# On Secrecy Capacity of Binary Beampointing Channels with Block Memory and Feedback

## TL;DR

This paper analyzes the secrecy capacity of a binary beampointing channel with block memory and feedback, providing bounds and a novel scheme that enhances secure millimeter-wave communications.

## Contribution

It introduces a closed-form upper bound and a joint communication and adaptive sensing scheme for secure mmWave systems with feedback.

## Key findings

- The upper bound is based on the main channel capacity.
- The proposed JCAS scheme improves secrecy rates.
- Simulation shows bounds converge with longer block lengths.

## Abstract

This paper investigates the secrecy capacity of the binary beampointing (BBP) channel with block memory and feedback, a simplified yet insightful model for millimeter-wave (mmWave) systems with beamformed transmissions and backscatter feedback. We consider a system where a legitimate receiver and a passive eavesdropper experience independent and uniformly distributed angular directions over transmission blocks, with the base station receiving noiseless, unit-delayed feedback from both, under the per-symbol input cost constraints. We establish a closed-form upper bound on the secrecy capacity, which is based on the main channel between the base station and the legitimate receiver. Moreover, we propose a joint communication and adaptive sensing (JCAS) scheme and derive its achievable secrecy rate. Simulation results show that the gap between the inner and outer bounds narrows as the number of block length increases. This reveals the efficiency of this JCAS scheme, which strategically leverages feedback to balance the demands of sensing the legitimate user and preventing information leakage to the eavesdropper.