XPIC, or cross-polarization interference cancelling technology, is an algorithm to suppress mutual interference between two received streams in a
Polarization-division multiplexing communication system.
The cross-polarization interference canceller (known as XPIC) is a signal processing technique implemented on the demodulated received signals at the baseband level. It is typically necessary in
Polarization Division Multiplexing systems: the data sources to be transmitted are coded and mapped into
QAM modulating symbols at the system's symbol rate and
upconverted to a carrier frequency, generating two radio streams radiated by a single dual-polarized antenna (see feed pattern of
Parabolic antenna
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or par ...
). A corresponding dual-polarized antenna is located at the remote site and connected to two receivers, which
downconvert the radio streams into baseband signals (BB H, BB V).
This multiplexing/demultiplexing technique is based on the expected discrimination between the two orthogonal polarizations (XPD):
* an ideal, infinite XPD of the whole system guarantees that each signal at the receivers contains only the signal generated by the corresponding transmitter (plus any thermal noise);
* any real, finite, level of XPD instead manifests itself as a partial recombination between the two signals, so that the receivers observe an interference due to the cross-polarization leakage. Some of the factors causing such cross-polarization interference are listed in
Polarization-Division Multiplexing.
As a practical consequence, at the receiving site the two streams are received with a residual mutual interference. In many practical cases, especially for high-level M-
QAM modulations, the communication system cannot tolerate the experienced levels of cross-polarization interference and an improved suppression is necessary. The two received polarizations at the antenna outputs, normally linear horizontal H and vertical V, are routed each to a receiver whose baseband output is further processed by an ad-hoc cross-polarization cancelling scheme, commonly implemented as a digital stage. The XPIC algorithm attains the correct reconstruction of H by summing V to H to cancel any residual interference, and vice versa.
The cancelling process is typically implemented using two blocks: a baseband
equalizer and the baseband XPIC. The output from the latter is subtracted from the former and then sent to the decision stage, responsible for yielding the estimation of the data stream. The equalization and XPIC blocks are normally adaptive for a correct tracking of the time-variant channel transfer function: XPIC must provide a shaping of the received cross signal equal to the portion of the cross interference affecting the main one. The feedback control to drive the adapting criteria comes from the measure of the residual error across the decision block.
In the example, both blocks are based on the typical structure of the
Finite Impulse Response digital filter
In signal processing, a digital filter is a system that performs mathematical operations on a sampled, discrete-time signal to reduce or enhance certain aspects of that signal. This is in contrast to the other major type of electronic filter, t ...
and whose the coefficients are not fixed, but
adapted to minimize a suitable
functional
Functional may refer to:
* Movements in architecture:
** Functionalism (architecture)
** Form follows function
* Functional group, combination of atoms within molecules
* Medical conditions without currently visible organic basis:
** Functional sy ...
while multiple delays
act on the input signal.
Given:
*
: residual complex error at time instant
,
*
: baseband main received signal complex sample at time instant
,
*
: baseband cross received signal complex sample at time instant
,
*
: complex coefficient of baseband equalizer on the tap j and time instant
,
*
: XPIC complex coefficient on tap j and time instant
,
*
: index of tap
*
: result of cancelling action feeding the decision device at time instant
,
*
: estimated transmitted data at time instant
, so
=
-
*
: step-size or compression factor for adaptativity,
if the function to minimize is for example the mean power the residual error,
, the adapting
Gradient descent, gradient algorithm prescribes that the coefficients are
updated after every time step
k as:
*
C^_=C^_-\gamma\epsilon_k(s^_)^*;
*
C^_=C^_-\gamma\epsilon_k (s^_)^*;
where the asterisk denotes
complex-conjugation. No a-priori knowledge on the transmitted symbols is required with this basic scheme (
blind or zero-knowledge).
When the delay
D is equal to the symbol period, the blocks are denoted as symbol-spaced, while if
D is a fraction of the symbol period the blocks are said to be fractionally-spaced.
Other minimizing functions are least mean square LMS or zero forcing ZF while the architecture can be a Decision Feedback or further improved by means of known signals (
Pilot signal).
See also
Interference erasing system with independent receivers patentCross-polarization transmission system with asynchronous receivers patent*
Communications system
A communications system or communication system is a collection of individual telecommunications networks, transmission systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interoperat ...
*
Adaptive equalizer An adaptive equalizer is an equalizer that automatically adapts to time-varying properties of the communication channel. It is frequently used with coherent modulations such as phase-shift keying, mitigating the effects of multipath propagation ...
References
{{reflist
Data transmission
Digital signal processing>\epsilon_k, ^2/math>, the adapting
Gradient descent, gradient algorithm prescribes that the coefficients are
updated after every time step
k as:
*
C^_=C^_-\gamma\epsilon_k(s^_)^*;
*
C^_=C^_-\gamma\epsilon_k (s^_)^*;
where the asterisk denotes
complex-conjugation. No a-priori knowledge on the transmitted symbols is required with this basic scheme (
blind or zero-knowledge).
When the delay
D is equal to the symbol period, the blocks are denoted as symbol-spaced, while if
D is a fraction of the symbol period the blocks are said to be fractionally-spaced.
Other minimizing functions are least mean square LMS or zero forcing ZF while the architecture can be a Decision Feedback or further improved by means of known signals (
Pilot signal).
See also
Interference erasing system with independent receivers patentCross-polarization transmission system with asynchronous receivers patent*
Communications system
A communications system or communication system is a collection of individual telecommunications networks, transmission systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interoperat ...
*
Adaptive equalizer An adaptive equalizer is an equalizer that automatically adapts to time-varying properties of the communication channel. It is frequently used with coherent modulations such as phase-shift keying, mitigating the effects of multipath propagation ...
References
{{reflist
Data transmission
Digital signal processing