Joy Weiss, Ross Yu, and Jonathan Simon, Dust Networks Product Group, Linear Technology
Low power wireless technology is enabling substantial cost reductions for traditional wired sensing systems, and 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 utilize time-synchronized 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 anywhere–not just where power is available, but wherever the application requires sensor data.
Linear Technology, which includes the Dust Networks product group, has been at the forefront of innovation in the areas of highly reliable, low power TSCH-based WSN and energy harvesting technologies. 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 recent study by ON World shows that the two attributes of a WSN that matter most to industrial customers are reliability and low power (see Figure 1). Cost is third on the list: without solving the reliability and power issues, cost is not yet a customer priority.
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Figure 1. Perceived Importance of WSN Attributes
It is clear that the combination of precisely synchronized time slotting, channel hopping and an ultralow 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 multichannel 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 multichannel 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 Synchronization Enables Power Savings & Channel Hopping
The original 802.15.4 MAC necessitates that the nodes in the mesh network that route information from neighboring 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 synchronization, the less time the routing node radios must be in an ‘on’ state, which minimizes power consumption. Best-in-class TSCH systems synchronize 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 Mitigates Interference & Multipath 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 transmitters (the “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-band--a common occurrence in the 2.4GHz band shared by Wi-Fi, Bluetooth, and 802.15.4.
A second, unpredictable phenomenon called multipath fading can prevent successful transmission even when the line-of-sight link margin is expected to be sufficient. This occurs when multiple copies of the transmission bounce off objects in the environment (ceilings, doors, people, etc.), with each reflected copy traveling a different distance. When interfering destructively, fades of 20-30dB are common. Multipath fading depends on the transmission frequency, device position, and on every nearby object; predicting it is practically impossible. Figure 2 shows the packet delivery ratio on a single wireless path between two industrial sensors over the course of 26 days, and for each of the sixteen channels used by the system.
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Figure 2. Packet Delivery across 16 Channels over 26 days
By time synchronizing and scheduling the network into slots, transmissions can be precisely scheduled on specific known channels, and the choice of channel can change with every transmission. Furthermore, scheduling network transmissions solves the “hidden terminal problem” and virtually eliminates in-network collisions. Such a mechanism is field proven in the more than 10,000 WirelessHART networks in the field, which routinely achieve multiyear battery life and > 99.9% reliability.
Energy Harvesting Considerations
Once the power requirements of the WSN are suitably minimized, the choice of power source broadens. Ambient energy is everywhere: light, vibration and heat are but a few examples of energy that can be freely sourced and converted to sufficient electrical energy to run a low power TSCH WSN. The following examples illustrate some practical energy harvesting technologies that generate more than 150µW of power–more than enough to run a typical IPv6 routing node in a 802.15.4e network (for example, Dust Networks’ SmartMesh IP).
Most areas of a typical office building have sufficient indoor light to run a low power TSCH WSN. According to the United States General Services Administration, which sets the guidelines for U.S. public buildings, the more brightly lit areas, such as workstation areas and reading surfaces, have 500 lux of lighting. Even in areas considered “normally lit” such as lobbies, stairwells, and mechanical and communications closets, there are at least 200 lux of light and 300 lux is common for most conference rooms. With 200-300 lux of light, there are a number of indoor of small photovoltaic cells available that can supply sufficient power to operate an IPv6 router in a 802.15.4e TSCH network.
Thermal Electric Generators (TEGs) produce power from the heat dissipation from hot surfaces, such as waste heat from common devices normally thought of as very warm (e.g., computer monitors or high-current motors). As wireless solutions become more power efficient, the energy produced from commonplace temperature differences of as little as 10ºC become usable as an energy source. For reference, the typical difference between internal body temperature and room temperature is about 15ºC.
Many energy harvesting transducers produce only a few hundred millivolts of output, so step-up voltage DC/DC converters are often required to convert to a usable supply voltage range. ICs such as the LTC3105 from Linear Technology combine maximum power point control, so that the transducers operate at peak efficiency. The LTC3105 also enables the addition of a battery backup to the circuit. Since the batteries in these circuits are only used when the ambient energy source is insufficient or absent, battery life can be extended dramatically
The realization of the Internet of Things is accelerated by making it practical and easy to deploy sensors ubiquitously. Low power, reliable wireless sensor networks translate to no wires/no worries for customers and developers alike. Time synchronized, slotted multichannel systems confer customer-critical benefits to WSNs: reliability and network-wide low power operation. The WirelessHART and 802.15.4e standards are excellent embodiments of this networking approach. Low power operation ensures great flexibility in the choice of power source, and offers the potential for perpetual power. These factors all add up to making IoT easier and more practical.