It is important to understand the strengths and weaknesses of the range of wireless test environments. Test environment refers to the setup or environment into which the device being tested and the test equipment are placed. The four primary types of environments are:
Faraday cages are usually large, hand-constructed, copper mesh wrapped boxes or rooms. Because of the expense of their construction, they are typically found in the labs of large equipment manufacturers, where they are shared for testing and quality assurance. Because Faraday cages assure a fairly noise- and interference-free environment, they are good for a wide variety of individual product tests, especially for antennas. However, test configurations of more than a few devices can quickly congest traffic in a cage. In addition, there may not be enough distance in the cage to test effects such as multi-path or diversity.
Test Boxes, or RF Chambers
RF Chambers are metal boxes with absorbing material lining the inside to dampen interference. They provide a controlled environment for much lower cost than a Faraday cage. Typically, the DUT is placed into the test chamber, and probes are used to couple signals to/from the DUT through cables to an external test system. In some cases the DUT and the test equipment are placed within the same test chamber, at which point this approximates a Faraday cage. At some point, it ceases to be practical to use chambers as opposed to a Faraday cage. Moreover, because spatial information is lost, some equipment cannot be tested in a chamber, e.g., smart antennas.
Multiple sizes of chambers may be required for proper testing in a fully-enclosed environment. The lower limit on the size of the chamber is dictated by the distance at which the RF near-field transitions to the RF far-field. Objects – including the walls of the chamber itself – that are placed closer than this distance to the unit under test have a significant impact on the radiation pattern and efficiency of the unit; hence it is necessary to ensure that the chamber dimensions are greater than this distance, otherwise the test results may prove to be either irreproducible or erroneous.
Cabled tests simply substitute a wired connection for the wireless connection, bypassing the antennas and directly connecting two pieces of equipment. As a result, cabled tests are inexpensive and easy to configure, and provide good isolation from interference. They are not limited to small configurations, like cages and chambers. However, because of the lack of interference, their results in configurations are idealized toward better performance than would actually occur in the randomness of an open air environment. In addition, properly performing cabled testing relies on the DUT itself being well-shielded, which may not always be the case in consumer or low-end enterprise equipment. In addition, equipment with integral antennas (where the antenna cannot be disconnected to gain access to a connector or other attachment point for a cable) cannot be tested using this method.
Open air is the only test environment that truly matches the way the customer will use the equipment. Like cabled environments, open air has no size limitations or limits on the number of pieces of equipment in a configuration. For some tests, it is ideal because it can test both the antenna and the protocol effects. Also it is the only solution for certain location-dependent tests.
Open air test environments can be separated into indoor and outdoor. Indoor environments are normally actual buildings, usually with furnishings and other accoutrements characteristic of typical office buildings. Outdoor environments are usually open spaces without obstructions, such as would be found at an antenna range. Of these two, the indoor environment is of the most interest, as it most closely approximates the conditions under which the equipment is expected to function. Outdoor environments are used for applications such as characterizing antenna patterns, setting baselines for range and rate, etc.
Summary of test environments
Complete testing requires a combination of test environments; a one-size-fits-all environment does not exist for wireless testing. Ideally, test equipment should be able to accommodate all environments. Click on the thumbnail below to see a summary table.
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