An Efficient Channel Modeling for Underwater Wireless Optical Communication Links Abstract---- Recently, the role of underwater wireless optical communication is increased considerably for underwater observation and sea monitoring. This communication technology has many applications like prediction of natural calamities, studying the climate changes, detection off natural resources etc. In underwater wireless optical communication (UWOC), multiple scattering may cause temporal spread of beam pulse, which is characterized by its impulse response. The temporal spread leads to inter-symbol interference (ISI) and degrades the system performance. In this paper, the optical characteristic of seawater is analyzed and some new parameters for accelerating the convergence speed and probability of mutant for maximizing the coverage rate are introduced. The bit-error-rate (BER) and bandwidth of the channel are further studied for various link ranges. The zero- forcing (ZF) equalization has been adopted to overcome ISI and improve the system performance. Harmony search Algorithm is proposed to model the channel impulse response and quantify the channel time dispersion under different conditions of water type, link distance, and the transmitter/receiver parameters. Comparing with the existing method, the proposed approach can achieve a better performance in coverage rate and convergence speed. HSA is used to optimize the phase shift parameters and to improve both the network connectivity and the harmonic phases. It is plausible and convenient to utilize this impulse response model for performance analysis and system design of UWOC systems. I. INTRODUCTION Underwater acoustic communications which utilizes acoustic waves to transmit information has been widely studied and implemented in the past decades. However, the channel bandwidth of underwater acoustic links is typically limited to kHz since the sound is decayed in the ocean proportionally to its frequency. And the lower propagation speed typically 1500 m/s leads to a large time delay for acoustic system. Meanwhile, the multipath reflection of sound may cause signal fading and security issues. UWOC systems can provide high security, low time delay and a much higher data rate up to hundreds of Mbps in relatively short ranges (typically shorter than 100 meters). In this paper, focuses is mainly on the temporal dispersion of UWOC links and investigate the effect of corresponding impulse response. There are lots of theoretical and practical studies have been done on the impulse responses of underwater optical links. Most of the prior works are used Monte Carlo approach to model the impulse response of UWOC channels and validated the Monte Carlo approach of modeling the UWOC impulse response by experimental measurement. However, to the best of our knowledge, these prior works have not provided simple closed-form expressions of impulse response of UWOC links. In this paper, focus is mainly on the impulse response of relatively turbid water types such as harbor water where the temporal dispersion cannot be ignored and will affect the system performance. The double Gamma functions has been firstly adopted in modeling the impulse response of FSO links through fog and then applied to model the impulse response in dispersive medium characterized by Henyey-Greenstein (HG) function in earlier work .Although seawater has different properties from the medium studied in the prior works, they are dispersive medium through which the light may suffer the effect of multiple scattering. The optical properties of the seawater is analyzed by (a) apply the double Gamma functions to model the impulse response of UWOC links which fits well with the Monte Carlo simulation results (b) investigate the valid region of this model. Then, based on the double Gamma functions model, study the system performance of inter-symbol interference (ISI), bandwidth of the channel and the bit-error-rate (BER). The results got suggested that the temporal pulse spread reduce the BER performance without equalization based on on-off keying (OOK) modulation for high data rate and long. To eliminate the detrimental effect of ISI and improve the BER performance, equalization has been employed in atmospheric FSO links and indoor infrared links. In this paper, the simplest and most widely used zero-forcing (ZF) equalization based on double Gamma functions model is done to enhance the error performance of the high speed UWOC system operating in seawater environment in the presence of ISI. II. LINK CHARACTERISTICS AND SYSTEM MODEL 1. Optical Characterization of Seawater The interactions between seawater and each photon undergo absorption and scattering in beam propagation. In absorption process photons lose energy in the form of heat when it interacts with water molecules and other particles. In scattering, the direction of transmission of photons will be changed by the interactions between photons and seawater, which lead to energy loss because number of photons that reaches the receiver will be reduced. The loss in energy by absorption and scattering can be evaluated by absorption coefficient a(λ) and scattering coefficient b(λ), Anjana J J ME Communication Systems Sivaji College of Engineering and Technology Manivilai, Kanyakumari, Tamilnadu, India Haneesh Sankar CDAC, Trivandrum Kerala, India Manivilai, Kanyakumari, Tamilnadu, India International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 http://www.ijert.org IJERTV6IS120120 Published by : www.ijert.org (This work is licensed under a Creative Commons Attribution 4.0 International License.) Vol. 6 Issue 12, December - 2017 316