IEICE Communications Express, Vol.4, No.6, 198–204

Experimental evaluation of
analog eigenmodetransmission for short-range
MIMO: array parameter
independency and wideband
characteristics
Kentaro Murata1a), Naoki Honma2, Masakuni Tsunezawa2,
Kentaro Nishimori3, and Hisashi Morishita1
1Graduate School of Science and Engineering, National Defense Academy,

1–10–20 Hashirimizu, Yokosuka, Kanagawa 239–8686, Japan
2

Faculty of Engineering, Iwate University,

4–3–5 Ueda, Morioka, Iwate 020–8551, Japan
3

Faculty of Engineering, Niigata University,

42–8050 Ikarashi, Niigata, Niigata 950–2181, Japana) ed14002@nda.ac.jp

Abstract: We proposed an analog eigenmode transmission technique,that realizes multi-stream parallel transmission in a short-range multipleinput multiple-output (SR-MIMO) communication using only 180-degreehybrid couplers. Provided the MIMO array is symmetric, this technique canorthogonalize MIMO channels independently of antenna spacing and Tx-Rx
distance. Furthermore, due to wideband characteristics of a hybrid, thechannel orthogonalization effect can hold over a broad band of frequencies.
In this letter, its array parameter independency and wideband characteristics
are quantitatively evaluated in term of channel capacity. It has been experimentally clariﬁed that this technique achieves approximately 90-% performance of ideal eigenbeam space division multiplexing (E-SDM) by digitalbeamforming despite the deviations of the above factors.
Keywords: short-range MIMO, eigenmode transmission, passive analog
circuitClassiﬁcation: Antennas and Propagation
References© IEICE 2015
DOI: 10.1587/comex.4.198
Received April 9, 2015
Accepted April 23, 2015
Published June 18, 2015[1] N. Honma, K. Nishimori, T. Seki, and M. Mizoguchi, “Short range MIMO
communication,” Proc. 3rd European Conference on Antennas and Propagation,
Berlin, Germany, pp. 1763–1767, Mar. 2009.
[2] K. Nishimori, N. Honma, T. Seki, and K. Hiraga, “On the transmission method
for short-range MIMO communication,” IEEE Trans. Vehicular Technol.,vol. 60, no. 3, pp. 1247–1251, Mar. 2011. DOI:10.1109/TVT.2011.2113196

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IEICE Communications Express, Vol.4, No.6, 198–204[3] J.-S. Jiang and M. A. Ingram, “Spherical-wave model for short-range MIMO,”IEEE Trans. Commun., vol. 53, no. 9, pp. 1534–1541, Sep. 2005. DOI:10.
1109/TCOMM.2005.852842[4] K. Sakamoto, K. Hiraga, T. Seki, T. Nakagawa, and K. Uehara, “Performance
evaluation of short-range MIMO using a method for controlling phasedifference between each propagation channel,” IEICE Trans. Commun.,vol. E96-B, no. 10, pp. 2513–2520, Oct. 2013. DOI:10.1587/transcom.E96.
B.2513[5] K. Murata, N. Honma, K. Nishimori, and H. Morishita, “Analog eigenmodetransmission for short-range MIMO,” IEEE Trans. Vehicular Technol., in press.
DOI:10.1109/TVT.2015.2397194[6] K. Murata, N. Honma, K. Nishimori, and H. Morishita, “Analog eigenmodetransmission for 2 × 2 short-range MIMO,” Proc. IEEE AP-S International
Symposium, Memphis, USA, pp. 476–477, Jul. 2014. DOI:10.1109/APS.
2014.6904569[7] D. M. Pozar, Microwave Engineering, 4th ed., Wiley, New York, 2012.
[8] G. R. Simpson, “A generalized n-port cascade connection,” Proc. IEEE MTT-S
International Microwave Symposium Digest, Los Angeles, USA, pp. 507–509,
Jun. 1981. DOI:10.1109/MWSYM.1981.1129978[9] K. Murata, N. Honma, K. Nishimori, and H. Morishita, “Bidirectional
matching technique for Tx and Rx ports in short-range MIMO,” 2014 AsiaPaciﬁc Microwave Conference (APMC), Sendai, Japan, pp. 498–500, Nov.
2014.
[10] http://www.mentor.com/pcb/hyperlynx/3d-em/.
[11] D. M. Pozar, “Microstrip antennas,” Proc. IEEE, vol. 80, no. 1, pp. 79–91,
Jan. 1992. DOI:10.1109/5.119568[12] http://www.ieee802.org/11/.

1

Introduction

Short-range MIMO, an emerging utilization form of MIMO technology, is apromising avenue to realize a high-speed large-capacity short-range wirelesscommunication by employing multiple antennas [1, 2, 3]. Currently, the simpliﬁcation of SR-MIMO hardware-software conﬁguration is being pursued for its
practical usages, and ﬁxed-weight analog decoding methods were developed
utilizing the uniquely-determined channel property [4]. However, the conventional
methods suffer from the two unavoidable problems, “capacity deterioration causedby array displacement” and “being valid only at optimal array conﬁguration,” that
exclusively conﬁne its applicable scope. In order to solve the problems, an analog
eigenmode transmission technique using 180-degree hybrid couplers was proposed.
It was theoretically proven that this technique could orthogonalize MIMO channels
regardless of antenna spacing and Tx-Rx distance [5].
In this letter, its array parameter independency is quantitatively evaluated byexperiment [6]. In addition, its frequency characteristics are newly reported.
Consequently, it has been experimentally demonstrated that the proposed analogbased E-SDM (AE-SDM) can achieve more than 90-% performance of ideal
digital-based E-SDM (DE-SDM) regardless of the above-mentioned parameters.
© IEICE 2015
DOI: 10.1587/comex.4.198
Received April 9, 2015
Accepted April 23, 2015
Published June 18, 2015

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IEICE Communications Express, Vol.4, No.6, 198–204

Fig. 1.

2

Conceptual sketch of the proposed technique.

TheoryThe theory of the proposed technique is brieﬂy explained by taking a 2  2
SR-MIMO system for example. As shown in Fig. 1(a), if a SR-MIMO array is
conﬁgured symmetrically with respect to the reference plane, a channel matrix, Schl ,becomes symmetrical; the channels between facing ports, S31 and S42 , are equivalent as well as those between diagonal ports, S41 and S32 . Therefore, by replacing
them with constants, α and β, respectively, the channel matrix is expressed as"# "#S31 S32 ¼:
ð1Þ
Schl ¼
S41 S42 
Next, hybrids are cascade-connected to both Tx/Rx sides as shown in Fig. 1(b).
The transmission matrix of an ideal hybrid, Shyb , is expressed as
"
#
j 1 1
Shyb ¼ pﬃﬃﬃ
ð2Þ
2 1 1and an ideal hybrid has neither reﬂection at each port nor couplings between inputs
and between outputs [7]. The complete channel matrix of the whole SR-MIMO
system including the hybrids, S0chl , is expressed as the product of the transmission
matrices of the Tx/Rx hybrids and the channel matrix of the initial SR-MIMO
system [8],
"
#

þ

0
:
ð3Þ
S0chl ¼ Shyb Schl Shyb ¼ 
0© IEICE 2015
DOI: 10.1587/comex.4.198
Received April 9, 2015
Accepted April 23, 2015
Published June 18, 2015

Equation (3) indicates that two parallel streams are formed between the pairs of thecorresponding Tx/Rx ports. And, on the condition that the symmetry of the array
conﬁguration is maintained, array parameters such as the type of antenna, antenna
spacing and Tx-Rx distance have no inﬂuence on the channel orthogonality.

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Although reﬂection and coupling components at both Tx/Rx arrays are omitted in
the derivation process of Eq. (3) due to space limitations, it was proven in ourprevious work that these factors had no impact on the channel orthogonalization by
the proposed technique [9].
3Experiment

3.1 Experimental setup and condition
Fig. 2(a) shows the SR-MIMO system conﬁguration used for an experiment. The
Tx/Rx arrays consisted of two microstrip antennas (MSAs) shown in Fig. 2(b).
The MSA was fabricated on a substrate with thickness of 1.56 mm and relative
permittivity of 2.2. The size of the MSA and its feeding point were optimized so it
was fully matched at the center frequency of 2.5 GHz. Just for reference, simulated
half-value angles of E- and H-plane far-ﬁeld radiation patterns of the MSA are 84.5
and 88.3 degrees, respectively. Regarding array parameters, antenna spacing, d, and
Tx-Rx distance, D, were deﬁned as the distances between feeding points ofadjacent elements and between Tx/Rx array substrates, respectively. As a hybrid
used in the proposed technique, a rat-race coupler fabricated on the same substrateas the MSA was employed (Fig. 2(c)). In the ﬁgure, eff (87.8 mm) is the effective
wavelength at the center frequency calculated with the substrate parametersconsidered. The width of the ring was determined for a characteristic impedancepﬃﬃﬃ
of 2Z0 (Z0 is 50 Ω.) In the proposed SR-MIMO system, two rat-races wereconnected to the Tx/Rx arrays and a measuring instrument via ﬂexible cables fromthe back sides of the rat-races.
In the experiment, a submatrix representing transmission from Tx to Rx arraysin the S-parameter of the SR-MIMO system was deﬁned as a channel matrix, andtwo patterns of channel matrices were measured. First, as the proposed AE-SDM,
channels were obtained when the rat-races were connected. Second, channels wereextracted when the Tx/Rx arrays were directly connected to the measuring instrument, and after that, DE-SDM was implemented through post processing. Finally,
channel capacities of AE-SDM and DE-SDM were calculated based on Shannon’s
theorem regarding a signal from a non-corresponding Tx port as interference [5]. In
calculating capacities, the ratio of transmission power to receiving noise level was
set to 40 dB.
For reference, numerical simulations were also carried out to verify experimental results. For electromagnetic analysis, IE3D was used [10]. In the simulation, the SR-MIMO array identical to that of experiment was designed, and achannel matrix was extracted in the same manner as experiment. Regarding a
hybrid part, an ideal (but frequency-dependent) rat-race consisting of distributedconstant lines was assumed separately from the array conﬁguration. A channel
matrix when hybrids cascade-connected was numerically analyzed based on the Sparameter theory [8]. Therefore, the structural inﬂuence of the hybrids were
excluded from channel characteristics.

© IEICE 2015
DOI: 10.1587/comex.4.198
Received April 9, 2015
Accepted April 23, 2015
Published June 18, 2015

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IEICE Communications Express, Vol.4, No.6, 198–204

Fig. 2.

© IEICE 2015
DOI: 10.1587/comex.4.198
Received April 9, 2015
Accepted April 23, 2015
Published June 18, 2015

Experimental setup.

3.2 Experimental results
3.2.1 On array parameter independency
Fig. 3(a) shows channel capacity versus antenna spacing. In the experiment, d wasvaried to 81, 112, 143, 174 and 205 mm, and D was ﬁxed to 120 mm. In the ﬁgure,markers of circles, squares and triangles represent the channel capacities of AESDM, E-SDM and without any beamforming (shown as w/o BF), respectively.
Also, simulation results are shown with different types of lines.
First, see the simulation results, and it is particularly notable that AE-SDM has
exactly same properties with DE-SDM. This proves that AE-SDM allows a perfecttwo-stream parallel transmission in theory. Then, focusing on experimental results,
it is conﬁrmed that a channel capacity is dramatically improved by AE-SDM
compared to w/o BF since interference channel components are suppressed byusing rat-races as expressed as Eq. (3). As a result, channel capacity increases 2.4and 3.7-fold on average and at maximum when d ¼ 81 mm, respectively. Incidentally, it is observed that the deviation between experimental and numerical results of
w/o BF becomes signiﬁcant as d increases. The cause of it is considered as follows;when d is small, the hybrids are blocked by MSAs, therefore, they have less impact
on channel characteristics. On the other hand, when d is large, the hybrids are202

IEICE Communications Express, Vol.4, No.6, 198–204

exposed and they generate more scattered waves. Especially, it appears that the ratraces work to suppress interference components when d ¼ 205 mm, that leads tosuch improvement in channel capacity. Likewise, it is thought that the slightdifference between increase-decrease tendencies of experimental and simulation
results of DE-SDM comes from such scattering effect. In comparison with DESDM, although AE-SDM should theoretically agree with DE-SDM, the capacity isdeteriorated as the whole. The possible causes are cited as follows; (i) the impurityof channel orthogonality by manufacturing error of rat-races and array displacement, and (ii) mismatching and insertion losses by rat-races, cables and connectors.
However, the capacity deterioration is retained only 8.4% on average over the
analysis points of antenna spacing.
Fig. 3(b) shows channel capacity versus Tx-Rx distance. In the experiment, dwas ﬁxed to 81 mm and D was automatically changed from 60 to 180 mm at 10-mm
intervals using a positioner. Comparing AE-SDM with w/o BF, the signiﬁcantcapacity improvement, a 3.6-fold increase on average, can be seen as the previous
result shows. Moreover, the capacity deterioration is retained 7.9% on average over
the analysis range of Tx-Rx distance. These results successfully demonstrate that ESDM can be realized independently of both antenna spacing and Tx-Rx distance by
the proposed technique.

© IEICE 2015
DOI: 10.1587/comex.4.198
Received April 9, 2015
Accepted April 23, 2015
Published June 18, 2015

3.2.2 On wideband characteristics
Fig. 3(c) shows channel capacity versus normalized frequency. In the experiment,
the analysis frequency range was 2:5% from the center frequency, comparable to apossible bandwidth of a general MSA [11]. As for array parameters, d and D were
ﬁxed to 81 and 120 mm, respectively. In capacity analysis, it was assumed that ideal
DE-SDM could be implemented at every analysis frequency point while a frequency response of the rat-races was taken into account for AE-SDM. In theﬁgure, the most interesting point is that AE-SDM keeps the capacities nearly equal
to those of DE-SDM against frequency deviation in the simulation even though thefrequency dependency of the rat-races was considered. Similarly, although thecapacity deterioration is conﬁrmed in the experiment because of the aforementioned
causes, the deterioration rates are almost constant (8:110:5%) over the analysis
range. This indicates that the frequency response of the rat-races has a negligible
effect on the capacity deteriorations. These results attribute to the wideband
characteristics of a rat-race (its bandwidth is generally 2030% [7]), and as aconsequence, the proposed technique can keep the channel orthogonalization effectover the broad band of frequencies, i.e. AE-SDM performance is only limited byantenna frequency dependency.
Fig. 3(d) shows a channel capacity with 5-% bandwidth of each scheme. Fromthe experimental results, it is found that AE-SDM can achieve a 3.3 times higher
capacity than w/o BF, and moreover, a 91.7-% performance of ideal DE-SDM. For
supplementary information, according to [12], channel widths of wireless LAN(WLAN) related standards IEEE 802.11n and 11ac are at most approximately 1.7
and 0.8% (40 MHz at 2.4 and 5 GHz bands, respectively) and 3.2% (160 MHz at
5 GHz band), respectively. These values can be sufﬁciently covered by the propose
technique, therefore, its applicability has been suggested.
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Fig. 3.

4

Channel capacity characteristics.

Conclusion

The proposed technique can realize multi-stream parallel transmission using only
passive analog components. Therefore, E-SDM can be performed by a fairly simple
hardware conﬁguration with neither digital beamformer nor active electrical requirements. In this letter, the two huge advantages of AE-SDM, array parameter
independency and wideband characteristics, have been experimentally evaluated.
On array independency, it has been demonstrated that AE-SDM achieves almost thesame level of the capacity of DE-SDM with only 8.4- and 7.9-% degradations on
average over analysis ranges of antenna spacing and Tx-Rx distance, respectively.
Furthermore, it has been clariﬁed that AE-SDM can achieve a 91.7-% performanceof ideal DE-SDM over the 5-% relative bandwidth, that covers the channel widthsof some existing WLAN standards. From these results, it has been conﬁrmed that
E-SDM can be successfully demonstrated by the proposed technique independentlyof the above parameters, and ﬁnally, the effectiveness of it has been experimentallyveriﬁed.
Acknowledgement
This research was partially supported by JSPS KAKENHI (25709030).

© IEICE 2015
DOI: 10.1587/comex.4.198
Received April 9, 2015
Accepted April 23, 2015
Published June 18, 2015

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