For years, US mobile carriers have been moving their networks to a more centralized architecture that saves on both capital and recurring costs. The CRAN (centralized radio access network) approach consists of aggregating base station base band units (BBUs) in centrally located facilities. From here, capacity and coverage are brought to venues in the area over fiber optic CPRI connections to remote radio heads (RRHs) in those facilities.
This architecture requires a distribution of GPS signals to the BBUs to provide network timing and sync. With some manufacturers, every BBU requires a GPS connection. Some, such as Nokia, can slave the sync signal for two or three BBUs from one BBU with a GPS connection. Because of this, smaller CRAN hubs may need only, say, 16 GPS connections which can easily be provided by an active coaxial GPS splitter.
However, with the commitment to deploy 5G with its high density requirement, the number of BBUs at the CRAN hub has jumped. Sites that needed 16 GPS connections now require over 100 GPS connections which is impractical for a distribution of heavy coaxial cables.
Optical Zonu Corp, whose GPS Fiber Transport is the only fiber solution approved by all US wireless service providers, offers a low profile GPS distribution solution that utilizes easily routed fiber optic cables and short, lightweight RF jumpers. Often, cellular carriers will deploy a CRAN hub in an existing wireline switching center. These facilities have strict rules regarding cable penetrations and routing and will not permit a massive coaxial distribution. Optical Zonu’s GPS Fiber Transport meets these requirements since the components are compact and are connected using only lightweight fiber cable.
For added reliability, most carriers deploy the auto-redundant configuration. A single, compact fiber optic antenna unit connects to two GPS antennas on the roof. Using WDM, both channels are transmitted over a single fiber to a rack mounted passive optical splitter which can be up to a 1×8 split. These outputs are then routed to fiber optic base units with 8 or 16 RF outputs. Each base unit internally selects one path as the primary and switches to the secondary if there is any alarm on the primary. This system is easily scalable and supports up to 128 GPS connections.
Note that this same architecture applies to neutral host DAS sites. The lightweight fiber cable makes it easy to route the GPS to racks dedicated to each carrier.
By Gary Grimes, Director of Sales