Using small cell units to serve DAS: few thoughts...

:: Abu Sayeed
From all indications in the industry, it appears that small cell will be the wave of DAS future. In the past 6 months, we worked on over 30 small cell projects to augment coverage in enterprise in-building environment. Most of the coverage areas were below 100,000 sq. ft. In all cases these small cells were used as RF source to serve passive coax based DAS. We have designed a few fiber based DAS where small cell units are the RF Source. In September, we found out about 5 fiber based DAS network nationwide where small cells have been successfully used. We have so far designed 2 fiber DAS with small cells, and expecting to deploy within next 60-90 days.

Small cell, by design, usually has limited capacity (at least as of today). For an example, AIRAVE Pro Connect (a.k.a. eFemto) supports 29 active 1x voice calls AND 32 active EVDO Rev A sessions.  A Fiber DAS, on the other hand, is usually designed for a large coverage area where we expect to have higher traffic. This presents a sort of design objective dilemma and contradiction. As a result, when we are specifying a small cell to work as a RF Source for a fiber DAS, we have to analyze the traffic projection. It is possible to break up the DAS head-end in multiple sectors and have each sector to be served by one small cell unit. Make sure that small cell you will use can actually do hand-off with other small cells. Few other factors you may have to keep in mind while designing a DAS with small cell:

• Is there any limit on how many small cells can be used in a cluster? A cluster of small cell units will have proper hand-off in between them without dropping calls.
• Can the small cell do a proper hand-off to macro network and vice versa? I hand-off is an issue 
• Most small cells will require GPS antenna - since these are like mini base stations. Find out from design guidelines, if there is any restriction on how many GPS antennas can be within certain radius. This may impact a multi-sector DAS design where you were planning to place more than one small cell units in the same head-end room.