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The present paper defines diversity gain for stationary users. This deals in particular with gathering the received signal statistics over possible user positions and orientations in space rather than over time, and to define a meaningful diversity gain related to the cumulative improvement of the performances of the 1% users with the worst receiving conditions. The definition is used to evaluate diversity gain for some typical small antennas in an extreme environment with only line-of-sight (LOS). The LOS environment is regarded as user-distributed 3D-random LOS caused by the statistics of an ensemble of stationary users with arbitrary orientations in the horizontal plane (2D), and with arbitrary orientations of their wireless devices in the vertical plane. Thus, an overall 3D-random distribution of user orientation is assumed.Key Words: Antenna Diversity Gain, MIMO, Line-of-Sight, OTA Measurements. Ⅰ. IntroductionModern wireless communication systems with multi-port antennas make use of multiple input multiple output (MIMO) diversity and spatial multiplexing. There are new test methods under development that can characterize also the antenna diversity and spatial multiplexing. Such test methods are commonly referred to as over-the-air (OTA) tests in contrast to so-called conductive system tests without antenna, using a cable connected directly to the receiver input of the wireless device. The antenna diversity gain is properly defined in a rich isotropic multipath (RIMP) environment [1], and can be measured in a reverberation chamber that properly emulates a RIMP [2]. However, all environments are not rich and isotropic. Therefore, the present paper deals with defining diversity gain in a non-rich and anisotropic environment, see also [3]. The chosen non-rich anisotropic environment for this study is the extreme case of pure line-of-sight (pure-LOS).The statistical multipath environments in urban areas and inside buildings are normally rich and isotropic, where the latter means that the angles-of-arrival (AoA) of the incident waves on the wireless device are uniformly distributed over all directions in 3D space, with no LOS. At the country side, and close to base stations, there will often be significant LOS component, i.e. one dominant wave incident from a specific direction often in the horizontal plane. In order to study non-rich and anisotropic environments, we therefore in this paper choose the extreme case of a pure LOS. This is a classical environment present in all point-to-point communication systems with fixed antenna system installations. However, in wireless mobile communications we need to take the arbitrary orientation of the user into account, which is at least arbitrary in the horizontal plane (2D), and in particular the arbitrary orientation of the wireless device in the vertical plane, giving in total 3D orientation arbitrariness. Therefore, we will in the present paper study antenna diversity in a user-distributed 3D-random LOS environment, by studying an ensemble of users.Previous ...