Cellular base stations have come a long way since the first analogue (TACS) networks were launched two decades ago. GSM changed the ballgame considerably in the early 1990s, but now UMTS (3G) is also reshaping the industry.
Against this backdrop, Peter Jackson, product marketing manager for cellular antennas with AlanDick, describes the fast-moving world of cellular base station antenna design
GSM may have been around for the last 15 years, but the arrival of 3G on the cellular scene on March 3rd, 2003 — when the first 3G networks were launched in Europe — has changed the face of base station antenna design as we know it. This is because, although the 2.2 Gigahertz frequencies involved with 3G are not that much higher than the GSM 1800 waveband increasingly being seen across Europe, the industry is moving steadily towards multi-standard — as well as multi-frequency — base stations.
What is happening is little short of a revolution in antenna and associated base station electronics terms. The revolution is being driven by demand from the operators to get ever more from their base station resources, as well as seeking to cut costs on the installation and ongoing maintenance fronts.
Installing multi-band and multi-frequency antennas is only part of the equation when it comes to satisfying the needs of the operators. The industry now also has remote electrical tilt technology at its disposal. Remote electrical tilt antennas build on the principal of electrical tilt systems — introduced a decade ago to allow GSM operators to refocus their signals to meet changing traffic needs — and allow engineering staff to change the tilt of the antenna from the tower base or the network management centre.
In the early days of GSM, changes in the tilt angle of an antenna could only be carried out when riggers visited the site and physically changed the tilt of the antenna. Electrical tilt technology allowed for a more dynamic approach to the focus of an antenna, but remote electrical tilt means that the capacity of a base station can be changed dynamically and directly in response to the changing call traffic levels in the cell site’s vicinity. Remote electrical tilt technology is especially important with CDMA (3G) base stations, as the optimisation of CDMA-based systems needs to be continuous to cope with the trade-off between capacities and call quality.
CDMA cells differ markedly from GSM in that all the cells use the same frequency, suppressing interference through the use of coding gain technology. With GSM, installers have the luxury of being able to re-use the available frequencies over large areas. With CDMA, the approach to minimising interference between adjacent cell sites has to be different. This doesn’t mean that interference between CDMA cell sites is non-existent, just suppressed. And if you increase the coding gain — to improve the bit-error rate — you decrease the throughput.
The importance of
Remote electrical tilting of antennas is very important to CDMA operators, especially now that many 3G sites are co-located with GSM base stations. From an engineering perspective, it’s important to realise that the site hand-off regions are quite different between GSM and CDMA networks, despite the fact that some of the frequencies involved with GSM are quite close to those of CDMA.
Another issue of significant interest to designers at the moment is base station hoteling, the term that describes the co-location of base stations and the transmission of the RF cellular signals to convenient antenna sites that can be some distance away. To support base station hoteling, the industry has developed fibre optic delivery systems to allow a single set of base station electronics to feed multiple antennas, which can be located several kilometres apart.
As CDMA networks increase in importance, we expect base station hoteling of both GSM and CDMA cell sites to increase, with the electronics of a multi-standard base station housed at one point, and the GSM and CDMA antennas sited at multiple points. This approach allows a given area to have a cluster of GSM 900/1800 antennas, and a more dense set of CDMA antennas operating at the higher 2.2 GHz frequencies.
This separation of the base station electronics from the actual antennas will help engineers move steadily towards their ultimate target — in cellular terms — of a single box approach to base stations. If a multi-standard base station is based around a smaller number of boxes, then this helps to reduce operating expenses, especially, as is often the case, where a network operator is paying rental to a third party organisation for the use of the site in question. This is because most site rental agreements are based on the cellular facilities and/or boxes installed at the site.
Cost not the only factor
Cost issues are not the only factor driving the industry towards base station hoteling. There is also the fact that a single box approach means fewer jumper cables to install and maintain, which in turn makes life easier for the hard-pressed site engineer. Another feature that the cellular antenna industry is starting to
see being tested is convection-cooled RF head-ends, thanks to improvements
that have been made in amplification over the last few years.
Most of the cellular amplifiers seen in base stations to date have been around 15 per cent efficient in power/output terms. Now the industry is starting to double these efficiencies, a move that helps to keep temperatures down. Thanks to these enhancements, convection cooling of base station amplifiers has at last become possible. We’re starting to halve the power being used on base stations, which helps to improve reliability and operational efficiencies.
Inextricably linked
By now, readers will have realised that the process of cellular antenna advancement is as evolutionary as it is inextricably linked with the development of base station electronic design. Our observations suggest that we will soon start to see amplifiers on the head-end, rather than having lengthy — and power-sapping — cable runs between the amplifier and the antenna. We also expect to see the development of innovative fill-in GSM and CDMA sites, which are aesthetically pleasing, as well as being multi-standard and capable of intensive sectorisation. Sectorisation is the process of splitting a cell into two or more sectors, with the signal to each sector being radiated by multiple antennas. Up to six sectors are normally seen in a multi-sector cell, with three sectors being the norm in most city and motorway high call traffic areas.
Multi-sector base stations are traditionally associated with main cell sites, but we have come up with our own multi-sector (and multi-standard) solution for urban and country areas — the Telecoms Timepiece.
The Telecoms Timepiece, which has been on extended trials in the grounds of the company’s Cheltenham headquarters over this summer, is a mini base station designed to be aesthetically pleasing and look like a village or town centre clock — and with a working mechanism. The antennas of the base station are located inside the triangular headpiece of the base station, which can also house some of the amplifier and associated electronics. The rest of the base station electronics are housed in the hexagonal seating area unit, arrayed around the bottom of the steel monopole holding the elevated clock head aloft.
The Telecoms Timepiece is one of many aesthetic concepts and can also be adapted for use in many different styles or locations — for example, in a country location or narrow street by the removal of the hexagonal seating unit and its replacement with a series of hanging baskets arrayed around the monopole in an aesthetically pleasing way. Conventional cellular antenna designers might baulk at the prospect of hanging baskets interfering with the propagation of signals from the main signal radiators, but by skilfully arranging the baskets around the lower half of the base station, any signal attenuation is minimised.
Miniaturised electronics
This interesting rural cell site solution has been made possible through the use of miniaturised base station electronics and leading edge antenna design. The antenna design is such that up to six sectors can be accommodated by the mini base station. It’s even possible to install multi-standard antennas inside the triangular headpiece of the station, allowing both GSM and 3G signals to radiate from a single structure.
Further up the base station scale, we are also starting to see standardisation of the RF interface on cell sites, even where multiple standards and frequencies are involved. Thanks to industry moves like this, we expect to see the arrival of intelligent antenna systems, which are capable of responding dynamically to the changing needs of the networks.
Intelligent antenna systems will go a long way to meeting the needs of European GSM and 3G networks to dynamically change their network coverage to cope with major incidents and/or base station outages.
Currently, if a major incident occurs on a European motorway, autobahn or autostrada, or a cluster of cell sites go down for any reason, most networks struggle to continue to fully meet the call requirements of users. This is because most modern major cell sites are operating at close to their operational capacity. Gone are the days when GSM operators could change the dynamics of their coverage to meet such situations.
Intelligent design
An intelligent antenna design changes things considerably, with technologies such as remote electrical tilt and the ability to switch in (or out) various radiator antennas at a given cell site allowing the capacity and coverage of a GSM and 3G base station to be altered in real time to meet the changing needs of customers and their calls. This aspect of cell site design is something you can expect to hear more about as cellular handsets increasingly support mobile data applications. This is particularly topical, now that 3’s UK network has started to move away from its walled garden approach to the mobile Internet and is following the lead of its European sister networks in offering open access to the Internet. 3’s new tariffs — which were rolled out to the network’s UK customers in mid-September — allow most handset users access to the mobile Internet for £2.50 a month, as well as up to 512 megabytes of mobile data in return for £45 a month.
The fact that 3’s CDMA network now spans 85 per cent of the UK — and its sister European networks in Austria, Denmark, Ireland, Italy and Sweden are moving swiftly to achieve similar coverage levels — means that the ratio of voice to non-voice calls on its various networks looks set to change markedly in the near future.
This trend looks set to be mirrored on other 3G networks across Europe and beyond, meaning that cellular antenna designers will have to continue to rapidly evolve their technology to meet the changing needs of the operators. And that’s before we even begin to start thinking about 4G base station and antenna design…