A Systems Approach to Microcell Repeater Deployments
By David J. Porte, CEO OpenCell Corp.
As Wireless Service Providers in the US struggle to catch up to the quality and density of wireless deployments in Europe, they are learning from the European experience and using a new breed of technology to make a significant leap forward.
A ClearControl network, comprised of fiber-fed microcell Radio Access Nodes (RANs)
Europe has relied to a much greater extent than US operators on repeater technology and microcell base stations to improve coverage and capacity in urban cores. These technologies have allowed lower-impact deployments in culturally/environmentally sensitive areas, better RF coverage, and higher capacity through improved density of coverage.
In Europe, mature analog-over-fiber systems have been in broad deployment and have been based around a point-to-point deployment approach. In this approach, a sector is remoted from the base station by converting the analog signal at the base station PA to analog light, and then reconstituted into RF at the remote node. Systems like these were used in deployments such as the Olympic venues in Athens.
While US operators have missed the first wave of fiber repeater deployments, they stand to reap the benefits of a new wave of fiber repeater technology that breaks the point-to-point limitations of the analog technology and can leverage the emerging base station standards that will provide a digital output from the base station. This new technology is called dDAS, or digital distributed antenna systems.
With dDAS, the signal from the base station is output either as an analog signal (and converted to a digital format) or as digital signal directly to the dDAS platform. The dDAS platform then digitally replicates the signals from each channel or sector and simulcasts the signals across a group of microcells, creating a geographical grouping of microcells tied to one base station sector. The simulcast microcell assignments can be readily modified through a digital switching mechanism, allowing an operator to respond to changes in capacity requirements within the microcell network.
Multi-Access Distributed Antenna System
Core to this flexible approach is the ability to dynamically control system delay in the digital domain, rather than manually controlling delay by physically spooling fiber to equalize delays between nodes.
In contrast to analog repeater technologies, dDAS treats the radio access network as a dynamically interconnected system, managing the network at a system level, rather than as a collection of point-to-point circuits.
By centralizing base station capacity and being able to tailor capacity patterns, the amount of base station resources needed to cover any geographical area are greatly reduced. Trunking efficiency rises and telecoms costs fall. Maintenance, operations, and upgrade costs are reduced by having fewer resources, all in a single location.
In the near future, operators will be able to take advantage of the new breed of base stations that do not have integrated power amplifiers, further reducing the costs of the installed network. Software radio vendors have begun to take advantage of the feature set offered by dDAS platforms and are prototyping base stations with GigE digital interfaces.
Base station manufacturers have the opportunity to take advantage of the dDAS platforms with the planned introduction later this year of the software addressable simulcast switch. This allows the base station manufacturer's system to communicate to the dDAS system and shift capacity within the network on a dynamic basis.
There are obstacles to the broad based deployment of dDAS technology. The cost of fiber deployments in urban cores are high and fiber usage must be kept to a minimum. However, with newer digital transport technologies, a single fiber pair can support up to 128 nodes. Likewise, while dDAS technology does not require digital integration with a base station, the economics of a dDAS are hobbled with a sub-optimal analog interface.
dDAS technology may actually be the first true step toward a more intelligent RF network. dDAS technology has direct application to the macrocell world with the emergence of the remote radio head and likewise can be integrated into pico/inbuilding systems, providing seamless and dynamic coordination between the macro, micro, and pico segments of a robust network.
Like any new technology, dDAS is still in the early stages of deployments and the entire value of such systems have not been realized. The U.S. provides a good proving ground, given the unavoidable technology challenges faced by U.S. providers having to operate multiple protocols in multiple frequencies. While European operators have more monolithic networks, the easing of frequency restrictions predicted in the next few years should let the Europeans learn from the U.S. early adoption of dDAS technology.
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