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Use distributed antenna systems to complement small cells

John Spindler, VP Marketing and Product Management, Zinwave | Sept. 13, 2016
Using small cells and DAS together improves cell coverage within a building and overcomes the limitations of using either alone

This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter’s approach.

When small cells first appeared a few years ago, many pundits and some mobile operators proclaimed that distributed antenna systems (DAS) were dead.  Small cells promised inexpensive, easy-to-deploy mobile coverage that would become the primary means of enhancing in-building wireless coverage. But despite the hype, DAS solutions remain a prominent choice for mobile operators and venue owners.  In fact, small cells and DAS work well together for in-building wireless deployments.

DAS and small cells are different solutions to the challenge of improving indoor mobile coverage and capacity. A DAS uses a centralized RF signal source that feeds a system of distributed antennas connected by fiber, coax cable, or Cat 5/6 cable. Depending on the cable type used, a DAS can support tens of thousands of users and can extend for miles, which is why it is the solution preferred in stadiums, airports, transit stations, office towers, factories, and other large spaces.

Another advantage to a DAS is it supports multiple frequencies with a single system, so if a venue needs to support multiple mobile operators, a DAS can do it. Finally, a DAS can be configured multiple ways depending on the venue requirements, including simply creating one large cell, so user devices can roam throughout a covered area without handing off from one antenna to the next.

In most cases, a carrier-supplied base station provides the signal source for a DAS (in limited instances a bi-directional amplifier may be used). This signal source puts out as much as 40 Watts of RF power, which must be attenuated to bring it down to a level that is acceptable by the DAS head-end (typically .5 Watt).  This means that, along with the mobile operator’s base station and the DAS head-end, the customer must deploy racks of attenuation equipment. All of this equipment uses space, power, and cooling resources.

A small cell takes a different approach. Like a Wi-Fi access point (AP), a small cell combines the RF radio source and the antenna in a small, ceiling-mounted unit. These units are relatively inexpensive, easy to install and generally trouble-free, but since each cell is a separate signal source, there is the potential for interference between cells, and user devices must hand off from one cell to the next as the user moves throughout the building. A user can also experience a significant drop-off in data rates at the cell boundaries or edges between small cell coverage areas. 

Capacity is another issue. Each small cell can support on average 32 simultaneous users, so in a larger environment it is necessary to deploy multiple small cells. And, in areas of a building where user density is high (conference rooms, cafeterias, etc.), it will be necessary to install multiple small cells within a single coverage area just to provide the required capacity, particularly if data usage on the network is high.

 

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