Outdoor Enclosures for Wireless Base Stations
As wireless operators continue to build out their infrastructure to support ever-increasing bandwidth requirements, one commonly overlooked aspect that has a significant impact on operational cost is enclosures and cabinets needed to house electronics.
Outdoor equipment enclosures enable wireless operators to save substantial time, labor, space, capital and operational expense as compared to prefabricated buildings and shelters, while meeting their exact needs for equipment mounting, thermal management, coaxial and fiber cable management, power and battery backup. Similar to buildings and shelters, enclosures support multiple chambers, protect equipment in a wide variety of environmental zones, and can be mounted on concrete pads.
Image #1 Caption: Example of outdoor enclosure with thermal management system and RF cable access glands.
Typical equipment housed in the wireless enclosures include Base Transceiver Station (BTS), Base Band Unit, E-911 Location Measurement Unit (LMU), wireless backhaul, fixed microwave transceivers, and power and battery support. Listed below are the notable features that are relevant to wireless enclosure configurations.
Image #2 Caption: Enclosure installation at base of cellular tower with cable entry at the top of enclosure.
Temperature Range - Many of the older BTS and microwave transceivers are specified to operate from 0 to 40 C, while more modern BTS transceivers will typically operate from -40 to 65 C. This means that the enclosure manufacturers must provide thermal analysis and design services to ensure the wireless equipment stays within its prescribed operating range. Additionally, the enclosure manufacturer must offer a wider range of thermal management systems to support the tighter operating window of older transceivers. This should include direct air cooling, heat exchangers, air conditioners (with integrated heaters), and thermoelectric coolers (with integrated heaters).
Temperature Transition – A little known fact about BTS transceivers, both older and new models, is that they are very sensitive to quick changes in temperature and humidity. Although the older transceivers are rated to operate within 0 to +40 C and newer models are rated from -40 to 65 C, they operate best when the temperature changes do not exceed 10 C per hour within the chamber. Many of the cooling systems built by Purcell Systems for wireless operators strive to keep the electronics in +20 to +30 C range, and limit the temperature changes to 10 C per hour.
Indoor and Hardened Equipment – Many BTS manufacturers offer indoor and hardened versions of their BTS radios, and hardened equipment always commands a premium over indoor equipment because it is designed to accommodate severe temperature and humidity environments. Historically, hardened equipment was usually deployed in unmanaged outdoor enclosures. However, the deployment of indoor BTS equipment in thermally managed outdoor enclosures is a viable and cost effective alternative for the operator. Some BTS manufacturers may require testing and validation of their equipment in the enclosure before they warranty their equipment in that environment.
Telcordia GR-487 - The requirements of Telcordia GR-487-CORE specify a suitable environment for active electronics and passive equipment housed in above ground cabinets or enclosures in an Outside Plant (OSP) environment. Although originally developed for wireline deployments, many of the wireless carriers have adopted GR-487-CORE as a standard for their wireless deployments because it ensures that enclosures adequately protect their enclosed electronics, especially if the equipment is indoor equipment (i.e. not hardened).
Cable Entry – Most enclosures typically have cable entry at the side or bottom of the enclosure. Wireless operators prefer antenna cables entering near the top of the enclosure, as well as backhaul cables exiting through the side or bottom of the enclosure. “Cable raceways” organize and distribute antenna cables entering near the top to different sections of the enclosure.
Image #3 Caption: Cable raceways distribute antenna cables to different chambers within the enclosure.
LTE Transceivers connect to a GPS antenna to triangulate the location of any inbound E911 calls. The antenna cable for GPS antennas require an upper rectangular opening and knock-out to exit the enclosure. To simplify installation, wireless enclosures need some rectangular knock-outs in addition to the conventional round knockouts found in enclosures.
Partitioned Chambers – Since cellular towers can be shared among several operators, the chamber of a wireless enclosure may be physically partitioned so multiple operators can house their equipment in their own compartment (without access to the adjacent compartments). A wireless operator may lease excess chamber space within the enclosure to another wireless operator.
To support the phased equipment loading of an enclosure, the mounting scheme for thermal systems is externally mounted on the front and/or back doors. Field upgrades of thermal systems, which will be required when a new compartment is leased out, are accomplished by exchanging doors. Quick-release hinges allow fast and efficient changes to the thermal management systems that can typically be accomplished without having to remove the cabinet or electronic equipment from service. This exchange can be accomplished in minutes by a standard craft person.
Security - Security is a critical concern for any wireless operator, especially as larger quantities of smaller micro-sites are deployed in suburban and urban neighborhoods. Unauthorized access to equipment can quickly lead to network downtime. Enclosures and cabinets should have numerous features that provide uncompromising security.
Interior door hinges on all access doors must be only accessible from inside the enclosure, and reside inside the exterior gasket area of the enclosure so they are protected from exposure to the elements. Unlike interior hinges, exterior hinges (i.e. piano hinges) can be easily knocked off the enclosure, thus allowing the removal of the door and access to the enclosure interior as well.
All exterior doors must be locked with a multi-point latching mechanism to ensure that all door corners and edges are secured. The latching mechanism must be captive, such that the latch effectively bonds the door to the cabinet when locked. In the event that the exterior handle is compromised, the latching mechanism must stay in the locked position and be inaccessible to external tampering.
Any penetration of the enclosure wall by firearms can compromise the enclosed electronics. As an example, the Telcordia GR-487-CORE standard specifies that enclosures must be capable of withstanding a point blank 12-gauge shotgun blast without penetration of the cabinet wall by any pellets.
Intrusion alarms must provide signaling of unauthorized or unscheduled door movement (i.e. open/close) to a centralized alarm monitoring center. Alarms must work with all main chamber doors, as well as the side chamber.
Vertical Stacking – Shorter enclosures should offer vertical stacking features. Vertical stacking of enclosures adds capacity to an existing enclosure footprint, which is typically leased by the wireless operator. When enclosures are stacked, a key consideration is still meeting the local seismic zone requirements.
Image #4 Caption – Vertical stacking of enclosures maximizes utilization of available real estate
October 2, 2012