Mobile communication has become a firmly established part of our lives. It is difficult to imagine business and private lives without mobile communication, it provides independence and increased personal safety.

Mobile communication networks consist of a honeycomb of transmission cells, which are more or less evenly distributed across the country. Each transmission cell is supplied by sending and receiving equipment, the so-called base station.

These base stations are predominantly located outside, in town often on roofs of high buildings. The technology has to be protected from climatic influences by being housed in cabinets.

E-Plus Mobilfunk GmbH was founded in 1993 with the aim to establish and operate a digital cellular mobile transmission network on the basis of GSM 1800, GPRS and UMTS. In the meantime E-Plus has established itself on the German mobile transmission market as an innovative company and is a forerunner in this field in many business areas.

High demands on stability

The technology and network architecture of the base stations is developed at the E-Plus headquarters and manufactured by large systems technology suppliers such as EA-Elektro-Automatik in Viersen. In the past EA manufactured mainly small power supplies for the industry and laboratory sectors. Today the company has a large product range of power supplies that covers all demands with standard and special solutions. Developed in 1993, EA manufactures and supplies special solutions for telecom applications, in particular in the area of back-up systems for the mobile transmissions area. For E-Plus EA manufacture so-called Site-Support Systems for the base station, which guarantee the power supply during a power failure with battery buffering respectively battery back-up time. Additionally the Site Support System provides installation space for link-up technology of the base stations into the next higher network element via radio link or firm network connections. As E-Plus' base stations are often outside (approx. 10,000 outdoor stations), the use of correspondingly robust outdoor cabinets for the Site-Support-Systems is necessary. In Schroff, EA has found a competent partner for such cabinet systems.

Firstly the technical demands on the cabinets relate to the installed electronics and for this high stability of the cabinets and doors, sensitive climate control systems, easy accessibility and special cabling are necessary. Secondly the cabinets need to have a small footprint, with an installation area as large as possible. The outer appearance of the cabinet should blend well and inconspicuously into the complete picture of its site. Only in exceptional cases e. g. an outstanding colour is requested. On the basis of the cabinet construction criteria (measurements, roof construction, cabling etc.) determined by E-Plus and the requested performance data of the inside electronics, EA and Schroff have developed a corresponding concept for the Site-Support-Systems at E-Plus. There is a so-called SSC1 (Site-Support-Cabinet) - right or left handed and SSC2, an extension rack. For the inside installation there is a basic configuration with power supply, batteries and a plug-in unit for a radio link or firm network connections, which corresponds to the number of base stations to be connected and can be extended or upgraded. For maximum network use, a redundant DC unit with a performance of 10.0 kW is available for the - 48 V power supply. This concept is used at E-Plus in various assembly modules and takes the place of a previous cabinet construction by another supplier.

Two construction modules as the basis

For outdoor applications Schroff has two standard construction units from their range of outdoor cabinets: "Modular" and "Unibody".

The modular version is based on an Aluminium extruded frame, which has high flexibility in relation to the cabinet dimensions and allows excellent access from all sides.

The more economical Unibody version is based on a one part inner case, which is particularly suited for small dimensions for wall, mast and floor fastening. Both cabinet versions protect the installed components from wind, rain, heat, cold and vandalism.

Flexible installation possibilities are possible with 19", ETSI and other, and also non-standard accessories. Rubber seals guarantee protection to IP55, combined with a metal weave and chromated insides they provide electromagnetic shielding (EMC) and can also easily be exchanged if damaged. The cabinets, which are based on standardised components to IEC 61 969, fulfil the guidelines and standards for shock and vibration stability which has been tested to IEC 68-2-6 and -27, of earthquake stability - tested to IEC 68-2-57 and offer protection against climate influences - tested to IEC 68-2-1, -2 and -14. The cladding is coated with a weather resistant powder coating. Due to the double wall construction of both cabinet versions, a certain amount of heat can be dissipated without additional cooling components (i. e. fans, heat exchangers and climate control systems). The overheating of the enclosure through direct or indirect sun is also reduced. The hinged roof construction enabled easy installation and cabling. Additional cooling components can be integrated here in order to increase the cooling airflow through the double walls. The cabinets are available with one or two doors. At request Schroff integrates basic electronic packaging components into the outdoor cabinets such as cabling, climate control, power supply and further EMC measures.

Cabinet construction: Modular and customer specific

The cabinets for E-Plus are constructed to the modular concept and consist of an Aluminium extruded frame. For the high demands for stability the top and bottom frames are welded and screwed to the uprights. With this frame construction it is guaranteed, that the cabinets can take the relatively high weight of the power supply and the battery packs without any problems. Through the design construction, an ideal combination of functionality, stability and low weight are achieved. The weight of the empty cabinet plays an important part, when it needs to be transported to roof locations. For good accessibility the cladding can be entirely removed from the outside, however, for security reasons this can only be carried out after the doors have been opened. Corresponding to the customer demands, the front and/or rear and also the side panels can be fitted as doors. In contrast to the conventional cabinet construction the weight of the empty cabinet, the installation volume, the roof assembly, the arrangement of the cabling as well as the in and outlets have been improved considerably.

Predominantly 19" assembly

The inner assembly by EA is carried out on a 19" basis to a total height of 23 U. For a standard cabinet a rectifier unit is integrated into the top area and equipped with corresponding network parts. The remaining 6 U are especially for E-Plus installations (link-up equipment). Depending on performance requirements, standard batteries are placed in the battery section in the bottom 19" assembly area. Further installation space is available at the side of the 19" assembly area, e.g. for AC/DC distribution and in the roof area. For larger routers the 19" assembly area for the link-up equipment can be extended by a further Site-Support-Cabinet (SSC2). The entire additional 23 U will then be available for further customer installations.

Always a pleasant climate

The subject 'climate control' is, of course, particularly important for outside locations, as here not only the loss of performance that is generated through the installed components, but also the changing outside temperatures (summer/winter) have to be taken into account. The double wall concept of the outdoor cabinets from Schroff provides a natural air flow between the outside and inside wall, so that considerably less measures are necessary for the additional cooling. 85 % of the heat created by direct and reflected sunlight is eliminated. With higher demands for cooling, such as with E-Plus, the heat exchanger concept from Schroff provides optimal temperatures. The heat exchanger is installed on the inside of the cabinet door, which ensures that E-Plus are not restricted in their choice of the location, as rear access does not have to be guaranteed. The heat exchanger works with plastic lamellae, which has distinct weight advantages compared with conventional heat exchangers. The heat is expelled to the outside by the heat exchanger unit, whereby the inner and outer air circuits are separated from each other. Additionally the heat exchanger is equipped with a 1000 W heater. In the cabinet, below the power supply, an additional 19" fan unit is integrated, which distributes this 'hotspot' evenly over the inner air flow. With these measures the temperature requirements of minimum +5 and a maximum temperature difference of 10 K between the inner and outer temperature can be guaranteed.

Notes to the Author:

Dipl.-Ing. Christa Weil is a Technical Journalist from Trebur.