CPRI and DAS

CPRI defines the publicly available specification for the key internal interface of radio basestations between the radio equipment control (REC or basestation) 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. The CPRI specification has gone through several revisions, and today it is at version 5.0.

The idea behind CPRI was to create an open standard for interfacing basestations with radio heads. But in reality, CPRI is neither common nor public, as it is not truly an open standard. Instead, similar to what happened with the Integrated Services Digital Network (ISDN) for public branch exchanges (PBXs), each manufacturer developed its own flavor of CPRI that works only when interfacing its own basestations with its own radio heads.

Another of the enduring challenges in matching up a DAS with a mobile basestation has been the need to use RF as the method of interface, which adds complexity and cost to the deployment. But to date, DAS equipment has not been able to use the Common Public Radio Interface (CPRI), which has been defined for basestations. Now, DAS equipment that does use CPRI is emerging, solving several key problems.

Since the major basestation manufacturers don’t make DAS equipment, DAS systems supplied by third-party OEMs until now haven’t been able to interface the DAS head-end equipment directly in the digital domain with basestations through CPRI because each BTS manufacturer’s CPRI interface is unique.

Instead, the DAS head-end interfaces with basestations through the RF signal. This has been true since the inception of DAS more than 20 years ago. However, there is a significant power mismatch between basestations and DAS head-ends that must be accommodated for this interface to work. A typical basestation puts out about 40 W, and a DAS head-end takes in roughly 0.25 W. Feeding 40 W into a DAS will destroy the head-end. As a result, the basestation’s power must be severely reduced before it can interface with the DAS.

There are several challenges with reducing basestation power output:

• Complexity: Basestation power is reduced with racks of passive equipment called attenuators. All of this external “plumbing” between the basestation (which can also include splitters, combiners, circulators, etc.) and the DAS adds to the size, 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 and operating expenditures to the overall deployment, worsening the DAS business case for mobile operators.

• Inefficiency: Mobile operators invest in large, hot, power-hungry amplifiers for their basestations, only to have their power substantially reduced in the actual deployment. Amplifiers are one of the biggest cost drivers in a basestation.

Interfacing directly with a basestation via CPRI instead of RF eliminates the need for all this plumbing, saving space, power, and cooling costs in the DAS deployment. All of these elements are critical when evaluating the practical and financial viability of DAS deployment. DAS manufacturers’ ability to use CPRI interfaces versus traditional RF will greatly improve deployment time and the business cases for mobile operators, increasing DASs’ market reach. Obviously, this will require direct cooperation from the basestation manufacturers, as custom CPRI interfaces will need to be developed to work with each major basestation manufacturer.