RF tests for wireless vehicle-to-x communications systems

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In wireless communications, RF tests on transmitters and receivers help to minimize the safety risk arising from poor transmission conditions, says Thomas Brueggen, project manager of RF test systems for intelligent transport systems, Rohde & Schwarz

Automated vehicles can safely navigate the road only if 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 information is supplied by using wireless technologies to connect vehicles. To ensure that safety-related messages are received even under poor transmission conditions, the transmitter and receiver must adhere to minimum standards. Adherence can be verified using RF tests.

Wireless communications systems can be affected by several types of interference. Many of these are known collectively as fading. This includes shadowing and interference caused by physical effects such as scattering, diffraction, refraction and reflection, which cause multipath propagation of the signal (Figure 1). Traffic participants are also continuously moving, which adds a time variant to the fading scenario. All receivers inside vehicles are confronted with continually changing conditions and signal quality. Safety-critical applications must function reliably under these conditions, especially when drivers have come to rely on warnings from a vehicle-to-x (V2X) system and do not expect it to fail.

Testing physical transmission

To minimize the safety risk arising from poor transmission conditions, the RF transmitters and receivers found in onboard 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 these requirements. The two lowest layers of the OSI model, i.e. the physical layer and datalink layer, factor into these tests because they are responsible for the physical transmission of the data message. Tests at the protocol level are not suitable for verifying RF characteristics. These tests check that the bit stream, which is generated from the received signal, is processed correctly. The RF signal at the receive antenna is ignored.

However, the RF module in the OBU must meet certain minimum requirements, e.g. with respect to the power and frequency accuracy and the packet error rate (PER). These characteristics can only be verified with RF tests and not protocol tests, because any interference in the transmitted signal is conducted to the receiver via the OBU’s RF module.

Radio over cable

The automotive industry tests automotive components and ECUs in the lab, on testing grounds and even on public roads. For wireless communications, this is the equivalent of field tests, offering a realistic environment for RF tests. However, other influences such as the weather can unpredictably change the RF characteristics of the radio link. The test setup and test sequence depend on the vehicles involved and the antenna locations, and often they can be changed only with considerable effort.

This is not practical for testing a new device that is still in the development stage. What is needed are alternatives that enable 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 reduce errors:

• The tests can be performed at any time and at low cost.

• The test conditions are clearly defined at all times and can be changed at any time irrespective of outside influences.

• Clearly defined test sequences, when performed under the same conditions, lead to comparable results.

• Reproducible and comparable results facilitate debugging.

• Parameters can be easily modified. This is in contrast to the great deal of effort required to modify the fading profile in a field test, for example.

• Multiple tests can be combined into test sequences for automatic long-term runs.

• Certain RF tests, such as error vector magnitude (EVM) and receiver sensitivity tests, only make sense as conducted tests. In a field test, uncontrolled noise and interference from external sources falsify the measurement results.

Depending on the selected scenario, channel simulation exactly simulates 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 (Figure 2).

Detecting RF problems

To be able to compare the test results of the various hardware and software versions of a V2X unit, all test procedures must be clearly defined. Some countries have therefore defined test specifications for V2X systems that include RF tests for transmitter (TX) and receiver (RX) characteristics.

Various plug tests for V2X have shown that testing TX out-of-band emissions and fading is particularly problematic for many devices under test (Figure 3). It is possible, however, to detect these RF problems during the development phase by using appropriate test instruments.

At present, various wireless technologies are under discussion for implementing V2X communications, in particular WLAN 802.11p, LTE and 5G. Regardless of which technology is used, T&M equipment manufacturers such as Rohde & Schwarz already offer the test solutions needed for V2X. For example, solutions based on the widely distributed LTE technology can be tested using the R&S TS8980 RF test system family from Rohde & Schwarz. The available tests are continually being updated based on each LTE development, making it also suitable for V2X.

For 802.11p the R&S TS-ITS100 RF test system contains the complete package of global 802.11p test cases for Europe, the USA and Japan.

The test system enables measurements up to 18GHz, accepts a variety of filters as needed for various regions and is already set up to handle diversity and multiple input, multiple output (MIMO).

Safety-critical messages must be transmitted reliably and quickly in every environment and every traffic situation. Protocol tests alone are not sufficient because they do not test the circumstances under which the transmitted signal arrives at the receiver. Only RF tests can ensure that the minimum physical requirements are met by OBUs and RSUs, so that lives can be saved during emergencies.

Figure 1: Example of fading due to multipath propagation without a line-of-sight path

Figure 2: R&S SMW signal generator showing a fading profile for V2X at 5.9GHz

Figure 3: TX out-of-band test: The transmit power (blue line) of an 802.11p unit exceeds the allowable limit (red line) at multiple points. The frequency range between 5,855MHz and 5,925MHz is reserved for V2X in Europe and the USA

23 May, 2016

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