## 5 Discussion  
In this section, we suggest a few research directions for energy conservation in wireless networking-based IoT. Though not conceptually new, these proposed directions can be interesting topics in the realization of IoT. Firstly, previous research on energy conservation in IoT mostly focused on single-radio data transmission. However, for a coverage extension purpose for IoT devices, IoT gateways may support the simultaneous operation of multiple heterogeneous radio interfaces to relay data to IoT servers. For example, in the latest IoT platform development by Open Interconnect Consortium [97] and AllSeen Alliance [98], gateways typically use a low power short range radio to communicate with nearby IoT devices and also use a long range radio to communicate with an IoT server. In a case that such gateways are battery-operated like Ericsson's capillary network [99], energy conservation in utilizing multiple radio interfaces may need further investigation along with co-channel and adjacent channel interference issues.  
For the coexistence of heterogeneous wireless networking standards for IoT, Software Defined Radio (SDR) can be a promising approach. [PERSON] _et al._[100] presented IEEE 802.11 ah-based Software Defined Radio (SDR) prototype. In that prototype, authors provided a demo for evaluating the efficiency of upcoming 802.11 ah WLAN protocol that supports long outdoor range (above 1 km) at moderate data rates (up to 100 Kbps). [PERSON] _et al._[101] proposed a wireless IoT platform based on SDR technologies. They demonstrated interference mitigation scenario at a base station by using a cognitive radio technology. [PERSON] _et al._[102] discussed that an SDR framework can overcome the heterogeneity ofIoT devices by seamlessly adapting their communication technology and creating wireless cells that provide access to heterogeneous devices. Energy efficiency for SDR-based IoT gateways and devices needs investigation.  
Another direction regarding energy conserving IoT is based on energy generation. One of them is energy harvesting [103; 104], in which IoT device energy can be fetched from outside environments like solar energy, wind energy, or some other types of energies. It can increase the lifetime of IoT devices up to years without any need of changing batteries. Another approach, in its early research phase, is battery recharging of IoT devices from radio signals [105].  
The support of IoT devices in cellular wireless mobile networks has recently drawn significant interests from major mobile network operators [106]. In addition, IoT is considered as one of the essential components in development of the next generation 5G mobile network standards (3 GPP Release 14 and later). The usage of cellular radio technologies can benefit from the exploitation of the well-established core network infrastructure without any concern about backhaul cost, but it comes at the sacrifice of power consumption for longer range transmission. Small cell solutions such as pico-cells and femto-cells that are expected to proliferate in future may result in significantly lower power consumption. Energy efficiency for the next-generation cellular radio-based IoT will also be an important research topic in future.