Channel measurements and channel modeling for the open sea

Abstract

Maritime communication plays an important role in the marine activities. However, insufficient knowledge of radio channel characteristics over the sea is a limiting factor in the development of wideband wireless communication systems for maritime applications. Therefore, in order to increase understanding of the radio channel over sea, two land-toship (L2S) radio channel measurement campaigns have been performed in Trondheim, Norway. Two scenarios are investigated in the MarCom project to cover a variety of maritime environments: 1) short-distance open sea environment. 2) long-distance (≈ 45km) open sea environment.

Channel characteristics such as path-loss, spatial channel correlation, power delay profile, mean excess delay and RMS delay spread obtained from the channel measurements are analyzed. The received signal levels (RSL) have been compared with the Okumura-Hata model, the COST 231-Hata model and the ITU-R P.1546-2 model. It has been found that the ITU-R model for open cold sea corresponding to field-strength value exceeded at 50% of the locations gives the best fit with themeasured results. By using two receiver antennas with 1.85 and 2.9 meters spacing for the two scenarios respectively, the radio channel is shown to be partly correlated (spatial correlation coefficient for the received signals from the two RX antennas varies from 0.3 to 0.9) when the distance between the transmitter (TX) and receiver (RX) is within 2 km, while the two channels are found to be highly correlated when the boat is far away from the shore. The ship turning and the shadowing effect caused by other ships might, however, reduce the spatial correlation between the two RX channels. Power delay profile (PDP) is studied by classifying the channels into two groups: frequency-selective channel close to the harbor and non frequency-selective channel at a long TX-RX distance. The mean excess delay and RMS delay spread have been investigated. The PDF of the mean excess delay and the RMS delay spread for the short-distance measurement can be modeled by using a three-term Gaussian model. Few reflected rays are found except for the reflected rays from the sea surface with a small difference from the LOS in delay, which means that there is no need to model the PDFs for the long-distance measurement due to fewer reflections from the shore.

Based on measured results on RSL and classic path-loss models, two path-loss models for the open sea environments are proposed: a) Round Earth Loss (REL) model and b) Quasi-Deterministic (QD) model. The REL model has been proposed base on a geometrical model of round Earth together with effects on the radio link that are the Free Space Loss (FSL), effective reflection from sea roughness, divergence and diffraction loss. The Quasi-deterministic model has been invented by utilizing the merits of both the ITU-R model and the Plane Earth Loss (PEL) model after comparing both of them with the longdistance path loss measurement results. Both models match the measured results very well.