Page 123 - ITU Journal Future and evolving technologies – Volume 2 (2021), Issue 2

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

LOW COMPLEXITY FULL DUPLEX MIMO SYSTEMS:
ANALOG CANCELER ARCHITECTURES, BEAMFORMING DESIGN, AND FUTURE DIRECTIONS

George C. Alexandropoulos
Department of Informatics and Telecommunications, National and Kapodistrian University of Athens
Panepistimiopolis Ilissia, 15784 Athens, Greece

NOTE: Corresponding author: George C. Alexandropoulos, alexandg@di.uoa.gr

Abstract – The hardware complexity of the analog Self‑Interference (SI) canceler in conventional full duplex Multiple Input
Multiple Output (MIMO) designs mostly scales with the number of transmit and receive antennas, thus exploiting the bene‑
its of analog cancellation becomes impractical for full duplex MIMO transceivers, even for a moderate number of antennas.
In this paper, we provide an overview of two recent hardware architectures for the analog canceler comprising of reduced
number of cancellation elements, compared to the state of the art, and simple multiplexers for ef icient signal routing among
the transceiver radio‑frequency chains. The one architecture is based on analog taps and the other on AUXiliary (AUX) Trans‑
mitters (TXs). In contrast to the available analog cancellation architectures, the values for each tap or each AUX TX and the
con iguration of the multiplexers are jointly designed with the digital transceiver beamforming ilters according to desired
performance objectives. We present a general optimization framework for the joint design of analog SI cancellation and
digital beamforming, and detail an example algorithmic solution for the sum‑rate optimization objective. Our representative
computer simulation results demonstrate the superiority, both in terms of hardware complexity and achievable performance,
of the presented low complexity full duplex MIMO schemes over the relative available ones in the literature. We conclude the
paper with a discussion on recent simultaneous transmit and receive operations capitalizing on the presented architectures,
and provide a list of open challenges and research directions for future FD MIMO communication systems, as well as their
promising applications.
Keywords – Analog cancellation, beamforming, full duplex, MIMO, self‑interference modeling, simultaneous transmit
and receive

1. INTRODUCTION stronger than the power of the received signal of interest
(which is transmitted from another radio). Consequently,
In band full duplex, also known shortly as Full Duplex
SI can severely degrade the reception of the signal of in‑
(FD), is a candidate technology for sixth Generation (6G)
terest, and thus SI mitigation is required in order to max‑
wireless systems because of the potential spectral ef‑
imize the spectral ef iciency gain of the FD operation. As

iciency gains and latency improvements that can be
the number of antennas increases, mitigating SI becomes
achieved through Simultaneous Transmit And Receive
more challenging, since more antennas naturally result

(STAR) operations within the entire frequency band [1,

in more SI components. For the case of a Single Input

2, 3]. An FD radio can transmit and receive at the same
Single Output (SISO) FD node, it has been demonstrated
time and same frequency resource unit, consequently, it [13, 14] that signi icant SI mitigation can be achieved via
can offer increased spectrum usage lexibility compared a combination of Analog and Digital (A/D) cancellation

to a half duplex radio. Current wireless systems exploit techniques, where an estimate of the received SI is sub‑
Multiple Input Multiple Output (MIMO) communications, tracted from the received signal (which is the sum of the
where increasing the number of transmit and receive SI signal and signal of interest). A straightforward exten‑

antennas can increase the spatial Degrees of Freedom sion of SI mitigation solutions used in SISO FD to the case
(DoF), hence boosting spectral ef iciency and link reliabil‑ of MIMO FD can be envisioned. However, the hardware

ity. Combining FD with MIMO communications can pro‑ resources required for analog SI cancellation become the
vide further spectral ef iciency gains [4, 5, 6, 7, 8, 9, 10]. main bottleneck, since they scale with the number of an‑
Thus, enabling FD MIMO technology, for small to large an‑ tenna elements. Speci ically, for the two most widely con‑
tenna array systems, is of high interest in order to achieve sidered analog canceler solutions, which are: i) the ar‑

the demanding throughput, latency, and sensing require‑ chitecture based on taps (a tap consists of analog com‑

ments of ifth Generation (5G) and beyond wireless com‑

ponents that implement delay, phase shift, and attenu‑

munication systems [11, 12].
ation) [13, 15]; and ii) the architecture based on AUXil‑
An FD radio suffers from Self Interference (SI), which is
iary (AUX) TX Radio‑Frequency (RF) chains (an AUX TX
the signal transmitted by the FD radio Transmitter (TX)
RF chain generates an analog cancellation signal from an
that leaks to the FD radio Receiver (RX). At the RX of the input digital reference signal) [14, 16], the hardware
FD radio, the power of the SI signal can be many times
© International Telecommunication Union, 2021 109