Experimental measurements of maritime radio transmission channels Yvon-Marie Le Roux, Jacky Ménard, Claude Toquin, Jean-Pierre Jolivet, Fabien Nicolas Institut Télécom - Télécom Bretagne, UMR CNRS 3192 Lab-STICC, Brest, France Email: [email protected] Abstract-This paper focuses on experimental measurements, the propagation channel: masking effects or reflected paths due carried out by Télécom Bretagne, concerning both maritime radio to the coastal relief and the islands, tide effects that modify the transmission channels and new radio technologies (mainly WiMAX, 802.16e). The aim of these studies is to reinforce the refraction conditions of the indirect path. This paper proposes quality and the robustness of such transmissions. Several and estimates also the efficiency of vertical space diversity for measurements were set up, with specific experimental devices maritime radio links. developed and implemented by Télécom Bretagne, to characterize of the propagation channel in maritime environment and to study the communication performances of WiMAX. The II. MEASUREMENTS experimentations were carried out at frequencies around 3.5 GHz (Licensed WiMAX Band in France) and 5.8 GHz (Free Band in A. Measurement Locations France). A previous paper [1] published by the same authors The measurements presented were carried out in the Brest showed that there is a good agreement between the theoretical harbour during two different campaigns realized in March values of field attenuation computed with the well known two-ray 2008 and April 2010 with similar boats. The navigation model and the measurements. New measurements presented in conditions were rather good. Indeed the climate was gentle and this present paper confirm the possible presence of deep fading and bring to light the benefit to exploit the channel diversity the waves not very high. Figure 1 represents the locations thanks to the use of two receiving antennas separated in height. where the measurements were carried out and an example of Furthermore, a comparison between the propagation two routes covered by the boat with transmitting equipments characteristics and the performances of a WiMAX transmission (in yellow the forward route and in white the backward route). are analyzed in order to have a better comprehension of the Geographical representations are from Google Earth. Point 1 functionality of the system. Some results concerning maritime HF and UHF transmission channels are also proposed in this paper. indicates the receiver location for all the experimentations. I. INTRODUCTION In the wireless world, the needs in wireless broadband communications concern quite particularly the ground zones not provided by ADSL. In maritime environment like harbours and coastal areas, needs are also important but the specificities of this type of propagation channel complicate the transposition of the ground solutions such as WiMAX and require serious studies. Similar studies [2] and [3] were already carried out, but they did not take into account all the environment characteristics. As mentioned in [3] and [4], a basic model being able at a first order to apply to the 5 Km propagation in maritime environment is the two-ray model. This model takes into account one direct path between the transmitter and the receiver and another path refracted by the sea. For this model, it is possible to calculate the level of the Figure 1. Example of routes (Max Tx-Rx distance = 27 km). received field at a given point, according to: the distance between the transmitter and the receiver, the relative heights of B. Measurement Equipments the transmitter and the receiver from the sea, the wave At transmitting site (the boat), two kinds of signals may be polarization, the working frequency, the conductivity, the generated: a continuous sine wave or a pulsed one. The system relative permittivity of the sea and the transmitted power. involves several generators providing the different carrier The present study aims at evaluating the levels of accuracy frequencies 968 MHZ, 3.5 GHz or 5.4 GHz. Installation of of this model and at examining other causes of modification of antennas was different for the two campaigns, because of the configuration of the boats and according to the expected III. MEASUREMENT RESULTS measurements. For the first campaign [1], two horn Tx antennas were A. Theoretical Results installed on the boat, one to 12 meters above water and the As indicated in the introduction a basic model being able to other one between 2.5 meters and 5 meters above water. For apply to the propagation in maritime environment is the two- the second one, omnidirectional narrow frequency range Tx ray model. It takes into account a direct way between the antennas where set up at 14 meters above the sea. At the transmitter and the receiver and a way refracted by sea surface. receiving site, several systems were available, namely a We consider this model for simulate the propagation over the spectrum analyzer controlled by a computer and a second sea, the figure 4 shows simulation examples. system constituted by PXI formatted cards. A controller, a Field attenuation, horizontal polarization RED, vertical polarization GREEN, signal analyzer, a frequency converter and a mass storage 3.5 GHz, hTx = 12 m, hRx = 52 m, sea 1/d slope BLUE, 1/d**2 slope CYAN device constituted this last one. This acquisition system can 20 record the received signal with 80 dB dynamics and a 20 MHz 0 bandwidth centred on a programmable frequency included between a few tens kHz and 6 GHz. -20 For the second campaign described here, an additional dual -40 channels acquisition system was used to store the signals provided by two Rx antennas separated in height by about 4.8 -60 Field (dB) meters. These Rx antennas were 12 dB gain horns with 120° -80 horizontal aperture and 30° vertical aperture, followed by a filter and a low noise amplifier. -100 A GPS equipped a PC on the boat. The time and the position -120 data were recorded for each trip. These data make it possible to 3 4 readjust the recordings of the signal received according to the 10 10 Length Tx-Rx (m) real distance between the transmitter and the receiver. These Figure.4. Simulation with two-ray model: level of the received field according data were confirmed by the equipment of the ship. The GPS to the length Tx-Rx information was also used for clock synchronization of the Tx an Rx devices. When looking at simulated results obtained from this The following pictures represent the devices used for the theoretical model we can deduce the following considerations: experimentations. • the average slope of path loss is proportional to the transmitter-receiver distance approximately up to the radio electric horizon, as for a connection in free space, • the average slope of path loss is proportional to the Figure.2. Transmitting devices squared of the transmitter-receiver distance, approximately beyond the radio electric horizon, • the horizontal polarization presents major and recurrent fading (from 20 to 50 dB) depending on the transmitter- receiver distance, • the vertical polarization presents major and recurrent fading (from 10 to 30 dB) depending on the transmitter- receiver distance, • the fading of horizontal and vertical polarizations are phase opposite until a distance of few hundred meters, • the distance interval between fading for both the Figure.3. Receiving dev ices horizontal and vertical polarizations exceeds the hundred meters for transmitter-receiver distances greater than approximately 1 km. These characteristics were verified during experiments and Throughout this paper, the presented results concern the the measurement results are presented in the following chapter. following frequencies: 868 MHz, 3.5 GHz and 5.4 GHz. B. Experimental Results Figures 5 and 7 represent examples of travel during which measurements were carried out. 1 k m 5 k m Figure 7. Example of travel where the measurements were carried out (18/04/2010. 07h36-08h05 U.T). Figure 5. Example of travel where the measurements were carried out 18 04 2010 07 36 00 - 08 05 00 f = 5.447 GHz a) hTx = 14 m, hRx = 47.5 m (17/04/2010. 10h11-12h45 U.T). -20 -40 Examples of result are shown on figures 6 and 8, comparing -60 the levels of the theoretical values, defined by the two-ray field (dB) -80 model, and the measured levels of the received signal provided 2000 3000 4000 5000 by two antennas separated in height by 4.8 meters. The third b) f hTx = 14 m, hRx = 42.7 m -20 graph shows the result obtained when the most powered signal -40 of the two antennas is chosen during the travel. The gain is -60 significant, but not optimal since the difference of height field (dB) -80 should be in this case of approximately 12 m. 2000 3000 4000 5000 Max[a),b)] -20 -40 17 04 2010 10 11 00 - 12 45 00 f = 5.447 GHz a) hTx = 14 m, hRx = 50.9 m -20 -60 field (dB) -40 -80 2000 3000 4000 5000 -60 length Tx-Rx (m) -80 field (dB) -100 4000 7000 10000 13000 Measurement Theorical Value Free space attenuation b) hTx = 14 m, hRx = 46.1 m -20 Figure.8. Comparison of the measured received signal level with simulated -40 values. In blue the measured signal, in green the 2-ray model. -60 -80 There is a good agreement between the theoretical values of field (dB) -100 field attenuation and the measurements. On figure 6, the level 4000 7000 10000 13000 differences at the beginning of the travel are mainly due to Max[a),b)] -20 antenna effects. Indeed, the Tx and Rx antennas has only 30 -40 degrees vertical apertures, and the receiving antenna at point A -60 was located at about 50 meters above the sea, whereas the -80 field (dB) transmitting antenna was onboard a small ship.