Half-Duplex APs with Dynamic TDD vs. Full-Duplex APs in Cell-Free Systems

TL;DR

This paper compares half-duplex with dynamic TDD and full-duplex access points in cell-free systems, proposing novel algorithms for pilot allocation and power control that enhance spectral efficiency.

Contribution

It introduces a new pilot allocation scheme and a joint power and mode scheduling algorithm for DTDD in cell-free systems, demonstrating improved performance over benchmarks.

Findings

DTDD can outperform FD in spectral efficiency with similar antenna density.

Proposed algorithms significantly reduce pilot contamination and optimize power control.

Numerical results show the superiority of DTDD with the new algorithms over benchmark schemes.

Abstract

In this paper, we present a comparative study of half-duplex (HD) access points (APs) with dynamic time-division duplex (DTDD) and full-duplex (FD) APs in cell-free (CF) systems. Although both DTDD and FD CF systems support concurrent downlink (DL) transmission and uplink (UL) reception capability, the sum spectral efficiency (SE) is limited by various cross-link interferences. We first present a novel pilot allocation scheme that minimizes the pilot length required to ensure no pilot contamination among the user equipments (UEs) served by at least one common AP. Then, we derive the sum SE in closed form, considering zero-forcing combining and precoding along with the signal-to-interference plus noise ratio optimal weighting at the central processing unit. We also present a provably convergent algorithm for joint UL-DL power allocation and UL/DL mode scheduling of the APs (for DTDD) to maximize the sum SE. Further, the proposed algorithms are precoder and combiner agnostic and come with closed-form update equations for the UL and DL power control coefficients. Our numerical results illustrate the superiority of the proposed pilot allocation and power control algorithms over several benchmark schemes and show that the sum SE with DTDD can outperform an FD CF system with similar antenna density. Thus, DTDD combined with CF is a promising alternative to FD that attains the same performance using HD APs, while obviating the burden of intra-AP interference cancellation.