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Backscatter? Don't you mean just the ambient radio waves? Not quite sure I understand the backscatter concept... but here's an analogy I hope you can help me w
by Swannie 13y ago
Backscatter? Don't you mean just the ambient radio waves?
Not quite sure I understand the backscatter concept... but here's an analogy I hope you can help me with.
Imagine you and I are sat next to a round pool of water. In the middle is bird that's swimming around, flapping it's wings, disturbing the water. We both observe very similar ripples in the water. However, if I place a very thin sheet of plastic in the water, a baffle, the pattern that you observe is now different! (It's thin so it doesn't cause it's own ripples).
If I take this baffle in and out of the water in a discernible pattern, I could send you a message. The hard part is for you to decode when my baffle is in or out of the water. You don't just notice one or two ripples are affected, but over a few you can tell when the baffle has been placed and removed.
What I don't quite understand is how you can tell. In the analogy, you can make assumptions about the normal type of pattern, but realistically, how does it work?
- beambot 13y agoThe water analogy is hard to understand. As I said below: At a fundamental level, backscatter modulation is little more than a reflector that modulates its radar cross section. You could imagine building massive spinning reflectors to serve the same purpose at very low datarates. Basically, you're looking for the presence (or absence) of reflections over time. Your "baffle" needs to be able to change it's state to change how much of the wave it's reflecting, and the receiver needs to be able to detect the minute changes in the reflections. On the receiving side, you'd have another "duck" that generates waves to exactly cancel out the incoming "big" (direct) waves through destructive interference (but this new duck's waves don't propagate to the rest of the system). Then, all you're left with at the receiver are the small reflection waves, which you measure changes in over time. [See, it's hard to visualize for water.] For RF signals, this is done by mixing with a local copy of the carrier, low-pass (or bandpass) filtering the baseband signals, and then running symbol correlators. For ambient backscatter... I presume they have to do carrier recovery for their own LO.
- Swannie 13y agoThanks... I appreciate the explanation. It's kind of what I thought. I still don't quite understand how I can cancel out the carrier. I have an incoming signal, that includes the backscatter. Do I average out the signal to get the carrier, use that averaged signal to cancel out the non-averaged signal - ending up with a version of the signal, and then apply a filter to this signal (to remove anything else that is causing weaker back scatter/multi-path interference), and hope that the intentional back scatter survives?
- beambot 13y agoWaves as sinusoids can physically (or electrically or mathematically) cancel each other out. Look up constructive and destructive interference.