RF tests ensure safety for wireless vehicle-to-x communications

Automated vehicles can navigate the road safely when they have precise knowledge of the environment and the traffic situation. A wide variety of sensors and cameras already provide some of this information. Additional data comes from wireless technologies that connect vehicles. To ensure that safety-related messages are received even under poor transmit conditions, transmitters and receivers must be verified using RF tests. Dr. Thomas Brüggen, project manager, RF Test Systems for Intelligent Transport Systems (ITS) at Rohde & Schwarz, tells us more

Wireless communications are affected by different types of interference, many of which are described collectively as fading. Fading includes shadowing and interference produced by physical effects such as scattering, diffraction, refraction and reflection, which cause multipath propagation of the signal. In addition, the motion of traffic participants adds a time factor to the fading environment. Throughout the continually changing conditions and signal quality, safety-critical applications must function reliably, because drivers have come to rely on warnings from their vehicle-to-x (V2X) systems. These are not expected to fail.

Testing physical transmission

To minimise safety risks caused by poor transmit conditions, the RF transmitters and receivers found in on-board units (OBU) and road-side units (RSU) of communications systems must exhibit certain characteristics. Developers and users wanting to integrate V2X components into their system can use RF tests to verify such requirements. In contrast, tests on the protocol level are not suitable for verifying RF characteristics: they simply check that the bit stream generated from the received signal is processed correctly.

Nonetheless, the RF module in the OBU must meet certain minimum requirements such as power and frequency accuracy and packet error rate (PER). These characteristics must be verified with RF tests, because any interference in the transmitted signal is conducted to the receiver via the OBU’s RF module.

Radio over cable

Automotive components and electronic control units are tested in the lab, on testing grounds and on public roads. These equate to field tests, providing a realistic environment for RF tests. However, other influences such as the weather can change the RF characteristics of the radio link unpredictably. Plus, test setups and sequences depend on the vehicles involved and antenna locations. Changes in this test environment can involve considerable effort, making field tests impractical for new devices in development.

Fortunately, there are alternatives that permit realistic testing in the lab. In wireless communications, conducted tests represent an alternative to field tests. A test system simulates the transmission channel, while a cable replaces the actual radio link. This has many advantages that make development faster, less expensive and more reliable. Specifically, tests can be performed at any time and at low cost. Test conditions are always clearly defined, and can be changed at will. Clearly defined test sequences, performed under the same conditions, give comparable results. Such reproducible and comparable results facilitate debugging and, since parameters can be easily modified, multiple tests can be combined into test sequences for automatic long-term runs.

RF tests such as error vector magnitude or RX sensitivity only make sense as conducted tests. These are not prone to uncontrolled noise and interference from external sources, which falsify measurement results. Depending on the selected scenario, channel simulation exactly replicates the physical attributes of the radio link. Today’s signal generators such as the R&S SMW200A can also simulate the special V2X fading profiles in real time.

Detecting RF problems

To enable comparisons between test results of a V2X unit’s varied hardware and software versions, all test procedures must be clearly defined. Some countries have therefore specified tests for V2X systems including RF tests for TX and RX characteristics.

Plug tests for V2X have shown that testing TX out-of-band emissions and fading is especially problematic for many devices under test. It is possible, nevertheless, to detect such RF problems during the development phase by using appropriate test instruments.

With several wireless technologies under discussion for implementing V2X communications, such as WLAN 802.11p, LTE and 5G, existing T&M equipment can deliver the test solutions required. For example, solutions based on LTE can be tested using the R&S TS8980 RF test system family. Since the available tests are continually being updated for each LTE development, these can be deployed immediately for V2X. Similarly, RF test systems like the R&S TS-ITS100 RF offer a complete package of global 802.11p test cases for Europe, USA and Japan. The R&S TS-ITS100 provides measurements up to 18 GHz, and accepts the variety of filters needed for various regions, enabling diversity and multiple input multiple output (MIMO) to be tested, too.

RF tests like these help ensure that minimum physical requirements are met by OBUs and RSUs, so minimising the risk of life threatening situations arising from poor transmit conditions.

www.rohde-schwarz.com

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