Effect of Propagation Environment Control Method Using Drone MIMO Relay Station

Naoki Matsumura1, Kentaro Nishimori1, Ryotaro Taniguchi1,

Tsutomu Mitsui1, and Takefumi Hiraguri2 1Graduate School of Science and Technology, Niigata University

Ikarashi 2-nocho 8050, Nishi-ku Niigata, 950-2181 Japan 2Department of Electrical and Electronics Engineering, Nippon Institute of Technology 4-1 Gakuendai, Miyashiro-machi, Minamisaitama-gun, Saitama Pref, 345-8501 Japan

Abstract – This paper evaluates multiple-input multiple-output (MIMO) transmission when a small autonomous unmanned aerial vehicle (drone) is used as a relay station. Using drones as relay stations, we verify that it is possible to obtain high channel capacity owing to the line of sight (LOS) environment with low propagation loss. We also investigate the effect of the propagation environment control method, which reduces the number of drones used and improves the characteristic degradation of the eigenvalue distribution of the propagation channel that occurs when drones are used for relay stations.

Index Terms — Drones, relay stations, MIMO, propagation path loss, propagation environment control method.

1. Introduction

To diversify wireless communication technologies, multiple-input multiple-output (MIMO) [1] transmission is widely used for improving the efficiency of frequency utilization. While MIMO transmission is effective in multi-path non-line-of-sight (NLOS) environments, the propagation loss is inversely proportional to the power of three to four of the propagation distance in general urban areas [2]. Therefore, we propose to use drones as a relay station for MIMO transmission. When drones are used as relay stations, communication is carried out in the LOS environment above the buildings, enabling an environment close to free-space propagation loss to be realized.

In MIMO systems, owing to the LOS environment, the spatial correlation is higher because of the influence of direct waves. To solve the issue, we focus on three-dimensional (3D) and flexible mobility, which is a feature of drones in the proposed method, and we improve the characteristics by operating the drones' flight and the number of drones used. In this paper, we compare the LOS MIMO system using the drone relay station as the proposed method and the NLOS MIMO system when used in the conventional approach. In addition, fewer drones are used, and the effect of transmitting the optimum amount of data is evaluated.

2. Proposed Method

Fig. 1 shows the proposed system model, which is a LOS environment that passes over obstacles and considers free-space propagation loss. It is a one-hop relay model, and the communication between the transmitting / receiving station and the drones is performed by time-division duplexing of two time slots. A linear array antenna is used for each transmitting / receiving station. Considering the basic performance, the drone is randomly placed within the arrangement circle in the figure. To separate interference signals between drones, block diagonalization (BD) is used between transmitting station and drones, and zero forcing (ZF) is used between drones and receiving stations [3]. Note that the system is arranged on a two-dimensional (2D) plane assuming the basic environment.

Next, we explain the propagation environment control method, which improves the characteristics of the propagation environment by adding control to the drone. Fig. 2 shows the system model of the propagation environment control method. The proposed method (1) involves reducing the number of drones used for relaying to less than the number of antennas of the transmitting / receiving station. This prevents the reduction of the minimum eigenvalue of the propagation channel matrix. Method (2) involves switching the beam of the antenna that is mounted on the drone. In the drone, the beam that maximizes the received power is selected and switched to improve the signal-to-noise power ratio (SNR) and reduce the spatial correlation. Method (3) involves selecting an arrangement that becomes a propagation path, preventing a rise in spatial correlation from among a large number of arranged drones. In this report, the effect in method (1) was evaluated by performing computer simulations.

Fig. 1. LOS MIMO system model using drone relay stations.

[WeC2-6] 2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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Fig. 2. System model of propagation environment control method.

TABLE I. Calculation conditions.

Parameter Value

Conv. Prop.

Propagation loss factor 3 2

Distance between transmitting and

receiving stations 500 m

Radius of drone placement circle 125 m

Frequency 2.4 GHz

Number of transmitter antennas 4

Number of receiver antennas 4

Number of drones (ND) 1～4

Propagation channel characteristics

i. i. d.

Rayleigh

fading

Geometrical

optics

approximation

3. Determining Effectiveness of the Proposed Method Using Computer Simulations

For the conventional method (Conv.) and the proposed method (Prop.), the channel capacity and the eigenvalue distribution were compared and the performance was evaluated. The conventional method is a MIMO system in an NLOS environment. Table 1 shows the main calculation conditions. For simplicity, the received power and antenna gain are normalized to 1. Using the conventional method, the noise power is defined so that the signal-to-noise power ratio (SNR) becomes 0 dB when the transmission / reception distance is 1,000 m.

Fig. 3 shows the cumulative density function (CDF) of the channel capacity when 1–4 drones are used according to method (1). In the proposed method, the channel capacity achievable with CDF = 50% can be improved by about 8.6 bps / Hz compared with the conventional method when the number of drones is four. In addition, it can be seen that the improvement is about 6.1 bps / Hz more than the case involving four drones, when the number of drones is three as the effect of the method (1). Fig. 4 shows a comparison of the condition number (the ratio of the maximum eigenvalue to the minimum eigenvalue) of eigenvalues of the propagation channel. In the proposed method, the graph shifts to the right as the number of drones decreases, and a reduction of the condition number can be confirmed.

Fig. 3. CDF characteristics of channel capacity.

Fig. 4. CDF characteristics of eigenvalue condition number.

4. Conclusion

In this paper, a MIMO system using a drone relay station was proposed and evaluated its performance. The channel capacity is verified to be improved when compared with the conventional scheme. Regarding the propagation environment control method, method (1), which reduces the number of drones, showed that it is effective for improving the characteristic of the eigenvalue distribution when using drone relay stations.

Acknowledgments

Part of this work was supported by the SCOPE #165004002, #185004002 and KAKENHI, Grant-in-Aid for Scientific Research (B) (17H01738, 17H03262).

References

[1] G. J. Foschini and M. J. Gans, “On limits of wireless communications in a fading environment when using multiple antennas,” Wireless Personal Commun., vol. 6, pp. 311-335, 1998.

[2] Y. Okumura et al., "Experimental Study of Propagation Characteristics in Land Mobile Radio Communication," Kenjitsuho, Vol. 16, No. 9, pp. 1705-1764, 1967.

[3] Q. H. Spencer, A. L. Swindlehurst, and M. Haardt, “Zero forcing methods for downlink spatial multiplexing in multiuser MIMO channels," IEEE Trans. Sig. Processing, vol. 52, no. 2, pp.461-471, Feb. 2004.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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