Papers for
antenna array designers
Papers whose findings have a practical use for this group, as judged from the abstract. Open a paper to read what it means in practice.
Channels rotate backpropagation gradients causing errors in training
Coherent but Discordant: Rotate, Don't Invert, to Backpropagate Through an Aging Channel
Abstract: A channel can be almost perfectly coherent and still teach a transmitter the wrong direction. In physical ("over-the-air") backpropagation, the receiver sends its loss gradient back through the reciprocal channel; when the terminal moves in between, mobility does not merely add noise to the returned gradient, it rotates it. Under a scalar conditional mean model the rotation is set by the phase of the temporal correlation, so coherence, which measures only its magnitude, is the wrong ruler: in simulation, channels with absolute value of rho greater than 0.9 drive uncorrected training to 51.7% accuracy, while phase correction restores 92.5%. Arrays add a twist. Near zeros of the trace correlation, scalar correction fails even with rho known exactly, yet most of the information is still there: noise has moved it onto neighbouring antennas. Re-indexing the ports by the displacement, with zero digital feedback, recovers the lost performance in simulation, and so does rotating the returned seed with the unitary polar factor of the conditional mean operator, never inverting it; aligning each antenna's phase with its own past makes things worse. An exact array identity explains all three. The lesson fits in one line: correct the rotation, not the stretch.
Wideband terahertz links face beam squint and phase control limits
Beam Squint and Aperture--Bandwidth Limitations in Wideband RIS-Assisted THz Links
Abstract: This paper analyzes the ergodic rate of a wideband reconfigurable intelligent surface (RIS)-assisted terahertz link under beam squint and finite-resolution phase control. Independent Rayleigh fading on the two RIS-assisted hops produces double-Rayleigh cascaded amplitudes. Exact second- and fourth-order channel moments are derived by incorporating the frequency-dependent array response and phase-quantization errors. Moment matching then provides a Gamma approximation for the received power on each subcarrier and a tractable closed-form approximation for the wideband ergodic rate. A first-null bandwidth approximation characterizes the aperture--bandwidth limitation of frequency-flat RIS control. Monte Carlo simulations validate the analytical framework.