GB3IM Linking Project

Project to Enhance Coverage Of The Island

Using VOIP and Internet Technologies

 

Background to the project 

The Isle of Man has been served by two co-located repeaters for many years (at least twenty to my recollection, and certainly more than that). During that time technologies have changed and appeared which make it possible to greatly enhance the effectiveness of our repeaters, particularly with regard to coverage of the Island itself. One often cited shortcoming of the Snaefell repeaters is the fact that, whilst the location offers superb coverage of the Irish Sea and surrounding lands, it often fails to provide adequate coverage of its immediate surroundings. This leads to the situation where off-island stations enjoy excellent service, whilst on-island stations often struggle to be heard. An additional drawback of Snaefell is ease of access. The only motorised access to the summit is either by means of the Manx Electric Railway, which is operated only during the summer season, or by "hitching a ride" with the Civil Aviation Authority's on-site engineers in their Diesel-powered rail car. Again, the CAA staff only visit the site on a few days in the week.

 

At all other times, the only other access to the site is by walking - not an enticing prospect when you're carrying tools and test equipment. The weather is also an issue with Snaefell. It can be a very hostile environment for both people and antenna hardware. Opportunities to effect repairs can be few and far between, especially in winter, which could lead to extended periods of "Downtime" in the event of an aerial fault. The Conclusion

Whilst keeping Snaefell on the air is highly desirable, the group feels that additional, supplementary, coverage is needed in order to maintain communications if the site should be lost for whatever reason (Weather, power failure, antenna fault etc). Furthermore, it is felt that Snaefell alone does not provide adequate coverage of the island. These requirements have led to the birth of this project.

 

 

The Proposal

The project was first floated at a meeting of interested parties in December 2007. This discussion re-inforced the already well-known views of many local amateurs that we needed to become less dependent upon Snaefell as our primary repeater site, and that infill coverage of major communities was required. During the following year, various technical discussions were held with the Emerging Technology Co-Ordination Committee (ETCC) of the RSGB to determine whether we might be permitted to achieve our aim of providing greater coverage of the island by using multiple internet-linked repeaters. As it turned-out, once the ball had started rolling it quickly gathered momentum. The ETCC were keen to encourage this project, especially as the design deliberately avoided extending the coverage of the repeater beyond the island's boundaries. The question was asked, and was quickly rejected, as to whether we could do anything on 2 metres. One of the reasons for rejecting 2M was the absurdly high price of duplexers but, additionally, recent studies by the ETCC have determined that new proposals for this band will not be readily accepted. Once it was agreed that 70cm was the way to go, the ETCC were exceptionally helpful in moving things along at breathtaking speed.

 

The initial plan is as follows:

  • Re-Work the configuration of GB3IM.
  • Retain Snaefell on its present frequencies.
  • Install two additional repeaters at strategically located sites.
  • Link the three units via the internet so that they operate as one repeater.

The primary node for the project is located at the Carnane Radio Site (GB3IM-C), located south of Douglas. This node will house the main computer hardware and the primary internet link. Snaefell will be reconfigured to be a secondary node, triggered by an uplink from Carnane. A third node will be added in the North of the Island to provide infill coverage in areas tradionally poorly served by Snaefell because of terrain screening. The site is about 3 miles north of Ramsey and is designated GB3IM-R. This node will be internet linked to Carnane. Carnane and Ramsey (GB3IM-C and GB3IM-R) will operate on wide-spaced channel RU66 (Output: 430.825MHz, Input: 438.425MHz). Snaefell will remain on RB5 (Output: 433.125MHz, Input: 434.725MHz)

 

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.

 

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.

  1. The co-channel transmitters' carrier frequencies are not synchronised, leading to the production of a "beat" frequency.
  2. 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.

 

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.