Tech Scan

February 11 , 2003
Road testing remote antenna tilt

Allan Alderson

Field trials of remote tilt antenna technology are under way across the globe. Significant throughput gains and savings in optimization costs are predicted for emerging 3G and established 2G cellular networks.

For well over a decade, antenna tilt, in all its variants, has proved to be a vital network optimization tool in cellular systems. The latest stage in tilt evolution is the development of remote tilt systems, providing the facility to adjust the electrical tilt of the antenna without riggers, either from the tower base or the network management system (NMS).

Remote tilt antenna technologies have developed in response to increasing demands for more dynamic optimisation routines and, in particular, a need to adjust tilt more frequently. To date, the limiting factor has been the labour and cost-intensive nature of the tilt operation, explains Antti Kuurne of Nokia Research and Design. "Traditionally, changes in tilt angle could only be carried out through site visits involving riggers, to physically change the antenna angle," he says. "This process is very cost-intensive, so operators tend to not normally make use of systematic antenna tilting."

This problem will be intensified as many GSM operators move to W-CDMA. The essential challenge is that the optimisation of CDMA-based systems requires a network planner to continuously make a trade-off between capacity and quality. "Cells use the same frequency in a CDMA system. Interference is suppressed using a coding gain -- but it still exists. If you increase the coding gain [to improve bit-error rate (BER)], you decrease the throughput," Kuurne says.

He points out that while GSM operators had the luxury of modifying frequency re-use factors to deal with non-optimum sites, CDMA operators (and emerging W-CDMA operators) rely far more heavily on tilt: Kuurne says: "It is generally agreed that tilting will be very important in W-CDMA networks. Each cell service area is critical as it directly affects the interference. The ways you can effect this are tuning the cell power, steering the antenna azimuth, or tilting the antenna."

Remote tilt trials going on
OEMs and operators around the world are now undertaking trials of remote tilt variant. The focus of these trials is not exclusively to prove its merit in 3G systems, but its potential benefit in mature 2G networks as well. RFS's area product manager Remi Deniel explains: "The challenge in making remote tilt a truly viable solution has been to achieve a genuinely scaleable and modular antenna tilt solution, so that the system can be deployed at a pace that suits the operator -- and his budget."

Remote tilt variant -- the Optimizer RT -- aims to achieve just that with a modular antenna control unit (ACU) that may be retrofitted to existing variable electrical tilt antennas. Coupled with a wireless modem interface (WMI) and control network interface (CNI), the system permits antenna monitoring and control via a local PC, at the network management center or via the CNI's conventional Web-browser interface, via the Internet.

In Singapore, a cellular operator has teamed with a global OEM to test the benefits of remote tilt, as the island-state prepares to move on to W-CDMA. "Remote electrical tilt makes tilting easy, " explains Denis Ng, RFS Singapore's technical sales manager. "Operators here recognise that the essential optimization tool they'll need for interference and traffic management in the new W-CDMA systems is the ability to do electrical down tilting. Remote tilt will clearly reduce costs. There is also clearly a demand for reduced upper sidelobe antennas for interference control."

Trials of a similar nature are currently being undertaken in central London, as one of the UK's five W-CDMA license holders seeks out the premium tilt performance required to support new generation cellular.

The Optimizer RT is being put through its paces in a completely different cellular environment in the US. A major operator in the Chicago area plans to trial Optimizer RT within its 2G cdmaOne network. David Kiesling, RFS's area product manager, Base Station Antenna Systems, explains, "They believe remote tilt can lead to significant efficiencies in their network."

The US CDMA operators, Kiesling explains, have long been aware of the importance of variable tilt as an optimization tool, not only to provide optimal BER performance, but also as a means of addressing pilot pollution and hard handoff problems in multi-frequency CDMA systems. "It will provide them the opportunity--in real-time--to make optimization trade-offs by increasing or decreasing tilt," he says.

Throughput improvements
Further east, in Finland, is the site of arguably one of the most exciting trials. Here, Kuurne's Nokia R&D team plan to work with a major Finnish cellular operator to conduct a live trial in an existing dual-band, co-located GSM 900/1,800MHz network. Planned to be carried out on around 25 to 30 base stations across a mix of urban and suburban sites, the trial's objective is to measure the benefits of remote tilt in a mature 2G environment.

Data, such as RF level statistics, RF quality and carrier to interference (C/I) distributions, will be collected and analysed regularly at the NMS. The antennas will then be remotely tilted from the NMS to the optimal tilt angle.

Kuurne explains that a key objective is to test the potential throughput improvements resulting from the remote tilt/optimization algorithm pair. He says: "Our simulations show that remote tilt could save up to 20 percent on W-CDMA equipment requirements to serve the same amount of users or traffic, if tilt is optimized on a cell-by-cell basis. We wish to test if significant gains can also be obtained in GSM networks."

The Finnish trial provides the opportunity to explore the potential improvements gained in mature 2G networks. Kuurne expects to see a lowering of the BER and a decrease in the number of dropped calls. He adds, "In GSM these are harder to translate into direct capacity gains, but quality of service is one of the important factors in a competitive environment."

Evolving interference
Kuurne points out that in the early days of 2G development, operators opted for very high sites designed to radiate as far as possible to achieve maximum coverage at minimum cost. As the number of cells increased, these tall 'legacy sites' became prime sources of cell-to-cell interference.

This scenario, Kuurne believes, will probably re-occur in W-CDMA. "I don't think operators will immediately deploy [W-CDMA] networks built for huge traffic--I think they will start off cautiously. To provide coverage they will have cells that extend quite far, then in future, as the network capacity and number of cells increase, they will have to do something about these old sites," he says.

This, he says, is the dual benefit that remote tilt offers -- providing a cost-effective means of handling the 'essential tilts' such as those at the tall sites, plus the means to realise network throughput and capacity improvements via the more advanced and emerging optimization routines. From this perspective, remote tilt looks set to become an essential element in the network management tool kit, at least in emerging 3G, and potentially in established 2G networks.

Contact:
Radio Frequency Systems India
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