Accelerating MIMO over-the-air device testing

There is a growing clamour for a broadband experience with faster data rates. This is driving manufacturers to develop new technology, such as multiple input/multiple output (MIMO) antenna designs, to improve the radio performance of future mobile devices.

Devices are consequently becoming more complex and therefore require more extensive testing prior to market introduction. MIMO Over-the-Air (OTA) performance testing is used to assess the end users experience accessing data services on a mobile device by replicating real world conditions. OTA testing is conducted in the laboratory and involves testing a wireless device without any connected cables, thereby incorporating the device antenna performance.

When a radio waves path interacts with an object, the radio wave is scattered, diffracted, reflected or absorbed. A radio channel emulator accurately simulates this behaviour and it essentially replicates real-world radio channel conditions within a laboratory environment. These conditions include multi-path propagation, such as per-path delay, the Doppler effect, angles of departure (AOD), angles of arrival (AOA), or polarisation; all effects seen by the base station antennas, as well as noise and interference. MIMO OTA testing uses channel emulators to accurately emulate different environments, including urban, suburban, rural, and indoor environments.

There are two types of MIMO OTA testing; one that uses an anechoic chamber and another that uses a reverberation chamber, both carried out in combination with a channel emulator. Recently, the industry has moved forward to ensure the most appropriate type of MIMO OTA testing is used in order to accurately assess the performance of a device.

Accelerate MIMO OTA device testing
CTIA, an international non-profit membership organisation, has represented the wireless communications industry since 1984. Mobile operators and manufacturers of devices and network infrastructure use their recommendations as part of their development and evaluation programs.

Anite has contributed channel emulation expertise to the CTIA MIMO OTA sub-group (MOSG), which has been investigating MIMO OTA performance since March 2011. The subgroup has recently agreed (with endorsement from all CTIA operators) to move forward with two different types of OTA activities for devices with multiple antennas:

1. The first activity will develop a test methodology specifically for testing 2x2 DL MIMO performance based on Transmission Mode 3 (TM3), using spatial channel models, called MIMO OTA tests.

2. The second activity will develop a separate test to evaluate the device under test (DUT) for low throughput and low-latency use cases such as voice over LTE (VoLTE), leading to transmit-diversity tests based on transmission Mode 2 (TM2), called Transmit-Diversity tests.

Considering the nature of testing required to objectively evaluate the DUT, the subgroup has recommended that MIMO OTA testing should be performed using a multi-probe (Multiple Cluster) methodology (AC-MC) in an anechoic chamber, together with a channel emulator, and for transmit diversity testing to be performed using either the multi-probe methodology (AC-MC) or an RC+CE test methodology, which uses a reverberation chamber together with a channel emulator.

Preparing for CTIA standardised MIMO OTA
The technology drivers and standardisation requirements outlined above have resulted in leading independent test houses in China, Taiwan, Europe and US creating anechoic MIMO OTA test laboratories that meet CTIAs full set of recommendations.