Passive Front- and Backhauling

While fibers are mostly being deployed in the backhaul networks, a new approach of building flexible mobile networks is being pushed forward where fiber is also used from the base station to the antenna, which is called fronthaul. Traditionally, the baseband unit (BBU) and the remote radio head (RRH) are collocated inside a cabinet close to the antenna and a coax cable is used to connect the RRH to the antenna located at the top of the cell site. With migration to fiber based connection the RRH is placed close to the antenna at the top of the cell site and connected to the BBU using the common public radio interface (CPRI) or open base station architecture initiative (OBSAI) protocol. Fiber overcomes the loss of coax and also excess energy wasted by heating of the copper.

Centralised fronthaul network
Due to the possibility of longer distances, one can design the fronthaul network with centrally located base station at a central office location equipped with a number of baseband equipment for several base stations. While the connection between the each RRH and BBU can be deployed with a dedicated fiber, the most efficient way would be via the employment of WDM over a single fiber.

An active solution in fronthaul would require the signal synchronisation to be transferred transparently. Additionally, space and power limitation would dominate the design of the active system based network. Passive WDM on the other hand provides transparent low latency solution. Network architecture can be optimised as per requirement employing single fiber or fiber pair, CWDM or DWDM, point-to-point or ring architectures with scalable capacity up to 88 wavelengths. Additionally, add/drop multiplexers make them flexible with respect to locations.

Increase capacity per fiber
Coloured transceivers are used directly in the RRH to provide the necessary WDM wavelength signal. A WDM unit is then used to multiplex multiple services into the same fiber to increase capacity per fiber and reduce the number of fibers to be deployed. WDM based fronthauling is totally passive requiring no power, thus does not generate extra OPEX. It is extremely compact and can support a distance up to 80 km. Due to this distance advantage, the BBUs can be collocated to form a centralised BBU. This aids in easy maintenance at the single location and provides improved security (no cabinets to break into). In LTE networks, the collocation of BBUs simplifies the X2 interface and also increases security over the BBU to RRH link.

As previously mentioned, traditionally, the BBU and the RRH (or RRU- remote radio unit) are collocated inside a cabinet close to the antenna. The interconnection between the BBU and the core network is called the mobile backhaul network. A coloured transceiver is in this case connected to the BBU and CWDM or DWDM can be deployed to connect the signal to the central offices passively. DWDM can carry 80 wavelengths (for C band), that can be extended to 80 more channels when considering the L band. In the case of central base station, since the base station and the central office are collocated, no backhauling network will be required.  

The Passive Advantage
As the mobile backhaul connection by fiber is predicted to reach 42% by 2016, it is important to select the technology that promises a good return on investment. The passive backhaul and fronthaul technology enjoys the following advantages,

• Economic advantage with lower CAPEX (50% compared to active) and lower OPEX (support/maintenance, site rental and energy conservation).
• Lower latency improving the maximum allowable distance.
• Requires fewer resources w.r.t. space, energy, cooling with centrally located BBUs.
• Robust and resilient for outside plant application.
• Complete transparency to carrier services, i.e., independent of transport, migration and simple to long term changes.
• Several operators can share the same fiber while remaining independent of each other.