Monitoring and measurement through ingestible, implanted, and body-worn wireless sensors can improve healthcare, and if those sensors can support continuous data streaming, more real-time information will be available for analysis.
Properties of the radio transceiver are a key consideration in such medical applications. Each application has different requirements from the radio, which makes it obvious that one size will not fit all.
When surgery is needed for battery replacement, power consumption in has to be minimised so radios are heavily duty-cycled and mostly in sleep mode where they require extremely low leakage current. They also require efficient wake-up mechanisms to initiate communications, and low operational power consumption.
Ingestible applications include pill cameras, which transmit thousands of high-quality pictures of the gastro intestinal (GI) tract, and other applications such as those that deliver drugs and monitor acidity in the GI tract. Such applications also require circuits that consume low power, due to constraints on battery size rather than device life.
Heart and brain electrical activity, or blood oxygenation, require rates on the order of 0.5 to 5kbit/s to extract meaningful waveforms.
Average power is almost inversely proportional to the link data rate: a 100kbit/s radio will consume almost half the power of a 50kbit/s radio for the same payload - when comparing RF transceivers, energy/bit is the a better indicator of power efficiency than current consumption.
High data rate radios are often those with higher peak current demand, which can mean large, and therefore potentially leaky, storage capacitors to supplement their small batteries.
A secure communication protocol is important. Standard protocols such as Bluetooth Low Energy or ZigBee have sophisticated link and network layers, and their stacks can account for a large percentage of radio power consumption. Most implant radios therefore use proprietary protocols to optimise power, communication efficiency and security.
External wireless sensors cover a wide range of applications from fitness monitors to diagnostic physiological parameter monitoring.
Today’s sensing and monitoring solutions for wireless personal area networks (WPANs) and wireless body area networks (WBANs) can support continuous data streaming with low power consumption.
While they previously required AA or AAA batteries, these systems can now run on micro-power batteries.
Amongst issues that must be considered when selecting a short-range radio transceiver for WPANs and WBANs, power supply voltage is particularly important. Most sensors run on a single battery cell, so sub-2V operation is preferable, and ideally the short-range radio transceivers must be designed to work down to 1.1V for design flexibility and to reduce power management constraints.
Output impedance is important as it has a major effect on power amplifier (PA) consumption, and any impedance mismatch can contribute to insertion loss of several dB.
Carrier frequency also influences power consumption. The two options for medical (ISM) radio band are 2.4GHz (Wi-Fi, Bluetooth and ZigBee are prevalent) or sub-GHz frequencies.
In lower-data rate medical monitoring applications, sub-GHz wireless systems reduce power consumption as well as provide longer range for given power due to lower free-space propagation loss. The quieter spectrum means easier transmissions and fewer retries.
Reghu Rajan is strategic marketing engineer at Microsemi
