1502 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 20, OCTOBER 15, 2011 Experimental Investigation of Wavelet-Based Denoising Receiver for LOS Indoor Optical Wireless Communications Links S. Rajbhandari, Member, IEEE, Z. Ghassemlooy, Senior Member, IEEE, and M. Angelova Abstract—This letter reports the experimental evaluation of the on-off keying (OOK) modulation technique for a nondirected line-of-sight (LOS) infrared optical wireless communication (OWC) link. Performance evaluation is carried out in a typical room environment in the presence of fluorescent light driven by an electronic ballast. The adverse effect of the fluorescent light inference (FLI) is demonstrated and a number of mitigating tech- niques including high pass filtering (HPF) and discrete wavelet transform (DWT)-based denoising are investigated. The study shows that DWT outperforms HPF for all data rates. The practical measurements are also verified using a computer simulation. Index Terms—Artificial light interference, optical wireless com- munication, wavelet transform. I. INTRODUCTION O PTICAL wireless communication is an attractive com- plementary technology to the radio frequency (RF) of- fering a number of advantages; including a large, unlicensed bandwidth, low cost, a smaller transceiver size and freedom from electromagnetic interference [1], [2]. Due to the band- width congestion in RF technology, as well as a high licensing cost, an alternative wireless technology using infrared frequen- cies has already been suggested and deployed especially for the point-to-point communication and indoor environment where high mobility is not the prime requirement [3]. Because of the availability of cheap transceivers and eye safety regulations, the operating wavelength of the indoor OWC system is normally selected in the range of 750–900 nm. How- ever, there is serious link impairment within this wavelength range, mainly caused by the optical interference introduced by artificial light sources. Among the number of artificial light sources employed for room illumination, the interference in- duced by the fluorescent light, driven by the electronic ballast, is the most severe due to the presence of: a) high optical spec- tral contents at the near infrared wavelength and b) electrical Manuscript received April 19, 2011; revised June 15, 2011; accepted July 16, 2011. Date of publication July 25, 2011; date of current version September 23, 2011. S. Rajbhandari and Z. Ghassemlooy are with the Optical Communica- tions Research Group, School of CEIS, Northumbria University, Newcastle upon Tyne, NE1 8ST, U.K. (e-mail: sujan.rajbhandari@northumbria.ac.uk; fary.ghassemlooy@northumbria.ac.uk). M. Angelova is with the Intelligent Modeling Lab, School of CEIS, Northumbria University, Newcastle upon Tyne, NE2 1XE, U.K. (e-mail: maia.angelova@northumbria.ac.uk). Color versions of some of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2162723 Fig. 1. Schematic diagram of experimental setup for indoor OWC link. signals with harmonics extending from DC up to 0.5 MHz [4]. In order to reduce the effect of FLI, a number of mitigating techniques have been proposed including filtering, differential receivers and angle diversities. In our previous study [5], we investigated a DWT-based denoising to alleviate the effect of FLI which showed an improved performance, compared to the HPF scheme, and reduced complexity. In this study, the experimental evaluation of the DWT-based receiver is carried out. An OWC system with an operating wavelength of 830 nm is deployed in a typical room environment. The performance of the link is evaluated in the presence and absence of FLI. The experimental results are then verified using a computer simulation. The letter is organized as follows: the experimental setup for the evaluation of the OWC system in the presence of FLI is discussed in Section II. The experimental results and analysis of the results are carried out in Section III. Finally, conclusions are drawn in Section IV. II. EXPERIMENTAL SETUP The nondirected LOS-OWC system is deployed in a typical 6 5 3m laboratory room environment with a receiver lo- cated at a height of 1 m above the floor. The schematic dia- gram of the experimental setup is given in Fig. 1 (note that it is not to scale). A laser diode (LD) operating at a wavelength of 830 nm with a maximum optical output power of 10 mW is directly modulated using an input data source. The pseudo- random bit sequence (PRBS) of bits is generated using an arbitrary waveform generator (AWG) and is converted into a non-return-to-zero (NRZ) OOK format prior to intensity modu- lation of a laser diode. A holographic diffuser of full-width half-maximum (FWHM) is used to ensure eye safety as well as increasing the optical footprint. The receiver is positioned at the centre of the optical footprint where the received optical power 1041-1135/$26.00 © 2011 IEEE