Carriers are under constant pressure to improve network capacity and enhance network speed all whilst lowering deployment and operating costs. Keeping costs in check is a top priority for mobile network operators as they densify their networks. But small cells on city streets often require a new, and potentially expensive network element: fiber fronthaul.
Fronthaul refers to the link between a centrally located base band unit (BBU) and the radio at a remote cell site. This network architecture is called CRAN, or a centralized radio access network, because a centralized BBU pool supports multiple cell sites. The majority of fronthaul links are fiber because fiber offers more bandwidth, capacity and lower latency than microwave.
Wave division multiplexing (WDM) is one of the enabling technologies for CRAN. WDM is a way to use a single strand of fiber to transmit multiple optical signals by using different wavelengths.
“It actually breaks them down into individual wavelengths or colors within the spectrum,” explained Wes Oxlee, Director of Wireless Connectivity Solutions at CommScope. Oxlee said WDM can reduce small cell deployment costs significantly for mobile operators. “If you imagine a carrier already has a fiber network running down a street, they may already have one or two strands of fiber in there,” he said. With WDM, one or two strands can suffice, because each strand can carry multiple channels. Oxlee said C-RAN deployments typically require 4-6 channels at each at each small cell location, and without WDM, carriers will need to consider installing new fiber cables.
“For the carriers to go along and install a brand new cable is an expensive and time consuming process,” said Oxlee. “They need to get permits, they may have to dig up a street to install new pathways and conduit and cable. No matter what, installing new cable is slow and very expensive.” WDM is a much faster and less expensive alternative, Oxlee said. “They can install a passive WDM device … they simply plug into their network and it’s an instant fix for their fiber shortage.”
While there are multiple types of WDMs, the two most commonly used for mobile fronthaul are C-WDM and D-WDM. C-WDM refers to coarse wave division multiplexing and D-WDM refers to dense wave division multiplexing. Oxlee said the devices that enable these two solutions run on different channels, but they are technically very similar.
Oxlee said that with a passive WDM device, one strand of fiber can transmit up to 18 channels using CWDM and 40 channels on DWDM.
“Commscope offers WDM solutions in a wide variety of configurations and packages that allow a carrier to seamlessly integrate to them into all parts of their networks, whether that be in a central office environment, a remote cabinet at a base station or within an external plant closure at the cell site that allows for hardened plug n play connectivity to the radio,” Oxlee said. “The case for WDM is compelling. … Carriers are able to fully utilize their investment in their fiber network by using WDM.”
