GB3IM MultiSite Project

Project Approval
The project to add two new sites to the IOM repeater network was approved by the ETCC and OfCom on 20th January 2009, clearing the way for the acquisition of the required hardware and the implementation of the project to proceed.
Technical Considerations Planning for this project needed to take into account a number of factors:
Required Coverage The technical complexity of achieving that coverage The need (or not) to put extra effort into covering areas with little or no population Future expansion		Maintainability and reliability Cost Inter-Site linking Equipment availability

Coverage		Coverage Overlap
In an ideal world, we would achieve blanket coverage of the island from a minimum number of sites. Of course, that is only likely to be achieved if we could literally move mountains and make the Isle of Man flat - but then, it would lose a lot of its charm and would present less of a challenge. So, we keep a lumpy island and look at how we can place repeaters on top of some of those lumps. From the repeater keeper's professional experience with PMR systems on the Isle of Man, the next best site to Snaefell, which gives good coverage of a large area, is the Carnane radio site, located a few KM south of Douglas. This site has the advantage over Snaefell in that it is readily accessible by road at any time. Additionally, Carnane has a robust power supply and good quality accomodation over which we have good control. Carnane will offer good coverage of the island to the West and South but, due to terrain, there is little coverage of the island to the North. This actually helps with site and frequency planning because the terrain screening allows the same channel to be used on a Northern site with minimal overlap in coverage. Because of the lack of coverage from Carnane in that area, consideration was then given to what supplemental coverage was required in the North of the Island. Snaefell gives excellent coverage of the Northern Plain which encompasses a large area. However, because of the nature of the landscape, the main road between Sulby and Ramsey is in the shadow of the hills and signals in that area are very poor (particularly between Kerrow Mooar and Milntown). There are many other "pockets" around the Northern hills where coverage from Snaefell is also very poor; Slieau Lewaige on the coast road into Ramsey is a good example. To address this, a site was identified which is able to "look back" into these shadowed areas and which can provide good coverage, thereby mitigating the problems. As it happens, this site is the repeater keeper's home. The intention is to use a directional antenna to "flood" the Snaefell shadows and to provide infill coverage for a significant stretch of the TT course between Sulby and the Mountain Box.		The ETCC coverage maps base their predictions upon omni-directional antenna characteristics at the radio site. Even so, the predicted overlap areas between the North and South sites are minimal and, in any case, fall within areas which are extremely well covered by Snaefell. In reality, we will use directional aerials at both Carnane and Ramsey, which will further reduce the overlap areas. Further mitigation of the consequences of overlaps is achieved by means of the "Capture Effect" which characterises FM transmissions. Predicted coverage overlap between Carnane and Ramsey sites assuming omni-directional aerials. Overlaps are shown in blue. As can be seen, the overlaps occur in areas which are swamped by Snaefell's coverage.

Site Linking
Many technologies exist for linking radio sites together and some of them are absurdly expensive, requiring microwave links and other equipment. Luckily, many radio amateurs have turned their hand towards taking advantage of cheaper and more accessible methods which use the internet and Voice-over-IP techniques. EchoLink and IRLP are widely used by groups all over the world to link repeaters to the internet, however both have their limitations and restrictions which make them less than appropriate for use in our application. IRLP, for instance, requires radio interface hardware which is somewhat expensive, also it is not as flexible as we would like. The Isle of Man project will use a computer-based network using the Asterisk PABX software and the associated App_Rpt application. Between them, these applications allow us to create a formidable and exceptionally flexible radio network at reasonable cost. In future, this software will also allow for linking to either or EchoLink and IRLP. All of the software is available free-of-charge, and the radio interface utilises USB sound adapters, based-upon the CM108 chipset, which can be purchased for around £10.00.		Both the Carnane and Ramsey sites are internet-connected, making the task of linking very simple. Snaefell is not, at present, linked to the internet but moves are in hand to address this shortcoming. Amongst the key characteristics of the Isle of Man network will be the unified behaviour of the sites. When a user transmits on the input of one site, all other sites will go into transmit and re-broadcast the received signal. There will be no requirement to enter set-up tones from a keypad. Access will be CTCSS only. This simplicity will make the system as easy to use as possible not only for locals, but for visiting amateurs as well.

More technical details will be posted here very soon. Please check back with us and, in the meantime, if you have any questions or want to offer to help, please contact us.

The Hazards of channel re-use
As was briefly touched upon in the previous section, there are problems associated with using the same channel allocation for multiple sites. Any station operating in the areas where the coverage of co-channel sites overlaps will receive a garbled signal. This is due to two factors. The co-channel transmitters' carrier frequencies are not synchronised, leading to the production of a "beat" frequency. The audio signals leaving each transmitter are not in phase, neither are their audio characteristics matched, which leads to distortion. These problems are addressed in Quasi-Synchronous radio schemes but, whilst it is feasible to ensure highly stable transmitter frequency matching, VoIP links do not lend themselves to the other requirement, which is that audio must leave each transmitter site exactly in-phase and with comparable frequency response characteristics. Because of the "Capture Effect" a mobile operating in the overlap areas will probably not have any real problems with the signals he receives. Conversely, however, that same mobile could cause problems to the system....		It is not only transmitter coverage which presents a problem when re-using channels. Receiver coverage must also be taken into account. When a mobile in the overlap area makes a transmission, his signal will be received by at least one of the sites, and perhaps by two or more (if there are more than two, of course). Since this signal needs to be relayed, the "system" needs to make a choice between the received signals from each site as to which one to use for re-broadcast. The system cannot simply add all received audio signals from all of the sites together and use the result because one or more of them might be noisy and, furthermore, due to the path taken by the signal at each site, none of them are likely to be in phase, resulting in severe distortion. Once again, Quasi-Synchronous schemes take this into account by using a technique known as "Site Voting". Each site, along with the received audio, passes a quality signal indicating the relative strength of the incoming transmission. The control site can then select the audio which has the best quality signal and use that for re-broadcast. Because of the complexity of implementing Quasi-synchronous techniques, we will not be using them. However,it is important to be mindful of the consequences of having overlapping coverage areas. The key factor to bear in mind is that, in those areas where two or more co-channel sites overlap, there should be a better quality service available from another site on a different channel. In the case of Carnane and Ramsey, a high quality coverage is available from Snaefell in the overlap areas and this site should be used in preference. This will avoid a station causing problems to the system by being received at multiple sites and will also prevent his receiving garbled signals. Obviously, where there is likely to be significant overlap between adjacent "cells", different channels must be used. It is desirable, however to minimise the number of channels used so as to reduce the need for "channel-hopping" by mobile operators.