NLOS UV Communication Systems Using Spectral Amplitude Coding Mohammad Noshad and Ma¨ ıt´ e Brandt-Pearce Charles L. Brown Department of Electrical and Computer Engineering University of Virginia Charlottesville, VA 22904 Email: mn2ne@virginia.edu, mb-p@virginia.edu Abstract—In this paper a novel design for improving the performance of non-line of sight (NLOS) ultraviolet (UV) com- munication systems using spectral amplitude coding (SAC) is proposed. The same technique as in SAC-OCDMA systems is used as a high-order modulation instead of as a code. Intersymbol interference (ISI) and receiver noise are considered as the main limiting factors. The NLOS channel is modeled to obtain the ISI coefficients. Two bounds are presented and compared with simulation results for different number of symbols and for various geometries. The proposed system can support longer distances for the performance compared with on-off keying (OOK) systems. I. I NTRODUCTION In the last two decades, free space optical communication has become an interesting alternative to wireless commu- nications due to its potential for providing high bandwidth transmissions. Ultraviolet (UV) communication has attracted the attention of researchers in recent years. In the UV band (200-280 nm), because of the filtering of sunlight by the ozone layer, background radiation is low and so weak signals can be detected by the receiver [1]. Meanwhile, due to the high atmospheric scattering in UV band, the receiver can receive more power from the transmitter that is not in its line of sight, compared to other optical bands, and this makes UV communications a strong choice for non-line of sight (NLOS) communications at short ranges [2]. The multi-path effect in these systems, which causes temporal dispersion of the optical pulses and leads to intersymbol interference (ISI), together with the low received power, limit the data rate [3]. Spectral encoding is a common technique used to increase the throughput of both optical and RF systems [4]. In optical communications and networks, spectral encoding is mainly used as a coding method for multiple access purposes. One of the most common techniques for encoding the optical pulses in optical code division multiple access (OCDMA) networks is spectral amplitude coding. This type of systems has gained considerable interest since the interference effect can be reduced by using special code-families with fixed cross correlation [5]. To analyze the NLOS UV communication system, we have to know the path loss and the received optical pulse shape. Most of the previous work in UV systems has focused on obtaining a precise model for UV channels and calculating the impulse response. A technique using a single scattering model was presented by Luettgen et. al. in 1991 [6]. Lately, a more accurate model incorporating multiple scattering interactions using a Monte Carlo method was proposed [7]. The theoretical performance of NLOS UV using these models is studied for on-off keying in [8] and [9]. Experimental demonstrations are described in [10]. In this paper we utilize fixed cross correlation codes as modulated symbols in order to increase the bit rate of the NLOS UV communication system. Spectral amplitude coding (SAC)-OCDMA encoder/decoder structures, based on diffrac- tion grating, are used at the transmitter/receiver for sending and detecting the symbols. At the receiver side, an avalanche photodiode (APD) array is used for extracting the received signal in each wavelength. A simple model of the NLOS UV channel is proposed and compared with simulation results. The performance of the system with different number of symbols is analyzed by considering the ISI and the thermal noise. The rest of the paper is organized as follows. In Section II, the structure of the transmitter is explained. The configuration of the receiver using an APD array is presented in Section III. The channel model of the UV links is studied in Section IV, and the ISI coefficients are described. Lower and upper bounds on the performance of the system are derived in Section V, and their performance is compared with simulation results. Conclusions are made, and future work is suggested in Section VI. II. TRANSMITTER STRUCTURE The geometry of the NLOS UV link is illustrated in Fig. 1. We denote the distance between the transmitter and receiver by d, transmitter beam full-width divergence by φ 1 , the receiver field of view (FOV) by φ 2 , and the transmitter and receiver elevation angles by θ 1 and θ 2 , respectively. The structure of the transmitter using spectral amplitude encoding is depicted in Fig. 2. In this transmitter, an ar- ray UV light emiting diodes (LEDs) (e.g. Sensor Electronic Technology, Inc. model TO-3; 15nm FWHM) is used as the source. The output light of the LEDs is collimated upon a UV diffraction grating (e.g. 3600 grroves/mm; 0.25 nm/mr disper- sion at 250 nm; LaserComponents). The diffraction grating reflects each wavelength with a specific angle that depends on the physical parameters of the grating and the angle of 2nd IEEE Workshop on Optical Wireless Communications 978-1-4673-0040-7/11/$26.00 ©2011 IEEE 843