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Wideband Distributed Antenna Systems (DAS): The Only Option For 100% Indoor Wireless Coverage
Peter Hartmann, Technical Applications Consultant - Zinwave
Struggling with in-building signal coverage?
Network operators strive to achieve reliable and ubiquitous coverage levels across their deployment areas. While reported coverage can be very close to 100%, customer experience often shows a different picture.
One of the main reasons is that operators measure their
coverage outdoors - for example, in the streets and parks of urban
areas - while modern lifestyle confines the majority of revenue generating
customers into buildings for most of the day. Indeed around 70% of
all mobile calls originate from within buildings[1].
Furthermore, radio signals are further attenuated by
building infrastructure, including exterior and interior walls, and
even furniture. A GSM900 signal passing through a 20 cm thick steel-reinforced
concrete wall, for example, is attenuated by a factor of approximately
500 (or 27 dB). To put this number into perspective, a signal propagating
in free space would experience this amount of attenuation only after
a 22-fold increase in transmission distance. For this reason in-building
signal coverage is found to be unsatisfactory, even at only a small
distance from the periphery.
At the same time mobile devices inside the building need to increase their transmit power in order to reach the base station. This not only reduces battery lifetime, but also increases RF emission levels.
While all wireless services can suffer from poor in-building coverage, this problem is particularly pronounced for the newer high-data rate applications such as 2.5G (EDGE) and 3G (W-CDMA). These services require a much higher signal quality (i.e. SNR) than their voice counterpart, such that the same amount of attenuation can lead to a much poorer quality of service (QoS).
Additionally, newer services tend to operate at higher frequencies, partly because of the scarce availability of free spectrum and partly because they occupy a wider spectrum, which is easier to deal with at higher carrier frequencies.
Just as the free-space attenuation increases for signals with higher frequencies so do the attenuation values of the materials through which the signals have to propagate. As a result the services that require the most signal strength are those affected most by propagation mechanisms. Subsequently, in-building coverage is often very poor or even non-existent.
Ways to solve the issue
So how can satisfactory in-building signal coverage be achieved? Currently, there are four solutions that address poor in-building signal penetration:
1. Outdoor macro site
The obvious solution may appear to be to increase the power of a nearby macro site. Unfortunately this has a serious impact on the carefully planned wide area network (WAN) as it changes the cell-size, and can lead to other issues such as an aggravated near-far problem. Additionally with growing public awareness of potential issues over mobile phone mast radiation it may not be the politically correct approach either.
2. Repeaters
Repeaters on the other hand allow the extension of the outdoor cell into the building without changing the macro network. As the repeater is fed from a macro cell via its donor antenna no additional costly network infrastructure needs to be installed in the building.
Because of their high gain of 60 – 90 dB (i.e. amplification of 1 million to 1 billion times), however, the bandwidth of repeaters is limited to one or two frequency bands only. When coverage for a variety of services is required (e.g. TETRA, PMR, GSM900, DCS1800, 3G), a number of repeaters have to be installed in parallel, each for a different technology. Furthermore, for buildings with high traffic demands a repeater solution is unsuitable since it does not provide any additional call capacity.
3. Pico-cells
An installation using pico-cells can support and deliver additional capacity within the enterprise environment. However, in large or awkwardly shaped buildings, and in such installations where very high capacity is required, a large number of these pico-cells need to be installed - each wireless service requires its own dedicated infrastructure, resulting in multiple installations, ongoing management burden, and high capital and operational expenditure.
4. Distributed Antenna System (DAS)
A DAS marries the benefits of the repeater and pico-cell solutions. Extra capacity can be easily added by connecting a dedicated base station to the input ports of the DAS central hub unit. But if only signal coverage is required then the DAS can be connected to the indoor port of a repeater. The flexible architecture of a DAS is best suited to the granularity required for an effective in-building design, be it in a high-capacity office building or for a large space warehouse.
The DAS is therefore a key component in the design of coverage solutions for buildings.
Approaches to DAS design
Though passive systems (large coaxial cables connected to antennae) have been popular in the past these are now being superseded by more flexible, active optical fibre-based systems.
Fibre-based DAS are particularly suited for long distance and high-frequency signal distribution, benefiting from the low and frequency independent loss of optical fibre transmission.
To date the active DAS systems available have been narrowband and developed typically for specific needs, mostly around fixing the issue of poor GSM (800-900MHz signal) mobile phone coverage within buildings. With the advent of 3G mobile phone service, these 2G based DAS systems now require upgrade to support both 2G (GSM) and 3G service, and typically this is achieved with additional components being bolted-on, or indeed with complete overlay DAS systems.
However, there is a better way to fix the problem of support for multiple services and frequencies over a DAS – the wideband DAS.
Why Wideband DAS
A wideband DAS is defined as a system supporting a wide range of frequency signals, from the lowest (security/PMR radio services) up to the highest (3G cellular and Wi-Fi), and covering all frequencies in between (not just specific bands).
A fully wideband system has the advantage that new operators or even new services can be added with minimal added cost to the infrastructure. This feature is very valuable to neutral host providers (NHP) who would like to offer their system to as wide a customer base as possible.
Network carriers can also benefit from the advantages offered from a wideband system. As a consequence of mergers and acquisitions, network operators are faced with managing a portfolio of fundamentally different frequency bands. Here a wideband DAS can provide a single solution instead of the traditional approach of overlaying a number of systems.
Conclusion
Fibre-based distributed antenna systems are the solution of choice for future-proof provision of in-building coverage. Though 90% of all modern buildings are equipped with multi-mode fibre (MMF), only a small percentage of in-building systems use this type of fibre. This is mainly due to the complex signal processing involved in the established systems, which is applied to overcome the limitations typically associated with MMF.
Zinwave has developed a unique type of DAS system that avoids the complex signal processing and supports the transmission of the RF signals at their native carrier frequency. Systems of this type can utilise the pre-installed fibre readily available in, for example, offices, shopping malls and airports, thus significantly reducing the cost of installation.
Because these systems are wideband in nature, compared to the established narrowband-only MMF systems, they support a wide range of signal frequencies. As no signal processing is required, the same system can simultaneously transport a multitude of signals different both in frequency and modulation format. In addition new services can be added easily and at low cost as most of the existing infrastructure can be re-used.
The Zinwave Wideband DAS
About Zinwave
Zinwave is at the forefront of wireless technology development, pioneering a new approach to in-building coverage.
Developing a new generation of easy-to-use, scalable, self-configuring in-building cellular and wireless infrastructure systems that overcome the problems of poor signal propagation and prohibitive cost of ownership, Zinwave’s unique approach supports the convergence of new wireless technologies, enabling organisations to introduce new solutions previously unavailable from any single wireless infrastructure.
[1] K. Butterworth, presented at the In-building Forum (IBF), Lisbon, December 2005.
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