Telecoms.com periodically invites expert third parties to share their views on the industry’s most pressing issues. In this piece Douglas Rankin, VP Cell Site Solutions, Europe at CommScope, looks some contemporary options for connecting remote radio units.

Surging demand for mobile data has made network modernisation a priority for network operators. To meet the demand while lowering costs, more operators are now deploying remote radio units (RRU) at the top of the tower. The debate over the best way to connect the RRUs to the DC power supply and baseband unit, however, is still up in the air.

For years, the common practice has been to use a separate “home run” of fibre and power cables for each RRU. This traditional discrete design is familiar to installers and uses standardised components. But adding at least two cables to each RRU—on towers already heavily loaded with existing cables—can be complex and time consuming. Cabling a three-sector site, with two RRUs per sector, can take a full working day.

The hybrid fibre feed approach

Over the past several years, an alternative to traditional discrete cabling has been growing in popularity. Hybrid cable combines power and fibre in a single ruggedised cable. The transmission lines are protected by corrugated aluminium armour, water-blocking elements and a tough outer jacket. Long used in outside-plant telco applications, hybrid cable can be configured and deployed to support a variety of applications, including fibre-to-the-antenna (FTTA).

Hybrid direct: In the hybrid direct model, multiple fibre and power cables are aggregated into a single large trunk cable that runs up the tower. The trunk cable terminates just below the RRUs with a preconnected breakout canister. From here, individual jumpers connect the appropriate fibre/power pair in the trunk cable to the corresponding RRU. Each hybrid direct trunk cable can carry enough fibre and power connections to support up to nine RRUs, making it well suited for today’s high-density sites. In this configuration, only three hybrid direct cables are required to feed as many as 27 RRUs, minimising installation complexity and conserving tower space.

For lower density sites, a second hybrid solution is available than can support three to six RRUs.

Hybrid sector: The hybrid sector option is similar to traditional discrete cabling except that it requires only one cable per RRU instead of two. Individual HFF cables—each containing power and fibre for one RRU—are deployed directly from the baseband unit to the RRUs. Compared to the traditional discrete FTTA solution, the hybrid sector cabling enables installers to cut the installation time nearly in half while offering a comparable price point.

Putting assumptions to the test

Hybrid cable has been gaining traction lately as operators look for new ways to reduce deployment time and costs. Using half as many cable runs should provide significant savings in deployment time. But, until recently, this assumption had yet to be verified or quantified.

In December 2015, productivity researchers from Scott-Grant Ltd conducted a time study, comparing the time required to cable a typical three-sector cell site using the hybrid direct, hybrid sector, and traditional discrete cabling models. The site selected was an existing tower located approximately 28 miles east of London. The tower had three sectors and was 27 metres (88 feet) tall with one passive antenna and two RRUs per sector.

From start to finish, the hybrid direct method was approximately 55 percent faster than the traditional discrete model, while the hybrid sector method was about 48 percent faster.

By extrapolating the time savings across hundreds or thousands of sites, the financial impact can be quite significant. In the UK, for example, using the HFF direct model instead of the traditional discrete model to cable a 500-site expansion project could save 1,442 hours—equal to 180 eight-hour days. Factoring in the local labour rates and availability of qualified installers, this can lower overall installation cost by about 25 percent.

These results also support the theory that, for lower density installations up to six RRUs, the hybrid sector solution provides a more cost- and time-efficient approach. For higher density sites involving more than six RRUs, the cost and time benefits of the hybrid direct approach increase as the number of RRUs increases.

Benefits beyond time 

As mentioned earlier, the hybrid direct solution provides benefits beyond installation time. Based on the site requirements, these benefits—regarding site sharing and scalability—can often outweigh the higher purchase price.

If one considers the design of the HFF direct cable, the applications for site and infrastructure sharing, commonly mandated in markets across Europe, become apparent. With the traditional point-to-point discrete cable, each fibre is dedicated to the RRU of a specific operator—eliminating the possibility of sharing. In the HFF direct cable, each fibre is still dedicated to a specific RRU, but the trunk configuration enables the installation of up to nine RRUs. Therefore, on a three-sector site, one cable can support three operators, each using one air interface.

Site sharing, however, is not limited to cell towers. Across Europe, an increasing number of government agencies are handing management of their in-building networks over to private sector companies. The hybrid direct cable enables management companies to provision multiple agency networks using a common trunk. The trunk cable terminates at a rooftop junction box. From here, individual jumpers are used to feed the separate networks.

Network managers can also use the hybrid technology to build in excess capacity by loading more fibres in the trunk than initially needed. This allows them to bring new antennas and technologies on line as needed, by simply running individual jumpers from the breakout connector to the RF device. At a time when many operators have already mapped out future spectrum upgrades and technology migrations, the ability to pre-provision the network in anticipation of new antennas can significantly accelerate their time to market.

When compared to the traditional discrete model, both the hybrid sector and the hybrid direct solutions offer savings on long-term OpEx. By reducing the number of cables needed—along with the associated hangers, clips and other supporting structures—network managers are able to decrease the tower load. These savings can translate to lower lease costs or enable more equipment to be added to the tower.

At the same time, both the hybrid direct and hybrid sector solutions minimise the number of delicate discrete fibre cables exposed to environmental threats like wind, ice, animals and installer traffic. This can have a substantial impact on the network’s productivity. For example: a network of 1,000 towers—each with four antennas and 12 RRUs—requires 24,000 individual fibre cables. A 5 percent failure rate in the connections would translate to 1,200 incidences of RRUs going offline during the year.

Conclusion

In the past several years, the wireless industry has seen a significant shift in cell site architecture. Perhaps one of the most important has been the relocation of the radio from the bottom of the tower to the top. It has helped pave the way for more progressive concepts, including the move to a centralised radio access network (C-RAN).

At the same time, the mass deployment of RRUs has created challenges regarding the best way to get the fibre and power to the top of the tower. Currently, there are two popular technologies—traditional discrete cabling and the hybrid approach, featuring hybrid sector and hybrid direct. For years, purchase decisions regarding most types of cable, FFTA included, have hinged on price. However, there are a number of important variables beyond price that will significantly affect the total cost of installation.

For sites with multiple layers, the time savings, wind loading and projected RRU fail rates due to poor fibre connections both hybrid approaches have significant advantages. As the number of RRUs increases, the time and cost advantages increase as well. Still, the transition from traditional discrete to hybrid will take time.

Much of the hesitancy of site managers to switch to the hybrid cable solution has to do with the simple fact that they’ve grown familiar with the current traditional discrete solution. Their installation scheduling and budgeting systems are built around the longer installation times. But persistent pressure to increase capacity, reduce cost and accelerate time to market makes it hard to ignore any promising alternative.

Douglas has been responsible for CommScope’s wireless product and service sales across Europe for the past six years. He has over 20 years of experience in the telecoms industry, 15 of which were spent in senior strategic roles in the EMEA region for Motorola Networks.