# MAC Layer Protocols for Internet of Things: A Survey
## 3 Long Range MAC Layer Protocols
### LTE--Long Term Evolution
* Physical Layer: This layer carries all the information from the transport channels through the air interface. With LTE-RACH fixed on the 1.08 MHz bandwidth, it is convenient to use LTE-eMTC channelization in multiples of its value, allowing compatibility with previous versions. The sub-frame structure of the LTE eMTC is the same as the LTE standard. From a frequency plane perspective, the 180 kHz bandwidth of a resource block is divided into 12 sub-carriers, each of them with 15 kHz bandwidth [105; 106]. In terms of coverage, the LTE eMTC's have a 155 dB link budget is achieved with the addition of 15 dB in channel prediction and resource tuning processes, enabling coverage of up to 11 km. These characteristics allow the LTE eMTC to have satisfactory communication even in conditions of excessive losses but transmission rates are affected [107]. In the downlink, the sub-frame structure of the LTE eMTC uses only a part of the LTE framework legacy. As LTE eMTC devices can be implemented with a narrow bandwidth, it is not possible to use the LTE control channels. Thus, it is necessary to create a MTC Physical Downlink Control Channel (MTC-PDSCH) and Physical Downlink Shared Channel (PDSCH) using LTE data channels. PDSCH transports higher data layers, segmented according to the transmission resources available. The LTE eMTC devices depend on the NPSS and NSSS of the LTE standard for the acquisition of the carrier frequency of a cell and frame time usage. LTE signals can be used unchanged by LTE eMTC devices even under challenging coverage conditions. To improve reception performance, LTE eMTC devices can accumulate data received in buffer. This happens because the NPSS and NSSS signals are transmitted periodically. In order to discover the communication channel characteristics, the NRS signal is used to give estimates of channel behavior. This signal, which has a known pattern, is transmitted by the base station and performs downlink quality measurements [92; 108]. For uplink data transmission, there is only one Physical Uplink Control Channel (PUCCH) that carries information such as scheduling requests, channel quality information, and transmission confirmations. The channel used by the user equipment to transmit data in uplink is the Physical Uplink Shared Channel (PUSH). Some sub-carriers can be used to assign resources to the user and are not used by the PUCCH or Physical Random Access Channel (PRACH) signals. To initiate the connection between the terminal and the base station and estimate the arrival time of the uplink message, the PRACH channel is used [106]. Figure 21 illustrates the LTE eMTC uplink channels. Power consumption on IoT devices consists of standby and active power consumption (Idle and Connected modes, respectively). The power consumption of standby mode depends on the design and technology used, and essentially should not differ between LTE MTC and NB-IoT (both technologies have a battery life of approximately 10 years). Active energy consumption differs between these two technologies. Operating on active power consumption for downlink transmission, LTE eMTC has substantially higher throughput (both bandwidth and higher order modulation) than NB-IoT. As a result, it is possible to obtain an estimated active energy consumption 50% lower than the NB-IoT [91]. Figure 20: LTE-eMTC downlink layers.