Page 136 - ITU Journal Future and evolving technologies – Volume 2 (2021), Issue 2

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 2

(a) = = 1. (b) = = 4.
Fig. 7 – Average rates versus DL TX power and for UL TX power = [dBm] − 20 [dB] for = = 4.

7. RECENT FD MIMO APPLICATIONS AND product of the number of transmit beams and the num‑
FUTURE DIRECTIONS ber of RX RF chains, and the interplay between digital BF
and analog cancellation for improving the design degree
In this section, we brie ly introduce recent indica‑ of freedoms was thoroughly investigated.
tive STAR schemes leveraging the presented FD MIMO Capitalizing on the presented reduced‑complexity STAR

hardware architecture with multi‑tap cancellation, and architectures, a transmission scheme exploiting chan‑

present open challenges and research directions for fu‑ nel reciprocity for joint downlink beamformed informa‑
ture FD MIMO communication systems. tion data communications and uplink channel estimation,

through training data transmission, was presented ini‑

7.1 STAR applications tially for a two‑way FD MIMO system in [39] and then for

multiple users in [40]. Very recently, the authors in [41]
The performance of the presented framework in the pres‑ leveraged the FD massive MIMO framework with hybrid
ence of hardware imperfections, TX IQ imbalance, non‑ A/D BF of [35] and presented a direction‑assisted beam
linear power ampli ication, and imperfect Channel State management scheme for millimeter Wave (mmWave

Information (CSI) was recently investigated in [25, 34]. communications. In this scheme, UL direction‑of‑arrival

In particular, aiming at suppressing the residual linear estimation and DL analog BF were realized simultane‑

and nonlinear SI signal below the noise loor, [25] pre‑ ously, aiming at reducing the control information ex‑

sented a digital SI cancellation technique that was jointly change overhead that impacts spectral ef iciency.
designed with the con iguration of the analog taps and
digital BF. In [34], the authors focused on two‑way FD 7.2 Open challenges and future directions

MIMO systems and designed BF schemes, considering
models for imperfect hardware and CSI errors. Very re‑ The performance of the presented architectures has been
cently in [23], a multi‑tap wideband analog SI canceler investigated for ideal selection networks in the analog SI
architecture, whose number of taps does not scale ei‑ canceler, i.e., for ideal MUXs/DEMUXs of arbitrary val‑

ther with the number of transceiver antennas or the num‑ ues for the analog cancellation units. Additionally,

ber of multipath SI components, was designed for wide‑ the available operation algorithms are suboptimal requir‑
band FD MIMO communication systems with Orthogo‑ ing an of line investigation for the adequate value for .

nal Frequency‑Division Multiplexing (OFDM). Therein, an Therefore, further studies are needed considering selec‑

adaptive digital SI cancellation technique was presented tion networks with practical losses and feasible num‑
that reduces the residual SI signal estimation parameters. bers for the MUXs/DEMUXs sizes, as well as realistic cor‑
The presented FD MIMO architecture with multi‑tap can‑ related channels for planar antenna arrays. Additionally,

cellation was recently extended in [35] for transceivers the insertion loss induced in practice by each analog tap
capable of realizing hybrid A/D BF [33]. The proposed needs to be accurately characterized, since this will de‑

analog SI canceler interconnects a subset of the outputs termine the feasible upper bound in the number of taps
of the TX RF chains to a subset of the inputs to the RX RF comprising the analog canceler. Furthermore, optimized

chains in order to ensure that the residual SI signal after algorithms with low computational complexity for multi‑
A/D TX precoding and analog RX combining remains be‑ user and/or multi‑cell simultaneous UL and DL opera‑

low the noise loor. Very recently, in [36, 37, 38], it was tions need to be derived. To this end, the FD operation,

shown that the number of analog taps can be scaled to the which inevitably contributes to the overall network

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