How well does a multi-antenna terminal work?

New technologies bring with them new challenges in testing. The use of multiple antennas at both ends of a radio link (MIMO or Multiple-Input Multiple-Output) is one of the fundamental building blocks in LTE to increase data throughput and cell capacity. Even in today’s HSDPA systems, simulations have shown that use of two antenna diversity in terminals could increase the network capacity by almost 50%. A smaller additional gain then comes from adding a second transmit antenna at the base station. It is clearly in the interest of a network operator to encourage the use of terminals that will maximise the capacity of the network and improve the experience of the end users. But how can we tell a good product from a bad product? Because of the difficulty of testing over-the-air performance, radio systems are traditionally tested using “conducted tests”. In such a test, the antenna in the terminal is bypassed and the internal electronics is connected directly to test equipment. While such a test is important, it only tells part of the story. It is also increasingly difficult to do, as modern phones seldom have external antenna connectors (mainly because hands free car kits have moved over to using Bluetooth and no longer need a physical connection to the phone). Radiated testing is generally harder to do than a conducted test, because of the need to recreate a controlled radio environment.A story from the early days of GSM illustrates both the importance and difficulty of radiated testing. In order to compare the performance of a range of new GSM phones, we set up a special base station designed to be used only by our own phones. We set up attenuators in both transmit and receive paths at the base stations. We then walked up and down the same street over and over again adjusting the attenuators and recording a subjective measure of the speech quality of each phone. This is not something you could do as a regular measure - we got funny looks from the local residents and a visit from the boys in blue to ask what we were doing. However, the result was very revealing. The worst phone at the time had receiver sensitivity 10dB (=10 times) worse than the best. To achieve the same degree of coverage for such a phone, you would have to build four times as many base stations as if everyone was using the best. That’s a lot of network investment you can avoid by controlling the quality of the terminal.

There is an even bigger testing challenge in the case of MIMO systems. The gain achieved by using multiple antennas is fundamentally reliant on multipath propagation, where the signal arriving at the receiver not only in a direct line from the transmitter but also via delayed reflections from surrounding terrain and buildings, at distances of up to a few kilometres. In the past, most radio systems have treated multipath as an enemy, as it causes signals to fade and drop out. With MIMO we can finally exploit the multiple paths to carry more data. The challenge in testing of MIMO systems is thus to reproduce a realistic and controlled multipath radio channel in a small space and via the built-in antenna of the device being tested.

A year ago, there had been little focus on how MIMO systems would be tested. We started talking to a few interested parties to encourage people to think about how this might be achieved. Many of the technical experts in this field meet up through the European group COST2100. Within the LTE standards process, the responsibility for testing lies with 3GPP RAN4. These two groups are now cooperating to develop an Over-The-Air (OTA) test for HSPA and LTE devices. In parallel, US-based industry group CTIA is also gathering its technical experts in this field and has set up two sub-working groups to study this topic. From a starting point where nobody knew how to test MIMO terminals, we have now reached a situation where there are several different techniques on the table. There is now a good likelihood of agreement on a standardised test in the near future. This will make it easier for us to ensure that our customers have access to good quality products which will also maximise the capacity of our networks.