Reader Forum: Picocells and DAS belong together


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Small cells like picocells and femtocells are promising to deliver localized coverage and capacity inside corporate buildings, and in-building coverage will be critical for quality of service in 4G networks. But small cells aren’t the beginning and end of the story. In this article, we’ll look at the role of femtocells, picocells and distributed antenna systems in delivering mobile network capacity and coverage within enterprises.
Coverage isn’t the main issue
DAS have been used to provide additional coverage inside buildings for many years. The DAS takes a mobile network signal and distributes it through a series of remote antennas to provide a blanket of coverage within the building. But as users have adopted smart phones, tablet computers, and other devices, the requirement inside buildings is shifting from mobile network coverage to mobile network capacity. Suddenly, all of those iPhones and other devices are straining the capacity of the in-building network and this is changing the way capacity is provided.
In the past, a DAS was fed by a rooftop antenna or repeater that pulled capacity from the local macro network and distributed it through the building. But now that network capacity is being strained everywhere, the DAS must have its own, dedicated form of capacity. Traditionally, localized capacity came in the form of a BTS supplied by the mobile carrier, but in many enterprise buildings a traditional BTS is overkill for the capacity needed and much of its capacity ends up going unused.
The age of small BTS in the form of femtocells and picocells provides a way to deliver the right amount of capacity in an enterprise. A femtocell delivers capacity over about 2,000 square feet of coverage, while a picocell can cover 5,000 square feet or more. But obtaining the right amount of coverage and capacity inside an enterprise isn’t just about putting up small cells everywhere – the limitations of small cells dictate a hybrid solution that includes DAS.
Limitations of small cells
Picocells and femtocells were designed to provide coverage and capacity over a specific area. There is a lot to like about small cells. They cost a small fraction of what a standard BTS costs (hundreds or a few thousand compared with $25,000 or more for a BTS); they have small footprints that make them easy to deploy in wiring closets, on ceilings, or in other areas without special HVAC or power facilities; and they can use DSL or enterprise Ethernet for backhaul rather than requiring a dedicated T1 connection. If the building is small enough to be served by a picocell or femtocell, these units are an ideal solution for coverage and capacity. But when the building can’t be covered by just one small cell, there are several problems with deploying multiple cells to achieve the desired coverage and capacity.
Each cell distributes one frequency. Femtos or picos are generally single-frequency devices, so if a building owner wants to provide coverage for multiple mobile operators he must deploy a separate set of small cells for each frequency to be covered. Ceilings or wiring closets in enterprises that want coverage for three or four carriers will be positively festooned with femtos or picos in such a scenario.
Multiple cells introduce interference. When more than one femto or pico is used, there is the potential for interference between them, causing performance problems that degrade the quality of service. Like Wi-Fi access points, femtocells must alternate channels to avoid co-channel interference and doing this requires carriers to use a lot of frequency in a small area given the relatively small coverage footprint of a femto or pico.
Devices hand-off between one cell and another. A mobile device that detects two relatively equal signal sources will hand off between the two, rather like a listener trying to monitor two conversations at once. Small cells have very low output power (milliwatts compared with 20 watts for a macro cell), so users will find their devices hunting between equidistant cells, or between the nearest small cell and the macro network outside the building. As devices hand off they provide poor quality of service and drain their batteries more quickly. It is far better to establish a single dominant signal throughout the enterprise so that phones can connect to it and stay there.
Over-provisioning is inevitable. Each femto or pico provides capacity for a small number of users, so conference rooms, cafeterias, and other areas with high user density will need to be over-provisioned to provide enough network capacity for peak usage times. These investments in extra cells will go to waste during times of low usage.
Combining DAS with small cells
The way to overcome the limitations of using femtos or picos alone while providing strong and consistent mobile service in the enterprise is to combine these devices with a DAS: they provide the capacity, and the DAS distributes it throughout the building. Here’s how this combination solves the problems of using femtos or picos alone.
DAS is multi-frequency. A DAS can distribute multiple cellular frequencies to serve more than one carrier, so just one set of remote antennas is required.
There is no interference. Since the DAS simulcasts radio channels throughout the building, there is just one large cell. This eliminates multi-cell interference along with the need to hand off from one cell to the next as the user moves about.
There is one dominant signal. One signal source means a single dominant signal. The DAS simply provides a uniformly strong signal throughout the interior of a building so user devices don’t hunt between signal sources.
There is no need to over-provision. All antennas in the DAS have access to all of the feeder cell’s capacity, so there’s no need to add new cells for higher capacity requirements in certain areas. If additional capacity is needed throughout the building, additional cells can be added in a central location at the DAS head-end.
Deployment is less expensive. It is much less expensive to deploy a DAS for in-building coverage than to deploy dozens or hundreds of pico or femto cells.
Operating expenses are lower. A DAS is pretty much a set-it-and-forget-it solution, so once deployed it needs little maintenance. With multiple small cells, the cells will require continuous adjustment to function in an optimal manner.
Backhaul costs are lower. Since the DAS connects to a single femto or pico cell it can deliver service using a single RF source and backhaul connection.
There is a lot of talk about femtocells or picocells permeating the enterprise, but their single frequency, limited capacity, lower power, poor frequency management and traffic engineering required makes multiple cells a poor solution for enterprise coverage. A far better idea is to combine the benefits of these devices and DAS to get lower costs, easier deployment, better quality of service and multi-carrier coverage.

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