By TONY LEFEBVRE, TE Connectivity -- For more than twenty years, interfacing a distributed antenna system (DAS) host unit with carrier base stations has been needlessly complex because of the requirement for using RF as the method of interface. To date, DAS equipment has not been able to use the digital Common Public Radio Interface (CPRI) that has been defined for mobile base stations. Now, DAS equipment is emerging that does use the CPRI interface to specific manufacturers’ base stations, and this development addresses several key challenges. In this article, we’ll look at the CPRI interface and see how it improves DAS deployments.
CPRI: The Facts
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. The CPRI specification has gone through several revisions, and today is at version 6.0.
The idea behind CPRI was to create an open standard for interfacing base stations with radio heads, but in reality, CPRI is not common as it is not truly an open standard. Instead, similar to what happened with the Integrated Services Digital Network (ISDN) standard for PBXs, each base station manufacturer developed its own flavor of CPRI that works only when interfacing its own base stations with its own radio heads. Since most major base station 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 with base stations through CPRI.
Instead, the DAS head end or host unit interfaces with base stations through the RF signal. This has been true since the inception of DAS over twenty years ago. The fundamental problem is that 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 ¼ 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.
RF Interface Challenges
There are several challenges with reducing base station power output when using an RF interface:
• 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. In addition, the deployment requires installation of remote radio heads (RRHs) to provide the RF signal.
• Space – Racks of attenuators take up floor space, making a DAS deployment much larger than it needs to be. In many cases, the RF interface equipment may be much larger than the base stations or DAS host units, and there may not be enough floor space at the intended facility to accommodate the entire deployment. In these cases, 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 rest of the aforementioned “plumbing”), and the need to invest manpower resources in designing and deploying all of 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.
Interfacing directly with a base station via CPRI instead of RF eliminates the need for all of this “plumbing” (including the radio heads), thereby saving space, power, and cooling costs in the DAS deployment. There are several key advantages.
Simpler Operations
DAS offers great flexibility by supporting multiple carriers over a single system to venues or outdoor areas. The CPRI connection enables the interface from the Base Band Unit (BBU) to the DAS to be a direct digital optical link. This eliminates the need for additional RF processing, often via high-power RRH and RF conditioning equipment known as Point-of-Interface (POI).
Elimination of this equipment simplifies the network design and provides the benefits of a single wireless delivery solution to address multiple bands, multiple protocols and/or multiple operators in addition to a single solution for 2G, 3G and 4G technologies. In summary:
• There is a direct digital interface to the DAS via a plug-in card in the DAS host, and fiber connecting the plug-in card to the BBU.
• The digital interface eliminates the need for redundant RF processing and conditioning.
• The space efficient solution reduces typical RRH and POI footprint.
• There is a single wireless delivery solution for multi-carrier applications and technology evolutions.
• The solution is backward compatible to fielded DAS host unit solutions with addition of a CPRI card (no changes are required to the DAS antenna units). In fact, many early deployments of CPRI cards are being implemented with existing DAS installations to eliminate the racks of RF attenuation equipment and RRHs and thereby make room for more base stations to add new services.
Figure 1: CPRI plug-in card in DAS host (top middle)
Network Design Flexibility and Improved Network Performance
The DAS host serves as an aggregation point for multiple services. The host can either be co-located with the BBU or located at a remote location up to 15 KM away from the BBU, where the CPRI interface acts as the remote radio head. The DAS host will aggregate multiple carriers, services or operators for efficient digital RF transport to the service area.
The CPRI interface card eliminates RF processing and conditioning prior to the DAS (which is a significant source of Error Vector Magnitude (EVM) degradation), and eliminates introduction of passive intermodulation (PIM). The CPRI card allows for a native digital baseband signal from the BBU to the DAS host unit, eliminating noise from analog RF sources. The CPRI card simplifies commissioning by eliminating the need for analog signal balancing. In short:
• The CPRI card with DAS digital transport enables pristine digital optical transport from the BBU to the RAU.
• The all-digital solution presents the full EVM budget and eliminates sources of PIM.
• The CPRI interface makes it easier to commission the DAS and eliminates the need to balance the analog uplink.
Improved Total Cost of Ownership
Studies by Bell Labs and TE Connectivity concluded that for high-capacity, multi-carrier deployments, the BBU CPRI interface to the DAS offers significant CAPEX and OPEX savings over traditional analog alternatives.
With the elimination of RRH and POI equipment, over 40% CAPEX savings can be realized. If the deployment requires construction of equipment shelters to house the DAS, the savings are significantly greater. The elimination of the pre-DAS RF processing and conditioning saves greater than 50% OPEX in lower energy consumption, cooling and rent.
In all, the study concluded that CPRI enables a greater than 50% Total Cost of Ownership savings over five years. The cost savings arise from:
• Greater than 40% CAPEX savings from elimination of pre-DAS equipment and shelters.
• Greater than 50% OPEX savings from lower energy consumption, cooling and rent.
Design flexibility, costs, and network performance are all critical when evaluating the viability of a DAS 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. 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 and DAS manufacturer.
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.
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