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EETE JULAUG 2012

DESIGN & PRODUCTS MEDICal ElECTRONICS Ultra-low power radio drives continuous medical monitoring with low-cost batteries By Reghu Rajan 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 continu- ous data streaming with extremely low power consumption. Today’s wearable medical systems are targeted at applications including on-site and remote patient monitoring, mobility thera- pies and the management of diseases such as sleep apnea, and are used in environments where frequent battery replace- ment would be difficult and expensive. While solutions for these Fig. 1: External sensing use cases and technology short-range applications previously required aa or aaa batter- requirements. ies, they can now be powered by a new generation of micro- power batteries – as long as power efficiency is optimized. Now, high-output-power, long-range applications, however, and re- the advent of ultra-low-power short-range radio transceivers sults in up to five times higher current consumption than higher- is enabling low-cost button cell or small lithium ion batteries to output impedance options that are more suited for short-reach support continuous data streaming in wPans and wBans for wireless interconnect applications. overall, assuming a similar up to two weeks before replacement. receiver sensitivity and PA efficiency, a high impedance 900MHz wPans occupy a network space around an individual that radio would use only 1mW in its PA to achieve the same range covers the living or working space nearby (typically up to ten as a 50-Ohm 2.4GHz radio using 25mW to 40mW of power. meters), and are implemented with protocols such as Bluetooth The choice of carrier frequency also influences power con- and Zigbee. wBans occupy a smaller wireless space of ap- sumption. The two available options within the medical (isM) proximately one meter around a person and are used for sensor radio band are 2.4GHz or sub-GHz frequencies. The most communication associated with the human body. applications prevalent 2.4GHz protocols are Wi-Fi, Bluetooth and ZigBee. have expanded from heavily duty-cycled spot measurement in low-power and lower-data-rate wireless medical monitoring to more data intense continuous links. There are a variety of applications, however, sub-GHz wireless systems offer several uses for this technology in hospital and clinical facilities, clinical advantages, including reduced power consumption, as well as home monitoring and ambulatory applications, and consumer longer range for given power. health and fitness - see figure 1. The Friis Equation quantifies the superior propagation Many issues must be considered when selecting a short- characteristics of a sub-GHz radio, showing that path loss at range radio transceiver capable of optimizing power efficiency 2.4GHz is 8.5dB higher than at 900MHz. This translates into a in these networks. among these, power supply voltage is 2.67 times longer range for a 900MHz radio since range approx- particularly important. Most sensors run on a single battery imately doubles with every 6dB increase in power. To match the cell depending on chemistry, so sub-2 volt supply voltages are range of a 900MHz radio, a 2.4GHz solution would need greater preferable. This means that short-range radio transceivers must than 8.5dB additional power. Another benefit of sub-GHz car- be designed for low-voltage operation – ideally, down to 1.1 V rier frequencies is that they reduce the risk of interference from in order to optimize design flexibility and reduce power man- airways that are crowded with colliding 2.4GHz Wi-Fi, Bluetooth agement constraints. in contrast, radios that operate at 2.5v and ZigBee signals used in in everything from wireless hubs consume twice as much power as those with the same current and computers to cellphones and microwave ovens. Sub-GHz consumption operating at 1.25V. Operating at higher voltage is isM bands are mostly used for proprietary low-duty-cycle links only required when output power in excess of 5dBm is needed. and are not as likely to interfere with each other. The quieter in short-range applications, output power rarely exceeds 0dBm. spectrum means easier transmissions and fewer retries, which other key power supply considerations include the ability to is more efficient and saves battery power. maintain transceiver and receiver performance, and the use of a Furthermore, the narrower sub-GHz bandwidth creates current profile without excessive peaks to fit supply impedance. higher receiver sensitivity and allows efficient operation at lower another key issue is peak current. almost all wireless-based transmission rates. For example, at 300MHz, if the transmitter sensor networks rely on some level of duty-cycling to save and receiver crystal errors (XTAL inaccuracies) are both 10 ppm power and restrict the usage of radio space, which generates (parts per million), the error is 3kHz for each. For the applica- peaks in the current consumption profile of the sensor. Low tion to efficiently transmit and receive, the minimum channel peak current consumption in the radio transceiver reduces bandwidth is two times the error rate, or 6kHz, which is ideal for constraints on the wireless sensor’s power supply. output narrowband applications. The same scenario at 2.4GHz requires impedance is also important, as it has a major effect on power a minimum channel bandwidth of 48kHz, which wastes band- amplifier (PA) power consumption. Most radios have output im- width for narrowband applications and requires substantially pedance below 100 Ohms. Low impedance is only required for more operating power. Carrier frequency also has a major impact on the average Reghu Rajan is Technical Marketing Manager in the wireless power budget at the network level. Zigbee and Bluetooth offer Machine to Machine group at Microsemi - www.microsemi.com highly sophisticated link and network layers, but these stacks 30 Electronic Engineering Times Europe July/August 2012 www.electronics-eetimes.com


EETE JULAUG 2012
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