Base station antennas
As the cellular base station antenna takes the next step in its evolution — one that sees the ‘antenna’ become an ‘antenna system’, with integrated functionality that transforms it far beyond its passive roots — we trace the techical developments occurring at the base station.
Next to the mobile handset, the cellular base station antenna is probably the most visible evidence of the relentless march of mobile communications. The sheer number of base station sites found in cities, towns and rural centres around the globe bear testament to the increasingly RF-nature of modern communications. Yet in many respects the base station antenna — essentially a passive element for transmitting and receiving RF — has changed little over the past few years.
“In effect, we have reached the limits of the laws of physics for the passive element itself,” explains Patrick Nobileau, Vice President Base Station Antenna Systems with wireless technology group, Radio Frequency Systems (RFS). “But this doesn’t mean development has stalled — quite the contrary. What we are seeing now is entirely new functionality being integrated into the base station antenna, so that the antenna — the core passive element — is evolving into a very powerful ‘antenna system’.”
This integration of active and intelligent elements with the passive has spawned the development of a range of important next-generation antenna systems. These include compact cluster assemblies combining multi-band passive and active RF elements, a wide range of so-called ‘smart’ antennas, the much-publicised multiple-input/multiple-output (MIMO) solutions, and even fibre-to-the-tower-top antenna systems (remote radio head (RRH)).
Integration drivers
The drivers behind the push to the next stage in base station antenna evolution are entirely market-related and are primarily aimed at wringing more capacity and coverage from existing base station infrastructure. Commercial pressures — particularly in the more mature wireless markets — are top of the list, according to Tero Mustala, who is Director of Industry Cooperation in the Strategy & Technology Division, Nokia, and Chairperson of the industry group the Open Base Station Architecture Iinitiative (OBSAI). “There is great cost pressure on everybody. The whole industry is seeking ways to be more cost competitive. Carriers are heavily looking at operational costs, but also demanding increasing efficiency in investments,” Mustala says. “From an OEM point of view, standardisation of the basic building blocks of base station architecture is a clear and important way of addressing this situation, as it helps reduce the R&D costs.” This, he points out, is a core objective of the OBSAI group, with the base station antenna being incorporated within this standardisation.
From this cost-reduction perspective, there is a general push from the carriers to “achieve more from what they have”, particularly with respect to base station sites and spectrum. This optimisation push is made all the more challenging by the ever-increasing need for more capacity from existing network infrastructure. “Firstly, the total number of subscribers in almost all parts of the world is still growing,” Nobileau says. “But there is also a shift from voice traffic to data services. Wireless data, by its very nature, is dramatically increasing the bits per second throughput demand on wireless networks.”
In addition to this increase in capacity demand, the move from voice to data has created a noticeable change in the specific nature of the mobile traffic being supported. “Voice is essentially statistically predictable in terms of throughput demand,” explains André Doll, RFS Vice President Product Management RF Conditioning. “High-speed wireless data, by contrast, creates unpredictable peak demands.” These peaks can lead to base transmitter station (BTS) saturation, if not adequately accommodated.
“It is a double-edged sword for today’s carrier,” Doll says. “This data-driven peak traffic represents revenue, but is also the source of an unpredictable capacity demand and network pressure. They [the carriers] need to find ‘tricks’ to optimise the use of their existing sites and spectrum.”
The new generation of base station antenna systems with integrated functionality is a vital part of such optimisation solutions.
Three distinct developments — two technical, one attitudinal — have paved the way for new antenna system functionality. The first is the long-awaited development of reliable, high-performance and compact outdoor electronics; the second is the dramatic fall in the cost of data processing power, underpinning increasingly elaborate signal processing algorithms. Last, is a wave of cooperation across the industry that is permitting BTS interface standardisation, both at the macro and elemental level.
The improvement in the performance of outdoor electronics over the past few years — most particularly component reliability — has caused a noticeable shift in the attitude of carriers to ‘active tower tops’.
“Carriers plan very carefully what they build in the tower structure. They look very closely at the reliability of products when they are mounted on the tower,” Mustala points out. “Historically, they have been very apprehensive about building active electronics at the tower top, because they know that if there is
maintenance needed, it will be costly.”
Key in this area has been the improvement achieved in power amplifier (PA) efficiencies, which have permitted new levels of miniaturisation and reliability. The power amplifier is central to tower-top active technologies, including tower-mount boosters (TMBs), and the remote radio head (RRH).
“Five years ago, a Multi-Carrier Power Amplifier (MCPA) had an efficiency of between eight and 10%,” says Doll. “As a result, a booster offering 20-watt RF output power would have required around 400 watts of power dissipation, considering the additional loss of the passive elements of the booster.” The inherent heat generated within such devices would have demanded over-sized casings, external cooling and made the mean time between failures (MTBF) unacceptably low.
Improvements in design efficiencies and electronics has resulted in PAs with efficiencies between 15 and 25%. “This means you can divide the power dissipation by three,” says Doll, “which means one-third the quantity of silicon, roughly one-third the unit size, and more than three times the reliability.”
This has permitted the development of compact tower-top active equipment, offering extremely high levels of reliability. It has also permitted the development of all-in-one antenna assemblies — antennas with built-in RF amplification and filtering, plus electrical tilt and azimuth beam control systems, all within a single radome. This, in turn, has inspired the development of advanced ‘cluster’ antenna assemblies, accommodating the active and passive RF elements required for a complete tri-sector tower top, in a low-profile and visually low-impact package.
The general improvement in the performance and packaging size of active tower-top components, coupled with compact multi- and broadband antennas, has facilitated the support of multiple bands at the tower top. “Quality base station sites the world over are increasingly difficult to secure, so the need to concurrently support multiple bands and multiple services from a single site is great. This is particularly so with the advent of 3.5-generation (3.5G) technologies such as WiMAX,” says Nobileau. “Multi-band active and passive RF antenna systems have truly relieved this situation.”
Industry cooperation
Streamlining and simplifying such site overlays is one aspect that drives the OBSAI industry group. The development of pan-industry groups such as OBSAI — which are founded on technical cooperation between traditional industry competitors — represents a powerful shift in attitude across the wireless industry. Inaugurated in late-2002, OBSAI set out to establish a more open base station market based on pre-determined standard modules and interfaces.
Today, around 130 leading OEMs, carriers, and technology suppliers to the wireless industry form the membership of this industry group. Its company members have produced and made available a set of open interface and module hardware connectivity specifications addressing the four key base station subsystems: transport, control and clock, baseband and RF/radio (see above). Importantly, the model defines not only the modules, but also the interfaces between these modules. The model is also ‘technology-neutral’, so aims to be equally applicable to the entire spectrum of wireless platforms,
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including CDMA 2000, GSM, W-CDMA and WiMAX.
In the simplest of terms, such a cooperative venture base station model helps cut the R&D development time and effort for all players involved in base station deployment and upgrade. This, according to Tero Mustala, is particularly important for the active base station component chip set developers.
“R&D investment at the silicon interface can be sizeable. If a vendor doesn’t see enough prospective sales volume for a chip set, it is hard to justify the R&D. As a result, one of the great successes of the OBSAI has been to bring in and assist the silicon companies,” Mustala says.
Remote radio head
Significantly, the OBSAI-inspired spirit of cooperation is behind one of the recent successes in the realm of antenna functionality integration—a remote radio head (RRH). This RRH is the result of a joint development initiative between four OBSAI members: RFS, programmable silicon solutions provider, Altera Corporation, broadband communications and storage semiconductor provider, PMC-Sierra, and measurement company, Agilent Technologies.
Working to the defined OBSAI model, the RRH provides ‘3-Gbps plus’ fibre optic cable connectivity between the base station and the tower top, by relocating a portion of the ‘RF Block’ module from the base station to the tower top. This permits BTS-to-antenna separation distances of up to 15 kilometres, and allows the BTS to be located in more easily acquired sites, remote from the mast and radio head/antenna assembly. In addition, the RRH fibre-optic link offers ongoing opex savings, courtesy of the almost loss-free ‘BTS-to-tower-top’ fibre link.
The new unit’s ‘3-Gbps plus’ total throughput capacity ensures that the RRH is well-placed to support powerful RRH networking topologies, such as ‘daisy chain’, ‘ring’ and ‘tree-and-branch’. According to Mustala, the OBSAI model was a vital element in allowing this particular development — and others like it — to proceed to completion.
“The OBSAI specifically devised the interface standard needed to support the RRH,” he says. “We initially had one version of the baseband-to-radio module interface for the classical construction—our RP3 interface. Then, two years ago, we devised what we called the RP3-01 interface, which made allowance for a remote tower-top mounted RF block, with all the data and controls identified through an optical interface. This was specifically with the RRH solution in mind.”
Getting smarter
The third, and potentially most important, base station antenna technical advancement is one that is actually far removed from the antenna itself — the improvement in data processing power and costs. “We are seeing data processing at the ‘back end’ of the base station being used to provide extremely powerful solutions — simply with the existing antennas that the carrier has in place — all courtesy of low-cost processing power,” Nobileau explains. This, he points out, is what is behind the new wave of MIMO antennas, smart antennas and so on.
“Today, every carrier has his own site for his own system. In the future, we can foresee a situation where these antennas are ‘mutualised’ between the carriers. By using data processing power, multiple carriers could obtain optimal performance from a select shared antenna group,” he says. MIMO, he points out, is an example of such reuse of multiple signals received by multiple antennas on a site. Multiple inputs and outputs are ‘regrouped’ — via software — to contribute to one optimal signal. The essential element of such systems will always be the complex signal processing algorithms at the antenna system ‘back end’.
The so-called ‘smart antenna’ is similar. Founded on equally elaborate back-end signal processing algorithms to that of MIMO, these emerging solutions optimise beam patterns from one, or a group of antennas, on a real-time basis to maximise subscriber throughput and minimise interference.
WiMAX the driver
A core driver of these more advanced, software-powered antenna solutions is WiMAX — a platform that, according to Andre Doll, will exhibit entirely unique coverage requirements and equally unique carrier profiles. “With WiMAX, carriers will seek ‘hot-spot’ rather than national coverage,” he says. “You are really trying to optimise where the people are going to use the service. Similarly, WiMAX carriers may not be traditional mobile phone carriers. From this perspective, they will not have the luxury of leveraging an existing suite of sites.”
These unique differentiators make WiMAX deployment an entirely different challenge in the wireless world, and set new demands in RF
optimisation. Almost certainly, there will be specific needs for RF boosters, RRHs and smart or MIMO solutions at
many sites. Further cooperation and joint venturing will also be a must —and is one that the OBSAI group is ready for. The most recent addition to its model is the WiMAX specific detail. The group is now looking to the future, where the next logical amendment to the model would be one supporting the 3GPP’s long-term evolution (LTE) platform. Emerging broadband wireless platforms, such as HSPA, WiMAX, and LTE, will almost certainly spawn currently unthought of levels of antenna system functionality and integration.
“My belief is that MIMO may actually compete with entirely new antenna standardisations, due to the emerging needs of such 3.5G technologies,” Nobileau says. The antenna ‘system’ is now clearly a reality, and in some respects, the stand-alone ‘passive antenna’ fast becoming a thing of the past, spurred on by the relentless demands of the wireless broadband world.