One disclosure of the present specification proposes a method for receiving a Narrowband Reference Signal (NRS) by a Narrow band Internet of Things (NB-IoT) device. The method may comprise receiving the NRS on at least one or more orthogonal frequency division multiplexing (OFDM) symbols. The one or more OFDM symbols are in a time division duplex (TDD) subframe. If the TDD subframe corresponds to a TDD special subframe, the one or more OFDM symbols for receiving the NRS is determined based on which TDD special subframe configuration index among a plurality of TDD special configuration indexes is used by the TDD special subframe.