Underwater wireless optical communication (UWOC) has been regarded as one of the promising solutions to underwater wireless communication systems due to its advantages of high bandwidth, fast transmission and good confidentiality. In UWOC systems, after emitting from light source, photons will be scattered with random deviated angles before propagating to the receiver plane. Hence, the light beam suffers a spatial angle spread at the receiver side. In order to improve the system performance with an appropriate receiver design, it is of significance to figure out the angle of arrival (AOA) distribution of the received light beam. In addition, the UWOC channel is susceptible to other effects such as absorption, turbulence and bubbles, especially when it is exposed to the complex ocean environment. Therefore, analyzing the impacts of these factors on AOA distribution is also essential to evaluate UWOC system performance. The existing studies of AOA distribution only considered single scattering component, which is not practical for turbid water. Furthermore, there are few studies focusing on multi-source scenarios. In this paper, we first present a simple expression for AOA distribution with single light source, taking both single and multiple scattering components into account. Then we extend the work to multi-source scenarios and derive the corresponding closed-form expression of AOA distribution. Compared to the traditional single scattering case, numerical results show that the proposed AOA distribution can fit well with Monte Carlo simulation results with various water types, link distances, and the characteristics of actual light sources.
KEYWORDS: Orthogonal frequency division multiplexing, Adaptive optics, Wireless communications, Berkelium, Receivers, Radio optics, Data communications, Telecommunications, Signal to noise ratio, Modulation
Orthogonal frequency division multiplexing (OFDM) has been applied to optical wireless communication to achieve high data rates and wide bandwidth and solve the issues of inter-symbol interference (ISI). Due to the non-negative and real-valued characteristics of intensity modulation/direct detection (IM/DD) signaling for optical wireless communication systems, some optical OFDM schemes have been proposed to reach the requirements. Among these optical OFDM schemes, adaptively biased OFDM (ABO-OFDM) reserves 1/4 of the subcarriers and utilizes Hermitian symmetry to generate non-negative and real values after invert fast Fourier transformation (IFFT) and adding bias in time domain. In this paper, we extend the original ABO-OFDM scheme and proposed a generalized ABO-OFDM scheme in which 1/m subcarriers are reserved for any positive integer m. It is demonstrated that the bias added in time domain can be counteracted exactly in frequency domain, which means it has lower implementation complexity at the receiver than most of the other optical OFDM schemes. This generalized ABO-OFDM scheme has higher frequency efficiency and lower peak-to-average power ratio (PAPR) than asymmetrically clipped optical OFDM (ACO-OFDM) and higher power efficiency than direct current biased optical OFDM (DCO-OFDM). We also evaluate the impact of parameter m on system performance in terms of PAPR and bit error rate (BER). Consider the comparison with other optical OFDM schemes and the tradeoff above-mentioned, the generalized ABO-OFDM is a potential scheme to facilitate optical wireless communications with a flexible parameter m.
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