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Open base station architecture
So, what’s the correlation between Charles Darwin’s theory of evolution and base station antennas, you may ask. Peter Kenington makes the connection...
In 1859, when Charles Darwin first published “On the Origin of Species...”, there is little doubt that he did not have wireless base-stations in mind. In reality, however, many of his ideas are equally applicable to this area of evolution, as they are to evolution in the natural world. One of the key elements that have enabled this to happen is the now widely accepted set of open specifications, set out by the Open Base Station Architecture Initiative (OBSAI), covering the interfaces between the main modules within a base station.
Darwin's key insight was in noticing that, in the natural world, the survival of a species is based upon its ability to adapt to environmental change and to competition from rival species that are also evolving. The key to survival is in finding a niche, be it large or small, within the ecosystem of our planet. Niches exist at all levels within the food chain, from that of a simple, low functionality existence (e.g. single-celled creatures, bacteria etc.) through to something of much higher 'performance' (e.g. tigers, dolphins and man). The same situation exists in the base-station arena, where simple, low cost pico and femto BTSs are beginning to emerge to fulfil low cost, short-range coverage requirements. These complement larger, more sophisticated designs for high-capacity, multi-carrier macro cells. In both cases, the key to survival rests in achieving acceptable or superior levels of performance, for the lowest possible ownership cost.
The natural world and the world of base-stations have one major difference from an evolutionary perspective, and that is that base-stations can now immediately take advantage of the latest innovation (in RF or baseband) through the designing in or substitution of a new RF, baseband, transport, clock/control or power supply module. The internal interface specifications, pioneered by OBSAI, enable this process for a BTS (Base-station Transceiver System). In the case of the natural world, however, the adoption of a new form of jaw, leg muscle etc. from another species would either take very many generations or prove to be impossible. This is, in many respects, how things used to be in the base-station world: if a particular OEM included a useful innovation in its new generation BTS product, at best, this would be copied by its rivals in a subsequent generation of BTS.
The adoption of the OBSAI standards represents a significant short-cut in this process, since innovations which are included in module products placed on the open market, can become a part of many manufacturers' products quite quickly. This potentially reduces the hardware development burden within the OEMs and allows them greater freedom to concentrate on developing aspects of their products, which will provide true differentiation from their competitors. Many of these areas fall within the domain of software and this will increasingly dominate in future BTS generations. It is this area more than most that will lead to the survival of the 'fittest' – which in a BTS context translates to the most innovative.
The OBSAI organisation consists of more than 130 component, module and base station vendors. Its aim is to create an open set of specifications for the internal modules required in a base-station, encompassing both interface and mechanical aspects. A good analogy in the PC industry is that of the PCI bus/card specifications. Many more unit-level options will become available in the future, as the technology develops to integrate modules into combined units. This will open up new location possibilities for base-stations due to the availability of smaller and more versatile architectures that are easier to site.
Revolution in evolution
The last 20 years or so has seen a revolution in the PC industry, in cost, yes, but also in capability and this has been brought about, in a large part, by the PCI bus. Most PC manufacturers offer a huge range of models, but these generally fall into a very small number of 'families' – often just one. PC manufacturers have achieved the seemingly impossible, in being able to offer a huge range of choice to their customers whilst employing a modest cost base for their organization. This has been achieved through minimizing inventory and design engineering services, through the use of standard packaging and 'modules' (e.g. graphics cards, DVD-drives, memory cards, motherboards etc.). These modules can then be selected appropriately to generate a large product offering appropriate for all tastes and budgets. Their success rests, in a large part, on them being able to provide the customer with exactly what he or she wants. This is a level of service that it has not, in the past, been economic to provide in the base-station area.
Changed landscape
OBSAI's announcement, in June 2006, that it had released a full set of interface, hardware and test specifications for the internal interfaces within a base-station has changed the landscape for the mobile radio base-station. OBSAI's specifications are compatible with all of the major current and emerging air interface standards, including GSM, GSM/EDGE, WCDMA, CDMA2000 and WiMAX and are available for public download free of charge (from www.obsai.com). These specifications allow module vendors to manufacture modules that are capable of operating in any OBSAI-compatible BTS, thereby reducing substantially the development effort and costs involved in the introduction of a new range of BTS products. They also enable a more PC-like model to be adopted in the design and construction of a BTS product – i.e. the selection of modules from a range of vendors at a range of capability levels and costs, such that the overall BTS closely matches the operator's requirements. Embracing this model will be a route, and a key, to survival in the emerging BTS marketplace.
A head in the clouds
The changes in the market landscape, in terms of planning restrictions, health concerns, acoustic noise objections (from cooling fan noise) and many other issues are making it increasingly difficult for operators to erect new cell sites. New BTS architectures are therefore emerging to try and address these problems; here again, the survivors will be those that adopt these new architectures and can make them work for their customers.
These issues have given birth to the remote RF head – a new form of BTS deployment that is fully supported by the OBSAI specifications. This architecture places the active RF electronics remotely from the rest of the BTS and its associated backhaul. The remote RF head itself houses all of the radio-related functions (transmitter RF, receiver RF, filtering etc.). This is then connected to the remainder of the BTS via fibre-optic cable.
The above arrangement allows main elements of the BTS (the digital and network interface modules) to be housed in a low-cost internal location, such as a basement. The RF head can then be situated on the roof of the building or on an outside wall. Another option is to site the remote RF head at the top of an antenna mast, with the remainder of the BTS being located at the base of the mast in a suitable hut or other enclosure.
Cheap hotels bring comfort
This principle can be extended to multiple remote RF heads, whilst still maintaining a single, central, location for the other aspects of their associated base-stations. This concept is usually referred to as a BTS hotel. The remote RF heads themselves can be located a substantial distance from the main BTS hotel site, due to the very low losses associated with the fibre optic cables used to connect them to the remainder of the BTS.
One of the main advantages of the BTS hotel architecture lies in its ability to provide cost-effective BTS redundancy. It is typically not economically viable to provide 100 per cent redundancy within a traditional BTS. However, in the case of a BTS hotel, N+1 redundancy can be used (i.e. the provision of one redundant BTS covering a number of active BTS systems within the BTS hotel location).
Significant disadvantage
The significant disadvantage with the BTS hotel architecture is, however, in the cost of the fibre optic links that run between the BTS hotel and its remote units. Installing new fibre – if there is none in existence already – involves significant civil works and is therefore extremely expensive. There are, however a number of examples of various types of BTS hotel in operation today, covering applications in city centers, at airports and for major sporting events.
So, natural selection has been a part of the evolution of the earth's species for billions of years and has proved to be a successful method of ensuring that the best-adapted species are available to maintain our ecosystem. In the world of base-station engineering, the same principles apply – however the timescales are dramatically shorter. The open specifications provided by OBSAI shine a powerful light on the future evolutionary path for the BTS – it will be interesting to see who is the fittest and, hence, survives this new dawn.
Peter Kenington is the Managing Director of Linear Communications Consultants and the Technical Chair of OBSAI. Email: pbk@linearcomms.com
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