We have produced an analysis of the communication concept based on frequency tuning to exploit atmospheric resonances. The concept is that, while free-space path loss increases at a constant rate of 20 dB per decade of propagation range, the attenuation due to absorption by water vapor molecules increases exponentially, and therefore can dominate the overall loss. A transmitter (Alice) can therefore choose her transmit power and carrier frequency (relative to the peak of one of the water vapor absorption resonances) in order to ensure that the intended receiver (Bob) can receive the signal, but that an eavesdropper (Eve) located along the same line of sight but farther away, cannot. We show that this method for securing communications is extremely effective as long as Eve is roughly twice as far from Alice as is Bob (or farther). However, the trade-off is that tuning close to an absorption resonance also impacts the channel capacity from Alice to Bob. As an example, for dEve/dBob = 2, a sacrifice of about 40% in channel capacity can result in a normalized secrecy capacity greater than 80% for Alice and Bob. This computational study is now being followed up by experimental studies. A manuscript is in preparation.
Publications
Zhaoji Fang, Malachi Hornbuckle, and Daniel M. Mittleman “Secure communication channels using atmosphere-limited line-of-sight terahertz links,” in preparation (2021).