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2008 年，T.D.C. Little、P. Dib、K. Shah、N. Barraford 及 B. Gallagher 四個人發表了一篇名為 Using LED Lighting for Ubiquitous Indoor Wireless Networking 的論文。主要是針對普及的、家用的 VLC 系統作一個概略性的介紹，

以及實驗的展示。而值得一題的是，該篇論文除了展示單向傳輸的 VLC 通訊系統 外，同時也實際展示了雙向傳輸的 VLC 系統，並且針對可能產生的情況作分析與 改進。

該實驗的架構如圖(3-22)所示，是一個非常簡單的架構，由兩台電腦分別連 接兩組儀器所構成。在第一個實驗中這兩個儀器一個是發射訊號的 Transmitter，

另一個是接收訊號的 Receiver。兩個都是以手電筒的外殼作包裝，在

Transmitter 端是由一系列的電路與 LED 構成的，並且與電腦連接以便控制訊號。

而在 Receiver 端則是由一系列的電路與光感測器構成的，同樣的與電腦連接以 便分析接收到的訊號。如圖(3-23)所示。

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I. Introduction

Wireless networking is dominated by the use of radio frequency (RF) techniques. The desire for low-cost, high-speed links has motivated recent interest in infrared wireless communication [1]. At the same time the rapid development of new LED materials in the visible spectrum, particularly “white”

LEDs motivates consideration of visible light as a communication medium since it is expected to be widely adopted as the next form of illumination. With its significant energy savings over incandescent and compact fluorescent technologies this adoption creates an opportunity in the replacement of devices. Because LEDs are easily modulated for communications, there is a corresponding opportunity to insert wireless LED-based communication into these illumination devices.

The LED-based visible-light optical channel becomes a very attractive complement or even substitute for RF techniques. It has advantages of large potential bandwidth (THz) with no regulation or license fee. Optical communication with lasers is successfully employed today; however, simple LEDs, as shown in this paper, can be used for the required distances and in the context of noise sources indoors, where we expect to apply LED-based lighting. Finally, in addition to lower energy consumption LEDs have a significantly longer lifetime and are safer for the environment as they lack the harmful ingredient of mercury.

To demonstrate the concept and prove viability, we have sought to create two prototypes to capture scenarios associated with future access points providing LED-based indoor networking. The first is a simplex channel characteristic of one-half of an asymmetric hybrid RF-FSO model in which data may be downloaded at high speed from overhead illumination to a receiver (e.g., HD video streaming to a laptop). In such a hybrid model we can isolate asymmetric traffic and benefit from the line-of-sight characteristics of light and its channel isolation. The second model is of a full-duplex channel on which we can support the construction of a network interface for multiple access embedded in ceiling lighting. A photo of the simplex channel is shown in Fig. 1.

Fig.1 Prototype of simplex using flashlight LEDs

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Using off the shelf components (hollow flashlights as packaging, low cost circuitry and LEDs), the prototype supports communication between two computers using a serial link (we used PuTTY [2]

and USB/RS-232). The transmitter and receiver are powered by 9V batteries.

Fig.2 System Block Diagram of simplex channel

The simplex channel permits transmission in one direction only. Fig. 2 shows the overall signal flow.

By connecting computers at each end we can demonstrate the transmission of ASCII characters from on terminal to the next using the visible light channel. The signal chain uses visible light through an array of 10 LEDs. The receiver employs an array of 3 photodiodes and corresponding OOK decoding circuitry.

In the remainder of the paper we describe related work in LED-based communications (Sec. II), modulation (Sec. III), frequency considerations (Sec. IV), performance (Sec. V), the duplex channel (Sec. VI). Section VII concludes the paper.

II. Preliminaries A. LED vs. Laser

LEDs are constructed from direct band-gap semiconductor materials whose band-gap energies correspond to frequencies in the visible spectrum. In order to make LEDs efficiently emit light, this is typically accomplished through forward biasing a p-n junction. The current flowing through the junction allows for the rate of the radiative electron-hole recombination to be large. The light emitted from this radiative recombination is in the form of spontaneous emission and is radiated uniformly in all directions.

Laser diodes are made from similar materials as LEDs however they are generally more heavily doped to allow the materials to act as a semiconductor optical amplifier. As with all lasers feedback is required. In laser diodes it can take the simple form of cleaved surfaces which take advantage of internal reflections from the difference of indexes of refraction or engineered Distributed Bragg Reflectors [6]. The light emitted from laser diodes is in the form of stimulated emission.

Typical characteristics of LEDs and Laser diodes that can be used as a basis for comparison are their respective spectral intensities, efficiencies, and output optical powers. The spectral intensity of LEDs is very broad in comparison with that of the laser diode which is very narrow as a result of stimulated emission. Overall laser diodes are also more efficient with respect to power conversion efficiency and the output optical power is generally much greater. This arises from the fact that the laser light is

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圖 3 - 24 LED 雙向傳輸電路架構示意圖

而這樣的雙向傳輸系統最大的挑戰就在於如何將手電筒內的反光鏡(用來集 中光線用的)與 Transceiver 結合且不會有 Cross talk 的產生。因為光感測器需 要偵測到一定強度的光線，所以我們會在 Transceiver 四周、也就是手電筒內加 modulation (e.g. fractional off keying), drivers (reduction in RC limitations), and clocking circuits (closed-loop signal recovery).

VI. Duplex Channel

A duplex communication system in our context is formed by two simplex channels. The support of bidirectional communication is essential for the support of network interfaces that permit multiple access and indoor networking.

Conceptually the duplex prototype involved laying components corresponding to the simplex channel in parallel but opposing directions. However, the selected (duplex) transceivers permitted reuse of some components at each end. A schematic of the duplex prototype is shown in Fig. 10. One of the challenges was to design a transceiver that would utilize the flashlight reflectors by combining both LEDs and photodiodes without crosstalk.

Fig. 10 Duplex Channel Communication Interface (Transceiver) A. Simplex vs. Duplex: LEDs & Photodiodes

To transition from a simplex channel to a duplex channel, it was necessary to reconfigure the components within each reflector. Rather than having one reflector completely populated with LEDs (an array of 10) and another with just three central photodiodes, both reflectors needed to contain some number of both LEDs and photodiodes. Given the success of the reflector when implementing the simplex channel, it was quickly decided that the optical design for the duplex channel should also conform to the reflectors.

(a) (b)

Fig. 11 (a) Flashlight reflector with 3 photodiodes mounted in the center for duplex channel (b) LED and isolated photodiode array

56 approximately uniform illumination. The circular arrangement of the LED’s within the reflector leads to the center of the beam having the highest intensity at some distance away from the source. These two physical considerations led to a duplex design where the two sides are identical, with 7 LED’s around the perimeter of the reflector and 3 photodiodes in the center. The LED’s could not easily be removed from the flashlight circuit, so independent LED’s were chosen and purchased.

B. Crosstalk

In order to create a compact design we included both the transmit and the receive arrays on the same reflector that we used for the simplex channel. This led to the problem of crosstalk which affects our ability to receive data. In order, to reduce this problem we proceeded by wrapping the arrays of photodiodes with black electrical tape to reduce the effects of the local transmitter (Fig. 11 (b)).

This setup was beneficial, however it did not completely isolate the photodiodes from crosstalk and the reflectors actually served to be detrimental. The oscilloscope plots are shown in Fig. 12.

(a) (b)

Fig. 12 (a) Crosstalk (b) No Crosstalk - Oscilloscope Outputs for duplex channel

The first picture shows the effect of crosstalk with reflectors and the second image shows the received signal without the reflector. It can be seen on the crosstalk is more sever with the reflectors and this is because the reflected light by the local transmitter would also be recognized by the photodiodes at the small distances of separation that we were testing. Without the reflector the crosstalk is extremely diminished as shown in the figure on the left.

C. Results

The full duplex circuits, based on their origin of the simplex design, were anticipated to have similar performance limitations. In practice, we observed lower data rates due to the limitations of the breadboard implementation. This configuration was inconclusive in terms of the noise introduced by the illumination component, the bidirectional crosstalk, and other potential effects.

VII. Summary

This paper considers the opportunity provided by the replacement of existing illumination systems with LED-based lighting and the potential to introduce free-space optical networking with LED transceivers.

We developed simplex and duplex transceiver prototypes as part our investigation and demonstrated their expected and achieved performance. In summary, using off-the-shelf LEDs and photodiodes we were able to demonstrate viable communication using visible light in the presence of normal room

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of the driver. Clearly there is an opportunity for improving performance with more elaborate modulation (e.g. fractional off keying), drivers (reduction in RC limitations), and clocking circuits (closed-loop signal recovery).

VI. Duplex Channel

A duplex communication system in our context is formed by two simplex channels. The support of bidirectional communication is essential for the support of network interfaces that permit multiple access and indoor networking.

Conceptually the duplex prototype involved laying components corresponding to the simplex channel in parallel but opposing directions. However, the selected (duplex) transceivers permitted reuse of some components at each end. A schematic of the duplex prototype is shown in Fig. 10. One of the challenges was to design a transceiver that would utilize the flashlight reflectors by combining both LEDs and photodiodes without crosstalk.

Fig. 10 Duplex Channel Communication Interface (Transceiver) A. Simplex vs. Duplex: LEDs & Photodiodes

To transition from a simplex channel to a duplex channel, it was necessary to reconfigure the components within each reflector. Rather than having one reflector completely populated with LEDs (an array of 10) and another with just three central photodiodes, both reflectors needed to contain some number of both LEDs and photodiodes. Given the success of the reflector when implementing the simplex channel, it was quickly decided that the optical design for the duplex channel should also conform to the reflectors.

(a) (b)

Fig. 11 (a) Flashlight reflector with 3 photodiodes mounted in the center for duplex channel (b) LED and isolated photodiode array

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