## 3 Energy Conserving Issues in Wireless Networking-Based IoT
### Energy Conserving Issues for WWAN-Based IoT  
An important issue in using 3 GPP LTE for IoT is attributed to a massive number of IoT devices. An overload or congestion problem in the radio access network as well as the core network arises when  
Figure 3: A taxonomy of IoT energy conserving issues for different types of radio access technologies.  
both user devices and IoT/M2M devices in LTE or LTE advanced networks try to access the network simultaneously for data transfer. The problem can be severer as the number of devices, either Human-to-Human (H2H) or Machine-to-Machine (M2M), will increase in upcoming years. The overload problem can indirectly affect energy consumption in IoT devices because the overload condition, and hence high network utilization may cause delay and loss for IoT data packets and retransmitting them may incur more battery power consumption [30, 31, 32, 33, 34, 35, 56, 91]. In addition to congestion-induced energy consumption issues, another important factor is an energy-efficient allocation of radio resources (in terms of time, frequency, and transmit power) to IoT devices [36, 37].  
To manage the power consumption of user nodes, mostly smartphones, the 3 GPP has defined a conventional mechanism called Discontinuous Reception/Transmission (DRX/DTX) cycles or paging cycles. Figure 4 illustrates a basic DRX mechanism. The DRX mechanism is further classified into Connected DRX for devices in a connected state and Idle DRX for devices in an idle state. On every On-Duration period, a device wakes up and checks Physical Downlink Control Channel (PDCCH) scheduling information in subsequent subframes (one subframe is 1 ms long). If the device is not scheduled, it goes back to a sleep mode for low power operation. Otherwise, it will stay in an active mode to receive or send its data and start the Inactivity timer. If the Inactivity timer expires with no data transmission or reception, the device enters short DRX cycles. An issue is that conventional duty cycling may not be adequate for IoT and may possibly cause battery drainage problems for IoT/M2M devices [41, 42, 43]. Conventionally, the 3 GPP has limited a maximum paging cycle to be 2.56 s. However, this limit is not adequate for IoT/M2M devices, as they require very infrequent data transmission compared with human type communications. Some DRX parameters may have major or minor impacts on the power consumption of IoT devices [44, 45, 46]. Paging messages sent from eNBs to UEs can also bring up energy consumption in IoT devices.  
In the 3 GPP standards, significant efforts have recently been made to develop standard mechanisms and protocols for MTC support in cellular networks [47, 50, 51, 52]. They discussed various MTC-specific issues (e.g., transmit power, bandwidth, duplex operation, peak rate, and downlink transmission modes) that can have impacts on device power consumption and specified solutions for them. Other issues may include the difficulty of synchronization or connection establishment between IoT/M2M devices and IoT/M2M gateways for proper data transfer, which could adversely affect power consumption [53, 54, 57, 58, 59], and the usage of multiple heterogeneous radio interfaces.  
Figure 4: A basic 3 GPP DRX mechanism.