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Traditional coaxial-based systems on cell towers are rapidly being replaced with fiber-to-the-antenna architectures. FTTA can help mobile service providers significantly increase the capacity of their radio access networks.
Proper fiber handling techniques are essential to ensure a robust installation and reliable FTTA network performance. The following recommended best practices can help contractors and installers hone their fiber-optic handling skills.
FTTA fundamentals
Cabling architecture: FTTA cabling architectures differ, but they all generally consist of a base band unit at the bottom of the tower and multiple fiber-fed remote radio units at the top. Standard half-inch coaxial jumpers connect the RRU to its antennas. Fiber and power cables run up the tower to the RRUs, which support advanced technologies, such as LTE/4G. These services often call for antennas to be multiple in multiple out, requiring two or more feeds per antenna.
Generally, FTTA architectures use either point-to-point or multiple-element (trunked) cabling configurations. Multiple-element topologies provide greater flexibility for future enhancements, and new solutions in tower-mounted terminals have emerged to support these architectures. In addition, new hybrid fiber/power constructions bundle all the cabling elements required for a functional FTTA system into a single cable. Regardless of the type of architecture, the basic principles of proper fiber handling remain the same.
Fiber handling recommendations
Bend radius: When installing fiber cables, tower hands should heed the manufacturer-recommended minimum bend radius. Standard jacketed fiber can maintain a minimum bend radius of about 15-times the cable diameter. Failure to observe the recommended bend radius can stress the fiber optic core, resulting in signal loss.
Tensile strength and support: Fiber optic cable has a tensile load rating that defines the maximum vertical rise the cable can withstand without additional support. The maximum vertical rise can be calculated using the formula:
Maximum vertical rise = (1/2 x maximum long-term tensile load)/cable weight)
Using this formula, a 24-fiber cable can be suspended approximately 1,700 feet.
Hoisting: For pre-terminated fiber cable, a pulling sock is used to provide the hoisting grip, transmit the tensile force to the cable, and protect the fiber connectors during transit up the tower. Never pull a pre-terminated fiber cable up by the connectors.
Tower clamps: Fiber optic cable manufacturers recommend clamping the cable every three to five feet on monopole exterior, self-support and guyed towers to prevent the cable from moving in the wind. Stainless steel or nylon cable ties are not recommended. Rigidly anchoring the cable subjects it to shear stresses from the compression of the cable tie and the vibration of the tower structure. Instead, fiber cable hangers that isolate the fiber cable from the tower and permit some vertical motion are recommended.
Fiber optic connectors: The most common type of fiber optic connector in FTTA installations is the LC connector, characterized by its small size and positive latching feature. It commonly appears in a duplex configuration with two connectors mated or snapped together.
Fiber optic connectors can be pre-terminated in the factory or field-terminated during installation. Pre-terminated cable assemblies have emerged as the standard industry solution. However, using pre-terminated cables poses potentially costly challenges:
–Delayed installation if the correct cable length is unavailable in the field.
–Discarded cable in the event of connector damage.
–Inventory cost of equipping each installer with multiple different cable lengths.
As a result, radio network equipment OEMs and installers are beginning to consider field-mount fiber connectors. Modern field-mount fiber connectors are quick and easy to install with minimal tools and training, and their operational life and performance closely match that of factory-terminated connectors.
Connector cleaning: Fiber optic connectors are equipped with protective dust caps. Always leave the dust cap in place until the connector is to be mated to the tower mounted terminal or RRU. If the factory packaging or dust cap has been removed prior to mating, clean the connector ferrule before inserting it into the RRU. A self-contained ferrule cleaning device, such as the CLETOP-S from NTT, makes cleaning on the tower top quick and easy.
Hybrid cable: Hybrid, or composite, cable is being used more and more on towers. It contains both fiber and power conductors. Hybrid cables can be arranged for single, triple or six-RRU feeds. Generally, the elements inside the hybrid construction are individually jacketed. The jacketed cables are covered by a protective corrugation overlaid with a polymer jacketing material. The over-jacket and corrugation must be removed prior to installation at the RRU to permit access to the interior elements. Aside from using great care in the armor removal process, tower hands should avoid allowing the fibers to drape over the exposed edge of the cut corrugation. Over time, this can lead to fatigue and failure of the fiber optic cable elements.
Slack storage: Excess fiber must be stored in a fashion that respects minimum bend radius and provides sufficient support to stabilize motion. Use of cable ties to secure fiber slack is not recommended. A slack storage bracket or enclosure should be used.
Repair and testing
Fusion vs. mechanical splicing: Damage to fiber optic connectors is unfortunately common in FTTA installations. If a fiber connector is damaged, either a new cable can be pulled or the damaged connector can be replaced.
Fusion splicing is not recommended for connector replacement because the required equipment is expensive and requires a power source, making tower-top repairs difficult. Moreover, the tower hand must be trained in fusion splicing techniques.
A field-mount solution with an internal mechanical splice offers an alternative method for repairing a broken fiber connector. Field-mount connectors with a mechanical splice are well-suited for FTTA repairs. They can be installed quickly and easily using an inexpensive hand tool.
Testing: FTTA systems are commonly tested for insertion loss and return loss. Both are measured in decibels. Insertion loss measures how much light signal is lost as it travels through the fiber optic cable. Return loss measures how much light is reflected due to impairments (microbends, macrobends, damaged or improperly installed connectors, etc.) in the fiber. An optical source and power meter measures insertion loss, and an optical time domain reflectometer locates sources of return loss.
Conclusion
Tower hands who are knowledgeable and skilled in fiber installation, testing and repair are essential for the construction and maintenance of FTTA architectures. By following the best practices discussed above, tower hands can bring their knowledge of fiber installation and handling up-to-speed for today’s mobile networks.
Stephen C. King brings more than 28 years of industry experience to his role as applications engineer for the wireless business in the 3M Communication Markets Division. King is responsible for numerous products successfully commercialized by 3M and has been recognized with 3M corporate awards such as the Technical Circle of Excellence and Golden Step Award. He holds a Bachelor of Science degree in Electrical Engineering from The University of Texas and is a Licensed Professional Engineer.
