Migrating to Cloud RAN Fronthaul

Escalating bandwidth demand in the mobile network due to the proliferation of smart devices is driving upgrades to the Radio Access Network (RAN). The cost to build, operate and upgrade the RAN is increasing, while the revenue is not growing at the same rate.

Power and equipment space are expensive resources at cell sites, so Mobile Network Operators (MNOs) are looking to reduce these ongoing expenditures and maximize Return on Investment by enabling more dynamic use of network resources. To reduce infrastructure costs, MNOs are migrating from expensive, high-power base stations to the Heterogeneous Network (HetNet) that includes Femto cells, Pico cells, Metro cells, Distributed Antenna Systems (DAS) and Cloud-based Radio Access Networks (C-RAN).

The traditional cellular network architecture is illustrated in Figure 1. The Backhaul RAN connects to the Base Station Transceivers (BTS) / nodeB with RF, Baseband and Transport Layer functions located at each cell tower. The BTS/nodeB is connected to the antennae with coax cabling.

Figure 1: Traditional Mobile Backhaul RAN Architecture

In the C-RAN architecture shown in Figure 2, the Base Band Unit (BBU) with Baseband and Transport Layer functionality is deployed with a Base Station Controller (BSC) at a consolidated location like a Central Office or BBU hotel.  The BBUs are connected to the Remote Radio Head (RRH) with fiber instead of coax, and this link is commonly referred to as fronthaul.  The fronthaul from the BBU to the RRH uses a Digital Radio over Fiber (D-RoF) protocol such as Common Public Radio Interface (CPRI) or Open Base Station Architecture Initiative (OBSAI). The RRH is collocated with the antenna at the top of the cell tower.

Figure 2: C-RAN Architecture with Fronthaul

The BBU functionality can also be consolidated at a nodeB macro tower, with fronthaul links distributed from the nodeB location to other macro towers in a hub and spoke or ring architecture.

Consolidating equipment at the BSC or at the central office lowers deployment costs and operations expenses by reducing the power requirements and the space constraints at the cell site. The collocated BBUs also simplify the X2 interface and improve the performance with no transmission delay among the BBUs. The C-RAN architecture also reduces the equipment costs at the DAS location.

CPRI Fronthaul over CWDM Products

CPRI over CWDM Fronthaul to Cell Towers

This C-RAN application example illustrates how to transport four Common Public Radio Interface (CPRI) channels over one fiber fronthaul link using Coarse Wavelength Division Multiplexing (CWDM).

Omnitron’s iConverter CWDM/X Multiplexers, CWDM/AD Add+Drop Multiplexers and xFF Transponders enable CWDM connectivity in a C-RAN over a fronthaul fiber link between Base Band Units (BBU) and Remote Radio Heads (RRH) located at two different cell towers.

The BBU is located at a BBU hotel or at a Central Office location. The fiber ports on line cards in the BBU support standard 1310 wavelengths.  Fiber patch cables connect the BBU line cards to iConverter xFF transponders installed in a high-density 19-Module Chassis.  The four xFF transponders convert the fiber with standard wavelengths to CWDM wavelengths (channels) with Small Form Pluggable (SFP) transceivers. Standard wavelength SFPs and CWDM wavelength SFPs are installed in each of the xFF transponders.  The CWDM SFPs support specific wavelengths to enable connectivity to the matching channel ports on the iConverter CWDM/X multiplexer with fiber patch cables (shown in different colors to represent the CWDM wavelengths).  Omnitron’s CWDM transceivers have color-coded latch handles for easy identification. The CWDM/X multiplexes the wavelengths that transport the four CPRI channels over the CWDM Common Fiber Line (fronthaul).

At the first cell tower, a 5-Module Chassis with a two-channel CWDM/AD Add+Drop Multiplexer and xFF Transponders is deployed. The CWDM/AD Add+Drop Multiplexer filters out the 1510nm and 1530nm CWDM channels for the CPRI data connecting to the Remote Radio Heads in the cell tower. Fiber patch cables (shown in purple and green to represent the CWDM wavelengths) connect the channel ports on the iConverter CWDM/AD multiplexer to iConverter xFF Transponders that convert the fiber with CWDM wavelengths back to standard 1310 wavelengths. The standard wavelength fiber connects to the two Remote Radio Heads at the cell tower.

The 1550nm and 1570nm CWDM channels pass through the Add+Drop MUX and travel over the CWDM Common Line to the second cell tower.

At the second cell tower, another 5-Module Chassis with a two-channel CWDM/AD Add+Drop Multiplexer and xFF Transponders is deployed. The CWDM/AD Add+Drop Multiplexer drop off the 1550nm and 1570nm CWDM channels and connect the CPRI data to the Remote Radio Heads at the cell tower.

NOTES:

• The two channels dropped off at each tower can be for different services. One channel can transport 3G, and the other channel can transport 4G/LTE to the tower.
• Up to eight CWDM channels can be transported over a single-fiber CWDM fronthaul link (or sixteen channels over dual fiber) using iConverter CWDM/X multiplexers.
• Remote Radio Heads support multiple fiber ports, enabling Wireless Carriers to connect multiple channels to each RRH at the tower.
• For rural applications where coverage is more important than capacity, iConverter 2-Module Chassis can be deployed at each tower with a one-channel CWDM/AD add drop multiplexer and an xFF transponder to drop off one CPRI channel.