A guide to distributed antenna system (DAS) optimization
DAS in a nutshell
Signals are coupled off from a nearby base station or off-air repeater to the OMU and then distributed via fibre to one or several remote units. A single fibre optic cable connects the remote units to the OMU. Different colors (wavelengths) are used in order to combine the uplink of different remote units in a cost effective way.
Growth in data traffic
Mobile operators face continuous challenges to drive increasingly more improvements to the services they provide. The extremely rapid growth of the mobile network places greater demands on coverage and capacity. In an in-building situation, these demands are not always fixed factors at the outset, and operators must make allowances for growth.
The constantly augmented sophistication of the devices through which subscribers access the mobile network also necessitates high quality wireless coverage. Mobile video traffic, for example, exceeded 50% of mobile data for the first time in 2012. Global mobile data traffic grew by 70% in 2012, reaching 885 petabytes (PB) per month by the end of 2012, up from 520 PB per month at the end of 2011 (1 Exabyte = 1,000 petabytes). To give this figure perspective, one PB is 1000000000000000B or 1015 B, equivalent to 1, 000 terabytes. These astounding trends are driving operators to address their networks with a particularly sharp focus on indoor coverage; a focus which derives from both the importance of the usage environment – it is now widely acknowledged that indoor use accounts for 80% of mobile data traffic, and forecasts that the pace of growth will not slow over the next five years (See Figure 1).
Growth in DAS sophisticationWith increasing indoor usage come increasing demands on the mobile network. Requirements relate not just to the world of business—high-rise, multi-tenant buildings and complex campus scenarios, but also to a range of venue and location types from university and government/public institution campuses, to tunnels, underground transportation networks and sports facilities. A stadium capable of holding 90,000 visitors, for example, can generate peak traffic equivalent to a city with 500,000 inhabitants; of these two, the stadium is by far the biggest coverage challenge since most of the visitors would be using their devices at the same time resulting in an exceptionally high level of demand for network service in a concentrated timeframe.
The demands for robust network connections of the highest order of reliability are further intensified by the communication needs of emergency services. Provisioning such services is critical in the environments previously mentioned, all of which can, through their very structure and often obstacle-intense characteristics compromise signal propagation. Obstacles range from man-made barriers to coverage, such as with neighboring buildings, to unavoidable location characteristics, such as being underground.
The flexibility of the distributed antenna system (DAS) and its ability to break down the coverage challenges into sectors provides both coverage and capacity solutions. DAS seamlessly channels mobile operators’ networks into buildings and other inherently difficult locations. Over a fibre optic cabling infrastructure the DAS brings the signal to the interior of the building and caters to every major wireless technology including GSM, WCDMA, and LTE. At its simplest, a DAS addresses effective single propagation by making the big challenge of indoor coverage no more daunting than a series of small challenges. Addressing each one specifically within its ‘sector’ passes the challenges up the line in progressive stages, from localized (remote) optical units (repeaters) through to head-ends (the optical master units which convert signals from RF to light in a fibre-fed repeater system), through to the base station (the base transceiver station, referred to as BTS). The BTS can be inside the building, at a base station ‘hotel’, or externally located up to several kilometers away. Another option is to feed the DAS through an off-air digital repeater.
The system now also extends to provide IP backhaul infrastructure for other devices needing IP backhaul support, including, for example, small cells or devices such as the surveillance cameras installed in many public venues.
As recently as eighteen months ago, operators were not optimizing their indoor systems for next generation technologies, typified at the time by the arrival of 3G. Technology has moved fast. These same operators are now engaged in renewed capital expenditure as they go back to underserved locations to add base stations, or replace entire systems to reduce noise levels in line with the requirements of new technologies. Today, such situations can be avoided, and no one can be blamed for not having anticipated the pace at which the demands of the mobile world would change. It’s a lesson from which the industry as a whole has learned—a flexible system enables operators to augment indoor coverage and capacity as required.
The design stage is now more critical than ever, as operators now give high priority to future-proofing their indoor coverage systems. Delivering the highest achievable performance depends on maximizing the desired factors, a relatively straightforward affair arising from rigorous RF design procedures. High performance depends also on minimizing undesired factors; interference and noise come at the very top of the list. Production of a robust architecture plan is now a well-established discipline. Interactive apps are available that drive a comprehensive checklist approach to the process. This is the stage at which decisions need to be made regarding the relative power of remote optical units, dictated by the requisite coverage sector, which then informs decisions on the number of units required together with the number of optical master units, propagating the signal from the remote and take it on to the BTS (See Figure 2).
Figure 2. The DAS architecture