Medical Electronics
the RF spectrum is relatively “less busy”. Nevertheless, this also acts as an interference source for the WRPM.
What happens if there is a source interfering with the
communication of the WRPM? The sources of the interference signals are smartphones, smart watches, smart home appliances, IoT devices (smart toothbrush, smart lights etc.), WLAN routers, car doors openers etc., which are running cellular and non-cellular services such as Bluetooth, WiFi hotspot services etc. Not to forget, medical devices may be also be subjected to intentional jamming attacks. When there is an interference source operating at the exact same frequency (i.e., overlapping RF interference signals) as that of the WRPM, the data transfer between the WRPM and the server will become slower (i.e., the data rate of the transmission will drop). In the extreme worst case, the communication will completely break down and absolutely no data will be transferred. If the interference signal is on an adjacent frequency channel, then depending on the power level of the blocker, the data rate will also be lower. A drop in the data rate means the monitoring station does not receive the patient health information instantly but with an added time delay. In the worst case, medical personnel are not informed about life-threatening changes in the vital conditions at all.
What can be done to protect WRPM from interference signals?
Even though LTE is designed with interference mitigation algorithms to identify and avoid blockers, WLAN receivers mostly suffer in the presence of one or multiple blockers. Therefore, the best approach is to design robust receivers with good filtering capabilities. In order to ensure receiver robustness for wireless medical devices in the US, the FDA recommends product compliance according to the ANSI C63.27 and AAMI TIR69 standards to ensure minimum performance.
However, technology evolves much faster than standards. It is therefore important in the product development phase to select test methods that future-proof a product for its lifetime. A failing product even a few years after market placement can still hurt the brand reputation and can be subjected to heavy fines and penalties.
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Regulatory requirements for wireless remote patient monitoring devices
The ANSI C63.27 specification is the only standard focused towards wireless coexistence testing. It is a mandatory requirement in the US, relevant devices must be compliant according to the guidelines provided in the standard. Canada, the EU and APAC countries do not have any special requirements for medical devices to be specifically wireless coexistence compliant. By the end of 2020, there will be 20 billion wireless connected products. This means that, with the growing number of wireless products crowding the RF spectrum, hospital authorities have more confidence in products that can demonstrate interference robustness due to compliance to a certified and
Figure 2: Replicating the hospital operating environment in the lab
recognized standard (i.e., the ANSI C63.27 guidelines). All hospital administrators are interested in conformance to guidelines as a way of reducing legal liability in case of product failure that can be traced back to a wireless coexistence issue.
Thorough testing will save lives The ANSI C63.27 standard describes the wireless coexistence test using four different types of test setup. The most reproducible and realistic way of testing any wireless receiver is by performing radiated over-the-air (OTA) testing inside fully anechoic chambers.
A test plan needs to be generated that takes into consideration the risk assessment analysis for the product based on the intended use case. A product that is categorized as high risk (i.e., in the case of malfunctioning due to a coexistence- related issue, resulting in bodily harm to the patient) needs to be tested using a more sophisticated interference strategy that better simulates real world worst-case conditions. The risk assessment should take into account the wireless technology supported by the device, including the frequency and the exact bands supported as well as channels available on the radio module. The worst-case operating condition needs to be defined for the product itself and a method of evaluating the functional wireless performance (FWP) in both ideal and worst-case operating
environment. The risk assessment outcome dictates how stringent the test for compliance needs to be.
The T&M challenges include, firstly, recreating the electro-magnetic environment applicable for the product’s intended use while performing coexistence tests to monitor the defined functional wireless performance (FWP) in a controlled measurement area. Secondly, receiver robustness and application-level testing for intentional and unintentional frequency jamming using realistic interference signals in a repeatable manner.
Test and measurement solution The ideal solution includes a radio communication tester, which can emulate all the wireless network technologies present in the typical working environment for the device (such as 3G, 4G, 5G, Bluetooth, WLAN, ZigBee). Such a radio communication tester is an extremely powerful instrument since it can be used to replicate the hospital network, and it gives the user full control over the RF parameter configuration of the intended network. Additionally, it also includes the ability to monitor various functional wireless performance parameters such as data throughput, PER, BLER, as well as track IP packet data flowing through the network. Additionally, it is possible to measure IP security with this device. Figure 2 shows how a radio communication
tester can simulate a commercial access point and replicate a real-world network in a controlled testing environment. The baseline FWP is determined in ideal conditions without any interference signals present using this setup.
Depending on the risk categorization of the medical device being tested, the number of interference signals need to be adjusted. For ANSI C63.27 compliance testing, up to three interference signal sources are recommended. However, given the vast number of technologies and the quantity of interference sources (smartphones, smart watches, etc.) around us, it is recommended for R&D labs to test using an even higher number of interference sources, in order to fully characterize the WRPM device performance when the receiver is in a “fully stressed” RF operating environment. Figure 3 shows a test setup with an interference station with up to eight fully calibrated interference sources. Vector signal generators are used for generating interference signals, and should be capable of generating wideband-modulated signals, and able to flexibly adjust to the centre frequency and the output power level of the unintended signals.
While performing coexistence testing, it is important to monitor the RF spectrum, as is listed as a mandatory step in the standard. A swept-tuned spectrum analyzer is adequate, but in some cases a real-time
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