12.2 A mm-Wave/Sub-THz Synthesizer-Free Coherent Receiver with Phase Reconstruction Through Mixed-Signal Kramer-Kronig Processing

High frequency wireless systems operating in the high mmWave and sub-THz frequencies can enable new applications in communication, sensing and imaging, if they can operate with low latency in high resource-constrained environments. In particular, for one-to-many network nodes where the receivers can be highly energy-limited, processing traditional spectrally-efficient signals that utilize both amplitude and phase modulation (e.g., QPSK, 16/64-QAM etc.) places a significant power burden on highfidelity frequency and phase synthesis, and Tx-to-Rx synchronization at the receiver (Rx) [1,2]. For such coherent communication, the Rx needs to implement its own phaselocked loop with integer/fractional frequency synthesis, carrier recovery and synchronization, and subsequent phase alignment. At high frequencies, the strict power and latency requirements for such complex synchronization make many critical low power applications infeasible. Non-coherent communication based on on-off keying alleviates the synchronization issue, but comes with a significant penalty for spectral efficiency. In addition to the burden of frequency synthesis, for directional links, LO distribution (and buffering) to all Rx elements can dissipate significant power (comparable to all elements combined). In this paper, we present a proof-of-concept, synthesizer-free, coherent mmWave/sub-THz Rx architecture that eliminates the need for frequency synthesis for coherent demodulation. By optimally designing the transmitted spectrum, we enforce an analytical condition on the amplitude and phase functions of the baseband signal. This condition, popularly known as Kramer-Kronig, allows us to estimate the phase of the signal from its amplitude information (via a simple envelope detector) through a Hilbert transform. While this was first proposed in [3] and demonstrated recently in optical/THz communication [4,5], the reconstruction was achieved with dedicated high-speed, power-hungry commercial components and DSPbased Rx, defeating the low-power nature of such Rx. Here, we demonstrate an end-to-end synthesizer-free Rx architecture with low power envelope detection, analog signal processing, and broadband Hilbert N-path FIR filter. With the proposed phase reconstruction scheme, we successfully demonstrate 16-QAM links operating at Gbps. The architecture is translatable to Rx operating close to and beyond fmax.