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    Tuned to the key of RF

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    As 2G cellular network operators focus on wringing higher performance out of existing assets, the key to network optimisation may well lie in tight RF footprint management coupled with sophisticated network monitoring tools. Ellen Gregory of Relate Technical Communications explains.

    After two decades of rampant development in the cellular industry, it might now be a period of consolidation and gradual transition; but it is also a period of extraordinary challenge. Not only do consumers and business practitioners have high expectations of the quality and availability of cellular services, but the demand for ‘vertical’ technologies — such as data services overlaid on existing 2G networks — is escalating. Operators too are regarding 2.5G services as a potential means of growing their businesses — particularly in mature markets where subscriber take-up rates have petered out.

    As capital investment is minimised and operators seek to maximise returns from existing assets, network planning and optimisation have become more critical than ever. Yet, the more complex the network, the more layers of services, the greater the number of parameters that have to be considered and made compatible. With traffic patterns shifting so readily and networks interfering with each other, the art of network optimisation has, to the uninitiated, started to look like ‘black magic’.

    “One of the earliest things that we learned in the days of analogue was that a cellular system never moves out of the design phase,” says wireless communications consultant, John Smyth, who spent 15 years as National Manager of RF Systems, Mobile Networks, for Australian-based telecommunications company, Telstra. “Infrastructure is being added, traffic patterns are continually changing — so you need to have flexibility within the system to cope with it.”

    System flexibility is perhaps the credo of network optimisation, which is a constant balancing act between coverage, capacity and the increasingly important quality of service. “Because of this demand for flexibility, operators need to keep constant track of network performance — twenty-four seven,” says Smyth. “This is the first step in optimising a network.”

    Head of radio planning systems for T-Mobile in Germany, Wolf Mende, concurs with this view. Key performance indicators (KPI) such as hand-off success rate, call drop rate, hold time, and congestion are continuously monitored to provide indicators of areas that might require tuning. “We also take dedicated field strength measurements of the involved base stations where they are required,” Mende says. “These provide an image of the real interference situation in a network, as opposed to what we might have predicted using models.”

    Co-channel challenge

    Both Mende and Smyth acknowledge that one of the greatest challenges of network optimisation is controlling RF interference. In GSM networks, co-channel interference degrades audio quality by masking low level carrier signals; whereas in CDMA-based networks, capacity is depleted by interference, which increases the noise floor. Either way, the result is inferior network performance, providing dissatisfaction to users.

    “In mature networks, it’s not about coverage. It really boils down to managing interference,” says Smyth. “The object is to put the signal where it’s wanted, and keep it from where it’s not wanted. So managing RF is one of the basic steps in planning and subsequently optimising a network.”

    “It’s a kind of puzzle,” adds Patrick Nobileau, Vice President of Base Station Antenna Systems for wireless technology group, Radio Frequency Systems (RFS). “With so many cells, if you want to optimise your network, you have to make sure that you transmit the energy without creating too much of an overlap. This is particularly important for CDMA systems, where the same frequency is used for each cell.”

    Careful frequency planning of networks provides a measure of transmission quality  on a macro scale; however, the troubleshooting of problem areas invariably leads to a spot of local tuning. In such cases, it is the antenna beam that can be adjusted, says Mende. “We use information from the live system for measurements. Once the results are analysed, we can decide how to change parameters such as the antenna direction, downtilt, and transmission power.”

    However, mere tilting of the antenna beam can be a cause, rather than a cure, of co-channel interference. Without rigid control of the RF energy generated by an antenna, the spurious side and rear lobes can be thrust in the direction of neighbouring or nearby cells, creating the potential for interference. In mature markets, where there are many coexisting — and co-located — services and operators, cell interference issues abound, providing many headaches for network optimisers.

    RF where it’s useful

    The need for improved control of RF energy has led to ongoing developments in antenna technology aimed at reducing spurious emissions and providing tighter control of the antenna footprint.
    “Antennas play a critical role in networkoptimisation — they are a major part of RF management,” says Vibhore Bharti, Manager RF Planning with Indian cellular operator, Idea Cellular. “Improving antenna efficiency — the ability to control frequency pollution — is a vital element.”

    “What is needed is a clean propagation of RF energy — putting the energy where it’s useful and not where it’s unwanted,” says Nobileau. “The suppression of side and rear lobes, and footprint tailoring using electrical tilt, are therefore very important. This is particularly so as cells become smaller and smaller — the more you tilt, the greater the potential for interference.”

    The impact of interference in GSM networks is generally measured as the ratio of carrier signal (C) to co-channel interference level (I) — or the C/I ratio — where minimum C/I values for acceptable voice quality are 9 to 10 decibels. It follows that reducing interference will improve C/I, and in turn yield improvements in audio quality and network capacity.

    Nobileau reports that studies show a strong correlation between C/I improvement and the magnitude of suppression of antenna upper side lobes. Maximising side lobe suppression has therefore become a focal point for antenna designers and manufacturers in the quest for interference reduction. Where once side lobe suppression was typically in the range of 12 dB, the target is now 18 to 20 dB — with RFS achieving typically better than 20 dB across the entire tilt range with its Optimizer antenna series.
    “The smaller the side lobe compared with the main lobe, the better the antenna will fight co-channel interference,” says Nobileau. “But if it’s not the first upper side lobe that potentially interferes, it could be the second — so every unwanted signal needs to be as small as possible.”

    Smyth agrees that the first step of RF management should take place at the antenna, citing electrical downtilt capability as an advantage for cell planning and management of modern mature networks. While mechanical tilting of the antenna beam is simple to implement, it has little impact on spurious side radiation, and may even increase interference from the rear lobes. Electrical tilting technology, on the other hand, tilts all lobes — main, rear and side — to the same angle. This means that side lobe radiation can be managed across all tilt angles, providing greater interference control.

    Point of contact

    “The base station antenna is the primary point of contact with the customer,” Smyth says. “It seems strange to me that operators would spend hundreds of thousands of dollars acquiring a site and developing it, only to quibble about the extra hundreds of dollars invested in antenna technology and its maintenance. What you’ve got at the base station counts for nothing until you actually launch it out into the ether and point it in the direction of the customer.”

    This raises an interesting issue to be considered by network operators: the merit of upgrading existing antenna technology to higher performance antennas. According to Nobileau, doing so provides an incremental improvement in capacity that defers the necessity of deploying next generation services for the sole purpose of meeting capacity demands.

    Smyth believes that this path should be attractive for operators seeking to maximise returns from existing assets. “By replacing existing antennas with higher performance antennas, you achieve the flexibility to cope with changes at a much lower cost. The level of general interference will go down and drop-out rate will reduce — holding times and utilisation of the network will go up.”

    Of a Manhattan USA operator that recently replaced all base station equipment for an entire network, he adds: “I would have liked the opportunity to prove that by spending about a tenth of the money in upgrading to more advanced antenna systems, they could also have achieved a significant improvement in service and increase in ultimate capacity.”

    On the other hand, T-Mobile’s Mende is more cautious: “Better antennas always help,” he says. “As new sites are deployed, it’s always good to look for the antenna solution with the best interference suppression. But it’s always a question of whether to replace existing antennas!”

    Evolving challenges

    As networks continue to evolve and new technologies emerge, the role of optimisation will only increase — in terms of both regularity and importance. For instance, it is anticipated that 3G services such as real-time video transmission will lead to dramatic and unpredictable cell traffic-loadings, adding coverage challenges to the optimisation puzzle.

    T-Mobile is soon to launch its German UMTS network, and Mende is keen to take note of its operation. “There are many challenges awaiting us,” he says. “We expect 3G networks to be more sensitive to interference than GSM because the network dynamics are different. We’ve computed it all theoretically, but now the time comes to see how close we are.”

    The anticipated optimisation challenges of the future have fuelled the demand for remote antenna tilting technologies — essentially, the ability to adjust antenna downtilt from locations other than the top of the tower.  According to Smyth, the benefits of remote tilting are many: from eliminating the cost of hiring equipment for tower access, to avoiding impacting other  operators with base stations at the same site, and streamlining regular tilting operations as might be required during network redesign.

    “The futuristic vision is for operators to dynamically adjust the network as traffic patterns change throughout the day,” says Smyth. “I envisage there might emerge a set of ‘presets’ for various traffic situations; where all antennas move in a coordinated fashion to a particular cell plan. This would apply particularly to CDMA-based systems.”

    Mende has a similar dream, where ‘closed loop’ control between network monitoring, planning and operation exists. “It would be advantageous to follow the network dynamics — seasonal and weekly changes in behaviour. We can see where the traffic goes and then optimise those areas. In order to achieve this quickly, we’d need remote control of the antenna systems,” he says.

    Nobileau’s view is that remote tilt is just another essential feature of the multi-functional high performance antenna. “First you need an antenna that can control the side lobes; second, you need to be able to activate it remotely; third, you need to be able to feed the antenna with the network management information needed for the best possible optimisation scenario,” he says.

    For the moment, however, the bottom line is that operators in many countries are having to deal with mature 2G markets where subscriber growth has flattened out, and this month’s balance sheet is the commercial reality. It then comes down to whether or not the network can handle the demands placed upon it — particularly as they are compounded by the additional demands of GPRS and EDGE services.

    The technology is available: not only high performance antennas for controlling interference, but also sophisticated network monitoring tools to complete the optimisation loop. The key is to take a long term view; the implementation of new technologies now will provide immediate results as well as equipping networks for the future.

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