IEICE Communications Express, Vol.6, No.2, 71–76

Experimental evaluation ofinterference reduction eﬀect;
Eigen-beamforming anddigital subtraction by using
MIMO-OFDM signals
Yoshiyuki Yamamoto1, Ryota Takahashi2†, Masakuni Tsunezawa2,
Naoki Honma2a), and Kentaro Murata3
1

Faculty of Engineering, Iwate University,

4–3–5 Ueda, Morioka 020–8551, Japan
2

Graduate School of Engineering, Iwate University,

4–3–5 Ueda, Morioka 020–8551, Japan
3

Graduate School of Engineering, National Defense Academy,

1–10–20 Hashirimizu, Yokosuka, Kanagawa 239–8686, Japana) honma@iwate-u.ac.jp

Abstract: In this letter, we experimentally validate an interference reduction technique that combines a digital subtraction technique, and eigenbeam-forming with an end-ﬁre arrangement of linear array antennas. The
eigen-beamforming signiﬁcantly reduces the self-interference at the cost ofa slight drop in the degrees-of-freedom of the antenna. We experimentallyevaluate the degree of interference reduction using two linear sleeve-dipole
arrays in end-ﬁre arrangement. This experiment uses MIMO (Multiple-Input
Multiple-Output)-OFDM (Orthogonal Frequency Division Multiplexing)signals to evaluate the realistic performance of this technique. The resultsof the experiment show the combination of eigen-beamforming and digital
subtraction can signiﬁcantly reduce, −76 dB, the interference.
Keywords: full-duplex, interference, beamforming, digital subtraction
Classiﬁcation: Antennas and Propagation

References© IEICE 2017
DOI: 10.1587/comex.2016XBL0175
Received September 29, 2016
Accepted October 25, 2016
Publicized November 22, 2016
Copyedited February 1, 2017[1] 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.
2014.6736751[2] D. Senaratne and C. Tellambura, “Beamforming for space division duplexing,”
Proc. IEEE Int. Conf. Commun., pp. 1–5, Jun. 2011. DOI:10.1109/icc.2011.
5962761[3] N. Li, W. Zhu, and H. Han, “Digital interference cancellation in single channel,
full duplex wireless communication,” 2012 8th Int. Conf. Wireless Commun.,†Ryota Takahashi sadly passed away on 17th Nov. 201671IEICE Communications Express, Vol.6, No.2, 71–76

Networking and Mobile Comput. (WiCOM), pp. 1–4, Sept. 2012. DOI:10.1109/
WiCOM.2012.6478497[4] T. Riihonen and R. Wichman, “Analog and digital self-interference cancellationin full-duplex MIMO-OFDM transceivers with limited resolution in A/D
conversion,” Proc. 46th Asilomar Conf. Signals, Syst. and Comput., pp. 45–49,Nov. 2012. DOI:10.1109/ACSSC.2012.6488955[5] J. I. Choi, M. Jain, K. Srinivasan, P. Levis, and S. Katti, “Achieving singlechannel, full duplex wireless communication,” Proc. 2010 ACM MobiCom.,
pp. 1–12, Sept. 2010. DOI:10.1145/1859995.1859997[6] M. Jain, J. I. Choi, T. Kim, D. Bharadia, S. Seth, K. Srinivasan, P. Levis, S.
Katti, and P. Sinha, “Practical, real-time, full duplex wireless,” Proc. 2011 ACM
MobiCom., pp. 301–312, Sept. 2011. DOI:10.1145/2030613.2030647[7] 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 & USNC/URSI National Radio Science Meeting, pp. 2511–2512,
Jul. 2015. DOI:10.1109/APS.2015.7305644

1© IEICE 2017
DOI: 10.1587/comex.2016XBL0175
Received September 29, 2016
Accepted October 25, 2016
Publicized November 22, 2016
Copyedited February 1, 2017

Introduction

Full-duplex communication is being studied recently for application to the 5G
wireless network [1]. Full-duplex communication establishes two-way communication using the same frequency and time resources [2]. However, full-duplex
communication cannot be realized without resolving the self-interference problem,i.e. the strong transmitting signal is captured by its receiving antennas whichinterferes with the desired receiving signals. Ideally, self-interference can beeliminated by digital subtraction since the self-interference is known signal [3].
However, the self-interference is signiﬁcantly stronger than the desired signal, andcauses RF (Radio Frequency) front-end saturation, which produces nonlinear signal
distortion. In this case, digital subtraction is unable to adequately suppress theself-interference [4]. Therefore, the self-interference needs to be suppressed before
the RF front-end at the receiver. To combat this, three methods have been studied[5, 6, 7]. Antenna cancellation uses an additional transmitting antenna to form a nullpoint at the receiving antenna [5]. Balun cancellation places a balun circuit between
the transmitting and receiving antennas to subtract the self-interference signals fromthe received signals [6]. Note that these methods cannot be directly applied to
MIMO systems because the interference among all combinations of transmitting andreceiving antennas must be considered and implementation is diﬃcult. To resolve
this problem, eigen-beamforming using linear array antennas in the end-ﬁre arrangement was proposed for self-interference reduction in the MIMO full-duplex system[7]. This method oﬀers very high interference reduction with the consumption of
only one degree-of-freedom of the antenna array; this antenna arrangement degenerates the rank of the interference channel to 1. Only the S-parameter of the antennas
has been used to verify the performance of this method, meaning that the totalcancellation performance, including signal subtraction, has yet to be evaluated.
This letter experimentally evaluates the interference cancellation performanceof the method proposed in [7] by using real MIMO-OFDM signals. A signal frame-72

IEICE Communications Express, Vol.6, No.2, 71–76

format is newly developed for this experiment, and the total performance of theself-interference reduction method including digital subtraction technique is investigated, where the remaining interference caused by the saturation of the RF frontend and imperfection of the antenna arrangement is evaluated.
2

Self-interference reduction method using eigen-beamforming anddigital subtraction

Fig. 1.

System model and MIMO-OFDM frame format

Fig. 1(a) shows a system model of the in-house-developed full-duplex testbed withlinear array antennas in the end-ﬁre arrangement. M and N are the number oftransmitters (Tx) and receivers (Rx), respectively, H is the interference channel,
and s is the transmission signal. Let us describe the eigen-beamforming scheme inthis experiment. The singular value decomposition of the interference channel H isgiven by
H ¼ U V H ;© IEICE 2017
DOI: 10.1587/comex.2016XBL0175
Received September 29, 2016
Accepted October 25, 2016Publicized November 22, 2016
Copyedited February 1, 2017

ð1Þ

where fgH represents Hermitian transpose. V and U are eigen-vector matrices ofTx and Rx, respectively. The number of eigen-values is given by73

IEICE Communications Express, Vol.6, No.2, 71–76

Ni ¼ minðM; NÞ;ð2Þ

V ¼ ½v1 ; v2 ;    ; vM pﬃﬃﬃﬃﬃﬃ
pﬃﬃﬃﬃﬃ pﬃﬃﬃﬃﬃ
 ¼ diagð 1 ; 2 ;    ; Ni Þ;ð3Þand V and  are deﬁned byð4Þ

where vi is the i-th transmission weight vector, and i is the i-th eigen-value. The
eigen-values correspond to the received signal strength of the eigen-modes. Eigenbeamforming is the interference reduction technique that uses the eigen-mode
weight vector described in (3). The i-th transmission mode is formed by the i-thtransmission weight vector. In the eigen-beamforming scheme, the eigen-vectors
excluding those corresponding to several of the largest eigen-values are used. This
lowers the degree-of-freedoms of the antennas, where the number is determinedby the number of suppressed modes. When transmitting and receiving linear array
antennas are placed in an end-ﬁre arrangement, the rank of the transmission modes
is expected to degenerate to 1. This means, the required number of the antennadegrees-of-freedom is minimized to 1, ideally. However, the rank of the interference channel in the actual antenna array cannot made 1 because of the channel
imperfections created by the mutual coupling among the antennas and the ﬁnite
distance between the transmitting and receiving arrays. The self-interference
remains as the interference channel is imperfect. The remaining interference canbe digitally subtracted because the transmitted signals are known. If transmitbeamforming is not used, the interference signals signiﬁcantly impact the RF frontend at its receiver, and subtraction become ineﬀective as the interfering signals arenonlinearly distorted.
Fig. 1(b) shows the frame format of the MIMO-OFDM signals. SP (ShortPramble) synchronizes Tx and Rx, and LP (Long Preamble) is used to estimate theinterference channel. LPi;j and DATAi;j are the signals for j-th eigen-modes
transmitted from the i-th antenna. The interference power is deﬁned by the DATA
signals observed at the receiver. In this scheme, the ﬁrst channel estimation(w/oBF LP) is performed to get the interference channel H. Eigen-beamforming
is realized by using the transmission weight matrix consisting of eigen-vectors,½v2 ;    ; vM . The second channel estimation (w/BF LP) is also performed toestimate the interference channel, where the transmission weight without the majorinterference mode is used at Tx side. The received signal after digital subtraction is
given byySUB ¼ y  Hi V~ 1 s;

ð5Þ

where ySUB is the received signal after digital subtraction, y is the received signal,and s is the transmitted signal.
3© IEICE 2017
DOI: 10.1587/comex.2016XBL0175
Received September 29, 2016
Accepted October 25, 2016
Publicized November 22, 2016
Copyedited February 1, 2017

Experiments using MIMO-OFDM signals

3.1 Experimental setup
Fig. 2 shows the measurement environment. The transmitting and receiving antennas are linear array antennas consisting of four element half-wavelength sleeveantennas. The center frequency is 2.47 GHz. For the linear array antennas, the74

IEICE Communications Express, Vol.6, No.2, 71–76

center-to-center distance between Tx and Rx is dTx-Rx ¼ 100 (0 : wavelength in avacuum), and the element spacing delement is changed from 0:250 to 1:50 . Sheetlike microwave absorbers are placed below the antennas in order to prevent
reﬂection from the ﬁxtures.

Sleeve antennas

dElement
dTx-RxTx

Rx
Microwave absorbers

Fig. 2.

Measurement environment

Fig. 3. Measured results© IEICE 2017
DOI: 10.1587/comex.2016XBL0175
Received September 29, 2016
Accepted October 25, 2016
Publicized November 22, 2016
Copyedited February 1, 2017

3.2 Simulation results
Fig. 3(a) shows the eigen-value distribution versus element spacing. The maximum
diﬀerence between the ﬁrst and second eigen-values is 32 dB at delement ¼ 0:250 ,and the minimum diﬀerence is 23 dB at delement ¼ 1:50 . From this result, the eigen75

IEICE Communications Express, Vol.6, No.2, 71–76

weight without ﬁrst eigen-mode can suppress the interference by more than 23 dB.
Further, the narrower the element spacing becomes, the more eigen-beamforming
suppresses the interference.
Fig. 3(b) shows the interference level versus element spacing. The interference
level is deﬁned as the level of the total received power at Rx side relative to thetotal input power at Tx side. Comparing the interference levels without eigenbeamforming (w/o BF) and with eigen-beamforming (w/ BF) shows that eigenbeamforming suppresses the interference by at least 20 dB for all element spacing’sexamined. We also found the minimum interference level, −64 dB, is achieved witheigen-beamforming at delement ¼ 0:50 . The combination of an eigen-beamformingand digital subtraction (w/ BF + Sub) can suppress the interference to −76 dB at
delement ¼ 0:50 , which is 12 dB lower than that without subtraction. This resultshows that digital subtraction further suppresses the self-interference power.
4Conclusion

Experiments conﬁrmed that our combination of eigen-beamforming and digital
subtraction, where linear array antennas in end-ﬁre arrangement are used, cansigniﬁcantly reduce the self-interference reduction. When the element spacing isdelement ¼ 0:50 , the interference level reduction achieved by eigen-beamformingis −64 dB, while with eigen-beamforming and digital subtraction the reduction is−76 dB. Further interference reduction is needed to implement this technique in
actual full-duplex systems and will be the target of future work.
Acknowledgments
This research and development work was supported by the MIC/SCOPE#155002002.

© IEICE 2017
DOI: 10.1587/comex.2016XBL0175
Received September 29, 2016
Accepted October 25, 2016
Publicized November 22, 2016
Copyedited February 1, 201776