Optical versus RF Free-Space Signal Transmission: a Comparison of Optical and RF Receivers based on Noise Equivalent Power and Signal-to-Noise Ratio

To compare the power efficiency of free-space signal transmission over an optical carrier with that of signal transmission over an RF carrier, we compare the signal to noise ratio and the noise equivalent power of various optical detection schemes with that of simple antenna reception of an RF signal. We point out that for direct optical detection schemes, the noise equivalent power for optical detection is orders of magnitude larger than for RF signal detection with an antenna, with only coherent optical detection requiring similar power levels at the receiver as RF detection with an antenna and mixer, for the same available bandwidth. This is often ignored in articles in which the relative advantages of optical and RF transmission are discussed. Optical transmission mostly has advantages when very high capacity is required and where the available RF bandwidth is small.