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Mobile terminals are often used indoor with the base station outdoors. At the mobile terminal the major part of the signal energy comes through openings in the building such as windows. Typically, only one of the sides in a room has windows, and seldom does a room have windows on all sides. Hence, the dominating signal can be expected to arrive at the mobile terminal from a narrow range of angles. Mobile terminal antennas used next to the head in the speaking position will be directional due to the fact that part of the radiation pattern facing the head will be attenuated and reflected. Having a directive antenna in a directive environment, the performance will depend on the orientation of the antenna in the radio environment. A new statistical spherical outdoor to indoor power spectrum model has been proposed to be able to calculate the directional performance of mobile terminals with a single or multiple antennas. The model consists of a major scattering area in one direction and more uniformly distributed minor scatterers in the other directions. A verification of the proposed model was performed and 60 data sets of spherical power spectrum measurements were collected in a typical urban environment. Using the new model, the directional performance of mobile terminal antennas including a human operator has been investigated through directional mean effective gain, branch power ratio, and correlation calculations using spherical radiation pattern measurements of a mobile terminal including the effect of 42 different persons. The accuracy of the calculated values was verified by directly measured values using 200 persons walking with the mobile terminal in the same office-like environments as where the spherical power spectrum measurements were performed.