ASPIRE Workshop 2024
1

Laboratory Experiment of Broad-Range Null-Steering forMillimeter-Wave V2I Multiuser MIMO
Sojin OZAWA† , Yuki SASAKI† , Ryo IWAKI†† , Student Members, Jin NAKAZATO†† ,Manabu TSUKADA†† , Members, and Kazuki MARUTA† , Senior Member

SUMMARY
This paper demonstrates the practical effectiveness of our
proposed broad-range null-steering (BRNS) scheme through indoor experiment using 28-GHz band 4-element array antenna transmission system.
For realization of cooperative automated driving, use of millimeter-wave
band and multiuser multiple-input multiple-output (MIMO) are essential.
However, inter-user interference is an challenging issue to perform spatial multiplexing under mobility environment due to outdated channel state
information (CSI). BRNS was previously proposed that additionally nullifies around interfering users based on geometrical information of vehicles
that can be obtained through cooperative awareness message (CAM). Its
effectiveness have been confirmed through computer simulations, supposing cooperative automated operation in V2X. Our laboratory-experimental
result confirms null-steering range can be expanded.
key words: Multiuser MIMO, Broad-range null-steering, Millimeter-wave,Laboratory experiment, V2X, Cooperative automated driving

1.

Fig. 1Introduction

Illustration of broad-range null-steering.

anechoic box.

Connected automated vehicles (CAVs) through the integration of communications is gaining momentum as part ofvehicle-to-everything (V2X) communication [1]. It requires
large amounts of sensor information, such as LiDAR, to be
distributed via wireless links. Millimeter-wave should beleveraged to deliver such voluminous data traffic; massivearray beamforming and tracking are indispensable to compensate coverage shortage [2]. Multi-vehicle support is also
required for further system capacity enhancement. It is essential to eliminate inter-user interference (IUI) through amultiuser MIMO precoding. However, vehicles are moving at high speeds and thus steered nulls based on channel
state information (CSI) are outdated. Null-space expansion(NSE) was proposed to improve the performance of multiuser MIMO transmission in such a time-varying channel
environment [3]. NSE utilizes past CSI and expand thenull-space; IUI can be well suppressed better than utilizingfuture time-varying channels. Suppose a line-of-sight (LoS)channel for moving vehicles, we previously proposed theangular-based NSE; broad-range null-steering (BRNS) [4].
Its effectiveness is clarified in computer simulation. This
paper demonstrates the efficacy of the BRNS through experiment using 28 GHz-band 4 array transmission system in an2.

Broad-range null-steering (BRNS)

Suppose multiuser MIMO downlink, the channel vectorhθ ∈ C1×Nt estimated from the angle of departure (AoD)
θ is expressed as follows,nono ih2πhθ = 1 exp −j 2πdsinθ...
exp−j(N−1)dsin
θ.
tλλ(1)
Nt , d and λ denote the number of antenna elements of thebase station (BS), the antenna element spacing, and wavelength, respectively. Assuming a two-user Line-of-sight(Los) channel, we consider designing a precoding weightW ∈ CNt ×2 that suppresses interfering signals when signals
arrive from two directions, θ1 and θ2 , representing AoDs of
the desired and interference signals, respectively. Generally,
precoding weight for the desired user is designed to perform
beamforming to AoD θ1 while null-steering to θ2 . As illustrated in Fig. 1, the proposed BRNS additionally nullifies
around θ2 , expressed as θ2+ , θ2− . Then the precoding weightis calculated by zero-forcing (ZF) on the extended channel
H1± ∈ C4×Nt , as in the following equation.
hi
W1± = w1±,1 w1±,2 w1±,2+ w1±,2−

HH −1
= H1±
H1± H1±,(2)hiT
H1± = hTθ1 hTθ2 hTθ2+ hTθ2−.
(3)†The authors are Faculty/Graduate School of Engineering, Department of Electrical Engineering, Tokyo University of Science,Tokyo, Japan†The author is Advanced Wireless and Communication Research Center, The University of Electro-Communications, Tokyo,Japan††The author is Graduate School of Information Science andTechnology, The University of Tokyo, Tokyo, Japan

The precoding weight vector w1± steers beam to θ1 and nulls
to the three directions θ2 , θ2+ and θ2− , respectively. The above
process is also applied to θ2 , and the resultant precoding

Copyright © 2023 The Institute of Electronics, Information and Communication Engineers
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ASPIRE Workshop 2024
2Table 1

Simulation and Experiment Parameters.

ParameterFrequency
Number of BS antennas
AoD of desired UT
AoD of interfering UTAntenna duration
Resolution of amplitude
Resolution of phase
Transmission distance

Values
28 GHz4×145◦-30◦5 mm
0.5 dB
5.6◦
0.6m(a) Simulation

Fig. 2Experiment setup.

weight can be obtained as follows.
hi
W = w1±,1 w2±,2 ,W ← W/∥W∥F .

3.

(b) ExperimentFig. 3(5)was expanded. It was also found that finer phase and amplitude resolution is required for more precise nullification
control.

Laboratory experiment
Acknowledgments

For the measurement on the actual device, we utilized theDevelopment Kit (BBoard) manufactured by TMYTEK, capable of radiating millimeter waves (28 GHz) with fourelement antenna array. Experiment is conducted using anechoic box. Measurement environment for the experiment
setup is shown in Fig. 2. We then examined the beam pattern of both the experiment and the simulation. Evaluation
conditions are detailed in Table 1. Since the radiation device
performs analog beamforming, phase and amplitude resolutions are discretized. Similar constraints were applied in thesimulation.
Fig. 4 presents the simulation result and Fig. 4 displays
the measurement one. In both scenarios, the null width isobserved to be broader and deeper when applying BRNS,
which suggests an enhancement in interference suppression
performance. Although the completely the same beam pattern cannot be reproduced, these results demonstrate theefficacy of BRNS in real-world environments.
4.

Beam pattern.

(4)This research is supported by the Strategic Information andCommunications R&D Promotion Programme (SCOPE) by
Ministry of Internal Affairs and Communications (receipt
number JP235003002), in part by JST ASPIRE Grant Number JPMJAP2325, Japan. The authors are sincerely gratefulto Mr. Osamu Morita, Mr. Akinori Saeki, Mr. Satoshi
Ogura and Mr. Kazuyoshi Ohnishi of MORITA TECH CO.,LTD. for their technical support on this work.
References[1] A. Yu et al., “Autowarev2x: Reliable v2x communication and collective perception for autonomous driving,” 2023 IEEE 97th VehicularTechnology Conference (VTC2023-Spring), pp.1–7, 2023.
[2] M. Kazuki et al., “Fast beam tracking algorithm for wireless backhaul in adaptive vehicular cell networks,” 2021 IEEE International
Conference on Consumer Electronics (ICCE), Jan 2021.
[3] T. Iwakuni et al., “Null-space expansion for multiuser massive MIMO
inter-user interference suppression in time varying channels,” IEICE Transactions on Communications, vol.E100.B, no.5, pp.865–873,
2017.
[4] Y. Sasaki, S. Ozawa, K. Arai, J. Nakazato, M. Tsukada, and K. Maruta,“Location-based broad-range null-steering in V2X multiuser MIMO
transmission,” IEEE Consumer Communications Networking Conference (CCNC2024), January 2024.

Conclusion

This paper verified feasibility of our proposed BRNS with
28 GHz band four antenna array through a laboratory experiment. Measurement result confirmed that the null region28