Due to the explosive growth of mobile data, network operators are facing tremendous challenges in providing capacity to cell sites. This article addresses these challenges, discussing alternative architectures and their relative merits. Specifically, it demonstrates how Coarse Wave Division Multiplexing (CWDM) can quickly and economically meet capacity needs and allow capacity for growth.

Breaking Down the Challenge in Architectural Planning and Design
When planning and designing capacity for cell sites, it is important to carefully consider architectural alternatives and how the architecture selected supports capacity growth in the long-term. In the outside plant (OSP) network, typically there is a feeder cable from the central office (CO) to a remote terminal (RT) near the cell site, as shown in Figure 1. The route is often several miles long with a limited number of fibers. There may be fiber or copper (shown) between the RT and the cell site.

The type of services planned for the cell site is one consideration for selecting the architecture. Wireless 2G and some 3G services require less bandwidth and use a few DS-1s at each cell site. These services can be supported with fiber feeding a copper extension as shown in Figure 1. However, 4G and LTE services are much more bandwidth-intensive and require fiber all the way to the cell site, as shown in Figure 2.

In this example, new fiber has to be installed in The Last Mile to the cell site and in the drops to the wireless service providers (WSPs). Network operators typically install larger fiber cables when replacing copper cables to allow for future growth. This replacement of the copper cable provides adequate capacity from the RT to the cell site. However, the feeder cable is undersized and needs reinforcing and is referred to as the critical section. If the feeder cable extends a significant distance it will be very expensive to reinforce. In this example, it might cost $100k or more (assuming an installed cost of $20k/mile) to reinforce the 5-mile critical section.

Figure 1. Typical fiber-to-the-cell-site (FTTCS) network.

Figure 2. New fiber to the cell site is needed for 4G and LTE services.

Breaking the Cycle With Alternatives: Service Multiplexer or CDWM
There are a couple of alternatives to adding new fiber in the critical section. One alternative is to place a service multiplexer in the CO and RT. (See Figure 3.) This increases the capacity on the existing fiber by increasing the bit rate and requires only 2 fibers out of the feeder cable.

Figure 3. Using a service multiplexer on the Critical Section.

The service multiplexer aggregates the signals of the smaller multiplexers used by wireless providers housed inside their enclosures at the base of the cell tower. The smaller multiplexers might be DS-1, SONET, Ethernet or a combination of all. This solution has several requirements:
• A RT to house the service multiplexer, or the timely acquisition of an easement if a RT doesn’t already exist, or has no additional room for the new service multiplexer.
• A service multiplexer hardened for the OSP environment.
• Power and a back-up power source.

There are situations where this solution is simply not viable. Regulatory issues may prevent the multiplexing signals of different WSPs on to 1 output, and sometimes WSPs request fiber for their own dedicated links.

Latency is also a consideration with this architecture since the processing of SONET overhead or Ethernet frames in the service multiplexers adds significant delay.

Getting a Break With CWDM
An alternative architecture is to use CWDM multiplexers to add virtual fibers to the feeder cable. CWDM multiplexers are placed at the CO and in a remote enclosure. (See Figure 4.) There are ample fibers in the new fiber cable to dedicate fibers from the CWDM enclosure right through to the multiplexers belonging to each WSP.

Figure 4. Using CWDM for the Critical Section.

A CWDM system uses 1 to 16 wavelengths based on the ITU-T standard grid. The transmission equipment at the cell site (DS-1, SONET, Ethernet) can utilize CWDM small form-factor pluggable (SFP) transceivers. If not, a separate CWDM transponder can be used to convert a low power 1310 nm signal to the desired CWDM wavelength. CWDM SFPs and transponders can support a loss budget of up to 28 dB which would, in turn, support transmission over fiber spans of 60 km or more.

A CWDM system can successfully scale as the demands of the wireless provider grow. Additional wavelengths can be delivered to a particular WSP at a cell site. The bit rate over an existing wavelength can be increased by the WSP without having to change or upgrade the physical fiber facility. This could also enable wavelength services. A wavelength service could also allow a customer to upgrade their rate without having to upgrade the physical facility or request another bit rate-dependent service from the network operator (such as an upgrade from DS-1 to DS-3). A wavelength service utilizes a dedicated wavelength from the cell site to a termination point such as a CO or mobile telephone switching office (MTSO).

In terms of the OSP, the CWDM multiplexer, as a passive hardened device, can be placed in any of the following enclosures:
• A pedestal which takes very little space and has no power or environmental control requirements.
• A remote terminal with more room and power.
• A controlled environmental vault (CEV) with power and temperature control.
• An equipment building with power and temperature control.

The options listed get progressively more expensive but offer more flexibility as well as the ability to offer 10 Gb/s over CWDM with an environmentally controlled structure.

Other characteristics of Passive CWDM include:
• It is weather-tolerant and not susceptible to electrical damage from lightning strikes or voltage anomalies that a network element with metallic components might be subject to.
• CWDM is highly reliable, having a mean time between failure (MTBF) value of 75 years or more since it is passive equipment with no electronics or software.
• CWDM multiplexers add only a few nanoseconds of latency to end-to-end circuits. Service multiplexers can add latency several orders of magnitude higher.
• CWDM provides a very cost-effective alternative that can be deployed within a few days or a couple of weeks of a service request.

A second type of architecture utilizing CWDM would be a linear chain of cell sites (4 in this case) using the add/drop capabilities of CWDM. One such example is shown in Figure 5. Here a CO serves 4 cell sites with 8 wavelengths. Two (2) wavelengths are dropped at each cell site. The cell sites could be up to 25 miles from the CO, so keeping loss to a minimum is an absolute must. Each cell site could be served by a simple pedestal with the CWDM add/drop multiplexer (ADM) mounted in the pedestal using a compact CWDM format. Fusion splicing could be used instead of connectors for these ADMs to significantly reduce the loss at each pedestal as well as the overall loss.

Figure 5. Linear architecture with intermediate add/drop nodes.

Break Through!
A CWDM system is a very quick, effective means of providing capacity to cell sites served by several WSPs. The rise in 4G and LTE service has created tremendous demand for bandwidth, and many WSPs require a wavelength for themselves. CWDM compares favorably to placing a large service multiplexer at an RT near the cell site, and requires very little space and no power. CWDM is a very cost-effective alternative to fiber placement and can be a vehicle to provide wavelength service to wireless providers at a cell site. CWDM optics (SFPs or transponders) can provide 60 km reach or more.

Figure 6. Comparison of Service Multiplexer and CWDM Multiplexer.

The key to meeting the explosive and often unpredictable demand of WSPs is to fully understand the technical capabilities of the CWDM system, the characteristics and limitations of the physical plant (fiber, equipment, buildings, etc.), and have an effective plan in place to meet these demands.

As Optelian’s AT&T Account Manager, Ed Purcell works daily with operations and staff managers in the Outside Plant and Interoffice. Ed has been in the telecommunications business for 38 years, spending 27 of those years serving in various Outside Plant positions at Southern Bell/BellSouth Telecommunications. For more information, visit www.optelian.com.

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