Ultra-Low-Power Radio Technology Enables Medical Wireless Sensor Networks

Today’s sensor and monitoring solutions for wireless personal area networks (WPANs) and wireless body area networks (WBANs) can support continuous data streaming with extremely low power consumption. This is critical for wearable medical systems that are used in environments where frequent battery replacement would be difficult and expensive. Now, systems that once required AA or AAA batteries can operate using 3 V coin cell batteries. Making this possible are ultra-low-power medium-range radio transceivers, whose circuit design has been optimized for power efficiency across several key parameters.

WPANs occupy a network area that covers the living or working space around an individual (typically up to ten meters), and are implemented with protocols such as Bluetooth and IEEE 802.15.4. 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 high-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, as well as consumer health and fitness.

Radio Requirements for Wearable Technology

Several factors must be considered when selecting a short-range radio transceiver capable of optimizing power efficiency in WPANs, WBANs, and wearable technology. Power supply voltage is particularly important. Many of today’s sensors can operate from a 3 V supply voltage that allows wearables to operate using a single low-cost, readily available coin cell battery.

Another key issue is peak current. All wireless-based sensor networks require operation at a predetermined duty cycle to save power and restrict radio space usage. Low peak current consumption in the radio transceiver reduces constraints on the wireless sensor’s power supply. Low sleep current is also critical for low duty extended battery life.

The choice of frequency also influences power consumption. Available frequency bands within the industrial, scientific, and medical (ISM) radio band are 2.4 GHz or sub-GHz frequencies. The most prevalent 2.4 GHz protocols are Wi-Fi, Bluetooth, and ZigBee. In low-power and lower-data-rate wireless monitoring applications, however, sub-GHz wireless systems offer several advantages, including longer range for a given power and reduced power consumption.

The Friis equation quantifies the superior propagation characteristics of a sub-GHz radio, showing that path loss at 2.4 GHz is 8.5 dB higher than at 900 MHz. This translates into 2.67 times longer range for a 900 MHz radio because range approximately doubles with every 6 dB increase in power. To match the range of a 900 MHz radio, a 2.4 GHz solution would need more than 8.5 dB of additional power. Sub-GHz ISM bands are mostly used for proprietary low-duty-cycle links and are not as likely to interfere with each other. The quieter spectrum means easier transmissions and fewer retries, which is more efficient and saves battery power.

Carrier frequency also has a major impact on the average power budget at the network level. ZigBee and Bluetooth offer highly sophisticated link and network layers, but these protocol stacks can account for up to 50 to 75 percent of the radio power consumption with larger overheads. For ultra-low-power systems, the “one size fits all” standardized option is rarely the optimum solution. Instead, designers developing solutions for ultra-low-power applications should consider using the protocol best suited for their needs.

Finally, link data rate is one of the most important factors influencing power consumption in duty-cycled wireless links. The average power is almost inversely proportional to the link data rate; for instance, a 100 Kbps radio will consume almost half the power of a 50 Kbps radio for the same payload. When comparing RF transceivers, “energy per bit” is a better indicator of power efficiency than current consumption. But high data rate radios are often those with the higher peak currents, and these are highly undesirable for most small batteries as they result in large, leaky storage capacitors.

Each of the aforementioned factors is critical for applications where power is at a premium and payload is greater than 10 bits per second. While previous body-worn wireless sensors could only be used for slowly varying parameters, new RF technologies can be used to help observe more rapidly changing physiological parameters (such as heart and brain electrical activity or blood oxygenation) that require data rates on the order of 0.5 to 5 Kbps to extract meaningful waveforms.

Balancing Power and Performance through Proper Circuit Design

Achieving the desired radio performance capabilities while meeting requirements for extremely low power consumption and small package size is a challenge. Careful choice of radio architecture and building blocks are critical to meet communication requirements and power consumption mandates. It is also very challenging to achieve these radio performance capabilities while meeting requirements for extremely low power consumption.

Today’s ultra-low-power RF transceivers achieve the highest possible gain from low current by using inversion techniques. Increases in gate voltage results create a depletion region and an associated concentration of surface electrons that are created from the inversion of normally P-type to N-type material. The goal is to operate in a weak inversion region where possible, in which free electrons now have enough energy to freely move, creating an ultra-low power, low voltage amplifier.

One example of a solution derived from a careful balance of these tradeoffs is the ZL70550 transceiver from Microsemi. Housed in an approximately 2 mm × 3 mm CSP, it has standard two-wire and serial peripheral interfaces for control and data transfer using any standard microcontroller; combined with the ZL70550 transceiver, the resulting solution can be used to develop a wireless sensor solution that can run continuously from a CR-series coin for up to a week. Similar power efficiency can be achieved with devices such as a 3-axis accelerometer or pulse-oximeter for patient respiration measurement, as well as a variety of other wearable health monitoring platforms. Devices like this enable 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.

The ZL70550 is ideal for low-power applications with an ultra-low current of 2.4 mA in Rx mode, 2.75 mA in Tx mode, and an industry-leading ultra-low sleep current of 10 nA. The ZL70550’s low-power performance enables low-cost button cell or small lithium ion batteries to support continuous data streaming in wearable devices.

The following illustration shows the Microsemi ZL7050 in a low-power application.

Click image to enlarge

Figure 2. A typical wireless sensor based on the ZL7550

With the advent of micro-power batteries combined with advances in ultra-low-power transceiver technology, it is now possible to build smart, flexible wireless sensors. Proper transceiver selection is critical for addressing a variety of key design issues so that wearable medical wireless devices can perform continuous monitoring of bio-signals for long periods while using a single small battery. Today’s ultra-low-power transceivers deliver a combination of performance and power efficiency by balancing the tradeoffs associated with the use of inversion techniques to achieve the highest possible gain from a low current.

Microsemi Corporation