format N2. Although sent over the NPDSCH, the SIB1-NB resources are mapped similar to the MIB-NB shown in Fig. 6.12; that is, exclud- ing the first three OFDM symbols, which is necessary because the UE obtains the start of the resource mapping from SIB1-NB. One of the key features of NB-IoT is coverage enhancement through signal repetition. For typical NB-IoT use cases, the signal-to-noise ratio (SNR) operation point is below 0 dB. Under this operating condition, the channel estimation error is the dominating factor affect- ing the receiver performance. It can be shown that the effective SNR after combining NRP repetitions is given as follows [18]: where Y denotes SNR per transmission, o2 represents the variance of channel estimation error, and NRP is the number of repetitions. 6.3.8 Narrowband Physical Random-Access Channel Similar to LTE, the narrowband physical random-access channel (NPRACH) is used by an unsynchronized UE to inform the eNB of its desire to establish a connection. The random- access procedure is a contention-based and collision-prone mechanism in which the UE selects and transmits a random-access preamble to the base station. The NPRACH preamble consists of four symbol groups, w