In a distributed antenna system deployment, about half of the cost of installation – trade labor rather than material, labor and electronics – lies in the head-end, where the DAS host unit connects to mobile operator base stations. It pays to optimize the head-end as much as possible to reduce the overall cost of the system. Head-ends require space, power and cooling, and the choice of head-end architecture can impact these costs dramatically. Fortunately, a new technology is emerging that can greatly reduce head-end capital equipment and operating costs.
Traditional DAS head-end architecture
Since its inception more than 20 years ago, a traditional DAS host unit interfaces with mobile operator base stations using a radio frequency signal. An RF interface requires the use of remote radio heads connected to each BTS to convert the BTS’ digital signal into RF. Each BTS can have up to six remote radio heads connected to it, at a cost of $5,000 per radio head.
Once the RF conversion is made, the signal goes to the DAS host unit, but there is a significant power mismatch between base stations and DAS host units that must be accommodated for the 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 host unit. As a result, the base station’s power must be severely reduced before it can interface with the DAS, requiring racks of power attenuators.
There are several challenges with using radio heads and reducing base station power output when using an RF interface in a DAS head-end:
• Complexity – Radio head/BTS power is reduced with racks of attenuators. All of this equipment (which can also include splitters, combiners, circulators, etc.) between the base station and the DAS host unit adds to the complexity and cost of the deployment. As previously mentioned, radio heads alone can run up to $30,000 per BTS. Attenuation equipment adds thousands of dollars more to the cost of deployment.
• Space – Racks of radio heads and power 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 – Radio heads and RF attenuators generate a lot of heat, making it necessary to spend more on air conditioning in DAS head-end deployment areas. Businesses are increasingly conscious of energy expenditures and minimizing operational costs and environmental impact. HVAC is important to network operators and equipment manufacturers to ensure optimal performance and electronic reliability.
• Cost – The need for radio heads and attenuators, and the need to invest manpower resources in designing and deploying all of this RF “plumbing,” adds capital and operational expenses to the overall deployment, worsening the DAS business case for mobile operators and neutral host 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.
Using CPRI
The other way a DAS host unit can interface with a BTS is through the common public radio interface. CPRI defines the publicly available specification for the key internal interface of radio base stations between the radio equipment control and the radio equipment (RE, or radio head). The companies cooperating to define the CPRI specification include Ericsson, Huawei, NEC, Nokia Siemens Networks and Alcatel-Lucent.
Interfacing directly with a base station via CPRI instead of RF eliminates the need for remote radio heads and power attenuators, thereby saving space, power and cooling costs in the DAS deployment. A study by Bell Labs showed that using CPRI instead of the RF interface saves 40% on capital expenditures and 50% on operating expenses.
There are several key advantages to using CPRI:
• There is a direct digital interface to the DAS via a plug-in card in the DAS host unit, and fiber connecting the plug-in card to the BBU. This greatly simplifies the process of connecting the head-end components.
• The digital interface eliminates the need for RF processing (radio heads) and power conditioning, simplifying the deployment.
• CPRI reduces the typical radio head and point-of-interface footprint, enabling smaller deployments.
• The solution is backward compatible to fielded DAS host unit solutions with addition of a CPRI card (no changes are required to the DAS’s remote antenna units). In fact, many early deployments of CPRI cards were being deployed with existing DAS installations to eliminate the racks of RF attenuation equipment and remote radio heads, making room in an existing head-end for more base stations to add new services.
Implementing CPRI requires coordination between BTS vendors and DAS vendors, and such collaborations are beginning to take place. In the future, CPRI may well become the standard method of interfacing BTS and DAS host units, creating fully optimized head-end deployments. The economics are very compelling.
During his more than 13-year tenure with TE, Mark Kerschner has held a variety of business development and product management positions with the company, including senior proposal engineer, product manager for copper connectivity products and his current position of senior product manager for outdoor wireless products. In this role, he is responsible for product development initiatives, market management and life cycle management for TE’s outdoor wireless products and the business expansion into global markets. Kerschner holds a Bachelor of Arts degree in business management from St. John’s University, Collegeville, Minn., and a Master of Science degree in telecommunications from the University of St. Mary’s, Minneapolis.
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