As users of mobile devices demand more bandwidth for their apps, network carriers have begun implementing LTE (Long-Term Evolution) technology into their networks. Part of the LTE infrastructure relies on MIMO (multiple input, multiple output) radio channels, which use several antennas to focus signals and boost performance. Because users of mobile devices are generally on the move, carriers and equipment manufacturers must test their networks and their devices by emulating the signal fading of an actual network's radio channel.
To test the performance of a cellular handset or chipset, manufacturers can measure BER (bit-error rate), BLER (block-error rate), and FER (frame-error rate) as a function of the signal-to-noise ratio. All of these measurements count the number of errors in a predefined amount of data. High error values mean that the mobile device doesn't adequately mitigate fading effects in the radio channel. Another typical performance measure is the data throughput vs. the signal-to-noise-ratio. This is typically measured in kilobits per second and defines how much data the system is able to correctly transmit in 1 s.
Figure 1. A radio-channel emulator creates a real-life radio environment in lab conditions.
Performance tests require a communications tester and a radio-channel emulator (Figure 1) for the device under test (DUT). A communications tester establishes a link to a cellular device by emulating a base station, but its features include only a small subset of those in a real base station. For a throughput measurement, the communications tester sends a known data pattern to the DUT. With a direct cable connection from the tester to the device, the test setup can achieve its highest possible throughput. A radio-channel emulator, placed between the tester and the DUT, will distort the signal, which causes a decrease in throughput. Figure 2 shows a typical test setup for a device with multiple antennas.
Figure 2. A typical test setup for devices with multiple antennas (2x2 MIMO downlink) shows four fading channels.
Typically, the main purpose of testing is to verify a product's compliance to a standard. Standard channel models are defined by the 3GPP LTE standard (Ref. 1). The standard also sets performance limits with a given model, ensuring a certain minimum performance for all LTE devices on the market. Most tests against the standard requirements are simply pass/fail measurements, so they do not measure the maximum performance of the device.
Performance testing in the lab provides a fully repeatable environment that allows manufacturers to debug problems in a design. After a design change, the performance can be checked against the same test conditions.Follow the jump directly to Test&Measurement World
for the rest of the article, which covers beamforming, MIMO testing, and testing of access points.
1. "3GPP Specification series
." About the Authors:
Janne Kolu is VP of Elektrobit (EB) Test Tools. He received his MSc degree in physics from the University of Oulu Finland in 2000. He joined Elektrobit, Finland, in 1999, where his activities have included channel-modeling research and radio-channel emulator development projects. He has contributed to EU COST projects and also to 3GPP standardization work. At EB he has held several different positions related to the development of advanced radio-channel-emulation tools. firstname.lastname@example.org.
Petteri Heino is product manager for EB Propsim F8 channel-modeling applications. He has more than 10 years experience from radio-channel research and emulator products. Before his current position, he was responsible for the EB Propsound channel sounder product, a role in which he executed channel-measurement and -modeling projects globally for several research organizations and companies. He has also been the Elektrobit delegate to 3GPP UMTS/LTE standardization projects. email@example.com.
Juha Määttä is product manager for the EB Propsim F8 radio-channel emulator. He has over 6 years experience on research and design activities involving radio-channel emulators. He has participated in numerous radio-channel test-system installations, trainings, and workshops globally for several research organizations and companies. He previously worked as project manager in multiple R&D projects for radio-channel-emulator products. firstname.lastname@example.org.