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Currently used by 1.5 billion mobile subscribers in 200 countries, GSM is by far the world's most widespread mobile system. Every month, 35 million new GSM users sign up, while existing subscribers are increasing usage in the wake of lower tariffs and attractive new data services.

Niklas Hallén, Product Development Unit GSM BTS at Ericsson, says: "Many operators are approaching the limit for how much traffic their networks are capable of handling, given existing configurations."

Changing network configurations and boosting network capacity are the obvious solutions – but there are many restrictions. Network bottlenecks are often found in urban areas, where the addition of sites and antennas may prove costly as a result of high property prices, or where installation is simply impractical or is unwanted for aesthetic reasons.

Mexican operator Telcel has solved the problem with Ericsson's Combining and Distribution Unit (CDU-F), which is being implemented in some 100 sites in Mexico's largest cities. This is the first use of CDU-F in a 1900MHz network (the frequency band used in the Americas), but CDU-F solutions are used in 900MHz and 1800MHz networks elsewhere in the world.

Hallén explains why a filter-combiner solution was the answer to Telcel's need to install new capacity at peak sites without adding hardware to the already cluttered Mexico City skyline. "With CDU-F, you can expand a site from four to 12 transceivers per cell sector by simply going from one to two or even three cabinets without installing any new feeders and antennas,” he says. “Capacity is increased by about 300 percent following the Erlang B dimensioning rules, which brings considerable maintenance savings and keeps OPEX to a minimum.”

Large cells and few antennas give the lowest cost per user. For large sites, it is important to have a Base Station Controller (BSC) that can handle many transceivers. "Ericsson offers the largest BSC on the market – handling up to 2048 transceivers – and thus serving more than twice as many sites as the competition," Hallén says.

Many operators seeking capacity expansion do not just have physical space restrictions to deal with – spectrum allocation may have been tight to begin with, so efficient spectrum utilization is also paramount.  

Ericsson CDU-F solution is able to achieve the same channel spacing as a normal combiner (minimum 400kHz channel separation), whereas competing solutions are only able to reach minimum 600kHz. This provides a 33 percent increase in effectiveness, thus increasing the capacity of the network.

Another important benefit of narrow channel separation is that it follows the same channel restrictions as other combiner types with regards to frequency allocation and planning. This reduces cost both for network planning and network tuning. A base station fitted with Ericsson's CDU-F filter-combiner solution therefore squeezes more traffic into a given antenna configuration without adding extra cost or network planning. “The secret lies in the integration of microwave electronics and patented engineering solutions," Hallén says.

On a radio level, CDU-F uses a technique known as “baseband hopping”, which means each transceiver unit sends on a fixed frequency, but the frequency hopping is provided by the traffic multiplexed out on different transceivers instead. Baseband hopping allows you to jump across as many frequencies as there are transceiver units in the base station.

"By achieving tighter channel spacing within each CDU-F unit, our solution is able to pack the frequencies tight enough to be applicable for networks with FLP frequency planning,” Hallén says. “We are probably the only vendor offering a filter combiner that can be implemented in an already synthesizer-hopping FLP-tuned network.”

CDU-F is part of Ericsson's Capacity Growth offerings, where hardware – as well as the use of advanced software features – caters for operators working under different spectrum conditions but faced with the same need: to release new network capacity. Even the most urbanized nations still have sparsely-populated regions that can harbor a relatively high proportion of new users.

“Some operators choose to combine the various capacity-boosting techniques of Ericsson's Capacity Growth offering in order to obtain a solution that optimally addresses every corner of the network,” Hallén says. “An efficient part of these offerings are Ericsson's filter combiners, with which we build high-capacity sites without adding feeders and antennas."