C. Quintana et al.: Time-Hopping Spread-Spectrum System for Wireless Optical Communications Contributed Paper Manuscript received June 11, 2009 0098 3063/09/$20.00 © 2009 IEEE 1083 Time-Hopping Spread-Spectrum System for Wireless Optical Communications C. Quintana, J. Rabadan, J. Rufo, F. Delgado and R. Perez-Jimenez Abstract — This paper describes an optical wireless system based on time-hopping spread-spectrum techniques, as a first approach, at infrared wavelengths. It can be used either for low-speed sensor interconnections or as a return channel in a Visible Light Communications system. The main advantage of this scheme is an improvement in the narrowband interference rejection capability and the number of simultaneous users supported, compared with other wireless optical standards. Moreover, a wide analysis is carried out about the mutual interference between time-hopping spread-spectrum systems and other infrared ones. Finally, some conclusions are drawn about some design parameters of the time-hopping system in order to allow its compatibility with preexisting optical devices. 1 Index Terms — Optical Wireless Communications, time- hopping spread-spectrum (THSS), Visible Light Communications (VLC). I. INTRODUCTION Recently, there has been a growing interest in the use of wireless optical communications in some well-defined indoor environments. This has led to the development of two main research areas: the use of LED lamps not only as illumination sources but also as emitters for broadband signals (e.g. video or audio), and the transmission of several low-speed channels for sensor interconnection, using either visible or infrared emission [1]-[3]. Wireless optical communications present certain advantages over RF transmission that make them suitable for some specific scenarios. The optical systems do not interfere with RF systems, and vice versa, so EM compatibility restrictions are avoided. Moreover, there are no current legal restrictions due to bandwidth allocation and, as radiation is confined by walls, they produce intrinsically cellular networks, which are more secure against deliberate attempts of unauthorized access than those in radio systems. Illumination based in LED lamps is now being intensely promoted by companies and administrations not only due to their high energy efficiency, but also because of their 1 This work was supported in part by the Spanish Government (Research & Science and Industry Ministries, TEC2006-13887-c05-04 and TSI-020100- 2008-660 projects. Authors are in the Photonics and Communications Division of the Technological Center for Innovation in Communications (CeTIC) , University of Las Palmas de Gran Canaria (ULPGC), 35017 Las Palmas de Gran Canaria, Canary Islands, Spain. They can be contacted by e-mail at rperez@cetic.eu , phone (+34) 928 452 870. durability (measured in years) and robustness against weather conditions (basically humidity). They can also be used as communication emitters without losing their main functionality as illumination devices. This technique (known as Visible Light Communications (VLC) [4]-[6]) presents important advantages over other wireless optical systems such as IrDA (Infrared Data Association), as the optical power transmitted is, by far, larger. Due to this, VLC avoids the necessity of a very strict pointing. Furthermore, in spite of the fact that these LEDs are, in general, slower than infrared ones, RGB white LEDs offer baud rates that can reach 10 Mb/s. As we said, these lamps can be used for broadcast transmission of a single channel or for providing several simultaneous low-speed channels (e.g. using OCDMA (Optical Code Division Multiple Access) techniques [7-8], as required for domotic applications installed in houses or hotels). In both cases, we shall need not only an uplink channel, but also links between sensors themselves. Infrared links are a good alternative so as to maintain the RF compatibility and network privacy. These links need to deal with several interference sources: illumination, other infrared systems (e.g. remote controls or IrDA devices) or other users themselves (when a sensor network is considered [9]). For this problem, THSS (Time-Hopping Spread-Spectrum) is a good alternative as it is easy to implement. Synchronization is a major concern for these schemes but it is easily solved by using the VLC system for sending a pilot signal [10]. THSS also reduces the effect of multipath propagation and provides the sensors with CDMA (Code Division Multiple Access) capabilities. We have developed and tested a THSS infrared system suitable to be used as an uplink for VLC transmission or for sensor interconnection. As a first approach, we have considered a Maximum Length Code (MLS) coding scheme, since it is a simpler one. Other coding schemes like Orthogonal Optic Codes (OOC) have been considered too. These codes are characterized by sending a constant signature when a “1” symbol should be transmitted. They have some advantages over the pseudorandom codes, like the improvement of the receiver synchronization capability and the provision of a better autocorrelation function, especially when a unipolar baseband scheme is used, which is the most common approach in the wireless optical channel [11]. However, as they generate the same signature for each symbol, the data encryption capacity at physical level is removed, differently from others TH codes. This paper is organized as follows: section II presents a system description and the basic features of the THSS scheme