Multirate RZ communications for dispersive wireless optical channels

Abstract

Free Space Optical Communications through cloud-obscured channels suffer from severe degradations due to multiscattering, which induces delay spread in the received signal. The delay spread of FSO channel can vary considerably according to channel conditions, varying from nanoseconds to microseconds for clear and cloudy channel, respectively. In this paper, we present the use of return-to-zero (RZ) modulation in conjunction with multirate modulation schemes as viable means for minimizing the dispersive effects of the channels. We show that multirate communications provide diversity against channel degradation and fluctuation, while RZ modulation is more robust to dispersive channels than non return-to-zero (NRZ) modulation, thus providing an overall improvement in system performance, reliability and availability. INTRODUCIONWireless optical communications is one of the most promising candidates for future broadband communications, offering transmission rates far beyond possible by RF technology. Free Space Optical communications through cloud obscured channels suffer from severe degradations due to the multiscattering, which induces delay spread in the received signal. Even in the presence of clouds, large bandwidth values are attainable as suitable for high rate communications, and this bandwidth can vary widely even within the same class of clouds [1]. The delay spread of FSO channel can vary considerably according to channel conditions, varying from nanoseconds to microseconds in clear and cloudy channel conditions, respectively. The change in channel conditions can occur gradually as in the case of a cloud overcast clearing off, or abruptly as in the case of scattered clouds.These observations provide a strong motivation to research wireless optical communications systems that can provide reliable communications under such conditions. In order to maximize channel throughput, powerful modulation schemes need to be employed and provide reliable communications. Multirate communications using fractal modulation has received wide attention in the research community, as an efficient mean to provide reliable communications. Additionally, optimum pulse shaping has been shown to be a crucial element in optimizing system performance.In this paper, we present a new approach to wireless optical communications, where we combine multi-rate communications with short-pulsed RZ modulation. We show that multi-rate communications provide diversity against channel degradation and fluctuation, while RZ modulation is more robust to dispersive channels than NRZ modulation, thus providing an overall improvement in the system performance, reliability and availability. This paper is organized as follows; in section 2 we present the outdoor wireless optical channel, in section 3 we present a wireless optical system using fractal modulation and investigate its bit error rate (BER) performance. In section 4, we introduce a return-to-zero multirate wireless optical system and investigate its performance. Finally, in section 5 we compare t...