1950 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 54, NO. 5, OCTOBER2005 A Novel Multipath Light Signal Dispersion Reduction Technique Based on Controlled-Polarization Optical Wireless Link Setup G. C. Giakos, N. Patnekar, S. Sumrain, L. Fraiwan, and V. Kumar Abstract—The detection characteristics of an indoor-optical communication system, which utilizes infrared radiation as car- rier, has been explored and enhanced for telemedicine and wireless local area network applications. The novelty of the presented tech- nique consists of the fact that multipath dispersion can be reduced under controlled polarization link setup. The design of such a network is based on the specifications set by the IEEE 802.11 standard. I. INTRODUCTION T HE INCREASING demand for an optical wireless local access network (LAN) has resulted from its attributes of being more secure, fast, and immune to electromagnetic interference (EMI). This technology is utilized in large scale in developing portable personal computers, mobile communi- cation, telemedicine, laptops, PDAs, etc. [1]–[4], [6]–[9], [11], [12]. These portable devices integrate computational power and mobility on a single platform and introduce the need for accessing communication networks without the restrictions im- posed by cables [4]. The increasing interest in the wireless local access network gave way to the international standardization of this technology. In June 1997, a standard for WLAN for different technologies including radio and infrared set by the IEEE 802.11 committee was approved [4]. An essential characteristic of the IEEE 802.11 standard is that there is one single medium access control sublayer for all the physical layers [4]. The medium access is based on a carrier sense multiple access with collision avoidance protocol (CSMA/CA)[2]–[4]. The physical layer deals with the actual transmission of the signal. This can be done either by radio or infrared technology. The infrared technology is very well suited for such a low-range application because it is cost effective, has high bandwidth for data transmission, and hence, offers faster electromagnetic (EM) interference-free and more secure transmission with low power requirements. According to the specification, the infrared physical layer can support two data rates: 1 and 2 Mb/s. The specification is aimed at allowing a smooth migration to higher data rates [4]. There Manuscript received August 21, 2004; revised December 19, 2004. G. C. Giakos and S. Sumrain are with the Photonics and Optical Communica- tions Laboratory, Department of Electrical and Computer Engineering, The Uni- versity of Akron, Akron, OH 44325-3904 USA (e-mail: giakos@uakron.edu). N. Patnekar and L. Fraiwan are with the Department of Biomedical Engi- neering, The University of Akron, Akron, OH 44325-0302 USA. V. Kumar is with the Photonics and Optical Communications Laboratory, De- partment of Electrical and Computer Engineering, The University of Akron, Akron, OH 44325-3904 USA, and also with the Division of Engineering and Applied Mathematics, The University of Akron, Akron, OH 44325-0302 USA. Digital Object Identifier 10.1109/TIM.2005.853559 is a different modulation scheme for each data rate: 16-PPM for 1 Mb/s and 4-PPM for 2 Mb/s. Other modulation schemes such as amplitude shift keying (ASK) and return to zero, in- verted (RZI) can also be implemented. Lasers or light emitting diodes (LEDs) can be used as a transmitter. However, for an indoor environment LEDs are preferred as optical transmitters over lasers because they can produce substantial launch power and yet be Class 1 eye safe [1], [6], [7]. Infrared radiation has properties similar to light, and hence indoor surfaces are good reflectors of infrared [4]. In line-of-sight geometry, an obstacle between the transmitter and receiver can introduce attenuation of the collected optical power, which is typically called shad- owing effect. In diffused geometry, there is a reduction in the collected optical power due to the multipath propagation of the signal. Propagation through multiple paths can give rise to dis- persion of the received pulses, which is called multipath disper- sion. These signals can be distinguished based on their state of polarization. Several mechanisms can contribute to dispersion. For in- stance, multipath dispersion can occur due to the reflection of the infrared beam from different surfaces. Also, local scat- tering multipath results due to difference in the phase of the received signal. There is random distribution of the phase due to difference in path lengths of approximately 1 m. As a result, multipath spread delay occurs due to reflections from walls and other reflectors. This gives rise to a signal distortion known as intersymbol interference. Multipath propagation causes fading and time-spreading of the received signal. Interestingly enough, dispersion can compromise the bandwidth of both analog and digital signals. Various techniques are used to combat multi- path dispersion, such as adaptive equalization, digital adaptive equalization, spread spectrum techniques, antenna diversity, and directivity. The bandwidth of the infrared link is determined by multipath dispersion [4]. Along with these interferences, the system is also vulnerable to fluorescent and incandescent light that flickers on–off at 120 Hz [9]. Modulating the infrared signal onto a carrier avoids the interference by florescent lamps. This can also be eliminated by using a suitable optical bandpass filter. Low-frequency noises can be removed by an electrical filter [9]. However, this noise can also be eliminated by using signal processing algorithms. A group of stations that can have direct communication are called the basic service set (BSS), and the area occupied is called the basic service area (BSA). The basic service area can overlap or be totally disjoint. Different BSAs are connected by a system called distributed system. A group of BSAs interconnected by 0018-9456/$20.00 © 2005 IEEE