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 user’s 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 wave’s 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 multipath 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 2×2 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 performance test plan
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 CTIA’s full set of recommendations.
CTIA recommends a ring of at least eight antennas in an anechoic chamber, which is ideal for testing mobile phones and small tablets. MIMO OTA testing also includes the testing of larger devices such as tablets and laptops, which require a ring of 16 antennas. Since each antenna requires signals from two channel emulator ports, the channel emulator will, in such cases, need to support up to 32 channels. Test set-up is much simpler and quicker with a 32 port channel emulator compared to a 2 x 16 port channel emulator.
Figure 1: Anite channel emulators (Propsim F32) with up to 32 ports.
We have partnered with several test laboratories, including China Academy of Information and Communications Technology (CAICT), China Telecommunication Technology Labs – Terminals (CTTL-Terminals), SGS in Taiwan, and AT4 Wireless to provide our Propsim F32 channel emulators for MIMO OTA testing. The following includes test set up specifics.
These test houses provide OTA testing with an anechoic or a reverberation chamber and incorporate a channel emulator to create a realistic radio channel propagation environment.
– In an anechoic chamber, multiple antennas are deployed at various angles to simulate the reflections. The anechoic chamber is a non-reflecting chamber (with no external interferences) that systematically eliminates multipath reflections and is therefore widely used in testing the performance of wireless devices. Anechoic chambers provide vector information as a function of the orientation of the antenna under test so that the antenna pattern can be determined under different set-up conditions in full compliance with CTIA OTA measurements. Anechoic chambers also have the ability to accurately reproduce spatial channel models (e.g. SCME channel models), which is important for MIMO transmission and, therefore, MIMO OTA testing. This is why CTIA has recommended the use of anechoic chambers for MIMO OTA testing.
– A reverberation chamber is a highly reflective environment where multipath signals fill the chamber with standing waves, which are mechanically stirred to expose the DUT to a highly variable propagation environment. Multiple samples enable the test engineer to characterise a statistical response of the DUT to this reflective environment. Reverberation chambers are compliant to CTIA recommendations as they are suitable for Transmit-Diversity testing. They also offer a simple and quick method for pre-compliance OTA performance testing and a first order assessment of the device performance.
MIMO OTA testing in an anechoic or reverberation chamber enables test houses, manufacturers, and mobile operators to verify the performance of a mobile device that is equipped with multiple antennas. MIMO OTA testing is important in order to ensure end-users have access to a truly mobile broadband experience with higher data rates.
About the Author:
James joined Anite in 2000 as Project Manager of the GPRS conformance product line. After managing the development of many of Anite’s products through the technology progression from 2G to 3G, he joined the company’s product management team in 2007. He has now director of product management, responsible for the portfolio management of Anite’s Handset Testing products. James is a Chartered Engineer and holds a PhD and MEng in Electrical and Electronic Engineering.
Editor’s note; In recent days Keysight Technologies has announced its intention to acquire Anite, see news item here.