Wireless sensor communications and low power go hand in hand. In fact, low power is just as important as the reliability of the communications itself. Before the advent of wireless sensor communications, low power was synonymous with low current consumption. The lower the milliamp figure, the better. To further reduce power consumption, the device was turned off when it did not need to communicate, and was awakened when an alarm situation was raised or a periodic status update was called for duty cycling.

Current consumption continues to be very important in wireless sensor networks. So not surprisingly, state-of-the-art wireless sensor communications components score well on power consumption and utilization of wake-up/sleep modes for duty cycling.

However, power consumption is only part of the solution. Several other factors must also be addressed in order to achieve low power in wireless sensor applications.

Peak current
When closely examining the power consumption behavior of electronic circuits, it becomes apparent that what initially looks like a flat current curve actually bears more resemblance to a mountain range, with peaks and valleys. When certain functional blocks become active, they draw peak current. When two functional blocks switch on simultaneously, the peak amplitude doubles.

The secret to reducing the peak power lies in carefully managing the turn-on and turn-off time for key functions so that double peaks can be avoided. Peak current needs to be kept as low as possible, as it can make or break the feasibility of using specific power sources. For example, because of the internal resistance of a battery, its lifetime depends largely on the amount of peak current an application draws from it.

Graceful power failure
When an energy source has dried out, such as a depleted battery or a solar cell at midnight, the electronics cannot communicate and are dead for all meaningful purposes.

In both cases, the power problem can be dealt with, provided the application is intelligent enough to detect the upcoming problem before the energy source has completely dried out. During this last breath, the device should perform a number of actions to inform its environment of the situation, transmit some critical data and put itself in a state that allows fast recovery when the power is restored.

The current consumption in three typical wireless sensor node states for a commonly used wireless sensor platform might have the microprocessor and transceiver in sleep mode (10µA) in state one; in state two, the microprocessor is switched on while the transceiver is asleep (10 mA); and in state three, both the transceiver and the microprocessor are awake (27 mA).

By using graceful power failure, the devices carefully monitor the state of the power circuits. As they encounter declining power levels, they raise different levels of alarms, ranging from early warning to near-death. The alarms are escalated and communicated to other parts of the system, thereby enabling the system to be placed in a state consistent with the alarm condition.

Low-power mesh routing
One of the most dramatic differences between wireless sensor communications technology and other well-known wireless technologies is the ability of sensor nodes to forward messages from other nodes located further down the communications chain. This technique, known as mesh routing or multi-hop networking, provides an effective and reliable means of spanning large infrastructures beyond the range of what a single wireless link can do.

For a node to forward a message received from another node, it needs to be in an awake and receiving mode when the original wireless message arrives.

Unfortunately, the reception mode requires so much power that it can drain batteries in a few days. The most straightforward solution, as specified by most industry standards, is to limit the multi-hop capability to the nodes that are permanently connected to the main power. In such a framework, low-power devices, which are assumed to be in a power-down mode most of the time, are not capable of retransmitting messages from other devices.

By using 'synchronized wake-up', it is possible to coordinate receiving activity in a way that eliminates the need for the mesh routing nodes to continually operate in receive mode, thereby significantly reducing power consumption. The graph depicts how low-power-routing works when Node A wants to send a message to Node C, through Node B. All nodes in the pictures are low-power nodes, sleeping most of the time.

These low-power devices, known as end devices, are located at the end or beginning of the communications chain.

This framework, which combines mains-powered mesh routing devices and low-power end devices, works for some applications. Take, for example, an office lighting application utilizing interconnected wireless luminaires and light switches. The luminaires connected to the main power source house the mesh routing communication nodes. The switches, which are not mains powered, are a natural place for the end devices.

Many other applications do not fit well in such a framework. In applications such as gas detection, fire detection, access control, precision farming, battlefield monitoring, perimeter surveillance and warehouse temperature monitoring, mains power is not readily available or even present. Running a power cable in these applications would be cost prohibitive, offsetting the benefit of wireless communication.

To address these applications, low-power multi-hop networking, or low-power routing, is required, in which all of the nodes, including the mesh routing nodes, operate in low-power mode.

"Synchronized wake-up" coordinates receiving activity in a way that eliminates the need for the mesh routing nodes to operate in receive mode continually, thereby significantly reducing power consumption.

By synchronizing the sleep/wake-up cycles of the nodes to each other, nodes wake up when they expect a message from a neighboring node. This enables the routing nodes to operate in a nearly powerless sleeping state most of the time, thereby achieving ultralow-power operation. Clearly, more wake-ups will occur than strictly required to carry the data, as neighboring nodes will not always have data to transmit. However, the additional power required for periodic wake-ups and synchronization is more than offset by the power saved by eliminating the need for continuous receive-mode operation.

Since its inception, wireless sensor technology has been inexorably linked with low-power electronics. Most low-power wireless sensor networks have been designed for low power, meaning that they consume little power when switched on. That is not enough. By using wireless mesh networks, enabling graceful power failure and controlling peak power demands, developers can create systems that don't need batteries and instead can utilize energy harvesting to power the sensor network from environmental power sources. n

Wim De Kimpe (Wim.de.kimpe@greenpeak.com) is chief technology officer at GreenPeak Technologies. He is a member of several tech standardization groups and has an MS in electrical engineering from the University of Ghent (Belgium).