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In the new architecture, the radio electronics are typically mounted as close to the antenna as physically possible in what are called remote radio units (RRUs). The RRUs are connected through fiber optic cable to the baseband electronics, which are still in a shelter on the ground. This new architecture minimizes the power and signal loss that existed in the old architecture that relied on coaxial cables. But change is hard and real-world deployment challenges are often present along the way.
In perhaps as many as half of RRU installations worldwide, the RRU remains mounted at the bottom of the tower and the transmission coaxial cable is still deployed. These cases are usually driven by lack of available real estate at the top of the tower or other local economic factors including leasing practices. The use of lower band transmission frequencies can also make mounting at the tower bottom the preferred method.
When RRUs are deployed near the antennas at the tower top, fiber optic cable connects to the baseband as described. Typically, two fibers, transmit and receive, are used with every RRU. In a few cases, operators use multiple fiber pairs when “multi-mode” RRUs are deployed. In addition to the fiber, a power cable must be run to the RRU. Fiber and power can be run separately or combined within a sheath, but both must be delivered to the RRU.
The power required is not trivial. Each RRU typically requires 300 watts of delivered power and some higher power units require up to 1,000 watts. The industry trend for wattage of macro radio sites is trending upward to service and improve coverage and data rates. RRU power levels are only going to continue increasing for macro sites, just as the complexity of radios and subscriber data demands continue to rise.
The power cable presents another specific challenge to successful RRU deployments. The direct current voltage typically supplied to the site is usually at the legacy voltage for the shelter, -48 VDC. It is important to note that market trends show that the power required in the new architecture can be much higher. The increasing power levels impact the “voltage drop,” which is a buffer between the cutoff voltage and the minimum voltage.
In order to address this engineering challenge, the power cable conductor size must increase. However, bigger sized power cables create practical issues for operators in terms of material costs, more weight on the towers and increased difficulty in handling during installation. The race is on to find more creative and efficient solutions for powering RRU deployments in the wireless industry.
If you have any questions or comments about the challenges of efficiently powering a cell site, leave a comment below, and I will be sure to respond.