Capacity of Two-User Wireless Systems Aided by Movable Signals

TL;DR

This work analyzes capacity limits of two-user wireless systems aided by movable signals that reconfigure the signal spectrum. It derives MAC and BC capacity regions and shows that a suitably chosen frequency f* can orthogonalize the two user channels, enabling independent per-user rates and simple power-splitting strategies. Achievability is demonstrated via matched filtering or beamforming and a frequency-based orthogonalization condition h_ul,2^H h_ul,1 = 0 (uplink) or h_dl,1 h_dl,2^H = 0 (downlink), with a practical method for frequency-range optimization when f is constrained. Numerical results reveal up to 45% sum-rate gains over fixed signals in limited-frequency scenarios, highlighting movable signals as a viable third dimension for smart radio environments.

Abstract

Movable signals have emerged as a third approach to enable smart radio environments (SREs), complementing reconfigurable intelligent surfaces (RISs) and flexible antennas. This paper investigates their potential to enhance multi-user wireless systems. Focusing on two-user systems, we characterize the capacity regions of the multiple access channel (MAC) and broadcast channel (BC). Interestingly, movable signals can dynamically adjust the operating frequency to orthogonalize the user channels, thereby significantly expanding the capacity regions. We also study frequency optimization, constraining it in a limited frequency range, and show that movable signals provide up to 45% sum rate gain over fixed signals.

Figures (4)

• Figure 1: Two-user multiple access channel (MAC).
• Figure 2: Capacity region of a two-user aided by movable signals, with $P_1/\sigma^2=P_2/\sigma^2=10$ dB.
• Figure 3: Capacity region of a two-user aided by movable signals, with $P/\sigma_1^2=P/\sigma_2^2=10$ dB.
• Figure 4: Sum rate versus the SNR $P/\sigma^2$.