DAS over CPRI is not just plug and play, but brings real benefits

Connecting DAS to C-RAN platforms promises to bring benefits for DAS players and operators, but it's not all straightforward. What are the key catches?

At Mobile World Congress TMN wrote about the looming battle of indoor coverage, a story given legs by two DAS vendors making announcements at the show that promised tighter integration to base stations using the CPRI standard.

The first, the ION-E from Commscope, was a product release that promised a lot of things, amongst them the capability to connect via CPRI to a baseband unit.

The second, from TE Connectivity, announced the company had achieved direct integration, over CPRI, with Alcatel-Lucent baseband equipment. That would allow DAS nodes to be fed by radio resource direct from Al-Lu base stations over a direct optical interface, in effect becoming an indoor (and already deployed) version of the remote radio head.

As we wrote at the time, TE’s view was that:
“Being able to distribute RF over a direct optical interface will be much cheaper than other DAS. This is because it eliminates the Point of Interface processing that conditions power levels via high power RRHs.

“The jag is, TE has integrated its CPRI-capable DAS directly with Alcatel-Lucent (with other vendors to come later) because it says CPRI iteration is not interoperable between vendors. So in other words, if you haven’t integrated your CPRI DAS direct with vendors, then you don’t really have CPRI support at all.”

Here, John Spindler, director product management, TE Connectivity, expands on both those points – the advantages of a direct optical connection and the need for integration to the base station vendor.

CPRI and DAS
By John Spindler
One of the enduring challenges in matching up a distributed antenna system (DAS) 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 (CPRI) 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:

Complexity
Base station power must be reduced from 40W to 0.25W with racks of passive equipment called attenuators as well as remote radio heads to process the RF signal. 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 RF 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 head end deployment areas.

Cost
The need for attenuators and the need to invest manpower resources in designing and deploying all this RF “plumbing” adds Capex and Opex 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.

CPRI eliminates these issues. By interfacing DAS directly with a base station via CPRI, the need for all of the attenuation “plumbing” is eliminated, thereby saving space, power, and cooling costs in the DAS deployment. In addition, a direct CPRI interface eliminates the need for remote radio heads in the DAS head end deployment.

DAS manufacturers’ ability to use CPRI interfaces vs. traditional RF will greatly improve deployment time and the business cases for mobile operators, thereby increasing DAS’ market reach.

In reality, CPRI is neither Common nor is it Public, as it is not truly an open standard.

In reality CPRI is neither common nor public
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, each manufacturer developed its own flavour 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 until now haven’t been able to interface directly with base stations through CPRI because each BTS manufacturers’ CPRI interface is unique.

As a result, CPRI interfaces between DAS and base stations will require direct cooperation between base station manufacturers and DAS manufacturers, but the economics of doing so are compelling.

Recently, Alcatel-Lucent and TE Connectivity announced a direct CPRI interface between the Alcatel-Lucent base station portfolio and TE Connectivity’s DAS. We can expect other manufacturers to follow.

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