International Civil Aviation OrganizationACP WG-F/31IP 04

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

31th MEETING OF WORKING GROUP F

Seattle, Washington, USA 6-10 October 2014

Agenda Item 6: 5 GHz Band Planning

NICT C-band Channel Measurement towards Deploying UAS for Disaster Mitigations

(Prepared by Kenichi Takizawa, Toshinori Kagawa, Lin Shan, Fumie Ono, Hiroyuki Tsuji, and Ryu Miura)

(Presented by Fumie Ono)

SUMMARY

This information paper is intended to inform ICAO WG-F of a summary of radio propagation measurement in C-band Control and Non-Payload Communications (CNPC) channels for unmanned aircraft systems (UAS).

1. INTRODUCTION

1.1 At the 30th Meetings of Working Group F, the information paper ACP-WG-F/30 IP03 provided a description of a plan including the measurement setup and the flight route by the National Institute of Information and Communications Technology (NICT) to conduct a measurement campaign on the propagation characteristics of the air-ground (AG) channel at C-band intended for Unmanned Aircraft Systems (UAS) Control and Non-Payload Communications (CNPC). The information paper also developed the channel models of the air-ground channels. The purposes of the campaign are to provide the characteristics of the channel between the ground station and small Unmanned Aircraft (UA) and also to realize applications for disaster mitigation with small-size UAs after a massive earthquake, tsunami, volcanic eruption, and so on. This measurement is being conducted as a part of R&D on Cooperative Technologies and Frequency Sharing between Unmanned Aircraft Systems (UAS) Based Wireless Relay Systems and Terrestrial Networks supported by Ministry of Internal affairs and Communications (MIC), Japan.

1.2 This paper presents an overview of a measurement campaign on radio propagation channels at C-band. Based on the expected applications in this project, flight patterns are based on a typical deployment scenario of small-size UAs. Section 2 shows a measurement setup for A2G radio

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channels. A summary of measurement results is given in Section 3, which shows en-route received signal strength (RSS) and channel impulse responses (CIRs).

2. MEASUREMENT SETUP

2.1 A block diagram of the measurement setup is shown in Fig. 1. At the transmit side, which is installed on a small-size manned airplane, a vector signal generator (VSG) outputs a designed waveform with frequency bandwidth of 20 MHz for channel sounding at the center frequency of 5060 MHz. The output signal from VSG is amplified by a power amplifier to transmit power Ptx of +30 dBm at the antenna terminal. The amplified signal is passing through a bandpass filter (BPF) in order to suppress its bout-band emission. The transmit side has two individual RF transmitters for alternately transmitting the signal waveform from the left-side and right-side, in order to avoid shadowing effect due to the airplane. The antenna gain Gtx at the transmitter is 2.1 dBi. At the receiver side on ground, signals received at a patch antenna with an antenna gain Grx of 6 dBi are first amplified by a low noise amplifier (LNA) followed by passing through BPF. Then, the amplified signals are down-converted into baseband signals at a vector signal analyzer (VSA), and the baseband signals are recoded at sampling rate of 50 MHz. The VSA has capability of recording the baseband signals over more than 1 hour. The combination of transmit and receive antennas is based on an actual UAS in which a small UA has a monopole antenna and its GCS has a patch antenna. Two types of receiving antenna, linear and circular polarization antennas, were used for comparison. Frequency synchronization between the transmit side and receiver side is achieved by using a Rubidium oscillator. Time synchronization is also maintained by GPS-1PPS. This measurement setup has receiver sensitivity S of -96 dBm at the output of the receiver antenna. Therefore, its acceptable pathloss reaches Ptx+Gtx+Grx-S=134 dB, which is equivalent to A2G distance of 25 km in free-space.

Figure 1 - Measurement setup for A2G radio channels at C-band.

2.2 A small-size airplane shown in Fig. 2 was employed in this A2G measurement. Since the airplane plays a role of the transmitter, measurement equipment including the VSG was loaded. Two transmit antennas with vertical polarization were attached to the both wingtips individually. These wingtips are formed by polyurethane covered by sheets of Kevlar, which is the same material as commercially available small-size UAs. During the measurement, an on-board flight recorder stored the information on the airplane including GPS time, latitude, longitude, altitude above ground level (AGL), posture angles, and so on.

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Figure 2 – Photograph of the airplane used in the radio channel measurement.

3. MEASUREMENT RESULTS

3.1 One measurement has been conducted by setting an on-ground receiver in Honolulu. The airplane with transmitter has flown over a part of Oahu Island. Measurement results are summarized in Fig.3. The flight route of the airplane is shown in Fig. 3(a). Distance and posture angles of the airplane are shown in Fig. 3(b). The largest distance reached 25 km in this measurement. Also, the bank angle changed in the range of ±40 degrees, which causes additional loss due to mismatch of the polarization. Measured RSS is plotted in Fig. 3(c), in which the change of measured RSS against elapsed time is presented. In this figure, expected RSS given by free-space path-loss is also shown. This figure shows that the trend of the expected RSS agrees with that of the measured RSS. One of the reasons that cause fluctuation of the measured RSS is multipath fading and so on. Example CIRs calculated from the measured baseband waveforms are shown in Fig. 4(a). The CIRs show that the noise floor of the measurement setup is -120 dBm, and several delay paths are observed. The delay path appeared in delay of 1 s, which is equivalent to difference of around 300 meters in travel distance, is considered as a reflection caused by the ground. Fig. 4(b) shows the change of CIRs during the flight, in which the horizontal axis and vertical axis shows relative propagation delay and flight time, respectively. The figure shows that several delayed paths are observed between 0 and 900 sec in the flight time. Around the flight time of 900sec, the A2G channels have many delayed paths since the airplane for the measurement was approaching to the ground where there are a lot of buildings possible to be reflectors.

(a) Flight route of the transmitter during the A2G channel measurement.

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(b) Distance and posture angles of the airplane.

(c) Comparison between measured RSS and calculted signal level based on free-space pathloss.

Figure 3 – Photograph of the airplane used in the radio channel measurement.

(a) Channel impulse resposes (CIRs) in A2G chanenl calculated from recored base-band waveforms.

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(b) CIRs during the measurement flight.

Figure 4 – Photograph of the airplane used in the radio channel measurement.

3.2 Another measurement for A2G channel was conducted, in which an on-ground receiver was located at the north side of Oahu Island. The airplane with transmitter flew over a west-north part of Oahu Island as shown in Fig. 5(a). Distance and posture angles of the airplane are shown in Fig. 5(b). As shown in this figure, the bank angle has also fluctuated in ±45 degs, which causes additional loss due to polarization mismatch. The largest distance between the transmitter and receiver has reached 15 km. Measured RSS is shown in Fig. 5(c), in which the change of measured RSS against elapsed time is presented. In this figure, expected RSS calculated from so called free-space pathloss is also shown. The figure shows that the trend of the expected RSS curve in some cases; however, there are gaps of more than 30 dB in some areas. This is due to the directional pattern of the receive antenna since the antenna has a main-lobe with width of 120-degs. Example CIRs calculated from the measured baseband waveforms are shown in Fig. 6(a). The observed CIRs show that the radio channels are almost expressed by the direct path and scatter components followed by the direct path. Fig. 6(b) shows the change of CIRs during the flight, in which the horizontal axis and vertical axis shows relative propagation delay and flight time, respectively. The figure shows that several delayed paths are observed.

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(a) Flight route of the transmitter during the A2G channel measurement.

(b) Distance and posture angles of the airplane.

(c) Comparison between measured RSS and calculted signal level based on free-space pathloss.

Figure 5 – Measurement results –case 2 (RSS).

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(a) Channel impulse resposes (CIRs) in A2G chanenl calculated from recored base-band waveforms.

(b) CIRs during the measurement flight.

Figure 6 – Measurement results –case 2 (RSS).

4. CONCLUSION

4.1 This IP presents a summary of a measurement for characterizing A2G radio propagation in C-band CNPC channels. The measurement was conducted to clarify the characteristics of the propagation based on the scenarios of deploying a small-size UAs. From the results, several characteristics of the propagation were obtained. For example, the maximum incoming path-delay time in the line-of-sight condition was within 1 sec. This means that the configuration of UAS communication system, which operates CNPC link in the 5 GHz band, should allow 1 sec delay time. The effects of the difference between linear and circular polarization antennas and the characteristics of channel models based on measurement results will be shown in the following meeting.

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