HARQ for the PDSCH is realized by adaptive and asynchronous timing transmission. Three different coverage levels are signaled via the System Information Block Narrow Band (SIB-NB): normal, robust, and extreme. The coverage level is defined by Narrow- band Reference Signal Received Power (NRSRP) thresholds. The thresholds depend on the deployment environment and on the system configuration. The number of repeti- tions and network parameters can be selected for each class. The target of the normal coverage level is to match General Radio Packet Service (GPRS) coverage, while extreme coverage will exceed GPRS coverage by 20 dB (Figure 14.10). The Narrowband Physical Random Access Channel (NPRACH) is based on single tone transmission with frequency hopping. It uses a subcarrier spacing of 3.75 kHz, and the cyclic prefix length is designed for cell sizes up to 40 km. The NPUSCH has one resource unit schedulable for data transmission. The length with 15 kHz is 1 ms for 12 tones, 2 ms for 6 tones, 4 ms for 3 tones, and 8 ms for a single tone, and with 3.75 kHz 32 ms for a single tone. One transport block may be mapped to 1-10 resource units. The maximum TBS size for the NPUSCH is 1000 bits, and 1 HARQ process is used. 14 Internet of Things Optimization NPDCCH with Figure 14.11 NPDCCH repetition. 4x repetition 1 PRB Table 14.5 NB-loT peak data rates. Downlink Uplink Instantaneous peak 170 kbps 250 kbps Sustained peak rate without 26.2 kbps 62.5 kbps considering common channels Sustained peak when common 19.6 kbps 56.3 kbps channel overhead is considered All NB-IoT channels can utilize repetition, and the repetition can be configured indi- vidually per channel. Each repetition is self-decodable, and one ACK is used for all repetitions. An example repetition of four for the NPDCCH is illustrated in Figure 14.11. The repetition factor can be much higher: the NPDCCH allows repetitions up to 128, and the NPDSCH even allows 2048 repetitions. Modulation can only be QPSK or BPSK for the single tone case. No higher order modulati