Human efforts to secure and enhance maritime safety are being diversified to address the problems caused by a growing amount of data to be exchanged. The e-Navigation strategy and implementation plans of the International Maritime Organization (IMO) have enabled the harmonized emergence of various e-Navigation services, which also requires more robust processing of data and information in traditional ship operations as ship safety and increased traffic efficiency. Furthermore, the advent of various e-Navigation services has led to research on new maritime communication by analyzing and leveraging the multiple on-shore radio communications(Jeong and Kim, 2008).

As one of the e-Navigation projects to implement services to the maritime users, the Republic of Korea has started the Korean e-Navigation R&D project for five years from 2016, which included the sub-project of developing the maritime high-speed wireless communication network in the coastal sea area, named as LTE-Maritime project. In the first phase of LTE-Maritime, the feasibility of LTE technology in the maritime field has been investigated in the test-bed of the Korean coastal sea area and verified the possibility of LTE communication coverage up to 100 km from the base station in maximum. Regarding the initial analysis result of the first phase of LTE-Maritime feasibility, Jo et al.(2018) has shown that LTE communication could propagate and provide meaningful service up to 100km from the field test result. However, there were also considerable differences between the prediction of the existing Propagation Loss Model(PLM) and measurements of huge sea areas.

For a potential high-speed maritime communication, several studies utilized telecommunication technologies being used on land. Lopes et al.(2014) showed a study to enable communication at sea using Wi-Fi technology in the 5.8 GHz band, and Zhao et al.(2018) proposed that LTE could be a suitable solution for long-distance communication at sea. For the maximum theoretical coverage based on LTE technology, Jo et al.(2018) have shown that LTE-based high-speed maritime wireless communication service up to 100 km could be practically possible with field test and measurement verification in the coastal area. Regarding the coverage extension of LTE communication in sea area, Park et al.(2017) has proposed that the maximum service limit of the LTE system could be extended up to 200km in maximum by shifting the time advance parameter of LTE when the base station of LTE might be located at high altitude enough to have a line of sight to the edge of coverage.

The studies, as mentioned above, of mobile communication technology in the maritime field only showed the practical possibility of LTE with a considerable difference of propagation level prediction of radio waves in huge sea areas. Jo and Shim(2019) showed and explained that there was a level of difference between field measurements of up to 100km sea area and legacy PLMs such as Free Space Path Loss(Klozar L. and Prokopec J., 2011), Two-Ray Path Loss(Enaya R. and Ivica Kostanic D., 2020) and Okumura-Hata model. It meant that legacy PLMs could not be directly adopted as a propagation loss model of a nationwide maritime network with 100km coverage in maximum. In the case of Park et al.(2017), to provide overwhelming coverage, they proposed a three-slope propagation model adjusted with the field measurements to acquire the matching propagation loss model for vast sea area. The Adjusted Propagation Loss Model(A-PLM) has shown the meaningful match between predicted and test results, but the result of this study could be helpful when two base stations should be selected so far from each other that the 0-100km section of the base station serving the 100-2000km area did not overlap with that of other base stations serving the 0-100km sea area. This propagation model could also be helpful only in different frequencies between the vicinity of stations.

Considering the lack of appropriate PLM for cell planning of huge sea areas with one available frequency for LTE-Maritime, empirically, A-PLM was proposed to develop propagation loss prediction for huge sea areas of target coverage.

Following chapter 1 as an introduction to explain why A-PLM was necessary for LTE cell planning of huge sea area, chapter 2 explains the actual cell planning methods and procedure. Then, the verification of the cell planning was shown in Chapter 3, and finally, it concluded in Chapter 4.

Considering the given development period, the A-PLM was developed using the signal quality information measured from the actual sea in each designated sea area.

The LTE-Maritime project has defined the target zone of its quality goal as two distinguished 0-30km from the shoreline as an Intensive Management Zone(IMZ) and 30-100km as an Interested Zone(IZ). The range of the distinguished zone was based on the statistics of domestic small and mid-sized ships navigating in the coastal area in aspects of maritime safety affair.

In aspects of station altitude, 100m, the figure was chosen as a separative reference to match the 50km line of sight distance. In the case of 100∼500m station height, a sole A-PLM could not satisfy the comparison with the actual measurement data over the whole range, so it was distinguished as two A-PLMs.

The A-PLMs, which reflected the differences between the reference model used in the cell simulation and the actual measurements for each combination of the two levels of zones and base stations with altitudes of 0-100m and 100-500m, are shown in Table 1.

In order to verify the cell planning, it was necessary to secure communication quality measurement data in vast range of actual sea areas. To this end, the Communication Service Quality Monitoring System (CS-QMS) was developed in the LTE-Maritime project, which collected LTE signal information from onboard equipment and analyzed the signal quality of the whole service sea area in the main control center.

Since the sea area to be verified was 180,944㎢, it was physically impossible to measure and verify the entire sea area at once. Therefore, LTE-Maritime onboard equipment was installed on tens of ships sailing across the entire service area to verify cell planning results for as wide sea areas as possible in a short period. The onboard equipment has been designed to send the quality information of the received signal to the LTE-Maritime control center located in Se-Jong city.

When installing the onboard equipment, it was preferable to install the antenna height as high as possible and set up the distance between antennas to be at least 1.5m apart, considering the function of the equipment operating as MIMO. In addition, the distance between the equipment's main body and antennas should also be apart more than 0.3m, and the equipment was powered through the PoE(Power of Ethernet) interface.

All information of signal quality received from the ships was processed by the unit grid size of 4.5km(lat)×5.5km(lon), which was designed by the unit transformation of the coordinate system.

To verify cell planning, the average value of accumulated RSRPs on each grid was chosen in ten months to consider the weak value due to any interference during the field measurement.

By applying the grid size of CS-QMS described above, it was analyzed the number of grids for IMZ and IZ sea areas and compared the quality information accumulated in each grid with the target quality goals in Table 3. Then, it was finally calculated the number of grids of success with the satisfaction rate shown in Table 5.

In order to establish a high-speed maritime wireless network based on LTE technology in huge sea areas up to 100km from the coast, the cell planning of the communication network was an essential step and should be processed with the PLM appropriate to be applied for the service area of sea. The legacy PLMs, which have been generally used, were unsuitable for long distances and far sea areas up to 100km from the coastline.

To overcome this issue, the basic model of the software to be used for cell planning was empirically adjusted using real-sea LTE signal measurement and developed three A-PLMs as a result. Finally, these A-PLMs were applied to the cell planning software to satisfy the quality goals of the LTE-Maritime.

As a result of verification, we found the quality level of IMZ(more than -95dBm) with a goal of 95% success rate was satisfied with the 95.7%, and IZ(more than -115dBm) with a goal of 90% success rate was also satisfied with the 96.4%. However, considering that those success rate figure was driven during the period of resolving the interference problem of the surrounding network and was obtained while performing network optimization, it would be expected to be improved through the actual operation of the network in additional time.

LTE-Maritime is the world's first high-speed communication network in the sea that guarantees LTE service to a certain level higher than the quality target for the sea area of up to 100km. While the internet on land produces massive value-added services for our lives, LTE-Maritime can be expected like an internet infrastructure to implement the fourth industrial revolution of the ocean. Furthermore, it is expected that the novel high-speed wireless communication network, e.g., LTE-Maritime and Maritime-S2X, will be expanded to be used for maritime autonomous surfaced ships and unmanned ships.