By Ted Abrams…
Fronthaul is easy to say, simple to hear. Eager to find a catch phrase, a shortcut to express the latest news about the magic of wireless infrastructure, writers latched onto the term fronthaul. A popular use of the term describes common public radio interface (CPRI) links connecting remote radio units (RRU) with a baseband processing unit (BBU).
That CPRI-related definition of fronthaul is widespread, on track for publication in future lexicons of telecom terms. However, the “graybeards” of wireless infrastructure know the term from prior use, with another meaning. A closer look at some of the technology will reveal that earlier meaning and will equip wireless professionals to building the reliable networks of the future.
Base radios are stationary, functioning to exchange signals with mobile radios embedded in wireless devices. Adjacent circuits of the base radio formerly resided in the same cabinet. Now, those radio circuits can be physically separated, arranged into several pieces (split-architecture). Moreover, an extension cord of significant length can join those pieces.
CPRI is that extension cord between radio circuits that permits flexible location of the parts. In a different place, on different real estate, each part of the radio can be located at a position chosen to be the most efficient – for license rights, energy cost, space rent, etc. Split-architecture is a remarkable improvement in radio design; standardizing the specification for the extension cord to be CPRI, is pure genius.
Before split-architecture, the parts of the integrated base transceiver system (BTS) were inseparable, connecting through factory-built wiring. That wiring, or harness, an FCC type-certified bundle of cables and plugs that restricts distance between radio parts, is sacrosanct. Field adjustment of harness length would distort radio function and violate FCC rules. From the base radio to the antenna is a large gap where distance is constrained by physics of the feedline. Larry Fischer and Phil Wala solved the feedline constraint problem with digital technology. Other wireless leaders such as David Cutrer and John Georges figured out diverse solutions for the same purpose, to span the gap between antenna and radio.
A system of circuitry and physical media that spans the gap is often a DAS. Where the radio includes a new style RRU or where the radio is a monolithic BTS, fronthaul that spans the gap between radio and antennas can be an excellent solution, especially in those contractual circumstances that favor multi-operator sharing of real estate, with cabling along the same path.
For a DAS, digital devices (TE Connectivity and recently Dali) or analog devices (e.g. SOLiD, Corning, CommScope) at remote locations and head-end locations may be purchased by the host. Those DAS devices connect with radio equipment (RRU or BTS) owned by operators. Depending upon length, fronthaul media for a DAS can be coaxial cable, fiber optic cable, twisted pair copper, free-space optics, microwave, mmwave, and as proven by demonstrations at Cisco, something so simple as strands of a barbed wire fence.
CPRI cabling between RRUs and BBU looks like regular fiber optic network cabling. Inside the cable, at the functional level, the similarity disappears. A CPRI link operates at an equivalent of 2.5 Gigabits per second, requiring dark fiber or at least 10 GigE bandwidth. CPRI is an extremely powerful link that can be used with radio building blocks to create a virtual machine with massive wireless service capability. The potential benefit to network performance from CPRI extension to distant RRUs is immense and unmatched by any previous network design. Principles that are baked into Sprint’s network vision, prudent guidance of FirstNet, and DISH spectrum plans including their H-Block play will drive increased use of split-architecture radio technology with CPRI.
Feasible business models for multi-operator DAS fronthaul are well established and thousands of successful projects are operating. Business models for RRU placement miles away from the BBU are in development. At present, the disconnect between operators and infrastructure developers regarding price points for cabling and real estate is slowing deployment of fronthaul in the CPRI context.
Society benefits from fronthaul for many reasons. To name a few, fronthaul reduces the amount of energy consumed for air transport, increases the speed of mobile devices and makes mobile devices operational at more locations. Fronthaul in context with CPRI joins circuits of the radio; fronthaul in context with DAS spans the gap from the radio to antennas. In every case, customers depend upon infrastructure professionals to translate these ideas into reliable networks.
Ted Abrams, P.E., is the principal of Abrams Wireless, Inc. (AWI), which serves clients seeking strategies for profit through wireless infrastructure.