Polarisation is a crucial characteristic of an antenna, describing the orientation of the electric field vector as it propagates through space. In simpler terms, it describes the directional movement of the wave as it travels.
It is an important parameter to understand as wireless communication requires transmit and receive antennas to operate on the same polarity. Misaligned antennas can result in significant signal losses, potentially as high as 20 to 30 dB under worst case scenarios.
Most transmissions are vertically polarised due to the simple construction of vertical polarised antennas, historical usage, and superior propagation through weather. A key exception tends to be UHF television transmissions, which historically have been horizontally polarised to provide some level of interference protection from overwhelmingly vertically polarised transmissions.
Modern day cellular, multipoint, and microwave communications utilise both polarisations simultaneously in the form of Dual Linear Slant polarisation. With improvement in technological design of both antennas and radios, dual-polarised transmissions allow a near doubling of data rates. Slant transmissions also provide 3 dB protection from vertical/horizontal transmissions, as well as improved performance in multipath environments where polarisation state can be altered from a reflection surface.
Technologies such as 4G LTE-A Pro and 5G implement Massive MIMO, consisting of 64 or 128 antenna elements in a spatially diverse configuration. Many systems implement dual-polarisation in their Massive MIMO array. Simulation results show that performance of dual-polarised Massive MIMO systems can be improved as high as 16% to 92% depending on traffic and environment[2].
Another common type is circular or eliptical polaristion, whereby the tip of the electric field vector is seen to trace out a helix or corkscrew as it travels away from the antenna. The helix will either travel with a left or right-hand rotation, aptly named Left Hand Circular Polarisation (LHCP), or Right Hand Circular Polarisation (RHCP) respectively.
Technically circular polarisation is a combination of vertical and horizontal waves with a 90° phase offset, which allows their transmission by relatively simple antenna designs. Circular polarisation is necessary for communications systems that are subject to antenna rotation, the most obvious of which is satellite where the orientation of transmit and receive antennas are not known.
Definition of Polarisation
The polarisation or polarisations of the electric field radiated by the antenna evaluated in the far field.
The nominal polarisation associated to the antenna port whose related radiators generate a wave polarised (nominally) along the same plane.
Specification Definition
When disclosing polarisation on test reports and datasheets, Powertec follow the BASTA reporting recommendations:.
- The nominal value as a type and direction for the reference polarisation of the antenna.
- Linear polarisation is typically defined as H, V.
- Slant linear polarisation is typically defined as +45 / -45.
- Slant value, specified in degrees, is measured using the positive vertical axis as the zero reference.
- Multiple polarisation naming follows standard metric prefixes, excepting dual (two) to avoid confusion between di-pol and dipole.
- Circular polarisation is typically defined as left-handed (LHCP), right-handed (RHCP).
Testing & Reporting
Polarisations are defined per antenna port based on the orientation of the internal radiating elements. For more complex planar/patch antenna designs, polarisation is determined through electrical engineering simulation.
The measure and reporting to describe how polar an antenna is, is known as Cross-Polar Discrimination, or XPD. XPD measures how well an antenna rejects transmissions of an orthogonal polarity. A highly polar antenna is necessary for successful MIMO and diversity communications, and essential for a high performing dual/tri/quad polarised antenna.
While there is no limit to combination of polarisation types as reported on an antenna datasheet, it is however important that a system be adhered to. BASTA specification definition has been expanded to permit the disclosing of slant angles other than 45°. For example, a more complex antenna might have additional ±30° and ±60° slants, positioned 30 and 60 degrees from the positive vertical axis respectively. The number of polarisations is disclosed using standard metric prefixes, excepting for dual-polarised, as the metric prefix di- could easily lead to confusion between di-pol antenna and dipole antenna.
Example values include:
- Linear Vertical (V)
- Linear Horizontal (H)
- Slant Linear +45°
- Slant Linear -45°
- Dual Linear V, H
- Dual Slant Linear ±45°
- Left Hand Circular Polarised (LHCP)
- Right Hand Circular Polarised (RHCP)
References
- NGMN, "Recommendations on Base Station Antenna Standards", NGMN Alliance, N-P-BASTA v11.1, Mar. 2019.
- Jun-Ki Hong, “Performance Analysis of Dual-Polarized Massive MIMO System with Human-Care IoT Devices for Cellular Networks,” Journal of Sensors, vol. 2018, Article ID 3604520, 8 pages, 2018.