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    RF: the key to 3G?


    AS UMTS finally takes off across Europe, network planners are exploring how best to meet projected 3G subscriber growth. And the technology at the base station RF interface holds many of the answers, says Jorg Springer, cmo, Radio Frequency Systems (RFS)

    Mid-2004 saw Europe’s third-generation (3G) cellular industry achieve arguably its most significant milestone: after 16 months on air, Hutchison’s 3 Italia network announced its one-millionth subscriber. After more than two years of Euro 3G speculation, the Hutchison milestone allowed the industry to breathe a collective sigh of relief. 3G cellular had indeed taken off in Europe. During the first seven months of 2004 it consolidated further, with 3G/universal mobile telecommunications system (UMTS) coverage in Western Europe growing from just eight networks over six countries, to 29 networks over 14 countries.

    There have been sound reasons for UMTS to find its feet in Europe. This technology should ultimately provide answers to many of the business challenges facing the region’s aging 2G/global system for mobile communications (GSM) networks. For one, while Western Europe’s GSM networks still enjoy steady subscriber growth, many are over a decade old, and face serious capacity limitations.

    Capacity headache
    UMTS is founded on wideband code division multiple access (W-CDMA) technology — a technology that promises relief from Europe’s current capacity headache. W-CDMA offers a capacity-per-MHz of spectrum far greater than that of time division multiple access (TDMA)-based GSM, plus reduced OPEX. It also promises to provide more established and sustained average revenue per unit (ARPU) growth — a powerful driver in an industry that is only now climbing its way out of a three-year ARPU slump. The earliest experiences with 2.5G data services suggest that the more advanced data services of 3G will be an important factor in industry growth.

    Although the European ‘3G beast’ is now flying, it is clear that there are unique challenges ahead from a network expansion and RF perspective. While some would draw parallels between today’s 2G-to-3G transition and the mid-nineties leap from first generation analogue services to 2G/GSM, this is simplistic to say the least. Western Europe in 2004 presents a very different cellular scenario to that of 1992.

    In the space of just over a decade, Europe’s cellular services have matured dramatically, with penetrations at around 85 per cent. The downside of this is that all Europe’s prime base station sites are well and truly occupied. Over the same period, Europe’s environmental requirements regarding site location and visibility have also ‘matured’ to become some of the world’s most demanding.

    The transition from time division multiple access (TDMA)-based GSM to a W-CDMA-based UMTS technology also presents changes to the world of network planning. Where TDMA planning strategies are based on minimizing co-channel interference by reusing a select number of channels over a group of cells, W-CDMA uses the full frequency band in each cell. Moreover, W-CDMA cells are said to ‘breathe’ — the size of the cell varies with the number of callers within the cell, the transferred data rate and so on. The resulting co-channel interference that can occur in the W-CDMA network increases the noise floor, and progressively depletes the capacity of the network. It presents a notoriously tougher network planning challenge when compared with GSM, particularly in addressing the interference resulting from adjacent cells.
    Perhaps most challenging of all is subscriber expectation with regards to quality of service (QoS). In 1992, Europeans were simply excited to make a ‘digital’ call — dropped calls, fades and so on were part of the experience. In 2004, this is no longer the case — Western Europe has arguably the highest cellular QoS in the world. The new UMTS services have much to live up to.

    Rooftop realities
    The majority of Europe’s urban cell sites are rooftop-based. Given the tough site acquisition conditions, the easiest 3G roll-out option (and the one largely chosen to-date by Europe’s UMTS operators) is co-siting.

    The situation on European rooftops today has much in common with a crowded early morning commuter train — no-one enjoys the congestion, there are established long-term disputes and rivalries between some ‘passengers’, but on the whole, the system works. To accommodate UMTS spectrum, new antennas are required, so the ‘train’ needs to be reorganized. The most popular strategy Radio Frequency Systems (RFS) has seen adopted to-date is the multi-band antenna solution. This is manifesting in strong demand across Europe for bi- and tri-band antennas solutions supporting combinations of UMTS 2100 MHz, GSM 900 MHz and GSM 1800 MHz.

    A further challenge is co-siting interference. When antennas operating at different frequencies are located in close proximity, there is potential for RF interferences. These are caused by intermodulation products or spurious emissions, which can in turn lead to BTS or Node B blocking. The most extreme cases of these occur when the core band spectral separation is narrow (a pair of UMTS 2100 MHz and GSM 1800 MHz services is a most obvious case), and the antennas are physically close.

    As a result, UMTS/GSM co-location isn’t always straightforward. In some cases, it simply isn’t practical, and the new UMTS operator is forced to opt for a site that is nearby, but ‘sub-optimal’. The RF challenge is to make the best of such a bad situation, and to optimize the RF footprint to suit the alternative location.

    RF flexibility
    The upshot of this highly constrained site location scenario — coupled with the exacting requirements of W-CDMA network planning — is that Europe’s 3G operators are demanding higher levels of base station RF precision and flexibility. First and foremost is the issue of antenna performance. To ensure minimal cell-to-cell interference, the 3G antennas most in demand are the new-generation ‘precision footprint’ breed, with diminished side and rear lobe radiation levels, improved null fill, and increased front-to-back ratios.

    In the face of arguably the greatest roll-out challenge in the region’s cellular history, Europe’s 3G network planners are arming themselves for the challenge. ‘Flexibility’ seems to be the primary goal, and this is being sought on a number of RF technology fronts — specifically in the control of cell footprint size, shape, direction and power. To compensate for CDMA-style cell breathing and often less-than-optimal site locations, variable electrical tilt (which permits continuous adjustment of cell footprint size) is de rigueur. Increasingly, this is accompanied by remote tilt control functionality, linked back to the network management centre (NMC) via industry standard communication protocols. Coincidentally, the earliest antenna tilt control — adjustable mechanical tilt — was unveiled in the early nineties to address the growing pains of Europe’s fledgling GSM network.  There is also an immediate demand for tower mounted amplifiers (TMAs) across the majority of Europe’s 3G sites. These provide amplification in the uplink signal from the terminal, which overcomes losses in feeder and co-siting components, decreases the system noise, and provides a measure of flexibility by increasing the cell size. The greatest demand is for the new-generation compact and lightweight units that are quickest and easiest to deploy. The need here again is for flexibility — a broad choice of amplification levels, plus dual and multiband configurations. For the same reason, there is also demand for a wide selection of antenna gains.

    In the most congested of sites, there is some demand for diplexers and triplexers. These permit a single feeder to support a mix of 2G and 3G frequency bands. Most European operators view this as the ‘last option’, as it presents numerous difficulties, such as increased attenuation and the risk of intermodulation products, reduced system redundancy, and a reduction in critical isolation between services.

    Lastly, there is increased demand for a broader selection of antenna apertures. Where the 65-degree tri-sector is the norm in 2G/GSM networks, the challenge of W-CDMA adjacent cell interference has created a demand for alternative apertures, such as 90-, 45- and even 33-degree. These permit the W-CDMA network planner to ‘break the symmetry’ of the final cell pattern, and thus minimize cell-to-cell interference.
    From coverage to capacity
    The near and not-too-distant future holds even more RF challenges, as the European 3G roll-out progresses, and evolves from a coverage-driven to a capacity-driven strategy. A recent industry forecast predicts an almost ten-fold increase in Europe’s 3G subscriber count between end-2003 and end-2004.
    This means that in the very short term, we’ll see UMTS operators continue to expand and enhance their 3G coverage across the major city and urban centres. This makes the greatest sense — these are the areas that present the greatest revenue earning potential, and are also those that are currently most voice capacity-challenged.

    While today’s 3G operators often rely on 2.5G technologies (such as enhanced data rate for GSM evolution (EDGE)) to bridge the gaps between the major city centres, this will be less acceptable as more GSM subscribers move across to UMTS. As the coverage strategy extends beyond the major centres, we’ll see greater use of high-gain antenna systems to help extend coverage across larger areas.

    More challenging, though, is the longer term. Analysts predict a 50-fold increase in Europe’s UMTS subscriber levels between end-2004 and end-2009. In essence, this suggests Western Europe will see 3G subscriber levels rise to almost equal those of the region’s current GSM count, in a time frame just over half that afforded to the evolution of GSM. This represents extraordinary subscriber growth, and presents unique challenges to Europe’s 3G network planners and its RF technology providers.

    Such growth will clearly be driven by strong migration of the existing GSM subscriber base to the UMTS networks. Meeting the fast-growing capacity requirements — specifically in the site-starved major cites — will be the long-term challenge to Europe’s 3G network planners. Clearly, the solution will be even more advances in base station RF technology.

    Need for active antennas
    These advanced RF solutions will almost certainly be founded on two key elements: advanced ‘hybrid’ (a mix of active and passive components) antenna solutions, and greater intelligence and control functionality built into the antennas. While today’s antennas are purely passive, it’s clear that tomorrow’s antenna will need to integrate active RF conditioning components, such as low-noise amplifiers, multiplexers and filters, with even greater levels of control. Similarly, by providing more onboard intelligence within the antenna, an increasingly broad range of antenna pattern parameters might be adjusted and controlled. The bottom line is that this will provide superior levels of flexibility to the network planner.

    Over time, we’ll also see a vast improvement in W-CDMA network simulation tools. This should ultimately result in more dynamic and intelligent network management strategies, and possibly lead to the realization of the so-called ‘dynamic antenna performance control’. Here, the adjustable parameters of the antenna components — both active and passive — could be corrected in response to the simulation tools, perhaps even in a closed-loop real-time configuration. It is these types of super-flexible base station RF solutions that will play a significant role in the establishment of UMTS across Europe in the longer term.