# Secure Massive MIMO Communication with Low-resolution DACs

## TL;DR

This paper analyzes the impact of low-resolution DACs on secure massive MIMO communication, deriving conditions and strategies to optimize secrecy rates despite quantization noise.

## Contribution

It introduces a Bussgang theorem-based model for low-resolution DACs and derives a SNR threshold and optimal power allocation to enhance secrecy performance.

## Key findings

- Low-resolution DACs can outperform high-resolution ones at low SNR.
- A closed-form SNR threshold determines when low-resolution DACs are advantageous.
- Optimal power allocation can mitigate secrecy rate loss caused by quantization noise.

## Abstract

In this paper, we investigate secure transmission in a massive multiple-input multiple-output (MIMO) system adopting low-resolution digital-to-analog converters (DACs). Artificial noise (AN) is deliberately transmitted simultaneously with the confidential signals to degrade the eavesdropper's channel quality. By applying the Bussgang theorem, a DAC quantization model is developed which facilitates the analysis of the asymptotic achievable secrecy rate. Interestingly, for a fixed power allocation factor $\phi$, low-resolution DACs typically result in a secrecy rate loss, but in certain cases they provide superior performance, e.g., at low signal-to-noise ratio (SNR). Specifically, we derive a closed-form SNR threshold which determines whether low-resolution or high-resolution DACs are preferable for improving the secrecy rate. Furthermore, a closed-form expression for the optimal $\phi$ is derived. With AN generated in the null-space of the user channel and the optimal $\phi$, low-resolution DACs inevitably cause secrecy rate loss. On the other hand, for random AN with the optimal $\phi$, the secrecy rate is hardly affected by the DAC resolution because the negative impact of the quantization noise can be compensated for by reducing the AN power. All the derived analytical results are verified by numerical simulations.