Abstract
A multicarrier-division duplex (MDD)-based cell-free (CF) scheme, namely
MDD-CF, is proposed, which enables downlink (DL) data and uplink (UL) data or
pilots to be concurrently transmitted on mutually orthogonal subcarriers in
distributed CF massive MIMO (mMIMO) systems. To demonstrate the advantages of
MDD-CF, we firstly study the spectral-efficiency (SE) performance in terms of
one coherence interval (CT) associated with access point (AP)-selection, power-
and subcarrier-allocation. Since the formulated SE optimization is a
mixed-integer non-convex problem that is NP-hard to solve, we leverage the
inherent association between involved variables to transform it into a
continuous-integer convex-concave problem. Then, a quadratic transform
(QT)-assisted iterative algorithm is proposed to achieve SE maximization. Next,
we extend our study to the case of one radio frame consisting of several CT
intervals. In this regard, a novel two-phase CT interval (TPCT) scheme is
designed to not only improve the SE in radio frame but also provide consistent
data transmissions over fast time-varying channels. Correspondingly, to
facilitate the optimization, we propose a two-step iterative algorithm by
building the connections between two phases in TPCT through an iteration
factor. Simulation results show that, MDD-CF can significantly outperform
in-band full duplex (IBFD)-CF due to the efficient interference management.
Furthermore, compared with time-division duplex (TDD)-CF, MDD-CF is more robust
to high-mobility scenarios and achieves better SE performance.