You’re reading this blog, so there’s a reasonable chance you loaded this page over a wireless internet connection. Can you see the router from where you are right now? How about the cellular base station?
Whether you do or don’t have direct line of site on the wireless access point, you’re probably surrounded by walls and ceilings. If you aren’t, then you must be reading this at the beach, so well done you; you’ve achieved the ultimate office space… Write a book, the rest of us will buy it.
All that means that this blog post probably got from your wireless access point to your device by reflecting off something that got in the way of the signal.
But what happens to a wireless signal when it reflects off a surface? Are the Ones and Zeros now backwards? What does that look like for FM modulation vs QPSK vs 64 QAM? How could you possibly convey the idea to students?
Prof Zoya Popovic at CU Boulder has an answer; give the students access to the basic building blocks of a wireless system and let them learn by experimentation. Transmitters, receivers, pole antennas, horn antennas, aluminium foil, attenuators, combiners, splitters – it’s the radio frequency equivalent of giving a child a gigantic box of Lego® and letting their imagination guide them.
Pictured here is one of Zoya’s students, Will Haines, who had set up an interesting experiment using one USRP to transmit another to receive. He had a 16QAM signal coming out of one horn antenna, bouncing around the inside of a 90-degree corner (made of a cardboard box lined with aluminium foil) and then being received on the other horn antenna. The system was flexible so that within seconds you could change the modulation from BPSK to QPSK all the way up to 64QAM for compare and contrast. The resulting reflection did some pretty weird and interesting things to the received constellation diagram.
Maybe we can use these learnings to create the ultimate tin-foil hat? Or, you know, characterise a channel for an upcoming wireless system deployment. You decide.