The objective of the subsequent study is to investigate the impact of cumulative interference from multiple UWB systems to a narrowband victim receiver. The investigations are restricted to cellular victim receivers, in particular to the downlink of GSM-900, GSM-1800, and UMTS voice service. Since indoor environments are considered a three-dimensional geometry for the spatial distribution of the UWB devices is assumed. To obtain a realistic scenario a model describing the transmission activity of the UWB devices is taken into account. The outcome of this study is a statistical model, i.e. a probability density function (PDF), describing the cumulative interference from multiple UWB devices. The main question to be answered with this model is: What is the maximum acceptable UWB-device density? The question is treated with respect to current FCC and ET8I regulations.