The Internet of Things (IoT) is an emerging key technology for future industries and everyday lives of people, where a myriad of battery operated sensors, actuators, and smart objects are connected to the Internet to provide services such as mobile healthcare, intelligent transport system, environmental monitoring, etc. Since energy efficiency is of utmost importance to these battery constrained IoT devices, IoT-related standards and research works have focused on the device energy conserving issues.
This paper presents a comprehensive survey on energy conserving issues and solutions in using diverse wireless radio access technologies for IoT connectivity, e.g., the 3rd Generation Partnership Project (3GPP) machine type communications, IEEE 802.11ah, Bluetooth Low Energy (BLE), and Z-Wave. We look into the literature in broad areas of standardization, academic research, and industry development, and structurally summarize the energy conserving solutions based on several technical criteria. We also propose future research directions regarding energy conserving issues in wireless networking-based IoT.
INTERNET OF THINGS: NETWORK ARCHITECTURE, DEVICE STRUCTURE, AND APPLICATIONS
Figure 1 provides a generic view of an IoT network architecture using different wireless technologies, in which diverse IoT components are being connected to the 3GPP network components for proper operation and data transfer. Figure 1 shows the formation of IoT networks, also called M2M area networks, and also connectivity to IoT gateways and servers. The architecture presented in Figure 1 also represents a kind of a capillary network architecture, in which all devices are transferring their collected data to the IoT server through an intermediate entity that is an IoT gateway.
Devices in an MTC domain typically transmit or receive a fixed amount of data in a fixed time interval. Inter-MTC device communication can be performed through wireless mobile networks or in an ad hoc network fashion. In Figure 1, we can also notice particular application scenarios, where IoT devices and networks are being used for applications like structural health monitoring, environmental monitoring, human health monitoring, traffic monitoring, smart homes appliances, and so on.
In this IoT network architecture, we look into the further details of the IoT device or gateway structure as shown in Figure 2, where the necessary in-device components for transmission and reception of sensing and actuation data are illustrated. Considering an increasing need to support multiple heterogeneous Radio Access Technologies (RATs) for enhanced coverage and flexibility, Figure 2 provides a generic structure of multi-radio IoT devices or gateways with diverse communication components and their interaction with each other.
ENERGY CONSERVING ISSUES IN WIRELESS NETWORKING-BASED IOT
To manage the power consumption of user nodes, mostly smartphones, the 3GPP 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 1ms 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.
Basic energy conservation for WiFi devices is achieved through IEEE 802.11 Power Save Mode (PSM) as illustrated in Figure 5. In Ad hoc Traffic Indication Map (ATIM) window after the start of each beacon interval, a node remains awake for the exchange of ATIM requests and responses. A node sends an ATIM request message if it has a packet to send to an intended receiver. The receiver sends acknowledgement and will stay awake during the rest of the beacon interval for packet reception.
Bluetooth Low Energy (BLE) is one of the most promising low power consumption wireless technologies in the wireless personal area networking domain for IoT applications. Figure 6 illustrates the duty cycle of a slave device by using the Sniff interval method during which it listens to a master device for a particular amount of time. At every Sniff anchor point (typically every 100 ms), the device wakes up to check if it has data to send or receive, and goes back to a sleep mode for the rest of sniff interval.
ENERGY CONSERVING SOLUTIONS IN WIRELESS NETWORKING-BASED IOT
Having briefly looked at important energy conserving issues for IoT in the previous section, we now discuss the solutions proposed in the literature for different wireless access technologies. The following three Sections will deal with energy conserving solutions for WWAN-based IoT, WLAN-based IoT, and WPAN-based IoT, respectively.
In each subsection, we will review solutions to issues with criticism, and also summarize the survey in a table form for a comparison purpose, in terms of categories, approaches, and various technical criteria such as schemes (types of algorithms and mechanisms used), metrics (performance metrics of interest), control (who controls the mechanisms), and evaluation (performance evaluation methods).
- Energy Conserving Solutions for WWAN-Based IoT
- Energy Conserving Solutions for WLAN-Based IoT
- Energy Conserving Solutions for WPAN-Based IoT
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 and AllSeen Alliance, 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, energy conservation in utilizing multiple radio interfaces may need further investigation along with co-channel and adjacent channel interference issues.
This paper has provided a comprehensive survey on energy conserving issues and solutions for battery-operated IoT devices from wireless networking aspects. The extant solutions have tackled various operational aspects of IoT devices, including the adjustment of duty cycles, collision/congestion avoidance schemes, mechanisms to manage device sleep time by switching off radios or increasing a standby time, efficient radio resource scheduling, the intelligent selection of heterogeneous radio interfaces, and so on.
The real adoption of the solutions onto IoT devices should consider a combination of incorporated wireless radio access technologies. This survey has examined the literature regarding emerging IoT technologies and their energy conserving issues from a specific perspective of wireless networking. It will add to other recent surveys on IoT such as energy efficient multimedia streaming, communication standard bodies, and IoT semantics.
Source: Dong-A University
Authors: Zeeshan Abbas | Wonyong Yoon