The polarisation of an antenna is the orientation of the radio wave (electric field) transmitted from the antenna. Antenna polarisation is often vertically polarised or horizontally polarised, but we also have polarisation at any other angle, e.g. ‘+’ (0, 90 degrees) or ‘X’ (-45, +45 degrees) as often used in MIMO.
When GSM started out in the 1990’s they used vertical polarisation (0 degrees) on all their cellular network sites, but as the requirements for capacity and number of base stations (cellular sites / towers) increased, they started using Cross Polarised (X-Polarised) antennas which are +45 & -45 degrees polarised.
This enabled the operators to implement receive/transmit diversity (a mechanism to improve received signal) within a single antenna, whereas previously they would require two or three antennas to achieve this.
X-Polarised antennas were also used in urban areas where implementation space was critical (aesthetical issues, etc.) and at the same time enhancing the decorrelation between the receive signals in the same antenna housing for receive diversity. Implementation costs and other factors were also part of the reason to implement X-Polarised antennas. With 3G and LTE, the X-Polarised antennas were an ideal choice for the networks to achieve MIMO, and therefore enhance the performance and throughput of the base stations.
Now, most urban and suburban areas use X-Polarised antennas (-45, +45 degrees). We are finding that older sites and very rural sites may still use vertical polarised antennas – this depends on country to country and operator to operator. Omni antennas are vertically polarised in cellular networks and are often seen along rural highways and smaller communities.
It is important to align the polarisation between the two antennas, specifically for backhaul links, such as microwave, but due to the signal correlation difference and the technology itself; the antenna polarisation alignment from your LTE enabled router (or mobile phone) is less critical – i.e. you can use a ‘+’ (0, 90 degrees) or ‘X’ (-45, +45 degrees) configuration for 2×2 MIMO.