I want to understand the bound on the amount of energy harvested. Logically, at most what we can harvest is the amount of energy transmitted (minus the pathloss). I want to understand the theoretical bounds in term of number of antennas at the energy harvesting side. I mean if I increase the number of antennas to infinite at receiver side then how can we derive the total energy harvested
Hi Mohd, when you mentioned increasing antennas the number of to infinity, it immediately brought to mind massive MIMO.
So, assume for now that we are harvesting the energy from one source. If you have a single receive antenna, and you have destructive interference along the (rayleigh) propagation path, you will harvest potentially nothing.
If you have multiple receive antennas, you may harvest the energy arriving from several directions, depending on the correlation matrix.
If you have a huge number of receive antennas, you get this "channel hardening" condition, where the path loss looks like deterministic, free space path loss. So that would be my answer. From any single source, you can harvest as much energy as determined by free space path loss.
There is the negative aspect of such energy harvesting, of course. Any receiver downstream of your location, in the path from the transmit antenna, will get a very weak signal. Consequently, this energy harvesting practice could be illegal.
RF energy harvesting is a fool's game at best. Put on your thinking cap here. Look at the power densities involved. They are pathetically low.
Maximum available harvested power, assuming all path lengths and delays are equal so that all collected signals add in phase, would be equal to the sum over all of the antennas of the power density at each of the antennas multiplied by each antenna's effective capture area (generally given by Gain*Lambda^2/(4*Pi)) which is usually somewhat less that its physical projected area).
NOTE: Nothing to do with MIMO which refers to using multiple transmit antennas (transmitting "orthogonal" or "separable" waveforms) and two or more receiving antennas each receiving all of the orthogonal or separable waveforms.
Thanks for sharing your insights.
Further, by RF energy harvesting, we are not intending to do large scale energy recycling but just to sustain sensor nodes consuming power in ranges of microwatt to milliwatt (no need to replace their batteries).
So the question is, if I place say infinite antenna, then how to derive the upper bound theoretically?
It might seem that your greatest chance of recovering energy would be at the lower end of the RF spectrum such as from AM and FM signals where the transmitter powers are huge compared to cell and wifi. tHowever this all depends on transmitter placements and whether or not there are nearby transmitters with directional area coverage. I would think it would be wise to first take a wideband spectrum analyzer and sniff the area you tend to use this equipment at for likely RF signal candidates.
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