BER Estimation for Laser Based Underwater Communication

Abstract

Apart from land and air, sea has also become very important from defense strategic point of view and is required to be integrated with network centric communication. In future it may play a very important role in network centric warfare. Free space laser communications provide wide bandwidth and high security capabilities to unmanned aircraft systems in order to successfully accomplish intelligence, surveillance, target acquisition, and reconnaissance missions(l) Laser based communication is emerging as a most promising solution for underwater communication. Most of laser cannot penetrate through the sea due to be absorbed by the sea, but the blue-green laser (the wavelength is about 470 ~ 570nm) has the minimum energy fading in the sea, whose fading rate is about 0.155~0.5dB/m, this feature of blue-green laser in the sea is said the window effect(2). Long- range links use directed laser beams to transmit data, and can be used for building-to-building, ground- to-aircraft, or ground-to-satellite communication (3). Laser communication throws quite different challenges in water in respect to air. Laser attenuation in water is key bottleneck for underwater communication as attenuation in water is roughly 1000 times as in air. This exponential variation plays a very important role in determining the type of laser (CW or Q switched laser) and place a restriction on the use of CW laser for longer distance because of impractical sizes. Turbulences and variation in salt concentration in different part of a sea and in different seas imposes a dynamic challenge for underwater laser communication. Issues at sea-air interface for aerial platform to underwater communication also plays crucial role for establishing an uninterrupted communication link. All these issues throttle the effective baud rate of transmission.