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In this paper, we present a comparative study of half-duplex (HD) accesspoints (APs) with dynamic time-division duplex (DTDD) and full-duplex (FD) APsin cell-free (CF) systems. Although both DTDD and FD CF systems supportconcurrent downlink transmission and uplink reception capability, the sumspectral efficiency (SE) is limited by various cross-link interferences. Wefirst present a novel pilot allocation scheme that minimizes the pilot lengthrequired 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 thesignal-to-interference plus noise ratio optimal weighting at the centralprocessing unit. We also present a provably convergent algorithm for jointuplink-downlink power allocation and uplink/downlink mode scheduling of the APs(for DTDD) to maximize the sum SE. Further, the proposed algorithms areprecoder and combiner agnostic and come with closed-form update equations forthe UL and DL power control coefficients. Our numerical results illustrate thesuperiority of the proposed pilot allocation and power control algorithms overseveral benchmark schemes and show that the sum SE with DTDD can outperform anFD CF system with similar antenna density. Thus, DTDD combined with CF is apromising alternative to FD that attains the same performance using HD APs,while obviating the burden of intra-AP interference cancellation.

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