The number of radio products has long since surpassed the number of people living on Earth by many times and continues to grow. The interference-free coexistence of these products is a problem, but test and measurement equipment can help manufacturers master this challenge. Mahmud Naseef, Solution Manager for Wireless Coexistence Testing, Rohde & Schwarz and Christian Reimer, Regional Manager Testing Inspection Certification, Rohde & Schwarz explain more.
Connected products must not emit electromagnetic interference into their surroundings and they must be able to operate smoothly when subjected to external electromagnetic interference. International ElectroMagnetic Compatibility (EMC) standards make it possible to comply with these requirements.
The test specifications for any particular product depend on its product group, frequency of operation and industry. Different test standards apply to medical devices, consumer goods, military equipment, automotive, aerospace and defence, and many other industries.
The CE mark in Europe indicates that a product complies with relevant standards. A corresponding FCC label is optional in the USA, but every product must be accompanied by a supplier’s declaration of conformity (SDoC).
In Europe, products with integrated radio components must also comply with the Radio Equipment Directive (RED). The RED expands the EMC rules to include requirements for transmitting and receiving equipment.
It is specifically intended to prevent radio products from interfering with each other and to enable coexistence. The incorporation of the RED regulations into specific test rules is the responsibility of standardisation bodies such as ETSI, which integrates these requirements in harmonised standards and is valid among the EU member states and EFTA countries.
A question of coexistence
EMC tests have been around for decades. They are based on standards and norms that have only gradually evolved. The measurement parameters use base electrical quantities such as (interference) field strength, current and voltage.
The signal forms of these quantities are not important. Susceptibility measurements can therefore be performed using very simple signal forms such as CW, AM and pulse.
However, the product landscape has changed dramatically in recent years. More and more products have wireless modules with internet connectivity. Over 20 billion wireless internet-capable units are currently in use around the world and the numbers are growing.
All are subject to the EMC standards specified for their product group. Conventional EMC measures and tests are not enough to ensure interference-free operation in harsh electromagnetic environments. The additional coexistence tests initiated by RED and similar regulations increasingly have become more and more important.
These tests require proof of a radio product’s compatibility with other wireless services in the intended field of application. The latter can be a significant specification with a large impact on test design.
Formal compliance with a standard may not be enough if the standard fails to consider the specific product’s operating conditions. Manufacturers must work with authorities to avoid product liability risks or simply to obtain market access, especially when products are used in the healthcare sector.
The problem with the blocking test
Blocking tests were identified as suitable for coexistence capability testing and have been included in European standards associated with the RED. Performing the test involves establishing a connection between the DUT and a radiocommunications tester with specified parameters such as frequency and signal level.
A defined interference signal generated by a signal generator is superimposed on the connection. A robust receiver can still operate as intended even in the presence of strong interference signals. Otherwise, follow-up work will be needed.
However, tests that simply comply with standards cannot ensure that a device will be as immune to interference in regular use as suggested by the test results. The reason is that standards cannot keep up with the market as it evolves and as rapid advances are made in radio technology, meaning tests only cover the bare minimum.
A completed and properly documented test may create legal certainty for major product faults, but the market will remain unimpressed and gives users false confidence in a faulty product. Actual product performance in real world EM environments is what ultimately matters.
ETSI EN?300328 V2.2.2 for devices operating in the 2.4 GHz band is one example of a European standard with room for improvement. Coexistence is especially critical in this densely occupied band in which WLAN, Bluetooth, household microwaves and other devices jostle for space.
The standard blocking test is fairly easy to pass, but hardly depicts the situation in the real world. This idealised interference scenario uses a CW interference signal with a constant signal level and a noise floor limited to twice the signal bandwidth.
However, these CW interference signals are not realistic and actual broadband modulated signals can degrade reception quality even at lower levels. Complete blocking of communications by interferers is unlikely.
Instead, data throughput drops because individual data packets get lost and must be requested again. This means a device can have insufficient interference immunity despite passing a coexistence test (Fig. 2).
Realistic approaches
Not all standards lag behind current requirements. One example is EN?303340 version 1.1.2. for DVB-T and DVB-T2 broadcast receivers. The standard demands the use of various types of interference signals, including simulation of a fully occupied LTE base station signal. The standard uses interference signals but has other weaknesses.
It is virtually impossible to judge how well a receiver works in practice simply based on whether it passes a test in line with this standard, since the primary value for measuring degradation is the frequency with which picture failure points occur.
If the picture failure points occur no more than every 15 seconds when interference signals are applied, the device is standard-compliant and can be sold. Whether the customer is happy with this level of quality is another question.
Ensuring coexistence in the IoT era
The growing omnipresence of wireless products mean coexistence will be a key issue for the industry (Fig. 3). However, all devices cannot be lumped together and differences need to be made. A wireless pacemaker or an automotive emergency call system need much more accurate testing and greater quality certainty than a WLAN controlled toy.
Another area where current standards also come up short is user experience. Many wireless connected devices have integrated screens and speakers. If interference can be heard on the speakers and seen on the screens due to poor coexistence behaviour of the 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 taken into consideration.
The urgent need for practical solutions has caused manufacturers to take the initiative and develop testing procedures that ensure the 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 experience they have gathered.
Modern coexistence test methods should include the following four aspects:
1. Estimate risk
Test requirements strongly depend on product group and operating conditions (Fig. 4). The greater the potential harm from product failure, especially harm to life and health of the users, the stricter the test conditions.
The requirements placed on medical technology are particularly high. In the USA, the Food and Drug Administration (FDA) defines medical device approval rules instead of the Federal Communications Commission (FCC). The FDA demands a declaration of conformity with ANSI C63.27 for the coexistence of wireless devices.
This standard refers to ISO 14971 for medical device risk management, which has an evaluation chart to help manufacturers estimate their product’s risk. The FDA and ANSI do not provide specific testing procedures for proof of conformity, but instead leave test criteria (KPI) 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. Rohde?&?Schwarz is now working with a test house to develop tests for a medical device manufacturer.
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 clearly noticed in devices with visual and/or audio interfaces.
Since users do not judge products by data sheet performance but everyday usefulness, a general analysis of product quality as part of coexistence testing makes sense, such as with the R&S AdVISE inspection software that captures every visual or audio signal irregularity. Audio and video quality testing also has become a routine task.
3. Take radio module installation location into consideration
Products with integrated radio modules behave differently than modules alone since radio characteristics are influenced by the housing and installation location. Some countries still do not require coexistence testing for fully assembled products.
The incoming direction of wanted signals and interference signals affecting the DUT also play a role. This makes varying the angle of incidence with DUT and antenna positioners important for reliable testing.
4. Choose the right interference signals
As mentioned above, 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. This 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, there should be nothing to prevent trouble-free product operation.
A typical coexistence test setup
Fig. 5 shows a test setup for low-risk devices that satisfies all the criteria. The setup includes a radiocommunications tester, a vector signal generator, a spectrum analyser, real-time inspection software and an optional power amplifier.
Measurements are performed in a completely reflection-free, electromagnetically shielded anechoic chamber. The number of interference signals can be significantly expanded for high-risk products by adding further signal generators and antennas to model complex signal scenarios.
First, the radiocommunications 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 (5G is not possible with the R&S CMW500 alone).
The power amplifier is needed to boost the signal level. For 5G, a combination of the R&S CMW500 and R&S CMX500 is required.
A functional test is performed without any interference signals and the results are recorded for all relevant physical and application layer KPIs (data throughput, PER, BLER, video and audio performance) to create a baseline performance threshold.
The power level of the wanted signal is reduced to the point where communications is just barely possible (10% PER on the wanted system), to reproduce the worst-case scenario or cell edge conditions. After the interference signal is activated, the measurements are repeated.
If the DUT generates visual and/or audio output, the inspection software monitors the DUT using a HD webcam and microphone to check whether the outputs have the desired quality. Deviations are documented with a timestamp and evidence data.
A spectrum analyser monitors the RF spectrum during the measurement process. The spectrum analyser can be used to check whether the wanted and interference signals have the right frequencies and levels and whether any external signals are present that could invalidate the test result.
Summary
Ensuring interference-free coexistence of radio products will grow ever more important in the IoT era. Rapid technological developments and changes in the market have left current coexistence test standards often unable to adequately represent actual operating conditions.
This is why manufacturers of particularly high-risk products work with test houses to go beyond the required tests and develop more realistic tests. These tests reduce liability risk while boosting user satisfaction and the quality brand image of their products.
With a few principles taken into account, appropriate tests can be easily carried out with T&M equipment from Rohde?&?Schwarz.
