A logical transition
We rely extensively on mobile networks in our daily life: to accomplish higher business efficiency, better communication with family and friends, for entertainment … and the list grows with our ability to innovate. This “everywhere and all the time” mobile networks model for a growing user base from 4.5 billion today to 7 billion by 2015, according to Infonetics, is not without its challenges. Operators are put to the test to deliver the full potential promised by mobile network technology. And, this is just the beginning; the evolution of smartphones, LTE and LTE-Advanced is rapidly expanding the technology horizon. Just as 3G has gained momentum, a new player promised 100-times higher data speeds with 4G using LTE and WiMAX technologies. The reality of the technology revolution, converging voice and data, puts network capacity in question. Third-generation and 4G networks are leading another change … bringing optical fiber closer to the edge of wireless networks.
Radio access network infrastructure plays an important role in supporting the mobile voice and data networks. When one thinks of radio access network infrastructure, the coaxial cable between base station and antenna usually comes to mind. However, mobile technology is a dynamic environment and what was once an ideal solution begins to show its limitations. Experts continue to investigate the different methods of maximizing the capacity and bandwidth, while minimizing the losses and power consumption. Progressing from 2G to 3G to 4G means adding new technologies to existing infrastructures such as towers and rooftops. This article explores the transition from coax to fiber in a wireless environment and its benefit for the wireless service provider, equipment manufacturer as well as the tower operator.
A future with coaxial cable?
Coaxial cable truly has helped enable the mobile network. Nevertheless, as 4G rollouts proceed, wireless service providers, equipment manufacturers and tower operators begin to worry. Wireless service providers are concerned about the reliability of coverage and network bandwidth to maintain the trust of their mobile subscribers. Equipment manufacturers are required to match the expected technology benefits with the outcome of their radio equipment. Tower operators are apprehensive because there are physical constraints to structural upgrades in the field and the number of operators that towers can support (Figure 1). Here is where the network infrastructure enters the game. Traditionally, coaxial cables have been the right fit for radio frequency signal transmission. However, to sustain this dynamic network, one starts to reconsider the traditional method.
The objectives of a good mobile network design today is to minimize capital expenditures and operating expenditures, weight and wind load on towers, power consumption for equipments and most importantly, maximize bandwidth while achieving the maximum reliability of the network. The choice of the active and passive infrastructure becomes an important deciding point.
Coaxial cables suffer from feed line losses (typically 3 dB) as a function of distance between the base station and the radio equipment on the towers. Higher frequency signals attenuate more than lower frequencies over the same distance. The signal needs to be amplified to achieve the coverage required. Additionally, high data speed is the requirement for 3G and 4G networks where networks are already experiencing congestion.
Optical cable has already gained widespread acceptance in mobile backhaul applications to relieve congestion up to the base station. Now, with remote radio head technology also gaining acceptance, fiber is taking the next logical step by carrying mobile traffic up/down the tower. With the changing requirements of modern wireless networks and the increasing use of fiber in this space, connectivity both to and within the cell site is effectively being redefined.
Figure 1: Coaxial cable: Structural challenges to expansion.
Evolution to fiber-optic infrastructure
The redefinition of connectivity at the cell site is driven by physical, financial and performance objectives. It is enabled by RHH technology. Whereas the traditional base station contained all functions – control, radio, power and backhaul connections, the new system carves out the radio function and moves it to the tower/rooftop edge in the proximity of the antennas. For these next generation mobile data-network architectures where the base station units are now distributed radio frequency units, fiber optics becomes the definite choice between the base station and the RRH. Fiber optic cables possess the unique ability to provide virtually unlimited bandwidth available all the way up to equipment placed on the tower. As the RRH and the antenna are in each other’s vicinity, only a very short coaxial cable is used to connect between the RRH and the antenna. More recently, the antenna and the RRH come as integrated units. These types of antennas are called the active antennae that further reduce the coaxial links.
Figure 2: Fiber to the antenna schematic.
Not only does the above architecture support the highest bandwidths, but it also facilitates reduced power consumption (low loss, less cooling at the base station). Governments impose regulations on wireless service providers and equipment vendors to reduce their energy consumption footprint. This number varies from country to country. One example is that “energy consumption” features in Europe 2020’s (a 10-year strategy proposed by the European Commission for smart, sustainable, and inclusive economy) five target objectives. The target is to decrease energy consumption 20% by 2020. Using fiber optic cables in the base station to antenna link alone saves power consumption of up to 50%. Less power consumption, in turn, provides large savings on operating costs for the operators.
Fiber to the antenna
Fiber to the antenna indicates a link from the base station to the RRH on the tower or rooftop. The use of fiber to reach the antenna is rapidly gaining acceptance by the wireless operators and is supported by the equipment manufactures. They are assured that their uplink bandwidth requirement is met using the high-speed data fiber optic link. However, this new decentralized approach that improves coverage and the bandwidth becomes a concern for the tower infrastructure managers. Typically, more antennas and new tower-top electronics (e.g., RRHs) are needed to support the new technologies on the towers. This generates several physical constraints. Space is one of them; weight of cables and equipment, and wind load among others. One may ask why these constraints are significant. Better coverage and bandwidth optimization requires new cell sites. New cell sites could prove to be very expensive for the wireless service provider. To control costs, some existing sites are being reused. These sites are quickly populated and with further technology upgrades, more and more equipment is added to the existing infrastructure.
Figure 3: Fiber installation on a tower (small and lightweight).
In a “day one” 4G deployment, for example, an average of three to six RRHs is mounted on a tower to sustain a single operator’s network requirement. The trend of cell site sharing is also increasing significantly in some parts of the world. In a traditional installation, each one of the antennas on the tower was connected with a separate coaxial cable. Coaxial cables are inherently heavy and bulky. Despite being shielded, interference can occur with nearby devices. Deploying FTTA for the same setup reduces the weight on the tower to a fourth compared to the coaxial solution (voax “one-and-five-eighth-inch” with Corning Cable Systems’ FTTA solution and Equipment vendor’s RRH). Tower operators consider wind load on structures such as towers as a vital metric. Wind load varies geographically and by season and it is defined as the force that wind puts on the structure. In FTTA solutions, the size of the cable and the number of cables are drastically reduced (Figure 3). Hence, the wind load is diminished by a factor of 1.5- to 2-times depending on the tower height. Additionally FTTA compared to coaxial cables is faster and easier to install. Factory-terminated and tested assemblies avoid the need to install connectors in the field. As displayed in figure two, today’s infrastructure uses terminal-based architecture, in which optical connection ports on the terminal are placed on the top of a cell site. Cable assemblies to each radio head make the final link. The new architecture using RRH as an enabler, addresses the weight, the wind load and space with a small lightweight optical cable for connectivity to the base station.
An all-fiber solution: What does it mean?
So as you are streaming your favorite movie on your iPhone, or perhaps playing a game with over a thousand people in your neighborhood, while simultaneously investing in stocks on your other smart phone and making business decisions on your voice network, you are probably enjoying the magic of light that the fiber optic cable infrastructure is providing. For the wireless service provider, this means a happy customer and many more subscribers in the future at lower capex and opex costs. For the equipment manufacturer, this confirms the optimum usage of the technology. Finally, for the tower infrastructure manager, it means easy and fast installation, low troubleshooting and better cable management. Fiber to the antenna is the next-generation passive infrastructure that allows mobile users to benefit from the full data and voice network for the coming years.
