Loading...

Relay-assisted Multiple-input Multiple-output Underwater Wireless Optical Communication Systems

Jamali, Mohammad Vahid | 2015

538 Viewed
  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 47754 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Salehi, Javad
  7. Abstract:
  8. Today’s growing interest to the underwater explorations and navigations demands to design appropriate underwater communication methods and systems. However, radio frequencies which are widely being used in free space communications, can’t be a good candidate for underwater wireless communications, due to their sever attenuation in water. On the other hand, underwater acoustic communication has been widely studied and implemented in the past decades, inspired by suitable propagation of acoustic waves through the underwater medium. Nevertheless, acoustic communication systems have limited bandwidth, lower propagation speed and lower security than optical counterparts, which suggests underwater wireless optical communication (UWOC) as a powerful alternative for today’s requirements of large data and high speed secure underwater communications. Despite of all the interesting features of the UWOC systems, they have only the ability to communicate through ranges that are typically less than 100 meters, which hampers on their widespread usage. This is mainly because of the absorption, scattering and fading effects of the channel on the propagating optical signal. This research is inspired by the need to increase the viable communication distance of UWOC systems. We propose schemes to alleviate the aforementioned impairments of the channel, with concentration on the mitigation of turbulence-induced fading. In recent years, fading effects of the free space optical communication (FSO) systems have been comprehensively studied and numerous techniques such as multiple-input multiple-output (MIMO) and relay-assisted FSO systems have been proposed to mitigate fading impairments. Nonetheless, to the best of our knowledge, the beneficial applications of the above mentioned techniques on the performance of UWOC systems yet hasn’t been studied, which is the main focus of this thesis. Propagation of light in underwater medium substantially differs from that of free space; which necessitates comprehensive study of the light propagation under water. Hence, we first study the effects of the underwater channel on the propagating optical signal and simulate the UWOC channel impulse response relying on Monte-Carlo numerical method. Then we show that MIMO technique by mitigating fading effects can provide a remarkable performance improvement, especially for more turbulent channels. Next we consider a relay-assisted UWOC system with bit detect-and-forward (BDF) strategy and demonstrate that multi-hop transmission provides a significant performance enhancement by alleviating all the impairing effects of the channel, namely absorption, scattering and fading. Eventually, we extend the relay-assisted topology to an optical code division multiple access (OCDMA)-based underwater network, and illustrate the beneficial application of multi-hop transmission in extending the viable end-to-end communication distance of the underwater users. For both the MIMO and relay-assisted techniques we analytically derive the system bit error rate (BER) expressions using photon-count approach and then verify the results with computer simulation
  9. Keywords:
  10. Bit Error Rate ; Underwater Wireless Optical Communication Systems ; Multiple Input-Multiple Output Transmission ; Serial Relaying ; Photon-Count Method

 Digital Object List

 Bookmark

No TOC