Broadband cellular systems are becoming commonplace in maritime environments, with public and private networks desired over large sea areas to cover needs which were once uniquely served by satellite. To take advantage of economies of scale, land mobile cellular technology is often simply transplanted into the maritime context. In principle that is fine, however, propagation planning principles common in land mobile should not not be applied applied without forethought.
For example, planning methods based upon propagation models such as COST231 Hata that are often used in land mobile for predicting coverage and frequency planning are not suited to the maritime environment. These models account for a combination of physical effects for specific to a particular type of environment, and are not sufficient when taken out of context. Model tuning is the cell planners usual tool to improve validity, but it doesn’t really help, as fundamentally these models account for different physical effects, so a rethink in modelling approach is required.
Of course the maritime environment is not characterised by building or vegetation clutter, hence any approach based upon them is likely to come unstuck. However, whilst the environment is largely uncluttered in terms of obstacles above the surface of the sea which would cause diffraction losses, the sea surface itself is an excellent source of scattering that can lead to undesirable effects, as well as a source of shadowing due to waves in high sea states. Furthermore high seas are the potential source of unstable antenna platforms. These issues and many others are well understood by designers of other types of radio system deployed in a maritime context, and something useful can be learned from them.
In the radar world, designers are familiar with sea clutter, which is the term used to describe the excess information that can appear on a radar display as a function scattering for a given sea state. Other issues include lobing effects limiting range in the elevation plane due to surface reflection in calmer seas. Much of a radar system processor is devoted to alleviating these effects without compromising the ability to detect valid targets. In the fixed link world, paths crossing larger bodies of water may suffer heavy fading, particularly in calm conditions and space diversity in the vertical plane is often used to mitigate fading.
So, for maritime mobile network some of the considerations and techniques can be borrowed from fixed link and radar planning. One of the most basic things is to understand the nature of reflections from the surface, and in fact early planners of land mobile systems were quite aware of this, and simply talked of plane Earth modelling. In this case the effects of a specular refection is considered from the surface, and the interference pattern predicted for typical path geometry. Unfortunately common mobile radio diversity techniques with small displacements between antennas offer less relief than in a cluttered land mobile environment, which means that the antenna height of the mobile system must be carefully considered to avoid the effects of severe fading.
The plot below shows an example of the fading effect from surface multipath for a system with a base height of 100 m operating at 1.5 GHz to a receiver antenna height in the range 0 – 40 m above the sea surface at a range of 10 km.
In this case significant fades are present at multiples of 10 m, so unless the fades can be tolerated, the antenna configuration would require optimisation to ensure that bad heights are avoided, or a diversity configuration could be chosen with optimal antenna separation. These issues are well understood and have a clear impact that can demonstrated whatever the sophistication of current mobile technology.
