Once hidden in the shadowy recesses of base station innards, antennas are increasingly changing their role from unsung workhorse to star of the show. As ever-larger MIMO arrays, and the advent of truly smart technologies like AAS (Active Antenna System), drive advances in cellular network performance, the technology is becoming a source of competitive edge for carriers.
While differentiation in recent years has focused mainly on applications and device exclusives, now it is shifting back to the RAN again, and many cellcos are consciously marketing themselves on the basis of having a faster, cleverer network. That started, of course, with LTE, and some cellcos still have the luxury of being the only provider in their country to offer ‘4G’. However, as multiple launches occur in many markets, operators are luring their customers with pumped-up speeds, better cell edge performance and larger data allowances. That, in turn, makes the move to deploy some elements of LTE-Advanced, such as carrier aggregation, more urgent (see separate item, but also places new strategic focus on antennas. And as carriers rely more on their RAN to set them apart from rivals, so they will seek greater control over antenna evolution.
This is seen clearly among the Japanese and Korean majors, always particularly keen to invest in their own cutting edge network technologies, and among the first to deploy double-speed 150Mbps connections along with elements of LTE-A. NTT Docomo brought reality to the fledgling AAS space last year with some important trials, while SK Telecom of Korea has announced a new alliance with test and measurement provider Rohde & Schwarz, for a joint R&D program centered on next generation antennas.
Under the new memorandum of understanding, the two firms will collaborate to create a testbed for AAS, conducting performance verification and other developments, the aim being to address real world deployment of this technology, especially in the context of SK’s particular approach to deploying LTE-Advanced. This is centered heavily on small cells and the operator’s ‘Super Cell’ idea, which aims to improve capacity and data speeds beyond the baseline enabled in vanilla standards, with active antennas being an important enabler.
AAS has mainly been seen as a way to squeeze more performance out of a macro layer before needing to supplement this with small cells, but SK wants to apply it also to the metrocells themselves as part of its Super Cell program. Among the functions of AAS is the ability to increase the number of radio channels between base station and handset by ‘several tens of time’, and to distribute radio functionality along the antenna elements, reducing footprint and power and increasing flexibility in network design.
The main benefits of AAS for the operator are:
‘ Significant reduction of site footprint and the cost (eg of tower space)
‘ The distribution of radio functions within the antenna provides reliability via built-in redundancy, and reduces maintenance cost
‘ Support for multiple spectrum bands, even eventually non-contiguous ones, as Nokia Siemens demonstrated on its Flexi platform last year. Ultra-wideband antennas like CommScope’s HW range (which covers the 1710-2690MHz range) have the flexibility even to support licensed frequencies that are not yet available.
‘ Flexible RF. Because RF components like power amplifiers and transceivers ‘ conventionally part of the base station ‘ are integrated with the antenna in AAS, the signals from each antenna element can be separately shaped and steered. This allows for flexibility in RF planning, to maximize capacity and coverage, and so reduce cost and potentially site numbers. With independent baseband control of individual carriers on each antenna element, operators using three or four carriers can tune each one independently, perhaps to meet high peak data rates by reducing interference. Or tuning can be done by air interfaces sharing the same frequency, an important issue where carriers are gradually refarming a 2G band for 3G or 4G. For instance, W-CDMA at 900MHz could be tilted to deliver more focused coverage than GSM at 900MHz. Where cellcos are sharing a RAN, AAS can tilt signals for individual operators, allocating resources independently to meet the needs of each partner.
As LTE roll-outs cover a rising number of bands and air interfaces, AAS architectures will evolve from single-band arrays to more complex combinations of active and passive antenna arrays. Typically, the active elements will be used to deploy new 4G services while the passive equipment will support legacy networks. Also, a carrier will often add AAS to support a new frequency band, alongside the traditional passive equipment in the existing frequencies. They will also increasingly come in wideband configurations which can support virtually the whole LTE spectrum range.
Another important evolving technology associated with AAS is 3D beamforming. Since the radios are distributed along the antenna, they can carry out different types of beamforming compared to their monolithic cousins, including splitting a sector vertically rather than horizontally, which allows transmit and receive activities to be separated. That gives a cellco new efficiency options, such as turning up the receive element to get a better signal from the edge of the cell, while keeping transmit low to reduce interference with an adjacent cell.
This can be significant for uplink performance and for delivering more consistent quality of experience throughout a macrocell, whereas in conventional antennas, the carrier has to make a difficult trade-off between signal strength/coverage and interference. Coverage can also be extended to higher vertical planes such as tall buildings, without altering transmit power (because the amount of energy delivered by the base station is more concentrated on the target user).
Carriers like SK Telecom are beginning to enquire how these technologies can be applied cost effectively to cells of all sizes within a HetNet, not just to macro base stations. There is still some way to go. Active antennas remain rare in commercial roll-outs as yet though they are increasingly prominent in vendors’ portfolios and all the large OEMs have AAS programs, supported by the antenna makers such as CommScope and Kathrein.
However, standardization work, rising acceptance of distributed RAN concepts, and the need to squeeze greater efficiencies out of existing cells will all accelerate adoption from late 2013. To see that happen, and gain broader operator acceptance, it is vital that industry standards emerge for AAS, to allow carriers to mix and match their network elements ‘ basebands, antennas and radio heads – to keep costs down and avoid supplier lock-in. In summer 2011, the RAN4 group of the 3GPP standards body approved a new study item on AAS, but even so, lack of agreed common antenna specifications has slowed progress by reducing interoperability and economies of scale, and making it hard for buyers to compare products on a like-for-like basis.
Standards ‘will facilitate more informed and accurate cost/benefit decisions on antenna selection and ‘.drive quality and improved performance into antenna products and result in positive impact to coverage, capacity and QoS,’ said the operator driven NGMN Alliance, which this month threw its weight behind the key emerging standard in this area, ETSI’s ORI (Open Radio Interface).
ORI is in turn based on the CPRI standard for connecting elements within the base station. As so often, NTT Docomo of Japan is driving the industry to adopt the new directions. The company recently announced an AAS test in which it connected its antenna to its base station via ORI. The operator already supports ORI in its heavily customized base stations, but its public support, and the NGMN backing, should help ETSI to see its interface universally adopted by base station vendors ‘ however they would love to cling to proprietary architectures. The Japanese carrier said ORI will allow it to install active antennas quickly and inexpensively without having to set up new base stations. It will also submit the results of its test to the 3GPP to contribute to the standardization of future active antennas within the LTE specs. Docomo’s antenna was jointly developed by Japanese antenna maker Dengyo Kosaku and Ubidyne of Germany.
Another important development in the antenna world in recent times has been FTTA (fiber to the antenna), which goes hand-in-hand with the increasing move to deploy remote radio heads at the top of cell towers ‘ close to, or fully integrated with, antennas and separate from baseband processing, which may even be centralized in the cloud. CommScope recently announced a turnkey FTTA solution which aims to standardize installation of remote radio units (RRUs) and associated cabling and power, while supporting a wide range of radio technologies and frequencies. It can even offer a fully pre-assembled tower-top solution (called Andrew SiteRise), which has already been deployed by lead customer Ooredoo of Qatar.
This is the world’s first factory assembled tower-top for remote radios and, together with Ooredoo, CommScope has created RRUs which are pre-assembled according to a single global design standard, leading to a radio vendor-agnostic system that does not require significant on-site remote radio modifications for future upgrades. In the past, CommScope said, the lack of a single tower-top design standard has made each upgrade a complex process. The tower-top solution will become Ooredoo’s standard cell site design across its markets in the Middle East, North Africa and Asia, and support a network modernization strategy of upgrading or replacing 15,000 macro base stations over the coming years.The tower tops are vendor agnostic and support virtually all current frequency bands and technologies, said the carrier.
Ooredoo’s CTO Paul Salmon said: “Ooredoo is embarking on a radical plan to upgrade network sites as part of our mission to stimulate human growth. Working with CommScope to develop a new tower top solution that is assembled in a factory will enable us to deliver a distinctly different approach to building and operating a wireless network.’
This is just one approach, but it epitomizes the approach of the new RAN, and the way that traditional economics will be thrown upside-down ‘ potentially with those vendors which fail to adapt sufficiently quickly. Standard designs and interfaces, enabling multivendor, multi-technology networks – with differentiation taking place in software and network planning ‘ will be the order of the future, leading to a model that will be closer to that of the enterprise.