September 29, 2016
Coverage testing for wireless systems is an important part of network and building management, whether the signals are generated by owned equipment located in-building or on-campus, or by provider-owned equipment outside. Coverage testing is a process that begins with planning and continues with equipment setup, test execution, data analysis, and corrective action. The testing process should be repeated periodically, and after major changes such as remodeling and construction.
As we approach the completion of the IMT-2020 requirements and the rollout of 5G networks, coverage testing will be increasingly important. The reason is that in-building propagation of RF signals above 24 GHz is very different than signals from wireless technologies field technicians may be used to testing.
Reasons for Testing
Service Assurance - The performance of a wireless network has an effect on several metrics. Customer satisfaction and loyalty, employee satisfaction and retention, management process efficiency, and IS/IT system performance are all influenced by network quality.
Public Safety & Building Codes - Local governments often require in-building testing of wireless networks for public safety reasons, and for tying the performance of these systems to building codes. The technologies used to construct energy-efficient buildings (primarily low-emissivity glass and radiant-barrier insulation) will attenuate wireless signals. In many locations, conducting an in-building test to confirm performance of radios for police, fire, and emergency medical is required to obtain a certificate of occupancy.
RF Signal Performance in Millimeter Wavelengths - In 2016 the Federal Communications Commission (FCC) assigned the 28 GHz, 37 GHz, 39 GHz, and 64-71 GHz bands for use by future 5G wireless systems, and the agency is considering additional spectrum above 95 GHz. These frequencies will present new challenges for performance of in-building wireless networks.
Lower frequencies behave like water in a stream – flowing around obstacles. Higher frequencies behave like visible light – casting shadows behind the obstacle and creating “dead zones.” The RF energy is absorbed by the obstacle, reflected back, or scattered – and this RF energy can combine with the original signal and other reflections to form complex patterns of constructive and destructive interference. This is especially true for obstacles made of metal, and to a lesser extent any material capable of retaining moisture, such as gypsum wallboard, wood, and even fabrics.
Equipment - It’s important to use coaxial cable and connectors designed for microwave frequencies. RG-174 coax cable is rated up to 65.6 GHz; higher frequencies require semi-rigid coax. 1.85mm and V-type air dielectric connectors are rated up to about 70 GHz. Above that point, 1mm air dielectric connectors are required for 5G bands. The test receiver’s antenna should be omnidirectional.
Environmental Simulation - RF signals above 24 GHz are often affected by objects in and around the space being tested. In an office environment, the position of PCs, LCD displays, desks, chairs, bookcases, shelfs, cubicle walls, potted plants, and even people, will affect the coverage. Simulating an in-building RF environment that closely matches an occupied building is hard, so conducting another coverage test after the building is occupied is recommended.
Managing Variables - In-building coverage testing should be done when the interior space is unoccupied to minimize randomization of the signal propagation and measurements. Coax connectors should be torqued to specification to avoid variance, and coaxial cable or semi-rigid cable should be secured to ensure that changes in loss from movement do not cause measurement variance. Directional antennas are not recommended as reflections from objects in the environment will cause measurement variances.
Anritsu has just published a white paper on this topic. It can be downloaded here.