Measure throughput of cellular and WiFi MIMO radios, part 2
In Part 1 of this series, we presented the background of how MIMO radios work. We examined wireless adaptation techniques and MIMO modes of transmission. We also outlined the factors that impact MIMO throughput. Now in Part 2, we compare the conducted measurement techniques to new over the air (OTA) techniques. We'll explore the challenges of MIMO OTA test methods and factors that can make measurements non-repeatable.
Controlled channel, repeatable tests
As outlined in Part 1, engineers testing MIMO radios need to create a range of channel conditions from which to validate a radio's adaptability and throughput performance. This can be accomplished using a MIMO fader such as Azimuth ACE, Spirent VR5, or Anite Propsim), or a multipath emulator, such as the octoScope MPE. A fader or a multipath emulator can connect two or more devices through cables to the device antenna ports or over the air. For devices such as smartphones, tablets, or sensors with difficult-to-reach internal antennas, OTA coupling is highly desirable.
When most wireless communication radios were SISO (single input single output) and antennas were external, you could test a handset or other device by disconnecting it's antenna and connecting a programmable attenuator or a fader between a Master (reference) device and a DUT (Figure 1) left). Testing of MIMO radios with internal antennas requires a MIMO OTA testbed (Figure 1 right) that support stable and repeatable MIMO measurements. Of course, challenges arise when we couple signals over-the-air in a shielded box.
Figure 1. Radio test-bed architecture is evolving from conducted SISO to MIMO OTA.
OTA signal coupling
There are two issues with OTA measurements inside a metal enclosure. First, the signal can reflect from the metal walls and form nulls completely altering the shape of the field and introducing positional variation of signal power. Anechoic chambers must be used to avoid this variation. Refer to octoScope white paper, Throughput Test Methods for MIMO Radios, for more details.
Second, when testing devices with internal antennas, device orientation with respect to the test antennas can have a significant impact on the measured throughput. The reason is that the radiated field is often non-uniform with strong lobes and nulls, as shown in Figure 2 on the right. The field of an internal antenna typically has nulls and peaks because the signal can be blocked by batteries, PCB ground planes, and other metal surfaces.
Figure 2: Left: An octoBox small anechoic chamber with a built-in turntable can rotate the DUT while measuring MIMO throughput performance. Right: An example MIMO antenna field pattern shows red areas indicating high antenna gain and blue areas indicating nulls in the antenna field.
When performing OTA tests, you should rotate the device with respect to the test antennas. That lets you average the performance versus orientation or you can find the optimum and worst-case DUT orientation.
Beamforming test using a turntable
The latest 802.11ac APs (access points) feature beamforming capabilities – the ability of the AP to adapt the power and phase on each of its MIMO antennas to direct signal energy so as to optimize the range of the link. Beamforming typically requires that a channel sounding be performed between the AP and the target client.
One possible way to test beamforming capability of the new 802.11ac devices is to use a turntable to rotate the DUT with respect to test antennas, first with beamforming on and then with beamforming off. The difference in performance versus DUT orientation, signal power, and multipath conditions will indicate how much impact beamforming has on throughput.
It's important to perform this test in the presence of multipath and path loss because the impact of beamforming may not be apparent in ideal conditions. Emulation of multipath and path loss in the testbed will be discussed in Part 3.