Originally posted Sept 6, 2013
We wave our hands and it shows that many of us miss something important
The demonstration described in the previous post is a good way to get an intuitive feel for the processing required for signal separation. That’s essential because signal separation is the fundamental thing that makes MIMO work.
The demo also highlights important aspects of RF transmission that affect not just MIMO operation but most other types of single- or multi-channel signal transmission.
One remarkable thing about the demo is its ability to continuously and simultaneously show the frequency response of the four signal paths, and do so as quickly as the display updates. A video clip from this demonstration is available on YouTube, and an example display from a single transmitter (rather than two) is shown below.
A multitone signal covering approximately 36 MHz and transmitted from one antenna is shown as received by two different antennas. The tone peaks trace out the very different amplitude frequency responses of the two transmit channels.
As shown in the previous post, it’s often easy to visually interpolate between the alternating sets of multitone signal peaks and trace out the amplitude response of each path.
The other remarkable thing is what RF engineers do when they see this live illustration of the transmission environment: They wave their hands. Specifically, they wave their hands in the air directly between the transmit and receive antennas.
The demonstration is often set up with two signal generators at one end of a table and a two-channel RF receiver—in the form of a signal analyzer or an oscilloscope—at the other end. Engineers expect to reflect and absorb some of the signal and they want to see the effects in real time. They move their hands and watch for changes in the measurements, but the modest effects usually leave them feeling a little disappointed.
Their fluttering hands cause small changes in signal levels, and those changes are generally flat over the frequencies involved. At the same time, larger changes in frequency response are also seen and generally seem uncorrelated with all the hand waving. These other changes are often very non-flat with frequency—deep nulls, obvious peaks, ripple—and are much more interesting than the hand-made attenuation. What’s going on?
In most cases, the most dramatic frequency responses don’t come from direct transmission but instead are the result of reflections, multipath or delay spread. The signal at any receive antenna is a combination of all possible paths and, in many cases, no single path is fully dominant, not even the direct one. The path length and resulting phase variations cause constructive and destructive interference or fading, and the results can be narrower in frequency than simple attenuation due to obstructions.
What this means for our hand-waving demo is that the observed frequency responses tend to vary widely and respond strongly to environmental elements other than the hands of engineers. Perhaps someone is blocking a reflective surface or reflecting signals themselves. Perhaps a door is being opened or closed. Who knows? The number of possibilities is nearly infinite in office environments and thus this demo produces frequency responses that are constantly changing and seemingly random.
Of course, the full MIMO demo, even if only 2×2, is considerably more complex and it can appear that the four paths are completely unrelated to each other. And that’s a good thing. Highly correlated RF paths will cause the whole MIMO scheme to collapse—and that’s a good topic for another day.