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Importance of low power sensing for IoT

Posted: 14 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Low power? wireless sensor networking? WSN? time-synchronised channel hopping? TCSH?

Low power wireless technology is allowing significant cost reductions for traditional wired sensing systems. It is also opening up new possibilities for sensor networking that simply could not be done with wires.

Low power wireless sensor networking (WSN) standards, particularly mesh architectures that utilise time-synchronised channel hopping (TSCH) enable every node in the network to run on batteries or harvested energy without sacrificing reliability or data throughput. This frees application developers to put sensors anywherenot just where power is available, but wherever the application requires sensor data.

These technologies go hand in hand to increase opportunities for application developers to deploy systems that require few, if any, battery changes, further reducing the lifetime cost of deploying wireless sensors and spurring the progress of the Internet of Things (IoT).

A 2012 study by ON World shows that the two attributes of a WSN that matter most to industrial customers are reliability and low power (figure 1). Cost is third on the list: without solving the reliability and power issues, cost is not yet a customer priority.

Figure 1: Perceived importance of WSN attributes.

Based on Dust Networks' years of research and development of TSCH, it is clear that the combination of precisely synchronised time slotting, channel hopping, and an ultra low power radio enables the lowest power, most reliable WSNs. This focus on low power enables all nodes to run for many years on low -cost batteries, and also opens possibilities for a variety of energy sources, including energy harvesting supplies.

Low power radios
The introduction of the IEEE 802.15.4 standard created an excellent radio platform for WSNs. IEEE 802.15.4 defines a 2.4GHz, 16-channel spread spectrum low-power physical (PHY) layer upon which many IoT technologies have been built, including ZigBee and WirelessHART. It also defines a medium access control (MAC) layer, which has been the foundation of ZigBee. However, the single-channel nature of this MAC makes its reliability unpredictable.

To improve reliability, the WirelessHART protocol, also known as IEC62591, defined a multi-channel link layer based on the 15.4 MAC to achieve high reliability (>99.9%), which is required for industrial WSN applications. In early 2012, a new version of the 802.15.4 MAC called 802.15.4e was ratified, and this MAC embodies multi-channel mesh and time slotting. The typical power output for 802.15.4 compliant radios is around 0dBm, with transmit and receive currents in the 15-30mA range. Best in class transmit current at 0dBm is 5.4mA, and best-in-class receive current is 4.5mA (based on Linear's LTC5800).

Time synching for low power and channel hopping
The original 802.15.4 MAC requires that the nodes in the mesh network that route information from neighbouring nodes are always on, while nodes that only send/receive their own data, often called "reduced function devices," can sleep between transmissions. In order for every node in the network to be low power, communications between nodes must be scheduled, and it is necessary to have a shared sense of time in the network.

The tighter the synchronisation, the less time the routing node radios must be in an 'on' state, which minimises power consumption. Best-in-class TSCH systems synchronise all nodes in a multi-hop mesh network to within a few 10s of microseconds. Once there is a shared sense of accurate time in the network, and a schedule of time slots for pairwise transmission between nodes in the network, channel assignment can be incorporated into that schedule, thereby enabling channel hopping.

Channel hopping to reduce interference and multi-path fading
The wireless channel is unreliable in nature, and a number of phenomena can prevent a transmitted packet from reaching a receiver; these can be exacerbated as radio power decreases. Interference occurs when multiple transmitters send simultaneously over the same frequency. This is particularly problematic if they cannot hear each other, yet the receiver can hear all the transmittersthe "hidden terminal problem".

Backoff, retransmission, and acknowledgment mechanisms are required to resolve collisions. Interference can come from within the network, another similar network operating in the same radio space, or from a different radio technology operating in-banda common occurrence in the 2.4GHz band shared by Wi-Fi, Bluetooth, and 802.15.4.

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