Under the internationally-agreed arrangements for use of the maritime radio frequencies (the ITU Radio Regulations, appendix 18) there are approximately 65 radio channels available for maritime use. These are all 25 kHz wide and are used for analogue FM voice communication (with the exception of channel 70, which is used for DSC signalling as part of the GMDSS and the two channels allocated for AIS). On the face of it, this would seem to be enough for most, if not all situations.
However, when it comes to allocating working channels for operating in crowded areas, such as major ports and estuaries, it soon becomes clear that with interference from adjacent transmitters and neighbouring areas this can be a significant problem. It is made worse by interference from channels which already have a defined usage and so cannot be re-used. Recent changes to the regulations have further exacerbated the issue, with the new ASM and VDES data services occupying up to 14 channels, thus removing them from the pool of available voice channels. Additionally, channel 2006 has recently been allocated for use by AMRDs which removes another channel from the pool.
A number of port operators have appreciated that the problem will only get worse, as voice communications will always be required between vessels and port control centres as well as from vessel to vessel and, in response to this concern, they have looked for better ways to utilise the existing channels. One possible solution has been to look at the evolution of voice communications in the land mobile areas, which have moved from 25 kHz analogue FM to 12.5 kHz analogue FM and now to digital modes which can improve the spectrum efficiency to up to 4 times that of the current systems by encoding the voice signal into a digital data stream. A particular system that sparked significant interest is dPMR, which has a number of advantages over other solutions considered:
- Uses a publicly available standard from ETSI
- Spectrum efficiency is improved by a factor of 4 – it uses a 6.25 kHz channel
- It requires no shoreside infrastructure, so can be easily re-used at sea between ships
- It is an FDMA system so has no special timing or synchronisation requirements
- It uses constant envelope FM, so the radio designs require minimal changes to support it and they can easily replace current hardware on existing antenna systems
- It has a similar operating range to existing analogue FM (actually, slightly better) so requires no major changes to existing frequency and coverage plans.
- Minimal cost implications on equipment.
- Allows additional services, such as SMS, to be implemented
A recent trial in the port of Rotterdam using equipment from two manufacturers showed how the system could work in practice, with stations mounted on a workboat and the VTS centre as well as portable units, providing a direct comparison to analogue FM operation. The dPMR system provided clear and intelligible speech coverage just beyond that currently available to the analogue FM system with the advantage of the audio not being degraded by background noise, especially at the edges of the coverage area.
With level of interest shown, ETSI have now started work on deriving a maritime-specific variant of the existing dPMR standards to optimise it for the unique conditions of the marine environment.
Having been discussed in a number of standards bodies (IALA, ECC-CEPT and ETSI), a number of documents supporting this system have now been sent to the ITU and IMO for their consideration, and hopefully adoption, as the basis of a new maritime digital voice standard.
