Underwater Acoustic (UWA) communication has become an active area of research due to a growing range of applications for scientific, military and commercial purposes. Since many of these applications involve deep water intervention, underwater Remotely Operated Vehicles (ROVs) are deployed to avoid human casualties. Some applications also require a video to be transmitted in real-time to a surface vessel. The transmission of such data over the Underwater Acoustic channel (UAC) is quite challenging due to its inherent characteristics, namely, frequency dependent attenuation, ambient noise, multipath distortion, propagation delay and Doppler effect. The bit rates that are currently achieved over long transmission distances are usually in the order of a few tens of kilobits per second (kbps). These are however not sufficiently high to transmit large data such as video in real-time. In this thesis, the feasibility of real-time video transmission in both horizontally and vertically configured UACs is evaluated. In order to make maximum usage of the limited acoustic bandwidth and simultaneously overcome the harmful propagation phenomena in an UAC, physical layer waveforms such as Orthogonal Frequency Division Multiplexing (OFDM), Filterbank Multicarrier (FBMC) modulation and Orthogonal Time frequency Space (OTFS) modulation are evaluated for both Single-User (SU) and Multi-User (MU) scenarios. To further boost the achievable bit rates, Multiple-Input Multiple-Output (MIMO) and massive MIMO systems are considered for the SU and MU scenarios, respectively. In terms of video compression, the H.264 Advanced Video Coding (AVC) standard is considered due to its good error resilience compared to other standards. It is demonstrated that FBMC provides robust performance in doubly-dispersive UACs and can even outperform OFDM. Furthermore, FBMC systems based on OFDM - Offset Quadrature Amplitude Modulation (OFDM-OQAM) achieve 100% bandwidth efficiency as no Cyclic Prefix (CP) is used, thus improving the overall bit rate as compared to OFDM. With the use of Forward Error Correction (FEC) codes such as Turbo codes, the error performance of the different systems is significantly improved in both horizontal and vertical UACs. It is further demonstrated that the massive MIMO technique allows multiple ROVs to communicate over the same time and frequency resources with a surface base-station (BS) with very good error and throughput performances. Finally, it is shown that OFDM-based OTFS systems outperform the conventional OFDM systems in a dynamic UAC. Fewer pilots are required in the delay-Doppler (DD) domain to estimate the sparse UAC as compared to the frequency domain, thereby improving the bandwidth efficiency of the OTFS system. Therefore, the OTFS modulation scheme not only achieves robust performance in UACs affected by severe Doppler effect but also enables a higher bit rate than conventional OFDM systems. In this research, theoretical bit rates between 70 kbps and as high as 240 kbps are reported for the various MIMO and massive MIMO systems operating over a 1 km time-varying UAC. It is shown that such bit rates are sufficiently high to transmit and receive video in real-time with adequate quality over the long distance UAC.
|Date of Award||1 Oct 2019|
- The University of Bristol
|Supervisor||Dimitris Agrafiotis (Supervisor), Angela Doufexi (Supervisor) & Andrew Nix (Supervisor)|