Use distributed antenna systems to complement small cells

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.

Finally, today each small cell can support one or at most two frequencies, so in an environment where multiple frequencies and/or mobile operators must be supported, it will be necessary to deploy multiple small cells in each coverage area.

DAS and small cell use cases

Small cells are being deployed in smaller buildings with low user density, where multi-operator support may not be desired. In these situations, the small cells’ capacity and frequency-carrying capabilities are well matched to the needs of the users. Typically, indoor small cell deployments have been limited to buildings of less than 100,000 square feet.

DAS deployments typically involve larger buildings where the main requirements are to support hundreds or thousands of users, and to support multiple operator frequencies. Simply put, there is no other way to simply and economically cover a large space requiring multi-frequency/multi-operator support than with a DAS.

So it isn’t a question of replacing DAS with small cells, but rather finding the best environments where the strengths of each type of solution can be best applied.  As with many things, you should always use the right tool for the job.

How small cells and DAS can work together

In fact, it’s possible to use small cells and DAS together to overcome the limitations of using either alone.  You can use small cells to provide the RF signal, and DAS to distribute that signal throughout the building. In those instances where a building requires more capacity than can be provided by a single small cell, or when more than one or two frequencies must be supported, multiple small cells can be centrally located and easily fed into the DAS head-end.

Here’s how this combination solves the problems of using small cells or DAS alone:

* Small cells are far less expensive than base stations. Mobile operator base stations cost $30,000 or more, and make up a significant part of the cost of a DAS head-end deployment. Small cells cost a couple of thousand dollars each.

* Small cell power doesn’t need to be attenuated. Since they output .5 Watts of power, small cells eliminate the need for racks of attenuation equipment at the head-end of a DAS, saving on space, power, and cooling. Instead, small cells can be plugged directly into a DAS head-end.

* DAS overcomes small cell frequency limitations. A DAS can distribute a range of cellular frequencies to serve more than one mobile operator, so just one set of remote antennas is required, rather than multiple small cells in each location. The user would simply deploy a series of small cells at the DAS head-end to supply the needed frequencies.

* Interference concerns are mitigated. Since the DAS simulcasts radio channels throughout the building, multi-cell interference issues are eliminated along with the need to hand off from one cell to the next as the user moves around the building.

* 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 small cells for higher capacity requirements in high-density areas.  If additional capacity is needed throughout the building, additional small 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 coverage and capacity in a large building requiring multi-operator support than to deploy dozens or hundreds of small cells. At the same time, using small cells as the RF signal source saves the cost of a base station and attenuators that would sit between the base station and the DAS head-end.  It also eliminates the need for any specialized HVAC and power, which is common for base station deployments.

* Backhaul costs are lower. A group of centrally located small cells feeding a DAS head-end can be combined to use a single backhaul connection. This contrasts favorably with needing a separate backhaul connection for each of several dozen or several hundred distributed small cells.

DAS solutions continue to be popular because they are the most efficient and economical way to provide multi-frequency in-building wireless coverage in larger venues. Small cells are a preferred solution for residences or smaller buildings, but given their current limitations they will not replace DAS at the higher end. Rather, small cells and DAS can work together to reduce the cost of deployments in terms of equipment, cabling and real estate.

Spindler has over 30 years of product management and marketing experience in the wireless and telecommunications industries.

This story, "Use distributed antenna systems to complement small cells" was originally published by Network World.