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Polarization-division multiplexing (PDM) is aphysical layer method formultiplexing signals carried onelectromagnetic waves, allowing two channels of information to be transmitted on the samecarrier frequency by using waves of twoorthogonalpolarization states. It is used inmicrowave links such assatellite television downlinks to double the bandwidth by using two orthogonally polarizedfeed antennas insatellite dishes. It is also used infiber optic communication by transmitting separateleft andright circularly polarized light beams through the sameoptical fiber.
Polarization techniques have long been used in radio transmission to reduce interference between channels, particularly atVHF frequencies and beyond.
Under some circumstances, the data rate of a radio link can be doubled by transmitting two separate channels of radio waves on the same frequency, using orthogonal polarization. For example, in point to point terrestrial microwave links, the transmitting antenna can have two feed antennas; a vertical feed antenna which transmits microwaves with their electric field vertical (vertical polarization), and a horizontal feed antenna which transmits microwaves on the same frequency with their electric field horizontal (horizontal polarization). These two separate channels can be received by vertical and horizontal feed antennas at the receiving station. For satellite communications, orthogonalcircular polarization is often used instead, (i.e. right- and left-handed), as the sense of circular polarization is not changed by the relative orientation of the antenna in space.
A dual polarization system comprises usually two independent transmitters, each of which can be connected by means ofwaveguide orTEM lines (such ascoaxial cables orstripline orquasi-TEM such asmicrostrip) to a single-polarization antenna for its standard operation. Although two separate single-polarization antennas can be used for PDM (or two adjacent feeds in areflector antenna), radiating two independent polarization states can be often easily achieved by means of a single dual-polarization antenna.
When the transmitter has a waveguide interface, typically rectangular in order to be in single-mode region at the operating frequency, a dual-polarized antenna with a circular (or square) waveguide port is the radiating element chosen for modern communication systems. The circular or square waveguide port is needed so that at least two degenerate modes are supported. An ad-hoc component must be therefore introduced in such situations to merge two separate single-polarized signals into one dual-polarized physical interface, namely anortho-mode transducer (OMT).
In case the transmitter has TEM or quasi-TEM output connections, instead, a dual-polarization antenna often presents separate connections (i.e. a printed squarepatch antenna with two feed points), and embeds the function of an OMT by means of intrinsically transferring the two excitation signals to the orthogonal polarization states.
A dual-polarized signal thus carries two independent data streams to a receiving antenna, which can itself be a single-polarized one, for receiving only one of the two streams at a time, or a dual-polarized model, again relaying its received signal to two single-polarization output connectors (via an OMT if in waveguide).
The ideal dual-polarization system lies its foundation onto the perfect orthogonality of the two polarization states, and any of the single-polarized interfaces at the receiver would theoretically contain only the signal meant to be transmitted by the desired polarization, thus introducing no interference and allowing the two data streams to be multiplexed and demultiplexed transparently without any degradation due to the coexistence with the other.
Companies working on commercial PDM technology includeSiae Microelettronica,Huawei andAlcatel-Lucent.
Some types of outdoor microwave radios have integrated orthomode transducers and operate in both polarities from a single radio unit, performing cross-polarization interference cancellation (XPIC) within the radio unit itself.Alternatively, the orthomode transducer may be built into the antenna, and allow connection of separate radios, or separate ports of the same radio, to the antenna.
Practical systems, however, suffer from non-ideal behaviors which mix the signals and the polarization states together:
As a consequence, the signal at one of the received single-polarization terminals actually contains a dominant quantity of the desired signal (meant to be transmitted onto one polarization) and a minor amount of undesired signal (meant to be transported by the other polarization), which represents an interference over the former. As a consequence, each received signal must be cleared of the interference level in order to reach the required signal-to-noise-and-interference ratio (SNIR) needed by the receiving stages, which may be of the order of more than 30 dB for high-level M-QAM schemes. Such operation is carried out by a cross-polarization-interference cancellation (XPIC), typically implemented as a baseband digital stage.
Compared tospatial multiplexing, received signals for a PMD system have a much more favourable carrier-to-interference ratio, as the amount of leakage is often much smaller than the useful signal, whereas spatial multiplexing operates with an amount of interference equal to the amount of useful signal. This observation, valid for a good PMD design, allows the adaptive XPIC to be designed in a simpler manner than a general MIMO cancelling scheme, since the starting point (without cancellation) is typically already sufficient for establishing a low-capacity link by means of a reduced modulation.
An XPIC typically acts on one of the received signals "C" containing the desired signal as dominant term and uses the other received "X" signal too (containing the interfering signal as dominant term). The XPIC algorithm multiplies the "X" by a complex coefficient and then adds it to the received "C". The complex recombination coefficient is adjusted adaptively to maximize theMMSE as measured on the recombination. Once the MMSE is improved to the required level, the two terminals can switch to high-order modulations.
Is a novel method for polarized antenna transmission utilizing a differential technique .
Polarization-division multiplexing is typically used together withphase modulation or opticalQAM, allowing transmission speeds of 100 Gbit/s or more over a single wavelength. Sets of PDM wavelength signals can then be carried overwavelength-division multiplexing infrastructure, potentially substantially expanding its capacity. Multiple polarization signals can be combined to form new states of polarization, which is known asparallel polarization state generation.[1]
The major problem with the practical use of PDM overfiber-optic transmission systems are the drifts in polarization state that occur continuously over time due to physical changes in the fibre environment. Over a long-distance system, these drifts accumulate progressively without limit, resulting in rapid and erratic rotation of the polarized light'sJones vector over the entirePoincaré sphere.Polarization mode dispersion,polarization-dependent loss. andcross-polarization modulation are other phenomena that can cause problems in PDM systems.
For this reason, PDM is generally used in conjunction with advancedchannel coding techniques, allowing the use ofdigital signal processing to decode the signal in a way that is resilient to polarization-related signal artifacts. Modulations used includePDM-QPSK andPDM-DQPSK.[2]
Companies working on commercial PDM technology includeAlcatel-Lucent,Ciena,Cisco Systems,Huawei andInfinera.