How does NB-IoT achieve the extended coverage compared to standard LTE?

NB-IoT (Narrowband Internet of Things) achieves extended coverage compared to standard LTE by trading data rate for link budget improvement through several techniques that collectively provide 15-20 dB more link budget, enabling communication in basement meters, underground structures, and remote rural locations that standard LTE cannot reach. The coverage extension techniques are: narrower bandwidth (NB-IoT uses a single 180 kHz carrier (one LTE PRB) versus LTE's minimum 1.4 MHz. The receiver noise is proportional to bandwidth: a 180 kHz receiver has approximately 9 dB lower noise floor than a 1.4 MHz receiver and approximately 17 dB better than a 10 MHz LTE carrier, directly improving the link budget), repetition (NB-IoT supports up to 2048 repetitions of the same transport block. Each repetition provides approximately 3 dB of processing gain per doubling (coherent combining). With 128 repetitions: the processing gain is approximately 10 x log10(128) = 21 dB. The trade-off: the effective data rate is divided by the number of repetitions. With 128 repetitions at 250 bps base rate: the effective data rate is approximately 2 bps), lower modulation order (NB-IoT uses BPSK and QPSK only (no 16-QAM or 64-QAM), which reduces the required SNR. QPSK requires approximately 1-2 dB Eb/N0 versus 15+ dB for 64-QAM), relaxed device power class (NB-IoT allows +23 dBm transmit power, same as standard LTE Cat-1. Some NB-IoT devices support +14 dBm (reduced power class) for lower-cost PA designs), and MCL (Maximum Coupling Loss) target (the 3GPP specification defines NB-IoT with an MCL of 164 dB, compared to 142 dB for standard LTE Cat-1. This 22 dB improvement enables communication through approximately 2-3 additional building walls or floors).