The polarisation of electric field states the orientation and magnitude of its field vectors and their alteration through the time. Polarisation is related to the transverse electromagnetic waves (TEM), in which directions and magnitudes of both electric and magnetic fields vary by time. Polarisation of EM waves from an antenna is classified into three main categories: linear, circular or elliptical polarisations. Furthermore, the direction of polarisation may be clockwise (CW, right-hand polarisation) or counter-clockwise (CCW, left-hand polarisation). For instance, the equation indicates a circularly polarized wave which consists of two components in the x and y directions. If polarisation of the receiver antenna does not match with the polarisation of incoming waves, the amplitudes of the received waves decrease. This polarisation mismatch will cause polarisation loss and reduce the power of the received signal. On the other hand, polarisation discrepancy can be employed to transmit two signals simultaneously at the same frequency-time resources using two different polarisations such as in satellite communication.
Antenna polarisation is another opportunity for MIMO systems. Because the vertical and horizontal polarisation may be utilized to increase the number of antennas in a given area. Single polarized and dual polarized 2\times2 MIMO systems have been compared [1], in which they show that if there is a high correlation between two antennas, then polarized MIMO have better performance over a single polarized system. In another similar study, they perform broadband outdoor channel measurement to verify the performance of 2x3 dual-polarized MIMO system at 2.5 GHz frequency and they achieved higher capacity using dual polarisation especially in the close range. On the other hand, aforementioned two studies demonstrate that if the distance between transmitter and receiver is large enough, then single polarised MIMO system performs better than the dual polarised system.