10.2 – Wireless Devices

Introduction to Wireless Devices

Wireless Fidelity (Wi-Fi), is a technology that allows electronic devices like computers or biomedical equipment to connect to the internet or communicate with each other wirelessly. Wi-Fi uses radio waves to transmit data which is why the concept of radio frequency (RF) is important to our discussion. Simply put: Wi-Fi is a wireless communication technology that operates over specific radio frequencies to transmit data between devices, providing a convenient way to connect to the internet without needing physical cables.

Health Canada is responsible for conducting the research and investigation to set recommended RF exposure limits in order to protect the health of Canadians. In the United States, the Federal Communications Commission (FCC) oversees the use of the public Radio Frequency (RF) spectrum within which RF wireless technologies operate. The FDA’s policies on wireless medical devices are coordinated with the FCC and provide medical device manufacturers with more predictability and a better understanding of regulatory requirements for medical devices that utilize these technologies.

All types of wireless technology face challenges coexisting in the same space. Because the airways are shared, the functioning of your wireless medical device may be affected (such as data loss or disruption) by other wireless devices near you. Most well-designed and maintained RF wireless medical devices perform adequately. However, the increasingly crowded RF environment and competition from non-medical wireless technology users could impact the performance of RF wireless medical devices. Mobile wireless equipment can transmit using either licensed radio spectrum or on an unlicensed basis in specified frequency bands. Licensed spectrum allows for exclusive, and in some cases non-exclusive, use of particular frequencies or channels in particular locations. Unlicensed radio frequency devices are subject to the conditions that no “harmful interference” is caused and any interference from primary users of the frequency band must be accepted. Mobile wireless equipment can also transmit on an unlicensed basis in frequency bands such as the Industrial, Scientific, Medical (ISM) bands. ISM bands include 900 MHz, 2.4 GHz, 5.2 GHz, and 5.8 GHz and are commonly used for cordless phones and wireless data network equipment.

Incorporation of wireless technology in biomedical devices can have many benefits, including increasing patient mobility by eliminating wires that tether a patient to a medical bed, providing health care professionals the ability to remotely program devices, and providing the ability of physicians to remotely access and monitor patient data regardless of the location of the patient or physician (hospital, home, office, etc…). These benefits can greatly impact patient outcomes by allowing physicians access to real-time data on patients (through patient monitors, see next section) without the physician physically being in the hospital and allowing real-time adjustment of patient treatment. Remote monitoring can also help special populations such as seniors, through home monitoring of chronic diseases so that changes can be detected earlier before more serious consequences occur.

What is RF and why do we use it?

When we think of electricity, we naturally think of wires. From high-voltage transmission lines to tiny traces on a printed circuit board, wires are still the fundamental means of transferring electrical energy from one location to another. But history has consistently demonstrated that human beings are rarely, if ever, satisfied with the fundamental way of doing things, and thus we should not be surprised to learn that the proliferation of electricity was followed by widespread efforts to free electrical functionality from the constraints of physical interconnections.

There are various ways to incorporate “wireless” functionality into an electrical system. One of these is the use of electromagnetic radiation, which is the basis for RF communication. However, it’s important to recognize that electromagnetic radiation is not unique in its ability to extend electrical circuitry into the wireless domain. Anything that can travel through a nonconductive material—mechanical motion, sound waves, heat—could be used as a (perhaps crude) means of converting electrical energy into information that does not rely on conductive interconnections. Let’s make sure we understand what electromagnetic radiation is.

Fields and Waves

You could spend years studying the details of electromagnetism. Fortunately, you don’t need that sort of expertise to successfully design and implement RF circuits. But you do need to have a basic idea of the mysterious energy being emitted from your device’s antenna.

As the name implies, electromagnetic radiation involves both electric fields and magnetic fields. If you have voltage—such as the voltage across the impedance of an antenna—you have an electric field (from a mathematical standpoint, electric field is proportional to the spatial rate of change of voltage). If you have electric current—such as the current passing through the impedance of an antenna—you have a magnetic field (the strength of the field is proportional to the magnitude of the current).

The electric and magnetic fields are present even if the magnitude of the voltage or current is constant. However, these fields would not propagate. If we want a wave that will propagate out into the universe, we need changes in voltage and current.

Figure 10.2.1 The electric and magnetic components of an electromagnetic wave are represented as perpendicular sinusoids.

The key to this propagation phenomenon is the self-sustaining relationship between the electric and magnetic components of electromagnetic radiation. A changing electric field generates a magnetic field, and a changing magnetic field generates an electric field. This mutual regeneration is manifested as a distinct entity, namely, an electromagnetic wave. Once generated, this wave will travel outward from its source, careening day after day, at the speed of light, toward the depths of the unknown.

Examples of Wireless Devices we have in BTEC 327 Lab