RTC Magazine

Hypertension, along with type 2 diabetes and high blood cholesterol, are manifestations of an unhealthy lifestyle dominated by overeating, lack of physical activity and excessive tobacco and/or alcohol consumption. Unfortunately, it’s a lifestyle that’s becoming increasingly prevalent in both Western and developing cultures, and health authorities are worried. Their concern comes not only from the death count, but also from the crippling cost of looking after people stricken with the cardiovascular disease (CVD) that typically results from these ailments.

The good news is that CVD is preventable. Eating well and in moderation, exercising, stopping smoking and lowering alcohol consumption—allied to regular monitoring of blood pressure, blood glucose and blood cholesterol levels—can ensure CVD risk factors virtually disappear.

Technology in the form of home medical monitoring equipment enables users to keep an eye on their health. Such equipment reduces the load on general practitioners and keeps people out of the hospital and off expensive drugs. But who wants to spend time checking their vitals signs, recording the numbers and trying to interpret the data—even if it is good for them? Fortunately, ultra-low-power (ULP) wireless technology is making the process much easier—in fact, as easy as pressing a button and sitting back while the data automatically makes its way to the physicians. But what exactly is this technology, and how is it being incorporated into new products?           

Finding the Missing Link

Mobile connectivity is so pervasive, it is hard to envisage modern life without it. Such connectivity has allowed us to manage many aspects of our lives—such as work, finance and friends—from virtually anyplace and at anytime. But one thing that’s lagging is management of our health. And that’s because, until now, there has been a missing link. Home medical devices such as blood pressure and heart rate monitors, weight scales and thermometers have remained stubbornly “disconnected.”

Technology to connect these products to the cellular network or the Internet does exist, of course. Classic Bluetooth wireless technology and Wi-Fi are two examples that have proven their viability in millions of products—notably cell phones and computers—across the globe. But both technologies are relatively expensive, require lots of power and are primarily designed for rapid transfer of a lot of data.

None of these characteristics matches the requirements of portable home medical devices. These products are price sensitive, powered by small batteries and generate low volumes of data infrequently. What’s needed is an inexpensive, low power consumption wireless technology that’s designed to send a few bits of information perhaps every few seconds up to once a day. That’s the perfect description of ULP wireless technology.

ULP wireless connectivity is not new. However, until recently it has been a proprietary technology. Wireless chips from one company can’t communicate with those from another. So, for example, a weight scale equipped with a wireless connection from company A can’t send its data to a cell phone that incorporates a wireless chip from company B.

Bluetooth low energy, which is a hallmark feature of Bluetooth v4.0, the latest version of the popular wireless technology, changes all that (Figure 1). Bluetooth low energy is an ultra-low-power wireless technology that promises to bring interoperability between home medical equipment and the latest generation of cell phones that are starting to incorporate Bluetooth v4.0.  (See sidebar “A Tale of Two Chips.”)

Conservative Sector

Wireless technology has been slow to penetrate the medical sector due to the stringent demands of the community. These demands are driven by the need to guarantee that a chosen technology is totally reliable and not likely to be detrimental to a patient’s health.

The technology first needs to be built to an open standard so that products from different manufacturers can communicate with each other reliably and incorporate simple pairing. Second, transmission of data must be safe and secure when travelling across the Internet and the cellular network. And third, conservative health care institutions need a convincing argument to take up new technology.

Bluetooth low energy meets all of these requirements and more. For example, the RF protocol stack is small so the radios consume ultra-low currents when transmitting or receiving and can hibernate in “sleep” states consuming just Nano amps. Moreover, Bluetooth low energy supports AES encrypted wireless communication.

The Technology of Ultra-Low-Power Wireless

There are three characteristics of Bluetooth low energy technology that underlie its ULP performance: maximized standby time, fast connection and low peak transmit/receive power. Switching the radio “on” for anything other than very brief periods dramatically reduces battery life, so any transmitting or receiving that has to be done needs to be done quickly. The first trick Bluetooth low energy technology uses to minimize time on air is to employ only three “advertising” channels to search for other devices or promote its own presence to devices that might be looking to make a connection. In comparison, classic Bluetooth technology uses 32 channels.

This means Bluetooth low energy technology has to switch “on” for just 0.6 to 1.2 ms to scan for other devices, while classic Bluetooth technology requires 22.5 ms to scan its 32 channels. Consequently, Bluetooth low energy technology uses 10 to 20 times less power than classic Bluetooth technology to locate other radios.

Note that the use of three advertising channels is a slight compromise: it’s a trade between “on” time (and hence power) and robustness in what is a very crowded part of the spectrum. With fewer advertising channels there is a greater chance of another radio broadcasting on one of the chosen frequencies and corrupting the signal. The specification’s designers are confident they have balanced this compromise—they have, for example, chosen the advertising channels such that they don’t clash with Wi-Fi’s default channels.

Once connected, Bluetooth low-energy technology switches to one of its 37 data channels. During the short data transmission period the radio switches between channels in a pseudo-random pattern using the adaptive frequency hopping (AFH) technology pioneered by classic Bluetooth technology, although classic Bluetooth technology uses 79 data channels.

Another reason why Bluetooth low-energy technology spends minimal time on the air is because it features a raw data bandwidth of 1 Mbit/s—greater bandwidth allows more information to be sent in less time. An alternative technology that features a bandwidth of 250 Kbit/s, for example, has to be “on” for four times as long to send the same amount of information.

Bluetooth low-energy technology can “complete” a connection, i.e., scan for other devices, link, send data, authenticate and “gracefully” terminate, in just 3 ms. With classic Bluetooth technology, a similar connection cycle is measured in hundreds of milliseconds. Remember, more time on air requires more energy from the battery.

Bluetooth low-energy technology also keeps a lid on peak power in two other ways: by employing more “relaxed” RF parameters than its big brother, and by sending very short packets. Both technologies use a Gaussian frequency shift keying (GFSK) modulation. However, Bluetooth low-energy technology uses a modulation index of 0.5 compared to classic Bluetooth technology’s 0.35. An index of 0.5 is close to a Gaussian minimum shift keying (GMSK) scheme and lowers the radio’s power requirements. Two beneficial side effects of the lower modulation index are increased range and enhanced robustness.

Classic Bluetooth technology uses a long packet length. When these longer packets are transmitted the radio has to remain in a relatively high power state for a longer duration, heating the silicon. This changes the material’s physical characteristics and would alter the transmission frequency, breaking the link unless the radio was constantly recalibrated. Recalibration costs power and requires a closed-loop architecture, making the radio more complex and pushing up the device’s cost. In contrast, Bluetooth low-energy technology uses very short packets, which keeps the silicon cool. Consequently, a Bluetooth low-energy transceiver doesn’t require power-consuming recalibration and a closed-loop architecture.

Bluetooth low energy is an open standard ensuring that sensors from different manufacturers can establish communication quickly and easily. And because Bluetooth low energy builds on the legacy of Bluetooth wireless technology, it can easily form personal area networks (PANs) comprising several sensors—for example measuring arrhythmias, blood pressure and oxygen levels—communicating with a single “master” device.

The Smartphone as a Health Hub

Hong Kong-based IDT International is a leading manufacturer of blood pressure monitors (BPM). The company’s latest product, a monitor equipped with a ULP wireless link (Figure 2), is currently undergoing final medical certification with the U.S. Food and Drug Administration (FDA) and European Medical Devices Directive (MDD).

IDT’s product is specifically designed to make testing blood pressure simple and to ensure the resulting data is interpreted for the user’s best benefit. “It can be used by anyone,” says Danny Leung, engineering manager of IDT’s medical and sports & fitness division. “The end user has to do nothing more than put the cuff on their upper arm and press a button.” The monitor gives immediate voice feedback, calibrated to World Health Organization (WHO) recommendations, on the current blood pressure reading—for example, “Your blood pressure is normal.”

Blood pressure naturally varies depending on a number of factors such as whether the patient is standing or sitting, has recently exercised or is under stress, so single readings aren’t a good guide to underlying health, although abnormally high readings should always be immediately reported to a medical practitioner. To detect a potential problem, or a gradual upward trend over time, readings should be averaged over several days.

IDT’s latest innovation ensures that such a series of readings reaches expert eyes. This innovation is the result of adding a Bluetooth low-energy wireless link to the BPM. The wireless link is powered by Nordic’s nRF8001 µBlue Bluetooth low-energy solution. The nRF8001 is a single-chip-connectivity solution fully compliant with Bluetooth v4.0 (Figure 3).

The blood pressure meter is the first such device in the world to utilize Bluetooth low energy and the Bluetooth Special Interest Group’s (SIG) recently adopted Blood Pressure profile. The profile is an additional layer added to the Bluetooth low-energy RF protocol stack that optimizes the operation of a specific application. The use of Bluetooth Smart in the BPM allows it to communicate with one of the Bluetooth v4.0 smartphones now appearing on the market. Compatible devices include Apple’s iPhone 4S and handsets from Motorola, HTC and NEC. The BPM measures the patient’s systolic and diastolic pressure and displays the average blood pressure from a group of recent measurements.

Data from the blood pressure meter, which also includes heart rate and notifications of heart beat irregularities, is transmitted from the monitor to the handset and from there, via the cellular network, to a remote server in the medical facility. Alternatively, the data can be sent via SMS or email.

Home Monitoring Cuts Costs

Taking responsibility for personal health is key to reducing the incidence of CVD—the number one global killer. But it can be difficult to encourage adults who habitually overindulge to change their lifestyle. Government-sponsored campaigns, aimed at educating the population in the benefits of healthy living, can work, provided participants remain motivated.

That motivation comes, in part, from seeing results such as weight loss, lowered resting heart rate and decreased blood pressure. Home health equipment can help that happen. And using ULP wireless technology to transmit the medical data to the cellular network via Bluetooth-enabled products keeps doctors informed. That limits the time patients need to spend with the physician, and enables medical staff to make well-informed decisions about when to prescribe drugs—saving health authorities a fortune.

Like weight, blood pressure is a good indicator of general health. If users have access to a simple-to-use blood pressure monitor, and the results of their periodic measurements are wirelessly transmitted to a health care professional, they can see how they’re doing on a particular day and be safe in the knowledge that if there’s a detrimental long-term trend it will be identified by the doctor.

A Tale of Two Chips

The operational mode of Bluetooth low-energy technology suits transmission of data from compact wireless sensors (exchanging data every half second) or other peripherals where fully asynchronous communication can be used. These devices send low volumes of data infrequently, for example, a few times per second to once every minute or even less often.

There are two types of chips that together form the Bluetooth low-energy architecture: Bluetooth low-energy devices and Bluetooth v4.0 devices. The Bluetooth low-energy chip is brand new to the Bluetooth specification—it’s the part of the technology optimized for ULP operation. These devices can communicate with other Bluetooth low-energy chips and Bluetooth v4.0 chips when the latter are using the Bluetooth low-energy technology part of their architecture to transmit and receive. Bluetooth v4.0 devices are capable of both “classic” Bluetooth and Bluetooth low-energy communication.

Bluetooth v4.0 chips will be used anywhere a classic Bluetooth chip is used today. The consequence is that cell phones, PCs, Personal Navigation Devices (PNDs) or other applications fitted with the new Bluetooth chips will be capable of communicating with all the legacy Classic Bluetooth devices already on the market as well as with all future Bluetooth low-energy devices. However, because they are required to perform Classic Bluetooth and Bluetooth low-energy duties, Bluetooth v4.0 chips are not optimized for ULP operation to the same degree as Bluetooth low-energy devices.

Bluetooth low-energy chips can operate for long periods (months or even years) from a coin cell battery such as a 3V, 220 mAh CR2032. In contrast, classic Bluetooth technology—including Bluetooth v4.0—typically requires the capacity of at least two AAA cells, which have 10 to 12 times the capacity of a coin cell and much higher peak current tolerance, to power them for days or weeks at most, depending on the application.

Nordic Semiconductor
Oslo, Norway
+47 22 51 10 50
www.nordicsemi.com

IDT
Hong Kong
(852) 2764 7873
www.idthk.com