Page 124 - ITU Journal Future and evolving technologies – Volume 2 (2021), Issue 2

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

requirements in MIMO scenarios are as follows. For the case architectures together with the presented joint design of
where the analog canceler is based on multiple taps, an analog cancellation and TX/RX digital BF are capable of
extension to MIMO requires at least taps with achieving higher rates with less hardware compared to

and denoting the number of RX and TX antennas, re‑ State‑of‑the‑Art (SotA) FD MIMO solutions. This paper’s

spectively, at an FD MIMO node . For the case where the contributions can be summarized as follows.
analog canceler is based on multiple AUX TX RF chains, an
• We provide a detailed overview of two recent ana‑
extension to MIMO requires at least AUX TXs. Con‑

sequently, depending on the number of TX and RX anten‑ log SI canceler architectures, one based on multi‑
ple taps and another one consisting of multiple AUX
nas at the FD MIMO node, the extension of SISO analog

TX RF chains. Both architectures include networks
canceler solutions to the MIMO case may be prohibitively
of MUXs/DEMUXs intended for ef icient signal rout‑

complex. Thus, some works have proposed only digital SI

ing between either the taps or AUX TXs and the
mitigation for FD MIMO [4, 7]. These approaches ex‑

transceiver antennas.
ploit the availability of multiple antennas at the FD node
in order to provide SI mitigation via digital BeamForming
• We present a general optimization framework for
(BF); such an approach is known as spatial suppression.
the joint design of analog SI cancellation and digital
However, as has been pointed out, spatial suppression ap‑
transceiver BF at FD MIMO nodes.
proaches often result in lower rates for both the outgoing
and incoming signals of interest, since some of the avail‑ • We present an example algorithmic design for the
able spatial DoF are solely devoted for mitigating SI.
analog cancellation parameters as well as the digital

In this paper, we present an overview of the recent ar‑

TX precoder and RX combiner that targets the maxi‑

chitectures of [17, 18, 19] for analog SI cancellation in
mization of the FD sum rate performance.

FD MIMO systems and present an optimization frame‑

work for jointly designing the analog canceler and the • Extensive simulation results incorporating realistic

TX/RX digital BF parameters. The irst architecture for models for non‑ideal hardware for both cosidered
analog cancellation consists of multi‑tap hardware, where analog canceler architectures are presented. We

the number of taps does not increase with the number compare both designed low complexity FD MIMO

of TX or RX antenna elements. The second architec‑ schemes with lately available ones in terms of hard‑

ture includes AUX TX RF chains whose number does not ware complexity and achievable performance.
again depend on the number of TX or RX antennas. The

number of taps in the one architecture and that of AUX • We discuss recent STAR schemes capitalizing on the

TXs in the other can be chosen line as a function of presented FD MIMO hardware architectures, and

size constraints, cost per tap and cost per AUX TX RF present a list of open challenges and research direc‑
chain, or other constraints on the analog canceler hard‑ tions for future FD MIMO communication systems, as

ware. Both simpli ied analog canceler architectures are well as their promising applications.
enabled via the use of MUltipleXers (MUXs) and DEMUlti‑
pleXers (DEMUXs), which allow lexible connectivity be‑ The outline of the paper is as follows. The considered sys‑

tween the taps or AUX TXs and the transceiver anten‑ tem and signal models are included in Section 2, whereas
nas. The settings of taps or AUX TXs and the con igu‑ Section 3 details two analog SI cancellation architectures.

rations of MUXs/DEMUXs is computed via the presented Our general optimization framework for FD MIMO oper‑

optimization framework. The lexible signal routing via ation is provided in Section 4, and Section 5 presents an
MUXs/DEMUXs enables the use of reduced taps or AUX example design optimization formulation together with a

TXs in an optimized way, since either of the latter will be detailed low complexity solution. Simulation results are

used between the subset of TX and RX antennas where presented and explained in Section 6, while Section 7 dis‑

they are mostly icial. The digital beamformer and cusses recent applications of the considered FD MIMO ar‑

analog canceler parameters are thus designed by taking chitectures together with open research challenges for FD
into account each other’s capabilities, hence, the burden systems. The paper is concluded in Section 8.
of SI mitigation is split between digital BF and analog can‑ Notation: Vectors and matrices are denoted by boldface
cellation. We note that the related work [4] has consid‑ lowercase and boldface capital letters, respectively. The

ered joint design of digital BF and analog cancellation, transpose and Hermitian transpose of A are denoted by
T
H
however these and related solutions [20, 21] assume un‑ A and A , respectively, and det(A) is the determinant
derlying analog canceler hardware as in [13, 6, 15, 16, 14], of A, while I ( ≥ 2) is the × identity matrix and

which scales with the number of transceiver antennas. 0 × ( ≥ 2 and ≥ 1) represents the × ma‑

For the JointNull solution proposed in [22], although the trix with all zeros. ‖a‖ stands for the Euclidean norm of
number of analog cancelers does not necessarily scale a, operand ⊙ represents the Hadamard entry‑wise prod‑

with the number of antennas, the underlying architecture uct, and diag{a} denotes a square diagonal matrix with
of the canceler (i.e., number of taps or AUX TXs) is not ex‑ a’s elements in its main diagonal. [A] , [A] ( ,∶) , and [A] (∶, )
,

plicitly taken into account in the BF design. As our sim‑ represent A’s ( , )‑th element, ‑th row, and ‑th column,
ulation results will show, the reviewed analog canceler respectively, while [a] denotes the ‑th element of a. ℝ

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