Multiple Antenna Terminals in Realistic Environments - A Composite Channel Modeling Approach

Sammanfattning: For evaluation of specific antenna arrangements in wireless communication systems we need physical channel models that take into account also the directional domain of the propagation channel. In this thesis we investigate, validate and propose a practical approach to wireless channel modeling and, particularly, to mobile communication systems. For this we make the assumption that the channel can be divided into separate parts, or regions that can be treated and modeled separately. The basic idea is that the antenna parts of the channel is the parts considered in the design of base station antennas and user equipments and can be characterized by a single measurement of each design, while the propagation part of the channel can be characterized separately, independent of the specific installed base station antenna or the user equipment, but based on generic channel sounder measurements with, as far as possible, open areas around the transmitter and the receiver antennas. For more complex antenna environments we may imagine intermediate scattering regions of the channel model between the antenna parts and the propagation part, that can or cannot be handled separately, e.g., the body of a mobile phone user, an office desk, a vehicle, surroundings of base station antennas in dense deployments, etc. A first step in evaluating such a composite channel modeling approach is to verify the validity of communication link simulations were the mobile phone antennas together with the user can be treated as a super-antenna with its aggregate far-field pattern to be combined with a directional channel model in a classical way. This is first presented in Paper II, and the method is in its extensible form here referred to as a composite channel method. It is found that this method, as we expected, work well for statistical performance evaluation of diversity and spatial multiplexing. An extension of the composite approach is outlined with an attempt to find a simple yet accurate directional scattering model for, firstly, the user body, and, secondly, a car environment with the user inside. A simple model that still catch the proper influence of antenna efficiency, fading statistics, and correlation at the mobile side. In Paper I a first investigation of user influence on an indoor 2x2 multiple-input multiple-output (MIMO) link is performed based on a narrowband measurement setup and the diversity performance is evaluated. In Paper II, the first step of the composite channel approach is evaluated with respect to MIMO by channel measurements including user influence in two static outdoor-to-indoor and indoor-to-indoor scenarios. The approach is verified for statistical properties such as antenna correlation and MIMO eigenvalue distributions. It is found, with extended detail given in Paper III that the presence of the user, apart from introducing hand and body absorption and mismatch that increases the path loss, also increases the correlation between the antenna branch signals and, thus, slightly decreases potential MIMO capacity. In Paper IV and Paper V the investigation is extended to the scenario were the user is located inside a family car (station wagon). In Paper IV a first analysis of an outdoor MIMO measurements campaign with the user outside and inside the car is presented. The results show an increased scattering inside the car that improves mainly the potential diversity gain, and to some extent also the potential MIMO capacity gain, to the cost of higher path loss (lower SNR) due to car penetration loss. An important observation is the dependency of this penetration loss on the directional properties of the outer propagation channel, which indicates a possible need for scenario dependent penetration loss in general channel modeling. In Paper V this is further verified with directional estimation of the propagation channel both outside and inside the car. We also find that the composite channel method with the inherit assumption of plane-waves impinging on the mobile terminal, actually does produce good results even in this close near-field environment inside the car with the nearby scatterers within the far-field (Rayleigh) distance of the probe antenna array.