Abstract
In this paper, we further explore the concept of phase-conjugated twin waves (PCTW) for nonlinear cancellation in space-division multiplexed (SDM) systems. Previously, we demonstrated that the PCTW technique can successfully provide nonlinear cancellation in SDM systems. In this paper, we investigate the cases where two and four spatial modes are copropagating in a multimode fiber, considering three link lengths (1000, 3200, and 8000 km). Weak-and strong-coupling regimes are also evaluated. Our numerical simulation results show an average performance improvement > 10 dB after a 1000 km transmission link.

Abstract

Abstract
In this paper we investigate the efficacy of applying the coupled field back-propagation algorithm as a post-compensation method for nonlinearity mitigation in a coherently detected fibre optic long-haul system using multi-band orthogonal frequency division multiplexed signals. Specifically, we analyze the impact of varying the band-spacing and the number of bands. We find that its efficacy is higher for largely spaced bands and a small number of bands. Additionally, we propose a method to include the Four Wave Mixing compensating term within the coupled field method to provide simplified means of multichannel compensation. We conclude that this method is more efficient in improving the performance especially for small band spacings. The coupled field method proves to be an interesting choice for the implementation of receiver-based real-time digital signal processing.

Abstract
The capability of relatively high-speed short-range communications of Autonomous Underwater Vehicles (AUVs) in underwater scenarios, for example, for communication between vehicles or when the AUV is approaching a docking station for downloading of data gathered during a survey mission, is becoming a relevant application in the context of sea exploration and mining. In this paper the analysis of the J-pole antenna and two of its configurations namely Super J-pole and Collinear J-pole antennas are presented, aimed at improving the propagation distance and data rates when such antennas are installed on AUV for onward usage in underwater communications. The performance of these three antennas is assessed through simulation in fresh and sea water, operating in the High Frequency (HF) band. These antennas are compared in terms of bandwidth and directivity which are important elements in the transmission and reception of electromagnetic signals. The results obtained show that these antennas will be desirable both for improved data rates and propagation distance in fresh and sea water. The antennas were designed with FEKO electromagnetic simulation software.

Abstract
Transmission and reception of high-speed short range signal is important for successful underwater water communications between an Autonomous Underwater Vehicle (AUV) or Remote Operated Vehicles (ROVs) and a docking station or underwater sensor nodes during a survey mission. The need for this form of application is currently receiving global attention from scientific groups and industries. Hence, underwater antennas are therefore required to provide these links and achieve good data rates and propagation distances for these applications either in fresh or sea water scenario. In this paper, the performance of loop antenna placed at a specified distance and parallel to the ground plane is assessed through simulation for usage in fresh water and operating in the High Frequency (HF) band. Three variations of this antenna namely; the circular loop, the square loop and the delta loop antennas has been placed on two different ground plane shapes (circular and square) for analyses of their performances. These antennas were designed in FEKO, an electromagnetic simulation software and their performance is assessed in terms of bandwidth and directivity. The results obtained shows that the antennas exhibit wideband and high directivity with square loop antenna placed on a square ground plane having slight advantages over the other antennas with respect to their bandwidth and directivity. Experimental results added for the same antenna, confirmed its performance in terms of the measured parameters are in good agreement with simulation results.

Abstract
Propagation of underwater electromagnetic signals for real-time transmission and reception of data gathered during a survey mission between an Autonomous Underwater Vehicle (AUV) and underwater sensor nodes or a docking station, continues to generate a lot of interest worldwide. To this end, underwater antennas will play a significant role in ensuring good data rates and propagation distances for various underwater applications. In this paper, the performance of two dipole antennas with different parasitic elements is assessed through simulation for usage in fresh water and operating in the High Frequency (HF) band. The antennas were designed in FEKO, an electromagnetic simulation software and their performance is assessed in terms of bandwidth, directivity and antenna-to-antenna distance. The results obtained show that these antennas exhibit wide bandwidth, which is important to achieve high data rates. Experimental results of the reflection coefficient of the fabricated antenna measured in fresh water are given that agree well with the simulation results.