Feature: Test and measurement
Figure 4: Example four-level risk classification of wireless connected devices, based on ISO 14971 (which has five levels) Another area where current standards also come up short is the
user experience. Many wireless devices have integrated screens and speakers. If interference can be heard on their speakers and seen on their screens due to poor coexistence with other devices, certification should take this into account. Standardisation is a lengthy process because so many different
parties are involved, where interests must be weighed and a wide variety of aspects considered. Te urgent need for practical solutions has caused manufacturers to take the initiative and develop testing procedures that ensure reliable operation of their products under operating conditions. Manufacturers have greater legal certainty for product liability and a competitive advantage thanks to the valuable user experiences thus gathered. Modern coexistence test methods should include the following
four aspects:
1. Estimate risk Test requirements depend strongly on the product group and operating conditions; see Figure 4. Te greater the potential harm from product failure, especially harm to life and health of the users, the stricter the test conditions. Te requirements placed on medical technology are particularly
high. In the US, the Food and Drug Administration (FDA) defines medical device approval rules instead of the Federal Communications Commission (FCC). Te FDA demands a declaration of conformity with ANSI C63.27 for coexistence of wireless devices. Tis standard refers to ISO 14971 for medical device risk management, which has an evaluation chart to help manufacturers estimate their product’s risk. Te FDA and ANSI do not provide specific testing procedures
for proof of conformity, but instead leave test criteria (KPI, or key performance indicator) formulation and the development of a suitable test procedure to manufacturers, placing a large responsibility on them. However, all manufacturer assessments and measures must be submitted plausibly to authorities together with a risk assessment, detailed test records and uncertainty analyses. To avoid having to start from scratch with each new product, the
industry is interested in developing test methods that cover entire product classes.
2. Take user experience into consideration Coexistence quality should cover more than just physical layer performance criteria; it should also examine the application layer and consider user experience. Coexistence problems can be most clearly noticed in devices with visual and/or audio interfaces. Since
30 May 2021
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users do not judge products by data sheets but rather by everyday usefulness, a general analysis of product quality as part of coexistence testing makes sense, such as with the R&S AdVISE inspection soſtware that captures every visual or audio signal irregularity; audio and video quality testing also has become a routine task.
3. Consider the radio module installation location Products with integrated radio modules behave differently than the modules alone do since radio characteristics are influenced by the housing and installation location. Some countries still do not require coexistence testing for fully-assembled products. Te incoming direction of wanted signals and interference signals affecting the DUT also play a role. Tis makes varying the angle of incidence with DUT and antenna positioners important for reliable testing.
4. Choose the right interference signals As mentioned earlier, the signals specified in some standards do not put the DUT under enough stress to exclude coexistence issues in real-world situations, making it necessary to apply worst-case interference signals. Tis includes the interference signal power level, bandwidth and spectral position compared to the intended signal and the quality of unintended signals. If all these factors are taken into account in the blocking test, nothing should prevent trouble-free product operation.
Typical coexistence test setup Figure 5 shows a test set-up for low-risk devices that satisfies all the criteria. Te setup includes a radio communications tester, a vector signal generator, a spectrum analyser, real-time inspection soſtware and an optional power amplifier. Measurements are performed in a completely reflection-free, electromagnetically-shielded anechoic chamber. Te number of interfering signals can be significantly expanded for high-risk products by adding more signal generators and antennas to model complex signal scenarios. First, the radio communications tester establishes an end-to-end
connection with a normal signal level to the DUT by emulating its radio interface, either as a local area standard such as WLAN or a cellular 2G, 3G, 4G or 5G network. A power amplifier is needed to boost the signal level. A functional test is performed without any interfering signals,
and the results are recorded for all relevant physical and application layer KPIs (data throughput, PER, BLER, video and audio
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