Multi-Antenna Designs Turn MIMO Testing Into Heavy Work

A couple of years ago, independent European test laboratory Broadband-Testing put six of the then most popular high-end smart phones to the test and was surprised by how greatly they varied in their ability to make and maintain voice calls under a range of everyday operating conditions.

“How many times have you blamed a mobile dropped call on a network operator, without considering it might be the phone that is to blame?” asked Steve Broadhead, founder and director of Broadband-Testing. “The results of this testing clearly highlight the contribution that the handset can make to these problems.”

The resulting test report from Broadband-Testing underlined the need to pay more attention to the handset when judging overall network performance. This is especially important for the service provider because, as Broadhead pointed out, when users have invested in a prestigious handset they will blame the network rather than admit they made a bad purchase.

The good news was that Broadband-Testing, in partnership with Spirent, had demonstrated a practical, repeatable, and rigorous process for testing mobile phone performance under realistic “real world” conditions in the lab that could have caught these issues before commercial release. Since then, the test processes have been further refined.

Now comes a far greater challenge posed by the application of multiple-input multiple-output (MIMO) techniques to increase data throughput and cell capacity for next-generation technologies such as Long-Term Evolution (LTE) and LTE Advanced.

The MIMO Challenge

At the service end, MIMO requires more antennas on the tower, where space generally isn’t a constraint. But at the device end, very sophisticated design and materials are needed to pack multiple antennas into a small space with enough physical separation for MIMO to be effective.

Bear in mind that, in addition to cellular, a typical 3G smart phone will contain antennas for Bluetooth, Wi-Fi, and GPS location. Multiple cellular antennas are a serious challenge, so initial LTE deployment is largely focused on 2x2 or 4x2 MIMO, with two or four antennas at the basestation and two at the mobile end. Future MIMO systems will, however, include more antennas on the device side as technology advances.

MIMO technology provides several benefits. First, it multiplexes data streams in the spatial domain, providing parallel streams that can be transmitted at the same time and at the same frequencies. Multi-antenna techniques also enable real-time shaping of the transmission beam, or “beamforming,” which helps to improve the range and capacity in 4G networks.

Spatial multiplexing is a common MIMO technique that’s designed to take advantage of any spatial diversity available in the wireless channel. This spatial diversity is a function of the channel correlation, which in turn is dictated by the antenna orientation and the propagation environment.

This means that the data throughput performance of a MIMO-equipped device depends on the physical orientation of the antennas and on how their orientation changes relative to each other. For example, consider scenarios where users are viewing streaming video on an LTE smart phone.

Moving the device just slightly will change the orientation of the antennas, which could result in the user experience going, for example, from excellent to marginal. Now consider the huge range of variations a device could experience while moving about (walking, driving, etc.), and you have a very complex situation that needs to be modelled in the laboratory.

Over-The-Air Testing

Most traditional device testing with a single antenna is performed with a conducted signal. Instead of sending test signals over the air, they’re sent over a cable plugged into the device’s receiver and transmitter via a temporary connector, effectively bypassing the device antenna.

This makes for a highly reliable and repeatable test setup. It’s inadequate for MIMO device testing, though, where so much depends on the antenna design and the precise behaviour of the signals received at the antennas. So today there’s a new emphasis on over-the-air (OTA) testing of devices to account for MIMO antenna performance.

Of course, the ultimate test is what happens in the hands of an end user in the field. But the manufacturer and service provider still must ensure that the ultimate test is passed with flying colours, and that means emulating those final usage conditions as accurately and repeatably as possible in the laboratory.

The first challenge is to model real-world signal behaviour as realistically as possible. Sophisticated wireless channel emulation instruments make it easy for users to take typical signals from a basestation or emulated network and modify them to imitate all sorts of conditions such as fading from building reflections, the Doppler effects of being in a moving vehicle, and the arrival of signals at the mobile device from particular directions. This can be done by recording actual conditions in the field or by inputting parameter values to emulate specific conditions.

So far the technique is similar to testing with a conducted signal. But during OTA testing, different setups enable the device to receive the generated test signals. Perhaps the ideal test environment is the anechoic chamber, where the walls, ceiling, and floor are padded with RF absorptive foam so they don’t reflect RF signals. This means that after passing through channel emulators, signals arrive at the device antennas directly from the transmitting antennas placed in the chamber, so they can be very tightly controlled.

A simpler, less costly setup uses a “reverberation chamber,” typically a smaller enclosure where signals reflect off the walls. This chamber approach can be very useful for MIMO OTA testing, though for more detailed testing an anechoic chamber enables better spatial fine-tuning.

Advanced channel emulation, employing accurate radio channel models that incorporate spatial characteristics, is a requirement for anechoic chamber methods. It also can be used to improve the performance of reverberation chamber techniques.

The anechoic chamber method is based on feeding faded signals to antennas in the chamber. Extensive research by Spirent has established that each antenna in the chamber can be used to contribute signal content for multiple clusters for the spatial channel model (SCM), which can simplify the scope and cost of the implementation. Using this approach, the resulting signal at the device under test has been shown to exhibit the expected properties of SCM, such as spatial correlation, angular spread, and angles of arrival.

Conclusion

With a very limited choice of devices on the market to date, many of which are data dongles, LTE is very much a work in progress, and this is well understood. As it gears up for mass deployment, however, it becomes critical to ensure that services not only add new functionality and superior bandwidth but also do so in a way that matches or exceeds the reliability and quality delivered by legacy networks.

Spirent’s work with Broadband-Testing brought attention to the importance of handset testing, and this is increasingly recognised. Witness the public commotion last year over the alleged antenna shortcomings of a particularly high-profile smart phone. But the move to MIMO is bringing a whole new level of complexity, in particular a need for extensive testing and increased analysis of signal propagation and reception over the air.

It is clear that further work will be needed as the industry gears up from two antennas per device today to maybe eight for LTE Advanced, and LTE Advanced development is already well underway!