Wireless Optical Feeder System with Optical Power Supply

Wireless Optical Feeder System with Optical Power Supply NOBUO NAKAJIMA and NAOHIRO YOKOTA Department of Human Communications The University of Electro-Communications Chofugaoka 1-5-1, Chofu-shi, Tokyo 182-8585 JAPAN Abstract: - Base stations of the cellular system increase according to the increase of subscribers and transmission bit rate. Most of the places suitable for the base station were occupied by the existing systems. Therefore, sometimes, new base station must be installed at the places where power supply or transmission cable is not available. The proposed feeder system is applicable for such case. The RF signals are fed to the repeater through the air by optical beams. And electrical power is fed by the optical beam, too. This paper describes the design principle and experimental results of the proposed system. By using 4 cm diameter optical beams for RF transmission and 26cm diameter beam for power transmission, available forward link RF power at the repeater station is mw in the case that the distance between the base station and the repeater station is 40 m. If 48 cm diameter reflector is used for power transmission, 0 mw RF power will be available for the forward link. Key-Words: - RoF, Wireless, Optical Transmission, Optical Power Transmission, Cellular System 1 Introduction Cellular base stations increases according to the increase of subscribers and transmission bit rates. It is getting difficult to build new base stations, especially in urban area. In some cases, the base station must be built on the place where neither transmission cable nor electrical power supply is available. The proposed system is applicable as a feeder in such a case. Currently, a Radio on (over) fiber (RoF) technology is applied for the in-building cellular systems. The advantage is that the size of the repeater stations equipped for each floor are very compact because they are composed of only E/O transducer, amplifier and antenna. A power supply-less RoF technology was developed to enhance flexibility for installing repeater stations [1][2][3]. Electrical power is fed to the repeater station through the optical fiber in the form of optical energy. A solar panel is applicable for the base stations as the power supply, too. However, in the rainy seasons, available power decreases. It is more reliable to get the power through the optical fiber. A fiber-space full optical connection is another unique optical technology [4]. Separated two fibers can be connected through the air using lenses which have tracking capability for air turbulence. The proposed system is based on these technologies [1][2][3][4]. In addition, new power supply technology by an optical wireless transmission was developed. This paper describes the concept, technology and experimental results of a wireless optical feeder system with optical power transmission. 2 System Configuration Figure 1 shows a brief description of the structure. There are two stations. One is base station and the other is repeater station. The base station is connected to a switching station in the cellular system. The repeater station transmits and receives RF signal through antennas. In this paper, the repeater station is supposed to be equipped on the place where both a transmission cable and a power supply do not exist. RX Low Noise 2nd AMP Light Base Station Lens 3rd Antenna Circulator TX Power 1st AMP Solar Cell Repeater Station TX: Transmitter, RX: Receiver, : Laser Diode, : Photo Diode Fig.1 Proposed System Configuration ISBN: 978-960-6766-79-4 52 ISSN 1790-5117

There are three optical beams between the base station and the repeater station. Two of them carry RF signals. These two beams can be converged into one beam if duplexer or half-mirror is applied. A 3 rd beam transmits electrical power by optical energy. Lenses or mirrors are used so as not to diverge the beams in the air. Optical waves emitted from (Laser Diode) is modulated by RF (Radio Frequency) signal come from TX. At the repeater station, transduces optical signal to RF signal. A power amplifier is used to increase RF power to the specified output level. As for the 2 nd beam, each component plays same role as that of the 1 st beam, except to employ a low noise amplifier instead of the power amplifier. A strong light source is used for a 3 rd beam. Received optical energy is converted to electric power efficiently by a solar cell and the obtained electric power is fed to the and amplifiers. Table 1 shows the technical requirements for the optical components used in Fig.1. Table 1 Technical Requirements Experiment was carried out using the equipment shown in Fig. 3. Figure 4 is an intensity distribution of the optical beam at the receiver position when the DS type lens is used. Desirable distribution is Gaussian but the experimental pattern is different. This is due to the aberration of the lens. The theoretical intensity distribution was calculated using Kilhihoff integral equation considering lens aberration. The calculation verified the experimental result. Measured insertion losses are shown in Fig. 2. When and DS type lens were used, the insertion loss of the optical part was 7.7 db at m distance. Whereas, the loss was decreased to 5.2 db when FS lens was applied instead of DS type lens. Although the intensity distribution was almost the same between two types of lenses, the SF type insertion loss was a little bit lower. Low Loss for E/O Transducers (, ) Low Loss for Optical Wireless Transmission High Optical Power Transmission 3 Optical Transmission through the Air Figure 2 shows an experimental configuration for evaluating insertion loss of the optical transmission through the air. Loss is caused mainly due to the aberration of the lens and miss-alignment of the optical components. Two kinds of lenses were tested in the experiment. They are Double-Sphere (DS) type and Sphere-Flat (SF) type. Reflection from the lens surface causes loss as well, but it is negligibly small (around 4% for each surface). m 40mm (a) (b) Insertion Loss 7.7dB 5.2dB : Single Mode Fiber Fig. 2 Experimental Configuration for Insertion Loss Measurement Intensity (db) 0 - -20-30 Fig.3 Equipment for Experiment Theory Experiment -20-0 20 Disntance (mm) Fig.4 Optical Intensity Distribution Figure 5 shows a long distance experimental configuration. The distance is 40 m which will be a typical distance when this system is applied as the base station feeder. Three kinds of structure were tested to compare insertion loss performance. They are (a) reception, (2) MMF (Multi Mode Fiber) reception and (3) direct reception. (a) is the best for RF performance but loss may be the highest. (b) is lower RF performance but loss will be smaller than (a). (3) is the best in terms of both loss and RF performance, but all the equipment must be integrated just behind the. The measured insertion losses are shown in Fig.5, too. The performance is same as those predicted. Huge ISBN: 978-960-6766-79-4 53 ISSN 1790-5117

loss of (a) will be due to the aberration of the lens and the alignment error. In the experiments, type (c) was selected because of the lowest loss. Wavelength=1550nm f=0mm 40mm 40m (a) (b) (c) MMF Direct Insertion Loss 38.1dB 17.7dB 5.7dB 0.1mmD Fig. 5 Loss Comparison among Receiver Structures 4 E/O Transducer Characteristics Figure 6, 7 is a measurement system of RF insertion loss between the transmitter and the receiver. Bias Tees feed driving currents for and. The impedance of the and do not usually same as that of RF transmission line (50 ohms). In order to reduce miss-matching loss between / and the transmission line, 3 stub is applied. The impedance is matched by adjusting the length of 3 line stretchers that are connected to the transmission line at a quarter wavelength spacing. DC Bias Bias Tee 3stub (L7551, HAMAMATSU) Network Analyzer 3stub DC Bias Bias Tee Fig. 6 RF insertion loss Measurement important for the repeater station because the consuming electric power must be as small as possible. Figure 9 shows the relationship between RF insertion loss and the power consumption of the surface emitting. 2 mw is enough electric power for laser oscillation. Figure shows the frequency response of the RF insertion loss. In this case, the minimum loss is 22.7 db at 1.17 GHz. Fig.8 Surface Emitting (VCSEL AS-0001 Fuji Xerox) Insertion Loss (db) 50 40 30 20 0 1 2 3 4 5 Power Consumption (mw) Fig.9 RF Insertion Loss vs. Power Consumption 0 20 22.7dB 30 40 50 60 70 80 90 1.1 1.2 Frequency (GHz) Fig. RF Insertion Loss of the Optical Parts when Surface Emitting is used Insertion Loss (db) 5 Optical Power Transmission Table 2 shows the requirements for the optical power transmission system. In order to drive the RF power amplifier, more than tens mw output is required. Eye safety is indispensable for commercial usage. Although a large lens is advantageous for reducing optical transmission loss, smaller lens should be preferred considering compactness. Table 2 Technical Requirements High Available Electric Power (mw-1w) Eye Safety Compactness Fig.7 Equipment for RF Loss Measurement A surface emitting laser diode is used for RF transmission (Fig.8). The feature is that the threshold power of the oscillation is extremely small. This is Figure 11, 12 shows the optical components for the power transmission. A high power LED and a tungsten halogen bulb are applicable as the optical power source. In this system, the tungsten halogen bulb is employed because of the higher output power than the LED. ISBN: 978-960-6766-79-4 54 ISSN 1790-5117

High Power Bulb Lens f L Reflector Solar Panel d h=(l x d) / f Fig.11 Configuration of Optical Power Transmission 240mm x 315mm Filament =4x1.5mm Fig.13 Diffusion of the Optical Power m 50 W (a) Solar Panel (SJJ Shell Solar Japan) (b) Tungsten Halogen Bulb 18cm f=2.3cm Filament Size = 4x1.5mm 24cm x 31.5cm 2 mw (c) Parabolic Mirror (180mm Diameter, 23mm Focal Length) Fig.12 Optical Components for Power Transmission As for the transmitter, the lens is not suitable because it collects small part of optical energy radiated from an optical source because the radiation is isotropic. A parabolic reflector can efficiently collect the radiated power than the lens. As for the receiver, a large solar panel (240 mm x 315 mm) is employed in the experimental system. A combination of lens and small solar panel will be low cost alternative. The optical beam diffuses even lens or the reflector is used. It is important to estimate the efficiency. The diffusion is caused by the filament structure of the bulb. If the filament were a point source (infinitely small), the diffusion would not happen. The size of the optical beam at distance L from the transmitter is approximately h=(l x d) / f as shown in Fig.13. Since the dimension of the filament used in this experiment is 4mm x 1.5mm, the beam size is 174cm x 65cm on the solar panel (Fig.14). Available electrical power at the receiver is calculated under the condition of light source power, reflector/solar panel dimension, distance and solar panel efficiency. Figure 15 shows the theoretical result. Experimental result is indicated as a dot on the graph under the conditions shown in Fig.14. Electrical power of 2 mw was obtained at m distance. Figure 16 shows a photograph of the transmitter. Fig.14 Experimental Configuration light Source Available Power (W) 1 0.1 Distance (m) 20 40 0.01 0 00 000 Reflector Diameter (cm) x Solar Panel Size (cm square) Fig.15 Size of Optical Components and Available Electric Power Fig.16 Tungsten Halogen Bulb with Parabolic Mirror 6 RF Link Budget Figure 17 shows the system structure and an example of the RF link budget. The distance between the base station and the repeater station is 40 m. RF amplifiers, an antenna and a circulator are used to compose the system. Low power consumption amplifiers were developed for this system. ISBN: 978-960-6766-79-4 55 ISSN 1790-5117

-28.4dBm Gain 38.4dB Distance=40m RF Power mw (36.3mW) RF Loss = 5.7+22.7=28.4dB RF Power Optical Loss=5.7dB 0dBm TX AMP (2mW) RX AMP Substantial RF Gain 15.6dB Gain 44dB ( 16.3mW) 26cm 26cm x 26cm 54.6mW ( ) : Consuming Power Base Station Repeater Station Fig.17 Example of the Link Budget The performance of the receiving amplifier (Fig.18) is, Gain: 22 db Power consumption: 8.2 mw. A transmitting power amplifier is made of a MES FET NE76984 (NEC) as shown in Fig. 19. The performance is shown in Fig. 20. Maximum efficiency is 50 % at mw output power.. Fig.18 Low Power Consumption RF Amplifier Fig.19 RF Power Amplifer (MES FET NE76084, NEC) The link budget is shown in Fig. 17. In order to transmit mw RF power from the antenna, 55 mw electrical power supply is required. In the case that 50 W tungsten halogen bulb is used, the size of the reflector and the solar panel is 26 cm. Output Power (mw) 4 2 1 0.4 0.2 0.1 Gain 8dB 1 2 4 20 Consuming Power (mw) Fig.20 RF Power Amplifier Efficiency If 0 mw RF output power is required and the power efficiency of the RF amplifier is assumed to be 30 %, then 390mW power is necessary. This power is obtained using 48 cm diameter reflector and solar panel. 7 Conclusion A wireless optical feeder system with optical power transmission was investigated experimentally. This system is applicable as the feeder of the cellular base stations in the case that neither power supply nor transmission cable is available, such at some of the building top or indoor spaces. All of the optical and RF components applied for this system are available easily and not expensive. Although the distance of the base station and the repeater station is limited, there will be many places where this system is indispensable. In order to make a compact system, efficient optical power transmission is important to reduce the reflector or solar panel size. A strong optical point source, such as an arc lamp, will be suitable to meet this requirement.. References: [1] N.Nakajima, ROF Technologies Applied for Cellular and Wireless Systems, 2005 International Topical Meeting on Microwave Photonics, Seoul, Oct. 2005 [2] H. Kawano, N. Nakajima, Simple and Low Loss Power-Supply-Less ROF Repeater, AP-MWP 2006, Postdeadline Papers, p. 2, ( April 2006) [3] N.Nakajima, et al., A Study on the Wireless Repeater without Electric Power Supply Using ROF Technology, Technical Report of IEICE, RCS2004-324, pp.127-132 (March 2005) [4] K.Kazaura, et al, Performance Evaluation of Next Generation Free-Space Optical Communication System, IEICE Trans. On Electronics Special Section on Evolution of Microwave and Millimeter-Wave Photonics Technology, vol.e90-c, no.2, 2007 ISBN: 978-960-6766-79-4 56 ISSN 1790-5117