Ultra-low power radios: key enablers in wireless sensor systems
Wireless sensor systems with ultra-small and low-power sensors that operate autonomously for a long period have a lot to offer. Their application can range from new patient centric health care, a healthier lifestyle, context aware support for business and entertainment purposes, future vehicles, monitoring of machines and processes, buildings and smart constructions, and the environment. However, their practical usefulness (functionalities, autonomy, form factor...) largely depends on how much power they consume.
Since most of the power is consumed by the wireless communication, the development of ultra-low power radio technologies is a key requisite. Equally important is the standardization of the devices, reducing the cost of system integration, realizing the connection to personal computing platforms and enabling device interoperability. We report the development of a multi-standard transceiver for wireless sensor networks, that, by careful selection of its architecture and circuit design trade-offs, achieves ultra-low power consumption.
Autonomous wireless sensor systems can be deployed for continuous monitoring: chronically-ill patients can be followed during their daily routine, machines and industrial processes can be monitored, smart buildings can be controlled. When compliant to wireless standards, the sensor systems can talk to each other and have plug and play functionalities with e.g. mobile phones and personal computing platforms. But today’s commercially available short-range radios, compliant to wireless standards such as Bluetooth or Zigbee, consume too much power for small devices running on small batteries (20 to 50mW). New, ultra-low power radio techniques are needed, which will also enhance the practical application and usefulness of the devices for reliable long-term monitoring.
Today’s wireless sensor systems operate on commercially available low-power radios such as Bluetooth Low Energy (BLE). The sensors themselves consume relatively little energy when functioning, as a result of power optimizations. However, the power consumption of the radio chip in the system is the weak factor. Although a BLE radio chip’s power consumption is highly dependent on the data rate of the application, around 50-85% of the power consumption of the total system is typically consumed in the BLE radio communication.
Imec and Holst Centre develop radio chips with significantly reduced power consumption compared to the off-the-shelf alternatives. The power consumption of our novel wireless communication chipsets is 10 to 100 times lower than the commercial equivalents. Such ultra-low power radio chips would decrease the overall power consumption of the complete sensor system and open up opportunities to add much more functionality to the device, for example processing functionality to reduce artifacts and local feature extraction. Alternatively, as the battery size is decisive in determining the size of battery-operated systems today, low-power radio chipsets could enable smaller batteries, leading to smaller devices.
New ultra-low power radio technologies ideally support the most commonly used short-range wireless standards for a certain application. Industrial monitoring, smart buildings, or smart metering applications are typically supported by standards such as IEEE802.15.4 (ZigBee) or proprietary protocols. In the context of smart homes, other standards in addition to ZigBee are of importance, such as BLE, for ease of connectivity with consumer devices such as smartphones and tablets. When monitoring body parameters such as EEG or ECG, body area network (BAN) standards such as the newly created IEEE802.15.6 standard are needed, which defines the next-generation of BAN wireless devices in line with the latest requirements of portable healthcare applications.
In order to support all multiple standards, the radio architecture is crucial and not straightforward. The radio should support non-constant envelope modulations, such as the non-constant envelope differential-phase-shift-keying (DPSK) modulations defined in the IEEE802.15.6 narrowband physical layer (PHY), which makes an energy efficient implementation a challenge.
The imec multi-standard transceiver supports these three widely used standards with one single IC. Moreover, for every individual standard, the imec radio chip consumes significantly less power than any of the commercially available single standard solutions.
The combination of multi-standard support and best-in-class performance is enabled by ultra-low power circuit design and the use of an advanced architecture. For example, techniques like two-point modulation of a fractional PLL and a sigma-delta digitally controlled power amplifier allow direct digital-to-RF envelope conversion in the transmitter. For the example of the BLE standard, this results in a power consumption of 5.7mW when transmitting at the maximum allowable output power. That is six times lower than the reported state-of-the-art power consumption.
In the receiver, a sliding IF architecture and the choice of a suitable frequency plan have been elementary to achieve a power consumption as low as 4.5mW when receiving at 1Mbps, for the example of BLE. This is almost 10 times lower power than the best available solutions today. Such power reduction is key to accelerate the deployment of low power wireless sensor nodes, especially so, in combination with the multi-standard support that enables interoperability and cost reduction. Imec is currently working on the next generation of this radio, as well as on implementations for other low power wireless standards. As these radios become mass deployed further power reduction and an even higher integration level will become needed.
Ultra-low power radios are crucial for the deployment of wireless sensor systems with long autonomy and small size across many markets during the coming years. By carefully designing an energy-efficient architecture, we have realized a record low-power radio that supports multiple wireless standards including BLE, Zigbee and IEEE 802.14.6. We have developed different solutions meeting different application requirements, ranging from industrial, building and home monitoring, to body and personal area networks.
At imec, we believe that the road to the internet-of-everything can only be fueled by smart circuit design innovation and ultra-low power radios, and we want to be at the forefront of this revolution.
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