The big news from Axell Wireless at TWC 2012 in Dubai was its partnership with UK public network operator Airwave to produce an enhanced repeater that will help to reduce the cost of building and operating networks.
‘We have long relationship with Airwave dating back 15 years,’ says Axell CEO Ian Brown. ‘We deployed hundreds of repeaters and coverage devices throughout the Airwave network in the UK, so we have a very close technological relationship as a result of that. Axell engineers support their network in the field.’
The enhanced repeater provides extra capacity, wider coverage and lower costs, especially during network rollouts or technology refreshes. Axell Wireless has supplied the core technology for the enhanced repeater, while Airwave has brought its design and network operational experience to the product.
Axell Wireless CEO Ian Brown tells Wireless the thinking behind the product. ‘We are looking at how we can help lower the operating costs of PMR networks. One way to do that is to have fewer base stations. But to do that you need to find a way to extend the coverage of your existing technology.’
A network generally has central base stations with a number of other base stations clustered around it. But to avoid interfering with each other they have to transmit on different frequencies. If they are on the same frequency inter symbol interference (ISI) can be created, which causes problems for radio users. ISI occurs where two or more cell sites overlap and the radio sees the same signal twice with different delays. It cannot resolve the signal and the service is dropped.
‘ISI means the radio cannot resolve the signals and you get dropped or very noisy calls,’ explains Brown. ‘We’ve been able to correct that by introducing variable delay into the product. By using an interface you can actually apply different levels of delays, so that all the signals arrive at the user’s terminal at the same time. The end user now sees the same signal from wherever they are. That way, the interference areas are managed and you avoid ISI and the call doesn’t get dropped. That is the revolutionary feature – and it is all done by using software defined radio (SDR) technology.’
What Airwave and Axell have done is to find a way to allow the base stations to transmit on the same frequency, but avoid ISI and oscillatory interference. Apart from improving the quality of calls there are two other key benefits to the new technology.
Using a conventional architecture, you might need eight base stations to cover an area and they would all have to transmit on different frequencies to avoid interference.
Cutting network costs
But now it is possible to cut the number of base stations to two by beefing up a central base station to provide a higher throughput and more horsepower and replacing the other base stations with enhanced repeaters.
The central base station and the enhanced repeaters can transmit on the same frequency by imposing micro-second delays to the signals at the base station to ensure the signals reach the radios at the same time through variable delay. By providing these signal ‘hops’ the coverage is extended via the enhanced repeaters and so fewer base stations are required.
‘The other benefit,’ says Brown, ‘ is that by getting rid of those base stations all transmitting on different frequencies and replace them with repeaters, they can now all operate on one frequency and that frees up spectrum.’
Network operators will be able to significantly reduce their operating costs as a result. Brown said: ‘The cost of running base stations is very expensive in terms of power consumption – around 650W is dissipated. With the enhanced repeaters just 180W is dissipated.
‘In addition, it is sometimes tricky to get power to isolated base stations. You cannot use solar or wind technology to generate enough power to run a base station, but you can use them to run an enhanced repeater, so there are more cost savings to be had there. And if you have fewer base stations in your network, your maintenance costs go down too,’ point out Brown.
The enhanced repeater is vendor agnostic so it can be used with any network suppliers’ equipment and it is ready to go to market now. Airwave and Axell will be taking the product to market in partnership with one or other company in the lead depending on how well they are established in particular countries.
‘We’ve done trials with Airwave in the lab and then in the field, which were very successful,’ reports Brown. ‘No one else has got anything like it. Our SDR technology is patented and there are not too many people that have patents on digital repeater technology.
Brown points out that all its repeaters are SDR repeaters, so all the functionality is driven by software – a big change from five years ago when if you wanted to change filters you’d have to physically pull them out, but now all filtering is now down by an FPGA (field programmable gate array).
Channel selective products
Another announcement in Dubai centred on Axell’s leading off air product, the CSR438 TETRA channel selective repeater, which has up to 8 channels and 8 filters. Using software, the repeater can now be turned into a band selective product.
‘You can switch it to channel selective to cover the whole band and back again if you want and for a lot of system integrators and operators that is a very useful feature,’ says Brown, ‘as they may not know which channels they are going to be operating with from day one. Historically, operators would need two separate products to do that.‘
Distrubted antenna systems
Axell was also displaying its DAS (distributed antenna system) in Dubai, which are used to extend the coverage of PMR and cellular systems
‘If you want to get coverage into a building there are only two ways to do it,’ says Brown. ‘You either pick the signal up off air by putting up an antenna and bringing it into a box, filter it, clean it and then propagate it around the building. Or you connect it directly to a base station, but you may still need a system for propagating it around the building.
‘At the heart of the DAS is a hub, which is connected to the base station via a fibre optic link. That could be local or 10km away, or you pick up the signal off air and feed it o the hub. The hub then communicates down to a bunch of remotes – repeaters that provide coverage in a particular area,’ explains Brown.
Axell produces a range of remote modes that support low, medium and high power, which are deployed according to the area that needs to be covered. The remotes support all frequency bands, so the Burj Khalifa tower in Dubai is using a TETRA system, but in the Pentagon we needed to support VHF, UHF, 700MHz and 800MHz – which required the deployment of a whole variety of remotes connected back to the base station.’
Brown is keen to point out that one of the major assets of its DAS systems is that it supports all frequencies from FM to 2.6GHz. ‘Not many people want to do that,’ says Brown, ‘but some do want to mix public safety with cellular bands. We didn’t do it in the Pentagon building itself, but we did in an extension, which houses 10,000 people. It has a system incorporating public safety and cellular frequencies all operating from the same DAS. We are seeing a lot more of this kind of thing.’
Brown reports that DAS project are now a huge business, because everyone wants in-building coverage to meet the data demand from smartphones and tablets. Good news for Axell it seems.