IEICE Communications Express, Vol.6, No.6, 363–368

Special Cluster on Antennas and Propagation Technologies
in Conjunction with Main Topics of ISAP2016

Rotationally-symmetricalarray for self-interference
reduction in full-duplex MIMO
Masakuni Tsunezawa1a), Naoki Honma1, Kazuya Takahashi1,
Kentaro Murata2, and Kentaro Nishimori3
1

Graduate School of Engineering, Iwate University,

4–3–5 Ueda, Morioka 020–8551, Japan
2

Graduate School of Science and Engineering, National Defense Academy,

1–10–20 Hashirimizu, Yokosuka, Kanagawa 239–8686, Japan3

Faculty of Engineering, Niigata University,8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950–2181, Japana) lab0392@gmail.com

Abstract: In order to realize a full-duplex wireless communication, theauthors have proposed a spatial self-interference reduction technique with an
end-ﬁre array (EFA) which consumes only one degree of the multiple-input
multiple-output (MIMO) freedom. However, this technique has a limited
interference reduction performance due to an insuﬃcient rank-degenerated
interference channel because amplitude diﬀerences of the paths increase therank of the channel in the EFA. In this letter, the authors propose arotationally-symmetrical array (RSA) suitable for our conventionally proposing scenario to reduce the self-interference, where geometrical symmetry of
the MIMO paths achieves a perfectly one-ranked channel and suppresses allof the self-interference in theory. Performance evaluation on the interference
reduction level PBF clariﬁes that the RSA reduces more interference than theEFA: PBF = 47.3 dB (RSA), 23.0 dB (EFA) while array length of the transmitter (Tx), element spacing of the receiver (Rx), and distance between theTx and Rx are 3.5 wavelengths, 0.5 wavelengths, and 5 wavelengths,
respectively.
Keywords: MIMO, full-duplex, interference reduction, rotationally-symmetrical array, eigen-beamforming
Classiﬁcation: Antennas and Propagation
References© IEICE 2017
DOI: 10.1587/comex.2016SPL0040
Received January 18, 2017
Accepted February 2, 2017
Publicized March 13, 2017
Copyedited June 1, 2017

[1] A. Sabharwal, P. Schniter, D. Guo, D. W. Bliss, S. Rangarajan, and R. Wichman,“In-band full-duplex wireless: challenges and opportunities,” IEEE J. Sel. AreasCommun., vol. 32, no. 9, pp. 1637–1652, Sep. 2014. DOI:10.1109/JSAC.2014.
2330193[2] S. Hong, J. Brand, J. Choi, M. Jain, J. Mehlman, S. Katti, and P. Levis,“Applications of self-interference cancellation in 5G and beyond,” IEEECommun. Mag., vol. 52, no. 2, pp. 114–121, Feb. 2014. DOI:10.1109/MCOM.

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2014.6736751[3] E. Everett, A. Sahai, and A. Sabharwal, “Passive self-interference suppression
for full-duplex infrastructure nodes,” IEEE Trans. Wireless Commun., vol. 13,
no. 2, pp. 680–694, Jan. 2014. DOI:10.1109/TWC.2013.010214.130226[4] E. Aryafar, M. A. Khojastepour, K. Sundaresan, S. Rangarajan, and M. Chiang,“MIDU: enabling MIMO full duplex,” Proceedings of the 18th Annual
International Conference on Mobile Computing and Networking in ACM,
pp. 257–268, Aug. 2012. DOI:10.1145/2348543.2348576[5] M. Tsunezawa, N. Honma, K. Takahashi, Y. Tsunekawa, K. Murata, and K.
Nishimori, “Interference reduction between SDD linear array antennas using
end-ﬁre arrangement,” 2015 IEEE International Symposium on Antennas andPropagation & USNCURSI National Radio Science Meeting, pp. 2511–2521,
Jul. 2015. DOI:10.1109/APS.2015.7305644

1Introduction

In-band full-duplex radio [1, 2], where terminals simultaneously transmit andreceive on the same frequency band, doubles channel capacity of the conventionalfrequency division duplex (FDD) communication by sharing spectrum resources
which are conventionally used for both transmitted and received signals. Fullduplex stations, however, suﬀer from an interference transmitted by themselves(self-interference). This loud interference hides whispering desired signals fromother terminals. Some spatial approaches try to reduce the interference by uses ofantenna directivities [3], and spatial compositions of signals [4]. [3] places transmitting and receiving antennas so that beams are not directed at each other, which
unfortunately separate the station’s coverage for transmission and reception. [4]controls phases of signals to be transmitted and urges them to cancel at thereceiving antennas. This is capable of multiple-input multiple-output (MIMO)
transmission; however, it wastes many degrees of freedom (DoFs) for MIMO
transmission because of making cancellation signals. To this end, the authors haveproposed a combination technique of an end-ﬁre array (EFA) and eigen-nullsteering, which reduces the interference by up to 75.2 dB while consuming only
one DoF [5]. Since this technique relies on the characteristics of interference
channel degenerated by the EFA, the defect in a rank-degeneracy causes a limited
interference reduction performance because amplitude diﬀerences among the paths
increase the rank of the channel. In this letter, the authors propose a rotationallysymmetrical array (RSA) for [5]’s scenario, where geometrical symmetry of pathsachieves a perfectly one-ranked channel and suppresses all of the self-interferencein theory. A principle of the proposed technique is discussed; its interference
reduction performance is calculated and compared with that of the EFA in orderto clarify an eﬀect of the symmetrical approach on the scenario.
2© IEICE 2017
DOI: 10.1587/comex.2016SPL0040
Received January 18, 2017
Accepted February 2, 2017
Publicized March 13, 2017
Copyedited June 1, 2017

Self-interference reduction technique using RSAFig. 1(a) shows a full-duplex system model dealt with in this letter, where theseparated transmitter (Tx) and receiver (Rx) of the base station use the same
frequency f0 . Signals via the self-interference channel Hi interfere with the uplink364

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signal at Rx from the user terminal. Fig. 1(b) describes a concept of a selfinterference reduction technique proposed in [5], where MT and MR denote anumber of antenna elements in Tx and Rx, respectively, Ni is the maximum numberof interference eigenvalues. wiTðnÞ (1  n  MT ) and iðkÞ (1  k  Ni ) mean the nthtransmitting weight and the kth eigenvalue in descending order. The techniqueinduces the rank degeneration of Hi e.g. by careful antenna arrangement, and thennulls the dominant interfering mode by Tx’s eigen-beamforming technique. This
scenario provides the signiﬁcant interference reduction with a cost of only one DoF.
Fig. 1(c) shows a model of a rotationally-symmetrical array antenna, which is a key
idea of this letter. An uniform linear array (ULA) as the Rx array is located on the yaxis, then an uniform circular array (UCA) as the Tx array is placed in the zx planeto form a circle around the y axis, where the circumference of the Tx-UCA and theelement spacing of the Rx-ULA are deﬁned by lTx and dRx , respectively, distance
between the Tx-UCA and Rx-ULA is DTx­Rx . By means of this, all of elements in
the Tx have a rotational symmetry about each element in the Rx. Assume all of theantenna elements are point wave sources in a line-of-sight (LOS) far ﬁeld eachother, and there is no mutual coupling in both Tx and Rx arrays. Element patternsof the Tx-UCA comprehensively make a rotational symmetry about y axis, and allof element patterns in the Rx-ULA also have the symmetry. Then an ideal selfinterference channel matrix Hi is written as32hið1Þ    hið1Þ
6 .
.. 77
ð1Þ
Hi ¼ 6

. 5;
4 ..
hiðMR ÞhiðMR Þwhere hiðmÞ (1  m  MR ) means a path from an arbitrary Tx element to the mth Rxelement. Because all of the components of the correlation matrix Hi H Hi are same,the eigenvalues are(MT ðjhið1Þ j2 þ    þ jhiðMR Þ j2 Þ ðk ¼ 1Þ:ð2ÞiðkÞ ¼
0ðotherÞ

© IEICE 2017
DOI: 10.1587/comex.2016SPL0040
Received January 18, 2017
Accepted February 2, 2017
Publicized March 13, 2017
Copyedited June 1, 2017

This claims that the ideal RSA absolutely degenerates the rank of the selfinterference channel matrix, and that an eigen-beamforming to null the ið1Þ
suppresses all of the interference. According to a structure of (1), the Tx’s 1st
weight vector wiTð1Þ has uniform components, which means that the dominantinterfering mode of the RSA is always realized by Tx’s in-phase excitation.
Therefore, all of the eigen-weight vectors to suppress the interference are orthogonal to the in-phase excitation. When the number of the Tx MT is a power of two,
the weight vectors can be picked up from Haramard matrices or discrete Fouriertransform (DFT) matrices, i.e. the eigen-null-steering can be performed by not onlya digital-beamforming (D-BF) whose weights are given by a singular value
decomposition (SVD) of a simulated/measured Hi , but also an analog-beamforming (A-BF) simply realized by combinations of 180-degree hybrid couplers andﬁxed phase-shifters for the self-interference reduction of the RSA. The A-BF has anadvantage to reduce a channel estimation process which takes some time and adds
some calculation loads.

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IEICE Communications Express, Vol.6, No.6, 363–368(a) Self-interference problem in fullduplex communication system.

(b) Proposed concept to reduce interference.

(c)
Proposedrotationallysymmetrical array antenna.

Fig. 1.

3

Conceptual sketches.

Simulation

3.1 Numerical parameters
Fig. 2(a)(b) show simulation models of (a) the RSA proposed in this letter, and(b) the EFA conventionally proposed in [5] for reference. Simulation is carried out
by the method of moments. The number of the Tx and Rx elements are set toMT ¼ MR ¼ 8, and all of the antenna elements are vertical dipole antennas. Thecircumference of the Tx-UCA and the element spacing of the Rx-ULA are set tolTx ¼ ð8  1Þ0 =2 and dRx ¼ 0 =2, respectively. Here, by regarding the circumference of the RSA’s Tx-UCA as the array length, the end-to-end distance of theEFA’s Tx-ULA is also given as lTx . The distance between the Tx and Rx-ULA(a) Roratationally-symmetrical array (RSA; proposed)© IEICE 2017
DOI: 10.1587/comex.2016SPL0040
Received January 18, 2017
Accepted February 2, 2017
Publicized March 13, 2017
Copyedited June 1, 2017(b) End-fire array (EFA; conventional).

Fig. 2.

Simulation models.

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DTx­Rx is varied from 0 (the Rx-ULA is in the circle made by the Tx-UCA) to 200in the RSA, and from 40 (one ULA is most likely to contact with the other) to 200in the EFA. In this letter, the interference power by the nth Tx port piðnÞ(1  n  MT ) is calculated bypiðnÞ ¼ jHi wiTðnÞ j2 ;ð3Þand the interference reduction level PBF is deﬁned byPBF ¼

pið1Þ þ    þ piðMT Þ:
pið2Þ þ    þ piðMT Þ

ð4Þ

3.2 Numerical results
Fig. 3(a) shows the interference power by each D-BF port in the RSA, where wiT isgiven by the SVD of the simulated Hi : piðnÞ ¼ iðnÞ . Curves corresponding to modes
lower than the 5th are too small to display in this range. This result points theextreme rank degeneration of the RSA. The more DTx­Rx increases, the more deeply
Hi degenerates. Fig. 3(b) presents the interference power by each A-BF port in theRSA, where wiT is the power-normalized Hadamard matrix without the in-phase
excitation. This result claims that the A-BF can be adopted to reduce the interference, however, and then leaks ið1Þ out to ports lower than the 1st. Increasing DTx­Rxsaturates the improvement of the interference reduction eﬀect by itself. Fig. 3(c)indicates the comparison of the interference reduction levels between the RSA andEFA, where the RSA expects scenarios using the D-BF and A-BF while the EFA
only uses the D-BF. Comparing their curves, the RSA is more eﬀective for theinterference reduction by the D-BF than the EFA. A beneﬁt of the A-BF is saturated
up to about PBF ¼ 12 dB. In DTx­Rx ¼ 50 , PBF by the RSA using the D-BF, A-BF(a) Interference power by each DBF port.

© IEICE 2017
DOI: 10.1587/comex.2016SPL0040
Received January 18, 2017
Accepted February 2, 2017
Publicized March 13, 2017
Copyedited June 1, 2017(b) Interference power by each A-BF port.

(c) Comparison of interference reduction levels between RSA andEFA.

Fig. 3.

Interference vs. distance between Tx and Rx.

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and the EFA using the D-BF are 47.3 dB, 12.1 dB, and 23.0 dB, respectively. The
PBF deterioration of the RSA in the small DTx­Rx is mainly caused by directional
element patterns in vertical (yz) plane.
4Conclusion

In this letter, the authors have proposed an RSA suitable for full-duplex station,
where geometrical symmetry of the paths achieves a perfectly one-ranked channeland suppresses all of the self-interference in theory. A principle of the proposed
technique was discussed with the derivation of the self-interference eigenvalues inthe RSA; its interference reduction performance was calculated using two scenariosof the D-BF and A-BF, and compared with that of the EFA using the D-BF. RSA
resulted PBF ¼ 47:3 dB (D-BF) and 12.1 dB (A-BF), whereas EFA resulted
PBF ¼ 23:0 dB (D-BF). This clariﬁed the eﬀect of the RSA on the interference
reduction performance of the scenario.
Acknowledgments
This research and development work was supported by the MIC/SCOPE#155002002.

© IEICE 2017
DOI: 10.1587/comex.2016SPL0040
Received January 18, 2017
Accepted February 2, 2017
Publicized March 13, 2017
Copyedited June 1, 2017368