Differential Amplitude Pulse-Position Modulation for Indoor Wireless Optical Communications
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Differential Amplitude Pulse-Position Modulation for Indoor Wireless Optical Communications Ubolthip Sethakaset Department of Electrical and Computer Engineering, University of Victoria, P.O. Box 3055 STN CSC, Victoria, BC, Canada V8W 3P6 Email: [email protected]
T. Aaron Gulliver Department of Electrical and Computer Engineering, University of Victoria, P.O. Box 3055 STN CSC, Victoria, BC, Canada V8W 3P6 Email: [email protected] Received 31 March 2004; Revised 28 August 2004 We propose a novel differential amplitude pulse-position modulation (DAPPM) for indoor optical wireless communications. DAPPM yields advantages over PPM, DPPM, and DH-PIMα in terms of bandwidth requirements, capacity, and peak-to-average power ratio (PAPR). The performance of a DAPPM system with an unequalized receiver is examined over nondispersive and dispersive channels. DAPPM can provide better bandwidth and/or power efficiency than PAM, PPM, DPPM, and DH-PIMα depending on the number of amplitude levels A and the maximum length L of a symbol. We also show that, given the same maximum length, DAPPM has better bandwidth efficiency but requires about 1 dB and 1.5 dB more power than PPM and DPPM, respectively, at high bit rates over a dispersive channel. Conversely, DAPPM requires less power than DH-PIM2 . When the number of bits per symbol is the same, PAM requires more power, and DH-PIM2 less power, than DAPPM. Finally, it is shown that the performance of DAPPM can be improved with MLSD, chip-rate DFE, and multichip-rate DFE. Keywords and phrases: differential amplitude pulse-position modulation, optical wireless communications, intensity modulation and direct detection, decision-feedback equalization.
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INTRODUCTION
Recently, the need to access wireless local area networks from portable personal computers and mobile devices has grown rapidly. Many of these networks have been designed to support multimedia with high data rates, thus the systems require a large bandwidth. Since radio communication systems have limited available bandwidth, a proposal to use indoor optical wireless communications has received wide interest [1, 2]. The major advantages of optical systems are low-cost optical devices and virtually unlimited bandwidth. A nondirected link, exploiting the light-reflection characteristics for transmitting data to a receiver, is considered to be the most suitable for optical wireless systems in an indoor environment [2]. This link can be categorized as either line-of-sight (LOS) or diffuse. A diffuse link is preferable because there is no alignment requirement and it is more robust This is an open-access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
to shadowing. However, a diffuse link is more susceptible to corruption by ambient light noise, high signal attenuation, and intersymbol interference caused by multipath dispersion. Thus, a diffuse link needs more transmitted power than an LOS li
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