Optical networks are commonly used in metro and core networks. However, optical networks are also now being considered for use in radio access networks, as mobile traffic demand increases and the structure of radio access networks is changing.
It is proposed that each radio access network (RAN) comprise a plurality of remote radio units (RRUs), distributed geographically, and a pool of baseband processing units, located at a centralised location, for example at a data centre a few kilometers away from each of the RRUs. This arrangement has the advantage that it allows the use of computational resources to be optimised. Optical networking, and in particular wavelength-division-multiplexing (WDM) optical networking, is considered a very promising technology for connecting the RRUs to the baseband processing unit pool.
However, a barrier to using WDM optical networks in radio access networks is that WDM networks use optical switches based on Wavelength Selective Switching devices (WSSs), which are based on complex free space optics. This means that WSS switches are costly, have a large spatial footprint and are difficult to mass produce. These are critical factors for radio access networks, which have lower throughput and shorter inter-node distances than metro or core networks.
An alternative to using WSS devices is to use fixed wavelength add drop devices (FOADM). A FOADM may simply comprise a cascade of thin film filters. However, use of this type of optical device has the disadvantage that complex inventory handling is required, since a FOADM with a different combination of add drop wavelengths is required at each node. Further, since FOADMs are passive devices they cannot be automatically reconfigured for example in the event of an optical fibre cut to provide restoration of the optical network.
Optical switches based on optical ring resonators have also been proposed for use in optical networks. For example, an optical switch using optical ring resonators is described in A. W. Poon, L. Xianshu, X. Fang and C. Hui “Cascaded Microresonator-Based Matrix Switch for Silicon On-Chip Optical Interconnection”, IEEE Proceeding 97, 1216-1238 (2009). The use of these optical switches, which may be manufactured using silicon photonics, has the advantage that these devices may have lower power consumption, higher minaturisation, and may be easier to mass produce at lower cost than other types of optical switch.