The advent of wireless personal area networks (WPANs) and wireless body area networks (WBANs) has created the need for sensing and monitoring solutions that can support continuous data streaming with extremely low power consumption.
Today’s wearable medical systems are targeted at applications including on-site and remote patient monitoring, mobility therapies and the management of diseases such as sleep apnea, and are used in environments where frequent battery replacement would be difficult and expensive. While solutions for these short-range applications previously required AA or AAA batteries, they can now be powered by a new generation of micro-power batteries – as long as power efficiency is optimized. Now, the advent of ultra-low-power short-range radio transceivers is enabling low-cost button cell or small lithium ion batteries to support continuous data streaming in WPANs and WBANs for up to two weeks before replacement.
WPANs occupy a network space around an individual that covers the living or working space nearby (typically up to ten meters), and are implemented with protocols such as Bluetooth and Zigbee. WBANs occupy a smaller wireless space of approximately one meter around a person and are used for sensor communication associated with the human body. Applications have expanded from heavily duty-cycled spot measurement to more data intense continuous links. There are a variety of uses for this technology in hospital and clinical facilities, clinical home monitoring and ambulatory applications, and consumer health and fitness - see figure 1
Figure 1: External sensing use cases and technology requirements.
Many issues must be considered when selecting a short-range radio transceiver capable of optimizing power efficiency in these networks. Among these, power supply voltage is particularly important. Most sensors run on a single battery cell depending on chemistry, so sub-2-V supply voltages are preferable. This means that short-range radio transceivers must be designed for low-voltage operation – ideally, down to 1.1 V in order to optimize design flexibility and reduce power management constraints. In contrast, radios that operate at 2.5 V consume twice as much power as those with the same current consumption operating at 1.25V. Operating at higher voltage is only required when output power in excess of 5 dBm is needed. In short-range applications, output power rarely exceeds 0 dBm. Other key power supply considerations include the ability to maintain transceiver and receiver performance, and the use of a current profile without excessive peaks to fit supply impedance.