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One of the enduring challenges in matching up a distributed antenna system with a mobile base station has been the need to use RF as the method of interface. Using RF as the interface adds complexity and cost to the deployment, but to date, DAS equipment has not been able to use the common public radio interface that has been defined for base stations. Now, DAS equipment is emerging that does use the CPRI interface, and this development solves several key problems.
CPRI defines the publicly available specification for the key internal interface of radio base stations between the radio equipment control (REC or base station) and the radio equipment (RE, or radio head). The companies cooperating to define the CPRI specification now include Ericsson, Huawei, NEC, Nokia Siemens Networks and Alcatel-Lucent. (Nortel contributed as member of the CPRI cooperation to CPRI Specification versions 1.4, 2.1, 3.0 4.0 and 4.1 and left the cooperation in December 2009.) The CPRI specification has gone through several revisions, and today is at version 5.0.
The idea behind CPRI was to create an open standard for interfacing base stations with radio heads, but in reality, CPRI is neither common nor is it public, as it is not truly an open standard. Instead, similar to what happened with the integrated services digital network for PBXs, each manufacturer developed its own flavor of CPRI that works only when interfacing its own base stations with its own radio heads. Since the major base station manufacturers don’t make DAS equipment, DAS systems supplied by third party original equipment manufacturers until now haven’t been able to interface the DAS head end equipment directly with base stations through CPRI because each BTS manufacturer’s CPRI interface is unique.
Instead, the DAS head end interfaces with base stations through the RF signal. This has been true since the inception of DAS over 20 years ago. However, there is a significant power mismatch between base stations and DAS head ends that must be accommodated for this interface to work. A typical base station puts out about 40 watts of power, and a DAS head end takes in roughly one-quarter watt of power. Feeding 40 watts into a DAS will destroy the head end. As a result, the base station’s power must be severely reduced before it can interface with the DAS.
There are several challenges with reducing base station power output:
–Complexity: Base station power is reduced with racks of passive equipment called attenuators. All of this external “plumbing” between the base station (which can also include splitters, combiners, circulators, etc.) and the DAS head end adds to the complexity and cost of the deployment.
–Space: Racks of attenuators take up floor space, making a DAS deployment much larger than it needs to be. In many cases, there may not be enough floor space at the intended facility to accommodate the entire deployment, so a separate, off-site facility must be built. This added expense can be a deal-killer for many mobile operators.
–Heat: RF attenuators generate a lot of heat, making it necessary to spend more on air conditioning in DAS deployment areas.
–Cost: The need for attenuators (and the rest of the aforementioned “plumbing”), and the need to invest manpower resources in designing and deploying all this RF “plumbing” adds capital expenses and operating expenses to the overall deployment, worsening the DAS business case for mobile operators.
–Inefficiency: Mobile operators invest in large, hot, power-hungry amplifiers for their base stations, only to have their power substantially reduced in the actual deployment. Amplifiers are one of the biggest cost drivers in a base station.
By interfacing directly with a base station via CPRI instead of RF, the need for all this “plumbing” is eliminated, thereby saving space, power and cooling costs in the DAS deployment. All of these elements are critical when evaluating the viability of a DAS deployment, both the practical and financial aspects. DAS manufacturers’ ability to use CPRI interfaces versus traditional RF will greatly improve deployment time and the business cases for mobile operators, thereby increasing DAS’ market reach. Obviously, this will require direct cooperation from the base station manufacturers, as custom CPRI interfaces will need to be developed to work with each major base station manufacturer.
At the end of the day, with all of the challenges currently facing mobile operators in terms of providing focused coverage and capacity, supporting CPRI interfaces to the DAS head end will make it easier and more cost-effective to deploy DAS and help meet those challenges economically. CPRI interfaces are clearly the way forward for DAS and its continuing role as a critical element of the small cell ecosystem.
