NB-IoT physical layer — the 180 kHz carrier

A Single 180 kHz Carrier

NB-IoT downlink occupies one 180 kilohertz carrier — twelve subcarriers at 15 kilohertz spacing. This matches exactly one LTE physical resource block, which is why in-band coexistence with LTE works cleanly: the eNodeB schedules one PRB to NB-IoT and the rest to LTE traffic. Modulation is QPSK only — no 16QAM or 64QAM. The downlink uses Turbo coding with rate matching, the same coding scheme as LTE but with smaller transport block sizes. Peak downlink rate is around twenty-six kilobits per second for Cat-NB1 and roughly 127 kilobits per second for Cat-NB2 with multi-PRB scheduling.