umerology. In this case, the UE can be allocated with 12, 6, or 3 tones. While only the 12- tone format is supported by legacy LTE UEs, the six-tone and three-tone formats are intro- duced for NB-IoT UEs, which due to coverage limitation cannot benefit from higher UE bandwidth allocation. Moreover, NPUSCH supports single-tone transmission based on either 15 or 3.75 kHz numerology. To reduce the peak-to-average power ratio (PAPR), single-tone transmission uses /2-BPSK or /4-QPSK with phase continuity between symbols. As shown in Fig. 6.19, NPUSCH format 1 uses the same slot structure as LTE PUSCH with seven OFDM symbols per slot and the middle symbol as the demodulation reference signal (DMRS), whereas NPUSCH format 2 consists of seven OFDM symbols per slot, but uses the middle three symbols as DMRS. The DMRS is used for channel estimation and coherent detection. The smallest unit to map a transport block is the resource unit whose definition depends on the NPUSCH format and subcarrier spacing. For NPUSCH format 1 and 3.75 kHz subcarrier spacing, an RU consists of one subcarrier in the frequency domain, and 16 slots in the time domain; that is, an RU has a length of 32 ms. For the 15 kHz subcarrier spacing, there are four options: 1, 3, 6, and 12 subcarriers over 16, 8, 4, and 2 slots, respec- tively (see Fig. 6.19). For NPUSCH format 2, the RU is always composed of one subcarrier with a length of four slots. As a result, for 3.75 kHz subcarrier spacing, the RU has 8 ms duration and for 15 kHz subcarrier spacing, the RU spans over 2 ms. Uplink multitone Uplink multitone (12x15 kHz) over 1 ms (3x15 kHz) over 4 ms Uplink multitone (6x15 Uplink single-tone (1x15 kHz) over 2 ms kHz) over 8 ms One schedulable resource unit with 15 kHz subcarrier spacing Subframe (1 ms) Uplink single-tone (1x3.75 One schedulable resource unit with 3.75 kHz subcarrier spacing kHz) over 32 ms Subframe (4 ms) 32 ms Uplink single-tone (1x3.75 kHz) over 2 ms (control) Uplink single-tone (1x3.75 kHz) over 8 ms (control) Uplink multitone (12x1