Generalized Kramers-Kronig receiver for coherent terahertz communications

Modern communication systems rely on efficient quadrature amplitude modulation formats that encode information on both the amplitude and phase of an electromagnetic carrier. Coherent detection of such signals typically requires complex receivers that contain a continuous-wave local oscillator as a phase reference and a mixer circuit for spectral down-conversion. In optical communications, the so-called Kramers-Kronig scheme has been demonstrated to simplify the receiver, reducing the hardware to a single photodiode(1-3). In this approach, a local-oscillator tone is transmitted along with the signal, and the amplitude and phase of the complex signal envelope are digitally reconstructed from the photocurrent by exploiting their Kramers-Kronig-type relation(4-6). Here, we transfer the Kramers-Kronig scheme to high-speed wireless communications at terahertz carrier frequencies. To this end, we generalize the approach to account for non-quadratic receiver characteristics and employ a Schottky-barrier diode as a nonlinear receiver element. Using 16-state quadrature amplitude modulation, we transmit a net data rate of 115 Gbit s(-1)at a carrier frequency of 0.3 THz over a distance of 110 m. The Kramers-Kronig approach is applied to high-capacity, free-space terahertz communications, bringing a greatly simplified receiver design.