User effect analysis on polarisation in multi-input multi-output systems

In this study the authors present the results of investigating the human head effects on the performance of a two-antenna system as applied to the polarisation of these antennas. For this, we define a 2 x 2 multi-input multi-output (MIMO) model constructed of two dipole transmitting antennas and two dipole receiving antennas. The user head effects are studied at three different frequencies: 700, 950 and 1880 MHz. The investigation is performed via full-wave electromagnetic simulations and lab experiments. Two antenna polarisation scenarios are investigated: vertically co-polarised (V-polarised) dipoles and cross-polarised (X-polarised) dipoles. The channel matrix that includes the antenna system on both sides of the communication link and the user's head is constructed and characterised. The channel power factor, the sub-channel weights and the channel capacity are all measured and analysed. The investigation showed that the presence of the human head decreases the channel power at all the examined frequencies and this effect becomes more sever as the frequency becomes higher. Also, the user head helped to improve the communication channel state in the V-polarised scenario, whereas it degraded the channel state in the X-polarised scenario. Therefore in the presence of the user, V-polarised antenna configurations would be the better choice if an increase in the channel capacity is the multi-antenna design objective. While X-polarised antenna configurations should be considered if an improvement in the signal quality and hence in the communication link margin is the MIMO design objective.