# Seamless 5G Multi-Hop Connectivity Architecture and Trials for Maritime Applications
## 2 Maritime Scenario and Problem Statement
### Coverage Status in the 3.5 GHz Band  
Using a 3.5 GHz carrier brings the operator multiple advantages and disadvantages. With regard to the advantages, we can name the following:  
Figure 3: Signal strength at 700 MHz along the ferry route from Vuosaari harbor. Greener color represents higher received signal strength and yellower color represents lower received signal strength. Black color represents power outages (i.e., received signal strength below \(-110\) dBm. Figure 2: Signal strength at 3.5 GHz along the ferry route from Helsinki harbor. Greener color represents higher received signal strength and yellower color represents lower received signal strength. Black color represents power outages (i.e., received signal strength below \(-110\) dBm. * The 3.5 GHz frequency band is well-suited for designing radios with phased-array antennas due to the wavelength characteristics of this frequency range [25]. With a shorter wavelength, it is easier to design radios with phased-array antennas and enable beamforming. This is because the size of the antenna elements in a phased array is directly proportional to the wavelength of the signal being transmitted or received. Higher-frequency signals, such as those in the 3.5 GHz band, can be accommodated with smaller and more closely packed antenna elements. This allows for the creation of a phased array with a larger number of elements, making it easier to enable beamforming and to leverage its technical advantages for optimal coverage and improved interference rejection along the ferry lines. * More bandwidth is available to facilitate even the most challenging use cases in terms of capacity (e.g., dense harbor areas). * High loss propagation allows for better control of the designated coverage with good flexibility for interference mitigation. * The commonly used time-division duplex (TDD) technique allows for the flexible configuration of up-link (UL) and down-link (DL) frequency resources based on the service type. The management of interference in non-synchronized deployments between different parties is challenging in practice. However, recent 5G advancements, such as the ability to mute specific transmission intervals and coordinate transmission between network nodes, make the use of this flexibility more practical. Regarding the drawbacks, the following are highlighted:
* Shorter wavelengths compared to low-frequency waves result in a reduced ability to penetrate through obstacles such as buildings, ships and other solid objects. This presents a challenge in terms of ensuring reliable coverage inside such environments. * The high propagation loss of high frequencies is a significant drawback when the required service distances are considerable, as is the case in the analyzed ferry use case. * Radio propagation is strongly impacted by heavy precipitation such as rain and snow, which are common in the region. These weather conditions can absorb, reflect or scatter the radio waves, leading to signal attenuation, interference and a decrease in the coverage area. As a result, the quality and reliability of the communication link may be degraded, making it challenging to maintain a stable and robust network performance. * Commonly, the TDD spectrum usage technique is used in the given bandwidth, which imposes a challenge in terms of precise network synchronization and requires a common timing source for all the operating parties due to side harmonics. Otherwise, a significant guard band is needed between the operating frequencies. * In addition, it is important to consider that the common TDD pattern required between different operating parties may limit the availability of services such as ultra-reliable and low-latency communications (URLLC). According to 3 GPP TR 38.913 V17.0.0 (2022-03), the target for user plane latency in URLLC should be 0.5 ms for UL and 0.5 ms for DL. * The prioritization of the DL transmission in the most commonly used TDD patterns can result in limitations to the UL transmission, leading to a negative impact on UL-intensive services.