Experimental Demonstration of Optical Camera Communications Based Indoor Visible Light Positioning System Bangjiang Lin(1, 2), Xuan Tang(1), Yiwei Li(1), Min Zhang(1), Chun Lin(1), Zabih Ghassemlooy(3), Yunfeng Wei(2), Yi Wu(2) and Hui Li(2) (1)
Quanzhou Institute of Equipment Manufacturing, Haixi Institutes, Chinese Academy of Sciences, Quanzhou, China,
[email protected] (2) Key Laboratory of OptoElectronic Science and Technology for Medicine of Ministry of Education, Fujian Normal University, China (3) Optical Communications Research Group, Faculty of Engineering and Environment, Northumbria University, Newcastle, U.K.
ABSTRACT We experimentally demonstrate an indoor visible light positioning system based on optical camera communications, in which the transmitted coordinate data are spatially separated and demodulated by a camera. The receiver’s position is calculated based on the coordinates of light-emitting diodes in the real word and in the image. Keywords: Visible light communications (VLC), Optical camera communications (OCC), light emitting diode, indoor positioning. 1. INTRODUCTION Indoor positioning systems (IPSs) using light-emitting diodes (LEDs) have attracted much attention from both academic and industry [1-2], due to the advantages of high positioning accuracy, license-free operation, no electromagnetic interference and low-cost front-ends, etc. The indoor positioning methods based on visible light communications (VLC) are mainly classified as: triangulation, proximity and image positioning. The image positioning method determines the position of the Rx based on the coordinates of LEDs in the real word and in the image. Most research works including [3-6] assume that the coordinates of LEDs in the real word are already known prior to determining the position of Rx. However, they are unknown in practical applications. Thus, it is important to establish the communications link between the LEDs and the Rx in order to acquire the coordinates of LEDs. In addition, the existing works reported in [3-5] are mainly simulation based with no experimental verifications. In this paper, to the best of our knowledge, for the first time we experimentally demonstrate a VLC-IPS
based on a commercial camera, in which the coordinates of LEDs are transmitted from LEDs to the camera by means of optical camera communications, and the position of the camera is calculated using the decoded LED coordinates and the estimated distances between the LEDs and the camera. The distance estimation method has lower complexity compared to the existing works [3-6]. The under-sampled phase shift keying (UPSK) modulation scheme is adopted to encode the coordinate data, which offers non-interference VLC with space division multiplexing capabilities [7-8]. We show that the proposed scheme for indoor applications offers mean positioning errors (PEs) of 5.0 cm for h of 120 cm. h is the distance of the camera from the ceiling. In addition, an error free transmission is achieved over a transmission distance of 1-6 m, which represents a typical indoor environment. 2. PROPOSED SYSTEM Fig. 1 shows the schematic block diagram of the proposed scheme composed of LEDs and a camera as the Txs and the Rx, respectively. We assume that LEDs are evenly distributed on the ceiling and each LED transmits its coordinate data via a free space channel to the Rx. A camera based Rx, which is composed of a lens and a two dimensional sensor, extracts the coordinates of the three selected LEDs (i.e., A (xA, yA, 0), B(xB, yB, 0), C(xC, yC, 0)) from the captured videos or photos. The coordinate of the center of the lens O (x, y, z) is considered as the position of the camera. For the lens being parallel to the ceiling, the projections of O on the ceiling and the sensor are O1(x, y, 0) and O’, respectively. The distances of O’A’ ( d O ' A ' ), O’B’ ( d O ' B ' ), O’C’ ( dO ' C ' ), A’B’ ( d A ' B ' ), B’C’ ( d B ' C ' ) and A’C’ ( d A ' C ' ) can be calculated based on the relative positions of received pixels and the size of captured photos. The distance of OO1 (h) can be calculated from:
arbitrary waveform generator (AWG) operating at 0.1-MS/s. The UPSK signals s(t) can be expressed as:
C
O1 y 0 z
O2
A
B
x h
θ
x’ y’
f
C’
A’ B’
A’
O’
O
θ
B’
O’ C’
Fig. 1 A system block diagram of proposed IPS with LEDs and a camera.
d d d h = AB = BC = AC , f d A ' B ' d B 'C ' d A ' C '
(1)
where f is the focal length of the camera, d AB , d BC ,
d AC are the distances of AB, BC and AC, respectively. Therefore, the z coordinate of O can be obtained from (1). The distances of O1A ( dO1 A ), O1B ( dO1B ) and O1C ( dO1C ) can be obtained from: d h d O1 A d O1 B = = = O 1C . f dO ' A ' dO ' B ' dO ' C '
ª (dO1 A − dO1 B + x + y − x − y ) / 2 º D=« ». 2 2 ¬ ( dO 1 A − dO 1 C + x + y − x − y ) / 2 ¼ (4) can be solved using the linear least square method, which is given by: X = (MT M) −1 MT D . (5) 2
2 B 2 C
2 B 2 C
2 A 2 A
an
s (t )
(2)
With dO1 A , dO1B , and dO1C , the x and y coordinates of O can be obtained using a set of three quadratic equations as follows: dO1 A 2 = ( xA − x)2 + ( yA − y ) 2 ° 2 2 2 (3) ® dO1 B = ( xB − x) + ( yB − y ) . 2 2 ° dO1C = ( xC − x) + ( yC − y ) 2 ¯ (3) can be rewritten in the matrix form as: (4) MX = D , where ª x − xA yB − y A º ª xº M=« B » , X = « y» , x − x y − y ¬ ¼ A C A¼ ¬ C 2
Fig. 2. Experiment setup for visible light positioning based on OCC (h: the distance of the camera from the ceiling, modules shown within Tx-A also applied to Tx-B&C).
2 A 2 A
3. EXPERIMENTAL SETUP AND RESULTS The feasibility of the proposed scheme is demonstrated experimentally as shown in Fig. 2. The coordinates of Tx-A, Tx-B and Tx-C are (33, 20.4, 0) cm, (49, 36.4, 0) cm, (49, 20.4, 0) cm, respectively, and the camera with a coordinate of (x, y, z) is located in the Rx plane with a distance of h from the ceiling. At the Tx, the coordinate data is encoded into UPSK symbols as shown in Fig. 3(a) and then uploaded to an
Fig. 3. Block diagram of: (a) coder, and (b) decoder.
°sgn ( cos(2π f h t ) ) ,0 < t
