  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.
