NB-IoT design exploits the basic functionalities of LTE. However, the signaling and control channels for NB-IoT are new. Furthermore, in NB-IoT, frequency division duplexing (FDD) half duplex type-B is chosen as the duplex mode whereas legacy LTE also supports full duplex mode 5. FDD Half duplex means that uplink and downlink are separated in frequency and the user either receives or transmits, i.e, does not perform both operations simultaneously. The brief frame structure of both downlink and uplink along with the control channels, extracted from 3GPP standard1, are as follows:

۲٫۱٫ Downlink Transmission Scheme

The frame structure of NB-IoT downlink is similar to LTE in the time domain with 10 ms length. Each frame consists of 10 subframes of 1 ms length and each subframe consists of two slots with a length of seven OFDM symbols. In frequency domain, it consists of one physical resource block (PRB) with 12 subcarriers having 15 Khz of spacing and normal cyclic prefix (CP). one sub-carrier × one symbol constitute one resource element (RE), the smallest transmission unit. Furthermore, unlike LTE, NB-IoT has two physical signals and three physical channels which are as follows:

• Narrowband reference signal (NRS)

• Narrowband primary and secondary synchronization signals (NPSS and NSSS)

• Narrowband physical broadcast channel (NPBCH)

• Narrowband physical downlink control channel (NPDCCH)

• Narrowband physical downlink shared channel (NPDSCH)