# Energy Consumption Analysis of LPWAN Technologies and Lifetime Estimation for IoT Application
## 4 Approach, LPWAN Communication Technology, and Energy Consumption Analysis
### LPWAN Communication Technology
#### 4.2.4 NB-IoT  
NB-IoT is a narrowband LPWAN technology under licensed frequency bands and uses the existing LTE deployed infrastructure [36]. It exists together with LTE or GSM with a frequency bandwidth of 200 KHz. NB-IoT supports different operation modes as: _Stand alone operation_ (utilization of GSM frequency bands); _Guard band operation_ (utilization of LTE carrier's guard-band that are unused); and _In-band operation_ (utilization of resource blocks of LTE carrier) as depicted in Figure 18. NB-IoT is a miniature form of the LTE protocol as it reduces the functionalities and enhances as per IoT requirement. It uses LTE physical and higher protocol layers by using LTE backend system. It uses QPSK and FDMA for uplink and OFDMA for downlink transmission with maximum throughput rate of 20 kbps and 200 kbps respectively. The maximum payload size for one message can be 1600 bytes. NB-IoT has power-saving states as shown in the Figure 18.  
The Radio Resource Control (RRC), is responsible for the mechanism of setup, modification and to release resources. It got two states: connected and idle state. All the communication happens in the RRC connected state. However, it consumes more energy, so once the communication is over the end-device moves back to RRC idle state, which triggers after network specific inactivity timer is over. RRC idle is constituted of two modes: Extended Discontinuous Reception Mode (eDRX) and Power Saving Mode (PSM) mode. From energy consumption point of view, eDRX is sleep mode which can vary from 0 to 186 min and PSM is deep sleep mode which can be up to up to 413.3 days. The eDRX mode consists of paging time window which is between 2.56 and 40.96 s. Initially, the end-device registers in the network and goes in a connected state. Thereafter, it enters the eDRX state where it can listen to the downlink control channel (Narrowband Physical Downlink Control Channel).  
Figure 16: Sigfox unidirectional and bidirectional message transmission with replica messages at different frequency.  
Figure 17: Scope view of a Sigfox data transmission captured using power analyzer for the measurement of energy consumption.  
This listening is termed as paging which ends when the end-device moves to PSM state. In this state, end-device keeps it registration state but turns off its radio to save energy.  
We have done the power consumption for the NB-IoT end-device as shown in Figure 19. It starts with the device activation and initialization state marked as state 1; then starts the registration process with the network service provider as shown in state 2. This state is specific to the network provider and is of 10 s. In our case, we had Orange as network service provider. In the subsequent state 3, we did the communication using AT commands to open a socket for the communication. State 4,6, is the eDRX state where the node does the paging to listen any downlink message. The eDRX mode can be configured to save the energy consumption. The timer value impacts the overall energy consumption and it is aligned with the application requirement. The state 5 is the transmission mode to send the message. The zoomed in scope view for the message transmission is shown in Figure 20. State 7 represents the PSM mode, where the radio gets switched off and becomes unavailable. Once the PSM state ends, the next state is the transmission state. The current consumption for each state is shown in the Table 6, to send one message. For our experiment, we used the AT commands (AT commands manual from SARA-N2 / SARA-N3 series) to add the device to the network. Post registration, we opened a new UDP socket for sending UDP data to a server. Following which sending a fixed payload of 5 bytes. The device had deep sleep mode enabled and the messages were sent at an interval of five minutes.  
Figure 19: Scope view of a NB-IoT data transmission captured using power analyzer showing all the states.  
Figure 20: Scope view of a NB-IoT data transmission and deep sleep.  
Figure 18: NB-IoT states and modes of operation.