# Multiple Access Wiretap Channel with Noiseless Feedback

## TL;DR

This paper investigates the secrecy capacity of a multiple access wiretap channel with noiseless feedback, proposing new bounds that enhance understanding of secure communication in such systems.

## Contribution

It introduces a decode-and-forward inner bound, a hybrid inner bound with secret keys, and an outer bound for the secrecy capacity region of the MAC-WT with noiseless feedback.

## Key findings

- The hybrid inner bound exceeds the decode-and-forward bound.
- The feedback enables cooperation and secret key generation.
- Bounds are demonstrated through a Gaussian example.

## Abstract

The physical layer security in the up-link of the wireless communication systems is often modeled as the multiple access wiretap channel (MAC-WT), and recently it has received a lot attention. In this paper, the MAC-WT has been re-visited by considering the situation that the legitimate receiver feeds his received channel output back to the transmitters via two noiseless channels, respectively. This model is called the MAC-WT with noiseless feedback. Inner and outer bounds on the secrecy capacity region of this feedback model are provided. To be specific, we first present a decode-and-forward (DF) inner bound on the secrecy capacity region of this feedback model, and this bound is constructed by allowing each transmitter to decode the other one's transmitted message from the feedback, and then each transmitter uses the decoded message to re-encode his own messages, i.e., this DF inner bound allows the independent transmitters to co-operate with each other. Then, we provide a hybrid inner bound which is strictly larger than the DF inner bound, and it is constructed by using the feedback as a tool not only to allow the independent transmitters to co-operate with each other, but also to generate two secret keys respectively shared between the legitimate receiver and the two transmitters. Finally, we give a sato-type outer bound on the secrecy capacity region of this feedback model. The results of this paper are further explained via a Gaussian example.