# MAC Layer Protocols for Internet of Things: A Survey
## 3 Long Range MAC Layer Protocols
### NB-IoT  
According to the LTE eMTC regional specifications, it can operate only within the bandwidth of an LTE carrier. NB-IoT systems can be implemented as autonomous systems in the Global System for Mobile Communications (GSM) band, employed in the LTE bandwidth carrier or in the LTE bandwidth guard band. Due to the reduction of the NB-IoT bandwidth to 180 kHz, low data rate devices can have extended coverage, complexity reduction, and low power consumption. For scenarios with coverage problems of cellular network operators, NB-IoT is seen as the future of IoT devices using mobile network infrastructure [88]. With the pressure of the growing IoT connectivity, the 3 rd Generation Partnership Project Agreement (3 GPP) launched the project called CAT-N. This project presents a set of categories that offer different air interfaces specifically dedicated to low-power systems. These categories also include different characteristics of MIMO radiating system usage and different values of uplink and downlink data rates by exploiting the available GSM spectrum [89]. The NB-IoT covers all important components for communication in the M2M/IoT systems: low complexity, low power consumption, and long range. Some key features of the standard include a 180 kHz bandwidth and uplink and downlink transmission rates of about 250 Kbps with a half-duplex operation. However, even being a new radio interface, NB-IoT implementations can be made in the band of existing LTE carriers. In addition to this mode of operation, the NB-IoT also supports the guard band mode of operation of an LTE carrier. These two modes of operation are shown in Figure 16. It is important to differentiate NB-IoT from LTE eMTC, which refers to the use of LTE evolution for use of MTC and IoT [90]. The third mode of operation of NB-IoT, which can be seen in Figure 17, is the deployment of NB-IoT using GSM carrier bands of the spectrum that has been assigned to legacy GSM cellular services [90]. Figure 16: NB-IoT operation bands. In the downlink, NB-IoT uses Long Term Evolution Orthogonal Frequency Division Multiple Access (LTE/OFDMA) structures with spacing between sub-carriers of 15 kHz, while the uplink is able to use SC-FDMA with sub-carrier spacing at 3.75 kHz [91]. For the 3.75 kHz spacing between sub-carriers, the frame structure shows little differences from the structure defined for the LTE standard. Each LTE frame slot becomes 2 ms, the frame NB-IoT is then composed of five slots, totaling a period of 10 ms. The NB-IoT technology features a maximum coupling loss extended to 20 dB over the 140 dB LTE. It is achieved through an increase in the number of time repetitions and (\(\pi/2\))-BPSK single sub-carrier transmission, providing a coverage radius for NB-IoT of about 15 km [90]. The NB-IoT contains the following physical signal and channel resources as shown in Figure 18:  
* Narrowband Primary Synchronization Signal (NPSS),
* Narrowband Secondary Synchronization Signal (NSSS),
* Narrowband Physical Broadcast Channel (NPBCH),
* Narrowband Reference Signal (NRS),
* Narrowband Physical Downlink Control Channel (NPDCCH),
* Narrowband Physical Downlink Shared Channel (NPDSCH). These channels and physical layer control signals in the NB-IoT are time multiplexed. Sub frames of the frame structure presented for the NB-IoT, are provided for different channels and physical signals. Each NB-IoT sub frame comprises a resource block in the frequency domain and 1 ms in the time domain. The NPSS and NSSS signals are used to perform time and frequency synchronization as well as cell detection. An NRS signal is used to provide phase reference in the demodulation of downlink channels. The NB-IoT supports up to two NRS ports [92]. The NPBCH channel, transmitted in each sub frame 0 of the frame, loads the main information block, called the Master Information Block (MIB). The NPDCCH carries scheduling information for downlink and uplink data channels,  
Figure 17: NB-IoT operation bands. Figure 18: NB-IoT operation bands.