The use of radio frequencies is governed by the International Telecommunication Union (ITU) Radio Regulations on an international scale, which are then adopted by regulatory authorities of the 193 ITU member states. The intrinsic goal of regulation is the efficient use of radio spectrum.
Looking at the two large regulatory schemes for radio devices, the Federal Communications Commission (FCC) has traditionally regulated transmitters to enable the efficient use of radio spectrum but has recently released a policy statement updating the FCC framework “… for a balanced spectrum management approach that comprehensively evaluates both transmitter and receiver performance…” [1]. The Radio Equipment Directive (RED) harmonised standards are already verifying receiver performance with RX sensitivity, blocking, adaptivity, adjacent channel selectivity and intermodulation test cases for various radio technologies. Receivers are also required to have a certain level of immunity to electromagnetic disturbance. (This is an electromagnetic compatibility topic and will be covered in one of my future articles.)
This article focuses on an important field of radio performance which is not covered by the regulatory testing addressed above: wireless coexistence.
Wireless coexistence can be defined as the ability of one system to perform a task in a given shared environment where other systems have an ability to perform their tasks and may or may not be using the same set of rules [2]. It is also described as a state of acceptable co-channel and/or adjacent channel operation of two or more radio systems (possibly using different wireless access technologies) within the same geographical area [3].
Wireless coexistence is of particular importance for radio communication use in critical systems that serve health & safety related applications, eg in healthcare, factory automation, transportation, defence, public safety and critical infrastructure. Radio system specifications implement coexistence management by protocol design or network topology, but there are very few standards that guide coexistence testing.
The wireless coexistence is most relevant to radio technologies that operate in the unlicensed 2.4 and 5 GHz frequency bands, recently extended by the 5.925 to 7.125 GHz band. Main radio technologies operated in these unlicensed bands and adjacent licensed bands today are Bluetooth, Wi-Fi, LTE and LTE-LAA.
The wide adoption of Bluetooth and Wi-Fi technologies in medical devices and their intensive use in various healthcare environments has resulted in the development of the ANSI C63.27 “American National Standard for Evaluation of Wireless Coexistence” triggered by a request of the US Food and Drug Administration (FDA). The standard was initially published in 2017 and later revised in 2021 to its current version C63.27-2021, which added LTE-LAA technology. ANSI C63.27-2021 is listed as an FDA Recognized Consensus Standard for Medical Devices [4].
The manufacturer of a piece of equipment under test (EUT) is responsible for defining the functional wireless performance (FWP). ANSI C63.27 defines the FWP as the subset of the total functionality related to wireless capabilities that could result in unacceptable consequences if degraded or disrupted during use.
The documented FWP is used to establish a pass/fail criteria for acceptable/unacceptable performance. Relevant key performance indicators such as throughput, packet error rate, error vector magnitude, latency, jitter, etc should be identified with pass/fail thresholds within the FWP specification.
Coexistence evaluation “test tiers” shall be defined to allow using the test results of ANSI C63.27 in a risk assessment for a medical device, eg in accordance with “wireless risk” categories as specified in Table 1 of AAMI TIR69.
The focus of ANSI C63.27-2021 is on radio technologies that operate in the unlicensed frequency bands as mentioned in section 1 above.
Annex A of ANSI C63.27-2021 is providing radio technology and band specific test guidance for the following radio standard coexistence configuration sets for wanted signals and unwanted signals:
In general, coexistence evaluation involves assessing the mutual effects of coexisting systems, termed as “two-way” coexistence. However, ANSI C63.27-2021 coexistence test guidance and test methods are focusing on the impact of an unintended network on an EUT, ie “one-way” coexistence. “Two-way” coexistence testing is a topic for future exploration.
ANSI C63.27-2021 specifies four different test methods in Table 1:
Each test method has different strengths and weaknesses that should be considered when selecting the best method for use in evaluating a specific EUT. From a test repeatability viewpoint, the test methods in Table 1 are ordered in terms of increasing variability.
While conducted RF measurements are generally found to be the most repeatable, most EUTs and especially medical devices do not provide access to the antenna port(s) and need to be tested in an unmodified configuration preventing the use of a conducted RF test method. Also, antenna characteristics can have a significant impact on the EUT performance. Over-the-air testing (OTA), used in radiated test methods, integrates antenna performance.
Also using a wireless network to generate the unintended signals in a reproducible way can be difficult if network management procedures change physical layer parameters to mitigate a degradation of the unintended communication link.
A resulting technical challenge to consider is the required monitoring of the unintended signals. The monitoring is becoming complex when a wireless network is used to generate the unintended signals as described above. Using a signal generator as the unintended signal source is significantly simplifying the monitoring and increases reproducibility.
At TÜV SÜD we have developed all required waveforms to implement the radio standard coexistence configuration sets for Bluetooth, 2.4 & 5 GHz Wi-Fi, LTE and LTE-LAA as specified in Annex A of ANSI C63.27-2021.
In addition, we have defined additional radio standard coexistence configurations and generated waveforms for Wi-Fi 6E / IEEE 802.11ax not yet contained in ANSI C63.27-2021.
Significant effort has been put into the accurate implementation and validation of key physical layer parameters including frequency band, frequency channel, channel bandwidth, modulation and coding scheme, signal amplitude and channel utilisation / duty cycle / resource block allocation. This includes a comprehensive review and validation of the required fixed reference channels for LTE and LTE-LAA where some flaws in ANSI C63.27-2021 have been observed and corrected.
Our preferred method is to utilise a vector signal generator to generate unintended signals for our pre-defined waveforms. This simplifies the monitoring of the unintended signals and ensures reproducibility as discussed above. Using our vector signal generators, we have the flexibility to tune physical layer parameters of the waveforms upon customer request.
Please contact us if you need to discuss details of waveforms proposed by TÜV SÜD.
Please find out more about our Wireless Coexistence Testing services.
[1] FCC Policy Statement No. 23-27
[2] IEEE Std 802.15.3™-2016, IEEE Standard for High Data Rate Wireless Multi-Media Networks.
[3] IEEE Std 802.16™-2012, IEEE Standard for Air Interface for Broadband Wireless Access Systems.
[4] Recognized Consensus Standards: Medical Devices (fda.gov)
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