Passive Intermodulation and Network Planning Challenges in Future Indoor Networks and Energy Efficient Buildings

Energy efficient building materials are these days being used to improve the thermal insulation and efficiency of modern buildings. However, such energy efficient building materials have been shown to block not only the unwanted heat radiation but also the outdoor radio signals penetrating into the buildings. Consequently, providing adequate coverage inside these modern buildings by using legacy outdoor deployment solutions, e.g. macrocells, is proving to be quite challenging. One solution to this problem is to deploy a dedicated indoor network, building on, e.g., indoor distributed antenna systems (DAS). However, a key problem in indoor DAS systems, supporting multiple operators, is the passive intermodulation (PIM), which has typically been considered to originate from loose connectors and damaged cables. However, it has been recently found out that the antenna environment can also play a significant role in generating PIM in multi-operator DAS systems. This paper examines the effects of antenna location in real apartment buildings through real PIM RF measurements. In particular, the aim is to find suitable guidelines for antenna placement/installation in cases where the DAS antenna has to located in a corridor, in the center of a building. We also evaluate the impact of measured PIM levels on the indoor network coverage and capacity in a realistic building setting through detailed ray tracing based analysis. Overall, the findings in this paper show that the coverage and capacity of DAS based indoor radio networks are very sensitive to the associated PIM mechanisms, and thus specific emphasis should be put on the location planning and optimization of the antenna units inside the buildings.