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Continuous zoom antenna for mobile visible light communication XUEBIN ZHANG, YI TANG,* LU CUI,

AND

TINGZHU BAI

Key Laboratory of Photoelectronic Imaging Technology and System, Ministry of Education of China, School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China *Corresponding author: [email protected] Received 27 August 2015; revised 29 September 2015; accepted 13 October 2015; posted 13 October 2015 (Doc. ID 248773); published 10 November 2015

In this paper, we design a continuous zoom antenna for mobile visible light communication (VLC). In the design, a right-angle reflecting prism was adopted to fold the space optical path, thus decreasing the antenna thickness. The surface of each lens in the antenna is spherical, and the system cost is relatively low. Simulation results indicated that the designed system achieved the following performance: zoom ratio of 2.44, field of view (FOV) range of 18°–48°, system gain of 16.8, and system size of 18 mm × 6 mm. Finally, we established an indoor VLC system model in a room the size of 5 m × 5 m × 3 m and compared the detection results of the zoom antenna and fixed-focus antenna obtained in a multisource communication environment, a mobile VLC environment, and a multiple-input multiple-output communication environment. The simulation results indicated that the continuous zoom antenna could realize large FOV and high gain. Moreover, the system showed improved stability, mobility, and environmental applicability. © 2015 Optical Society of America OCIS codes: (120.4570) Optical design of instruments; (080.4295) Nonimaging optical systems; (060.4510) Optical communications. http://dx.doi.org/10.1364/AO.54.009606

1. INTRODUCTION Visible light communication (VLC) is a wireless optical communication technology based on the source of light-emitting diodes (LEDs). Compared with wireless communication technology, VLC shows higher communication speed and is characterized by anti-wiretapping, anti-interference, and high reliability [1,2]. Combined with an existing wireless communication network, the technology is the solution to “the last 100-meter connectivity” and has become one of the hotspots of wireless short-range communication technology [3,4]. Previous studies were focused on the communication speed and sensitivity of VLC systems, which raised the requirements of simple and cheap transceivers [5]. During communication, a mobile receiving end can better meet user demands. Scholars from various countries studied the application of receiving optical antennas in mobile VLC [6,7]. Vehicular visible light communication (VVLC) network links [8] are tolerant of visible light noise and interference under working conditions and satisfy the requirements of stringent viability and latency in dense vehicle traffic conditions. The convex and concave reflector antennas and compound parabolic concentrator have large FOV and can improve mobile reception performance [9,10]. However, its optical gain is low. A spatial diversity antenna is composed of multiple small antennas and each small antenna is responsible for one viewing angle and direction [11]. In this way, 1559-128X/15/329606-07$15/0$15.00 © 2015 Optical Society of America

system mobility is increased, but the antenna structure is complex. Cell zooming [12] rearranges cell coverage regions in the network while maintaining the illumination required by the lighting system, but the cost is high. The beam-forming technology [13,14] can focus light on a desired target and improve the energy transmission, but its user number is limited and it is complex to implement. Most previous studies of optical receiving antenna were focused on the fixed-focus antenna. However, if the distance between the light source and the receiving antenna is changed, the actual communication performance will be correspondingly changed. In a VLC environment, it is assumed that light sources are uniformly and symmetrically distributed on the room ceiling and that the lamp placed on the desktop is used as backup source. If a fixed-focus optical antenna is used as the receiving end, the following problems may occur. First, when there are several signal light sources on the ceiling in a room and different lights emit different signals, the viewing angle of the optical receiving antenna is large enough to detect the signal light and stray light from other sources simultaneously. Moreover, if the FOV of an optical receiving antenna is larger than the source launching angle, it will receive more stray light. Therefore, the communication performance of the system will decline.

Research Article In a mobile VLC environment, the distance between the optical antenna and the signal light source may be changed. For example, after turning off the signal light on the ceiling, the table lamp is used as the signal light source. If the antenna– object distance is reduced after the signal light source is changed, it will disperse the spot on the detection surface, thus reducing the energy received by the system. If the multiple-input multiple-output (MIMO) communication technology is used to increase the channel capacity of the system, it is optically required that the detector array be able to accurately receive signals from the LED array and that each signal spot should be concentrated. If the fixed-focus antenna is used as a receiving antenna, it is necessary to ensure that the distance between receiver and transmitter is accurate and that the receiver cannot move. Otherwise, the spot position will be changed, thus resulting in the detection failure. Therefore, the application of the technology has been largely restricted. Aiming at the above problem, in the paper we designed an optical zoom-receiving antenna for mobile VLC. To the best of our knowledge, this is the first time an optical zoom-receiving antenna was applied in a VLC system. The designed antenna showed high stability and mobility while maintaining large FOV and high gain. The focus range of the system is from 4 to 8 mm, and the FOV is 18°–48°. The zoom ratio of the system is 2.44, and the optical gain of the system is 16.8. The system size is 18 mm × 6 mm, and the structure is simple and cheap. At last, in the paper, we established an indoor VLC system model in a room of 5 m × 5 m × 3 m and compared the received results of the zoom antenna and the fixed-focus antenna. Simulation results indicated that in a complex communication environment, the continuous zoom-receiving antenna showed high stability, mobility, and environmental adaptability and was suitable for mobile VLC. The paper is organized as follows. In Section 2, the basic structure and design method of the optical system are introduced. Detailed design and analysis are given in Section 3. In Section 4, the communication performances of the optical zoom-receiving antenna and the fixed-focus antenna are tested and compared. Finally, conclusions are provided in Section 5.

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same. This method can ensure the stable image plane position when the system is zooming continuously, but the method requires the complicated zoom cam structure. With the improvement of the precision instrument, the difficulty of the nonlinear cam curve is greatly reduced. Therefore, the mechanical compensation zoom system is increasing widely used [15]. In order to ensure good reception when the receiving antenna is zooming continuously and reduce the machining difficulty, in the paper we adopted a single zoom single mechanical compensation system as the initial structure of the optical zoom-receiving antenna. B. Theoretical Calculation

The diagram of thezoom antenna is shown in Fig. 1. The initial structure calculation steps are as follows [16]: f i is the focal length of lens group; βi is the magnification of lens group; βi0 is the magnification of lens group at the zoom position; M is the room ratio; d ij is the initial interval between each lens group; d ij0 is the interval between each lens group at the zoom position; f is the initial focal length of the system; f 0 is the focal length when the ratio rate is M ; x is the displacement distance of the zoom group; and y is the displacement distance of the compensation group. We obtained the initial magnifications of the zoom group and compensation group according to Eqs. (1) and (2): f2 ; f 1  f 2 − d 12 f3 β3  : f 3  f 2 1 − β2  − d 23 β2 

(1) (2)

The zoom ratio of the zoom system is f M  0: (3) f When the focal length is f 0 , the magnification of the zoom group is calculated according to Eq. (4): h i f 2 1−f  f 1  f 3 − 2 − d 23 − f −d 2331−f1 f 3  1 1  β20  f 3 −d 23 1−f 1 f 1 −1 2 M 1

2. STRUCTURE DESIGN OF THE ZOOM ANTENNA

Q2 ;   f 3 −d 23 1−f 1 f 1 2 − 1 M

A. Initial Structure

The optical zoom system can maintain stable image surface and good image quality while the focus length is changing. According to the compensation method, zoom systems can be divided into an optical compensation zoom system and a mechanical compensation zoom system. In the optical compensation zoom system, several lens groups are used as the zoom group and compensation group. The moving direction and speed of each lens group are the same, and a clear image is not available until each group moves to a certain position. The mechanical structure of the system is simple, but the system structure is large and the focal length cannot be changed continuously. The mechanical compensation zoom system compensates the movement of the image plane by moving a few compensation groups. The moving directions and speeds of the compensation group and zoom group are not necessarily the

Fig. 1. Diagram of the zoom antenna.

(4)

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where Q is calculated according to Eq. (5):  2 f 23 1 − f 1  Q  f 1  f 3 − 2 − d 23 − f 3 − d 23 1 − f 1   f 1  M f 23 −4 1  f 3 − f 3 − d 23 1 − f 1   f 1  f − d 23 1 − f 1   f 1 : − 3 M

Table 1. Design Indexes of the Continuous ZoomReceiving Antenna

(5)

The magnification of the compensation group after zooming is calculated according to Eq. (6): M β2 β 3 : (6) β30  β20 The displacement distance of the zoom group and the compensation group can be calculated according to Eqs. (7) and (8):   1 1 xf2 (7) − 0 ; β2 β2 y  f 3 β30 − β3 :

(8)

Then, we can get the interval between each lens group with the displacement according to Eqs. (9) and (10): 0 d 12  d 12  x; 0  d 23 − x  y: d 23

(9) (10)

If the system parameters, such as the system focus and the zoom ratio, are known, we can substitute them into the above equations. Through calculation, the distances among various lens groups can be maintained within a reasonable range (greater than zero). In this way, we can get the initial structure. The continuous zoom antenna in this paper adopts the negative–positive–negative structural type, which can adjust the angle of the chief ray and make the detected energy more even. As the optical antenna is used in VLC, it does not require good imaging quality. We are more concerned about cost, compact structure, and efficiency. Based on the above considerations, the entire lens in the antenna is a spherical lens, which can save largely on cost. In order to facilitate the use of the system and reduce the thickness of the optical receiving antenna, a rectangular reflecting prism is used to fold the space optical path so that the lens group moves along the length direction of the optical antenna. In this way, the movement space of each lens group is increased and more space may be used to install the driver assembly. 3. SIMULATION AND DESIGN OF THE CONTINUOUS ZOOM ANTENNA A. Design of the Continuous Zoom Antenna

Based on the requirements of mobility, high gain, and large FOV for the mobile VLC system, we designed a continuous zoom-receiving antenna. The antenna indexes are shown in Table 1. According to the above equations, we obtained the initial structure data. After optimizing the antenna based on the indexes above, we obtained the structure of the continuous zoom-receiving antenna with ZEMAX. The optical path is shown in Fig. 2.

System length System diameter System focal length Detector surface diameter FOV Relative illumination Wavelength range

Shot Focus

Medium Focus

Long Focus

13 mm 12 mm 3 mm 3 mm

13 mm 12 mm 6 mm 3 mm

13 mm 12 mm 9 mm 3 mm

25° ≥60% 0.48– 0.7 mm

15° ≥60% 0.48– 0.7 mm

10° ≥60% 0.48– 0.7 mm

Figures 2(a)–2(c), respectively, show a short focal length system, medium focal length system, and long focal length system. Lenses (1–3) are the fixed group, which can expand the FOV and fold the space optical path. Lenses (4–5) are the zoom group, and the distance is relatively fixed. The zoom group can change the system focal length by moving the zoom group back and forth. Lens 6 is the compensation group, which can compensate the detector surface movement by moving the zoom group back and forth. Element 7 is the detector. All the lenses in the system are spherical lenses, which can make the structure simpler and cheaper. The system size is 18 × 6 mm. The detection plane diameter is 3 mm. The focal lengths are 3.6, 6, and 8.8 mm. The room ratio is 2.44, which is the ratio of maximum focal length to minimum focal length. The FOVs are 48°, 32°, and 18°. The details of the optical descriptions for the elements in the system are shown in Table 2. The spacing is changed from long focal length to short focal length. The system length remains unchanged during continuous zooming, and the detection surface is stable. The system basically meets the design requirements. B. Simulation Analysis of the Optical Receiving Antenna

The illumination distribution reflects the uniformity of the antenna. The detection uniformity of the continuous zoom antenna is shown in Fig. 3. The illumination distribution on the detection surface is uniform. All the energy is concentrated within the plane with a diameter of 3 mm, and the spot flatness is good. The efficiency of the antenna is 92.6%. The relative illumination is the ratio of the edge field illumination to the center field illumination and indicates the detection efficiency. Figure 4 shows the illumination curve of each focal length. In the simulation results, all the relative illumination is higher than 90%, indicating that the optical antenna has high detection efficiency and that the illumination of the detection surface is uniform. Optical gain is an important indicator of the optical antenna in an optical communication system and is defined as the received optical power ratio of the detector with optical antenna to the detector without optical antenna [17]. For this antenna, the optical gain can be calculated according to Eq. (11) [18]:  2 n 0 ≤ ψ ≤ ψc : (11) gψ  sin2 ψ c 0 ψ > ψc

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Fig. 2. Light path diagram of continuous zoom-receiving antenna: (a) short focal length system, (b) medium focal length system, and (c) long focal length system.

In Eq. (11), n is the system refractive index; ψ c ≤ π∕2 is the system FOV. In this continuous room antenna, the optical gain is 16.8, which can satisfy the basic requirements of optical communication system. 4. COMMUNICATION PERFORMANCE ANALYSIS OF CONTINUOUS ZOOM ANTENNA In order to analyze the advantage of continuous zoom antenna in mobile VLC, we proposed an indoor VLC system utilizing Table 2. Actual Details of Optical Descriptions for the Elements in the System Spacing/mm Lens Radius of (LongCurvature/ Thicknesses/ Short) mm Lens mm (R1/R2)

Glass Types

1

—

K9

2 3 4

−75.64/ −71.69 — −4.72/6.21 3.61/−2.62

— 1.1 1.72

7 5 0.76–4.3

5 6 7

−2.14/−3.63 3.78/1.87 —

0.5 1.63 0.5

0.1 1.77–0.58 5.01–2.89

Mirror Qk3 NPSK53A ZF10 Qk3 —

1

white LED lights and compared the detection results of the optical zoom antenna and optical fixed-focus antenna. As shown in Fig. 5, the room size is 5 m × 3 m × 3 m and four groups of white LED arrays are symmetrically distributed on the room ceiling. Each group has 60 × 60 LED units [19]. The LED angular distribution of transmission power is shown in Fig. 6, and the power is 20 mW. The table in the room is 0.85 m high. The receiving antenna is connected to a laptop and placed on the table. The distance between the receiving antenna and the light source on the ceiling is 2 m. A table lamp is placed on the table, and the source of the table lamp is the same as the array LED source on the ceiling. The distance between the antenna and the table lamp source is 0.5 m. We used the avalanche photodiode detector (APD) as the detector. The detection surface diameter is 3 mm. The gain is 100 and the dark current is 0.05 nA. The excess factor is 2. A. Antenna Performance in Multisource Communication Environment

As shown in Fig. 7(a), the optical receiving antenna may receive light from several sources in a multisource communication environment. Source A and Source B are placed on the ceiling, and the communication distance is about 2 m. As Source A is closer to the antenna than Source B, Source A gives the stronger signal and becomes the signal light source, whereas Source B becomes a stray light source. If we adopted the fixed-focus

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Fig. 3. Detection surface uniformity analysis: (a) illumination two-dimensional surface and (b) Illumination Y scan.

Fig. 4. Relative illumination curve of continuous zoom antenna: (a) relative illumination curve of 3.6 mm focal length; (b) relative illumination curve of 6 mm focal length; and (c) relative illumination curve of 8.8 mm focal length.

antenna in this case, as shown in Fig. 7(a), it would be difficult to decrease the stray light from Source B. If we adopted the continuous zoom antenna shown in Fig. 7(b), the stray light from Source B could be decreased by the continuous zoom antenna. The signal noise ratio (SNR) of the VLC system can be calculated according to Eq. (12):

dark current without multiplication, and surface dark current; M is the gain and F M  is the excess factor; B is the bandwidth; T is the absolute temperature; R L is load resistance;

i 2p M 2 S  2 2 i q  i DB  i 2DS  i 2T N 

i 2p M 2 : (12) 2ei p  i D M F M B  2ei L B  4kT BR L  2

In Eq. (12), i 2p , i 2q , i 2DB , i 2DS , and i 2T are, respectively, the quadratic averages of light current, quantum noise current, ontology dark current, surface dark current, and thermal noise current; i p , i D , and i L are, respectively, light current, ontology

Fig. 5. VLC system communication environment.

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Fig. 6. Angular distribution of LED transmission power.

e is the electric quantity of elementary charge; and k is Boltzmann’s constant. In general, APD noise mainly comes from the device itself and thermal noise is small [19]. According to the simulation results, the SNR of the system in Fig. 7(a) is 12.87 dB and the SNR of the system in Fig. 7(b) is 24.95 dB. The SNR of the zoom system has been greatly improved. Simulation results showed that the continuous zoom antenna had higher environmental adaptability than the fixedfocus optical antenna and could be applied in a complex communication environment. B. Antenna Performance in Mobile VLC System

Fig. 8. Comparison of detection results of the zoom system and fixed-focus system.

spot size by adjusting the focal length, thus ensuring the high detection efficiency. In the same communication condition, the detection spot size of the fixed-focus antenna becomes large, thus reducing the detection efficiency of the system. The optical gain of the fixed-focus antenna is reduced to 8.4, and the optical gain of the continuous zoom antenna is increased by two times. Simulation results showed that the continuous zoom antenna had higher mobility and stability in the mobile VLC system.

In the indoor VLC environment, if the receiving end or the signal light source can move freely, it is convenient to users. Therefore, it is required that the optical antenna shows stable detection performance when the distance between the light source and the optical antenna is changed. As shown in Fig. 5, if we turn off the lamp on the ceiling and use the table lamp as the signal light source, the communication distance is changed from 2 to 0.5 m. The detection results are shown in Fig. 8. The spot on the left of Fig. 8 is the detection result obtained at a communication distance of 2 m. The spot in the middle of Fig. 8 is the detection result obtained with the continuous zoom antenna at a communication distance of 0.5 m. The spot on the right of Fig. 8 shows the detection result obtained with the fixed-focus antenna at the communication distance of 0.5 m. As shown in Fig. 8, if we change the communication distance, the continuous zoom antenna allows an unchanged

In MIMO technology, it is optically required that each detector in the detector array can accurately receive the corresponding LED signal and that the spot energy is concentrated. In order to facilitate comparative analysis, we used 1 × 5 LED arrays as signal sources and the receiving antenna moved back and forth within the distance of 1.2 m. The detection results are shown in Fig. 9. As shown in Fig. 9(a), when the position of the receiving end is changed, if we use the optical fixed-focus antenna to receive the signal, the position of each signal spot is significantly changed, and the detector arrays cannot detect the signal. As shown in Fig. 9(b), if we use the continuous zoom antenna in

Fig. 7. Receiving condition of the optical receiving antenna in the environment of multiple light sources: (a) receiving condition of fixed-focus antenna and (b) receiving condition of continuous zoom antenna.

Fig. 9. Received results of APD array detecting surface: (a) detection results of optical fixed-focus antenna and (b) detection results of continuous zoom antenna.

C. Antenna Performance in Mobile MIMO Communication System

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the same condition, when the position of the receiving end is changed, the detection surface can receive the signal clearly and the position of each signal spot remains unchanged. Therefore, the signal can be received by the detector arrays. Simulation results showed that the continuous zoom antenna could be better applied in the mobile MIMO communication system. 5. CONCLUSIONS Considering the complexity of the mobile VLC environment and the limitations of traditional optical receiving antennas, we designed a continuous zoom antenna as a receiving antenna in the paper. Simulation results indicated that the designed system achieved the following performance: zoom ratio of 2.44, FOV range of 18°–48°, system gain of 16.8, and system size of 18 mm × 6 mm. The designed structure is simple and cheap. In order to analyze the communication performance of the continuous zoom antenna, we established an indoor VLC system communication model in a room the size of 5 m × 5 m × 3 m and compared the detection performances of the optical fixed-focus antenna and continuous zoom antenna. Simulation results showed that the continuous zoom antenna had higher optical gain and SNR in the same condition. Therefore, the continuous zoom antenna can meet the requirements of large FOV and high gain and has better stability, portability, mobility, and environmental suitability. Compared to a traditional optical fixed-focus antenna, the continuous zoom antenna is more suitable for a mobile VLC system. Funding. The National Key Basic Research Program 973 Project (2013CB329202); National Natural Science Foundation of China (NSFC). REFERENCES 1. D. Kedar and S. Arnon, “Urban optical wireless communication networks: the main challenges and possible solutions,” IEEE Commun. Mag. 42(5), S2–S7 (2004). 2. C. Dominic, L. B. Zeng, H. L. Minh, G. Faulkner, J. W. Walewski, and S. Randel, “VLCs: challenges and possibilities,” in IEEE 19th International Symposium on Personal, Indoor and Mobile Radio Communications (2008), pp. 1–5.

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