CA1119255A - Circuit and method for reducing polarization crosstalk caused by rainfall by suitable manipulation of the signals prior to transmission - Google Patents

Abstract

CIRCUIT AND METHOD FOR REDUCING POLARIZATION CROSSTALK
CAUSED BY RAINFALL BY SUITABLE MANIPULATION OF THE
SIGNALS PRIOR TO TRANSMISSION
Abstract of the Disclosure A circuit and a method are disclosed for reducing the effects of polarization crosstalk, at a remote location, when the polarization crosstalk is due to the interaction of two common frequency RF signals, caused by rainfall. This reduction in the polarization crosstalk is achieved by suitable manipulation of the signals prior to transmission. The present invention functions by applying circuitry, to the transmitter, for pre-distorting the two signals being transmitted by the transmitter circuitry. This pre-distortion is produced by introducing a controlled amount of coupling between the two signals. This amount of coupling is controlled in magnitude and phase so as to ideally be equal in magnitude to, but 180 degrees out of phase with, the polarization crosstalk introduced by the transmission medium. The ideal result is that the polarization crosstalk introduced during transmission of the signals is cancelled by the coupling introduced prior to transmission. In the preferred embodiment, the magnitude of an RF signal originating at the remote location and received at the transmit location is used as the basis for controlling the amount of coupling introduced between the two signals prior to transmission. As a general rule, the less the magnitude of the received signal, the greater is the polarization crosstalk, and the greater is the amount of coupling introduced between the two signals prior to transmission. It should be appreciated that this is only an approximate correction scheme and will seldom, if ever, produce perfect correction.

- i -

Description

~i119ZS5 The present invention relates to rain depolarization in communication links, for example ground to satellite communications, and more particularly to a novel manner of reducing the effects of polarization crosstalk between two RF signals being received at a remote location by suitable manipulation of the signals, at the transmit locations, prior to transmission to the remote location.
As the frequency spectrum becomes more and more crowded, various ways and means are being devised to make more efficient use of the available spectrum. One of these methods, employed in ground to satellite communications, is the use of dual-polarized communication links, also referred to as "spectrum re-use". In spectrum re-use, the available bandwidth is re-used by transmitting two independent signals on a single radio frequency by using dual polarization. Dual polarization comprises transmitting two orthogonally oriented signals (e.g. one signal having horizontal polarization and the other having vertical polarization). This theoretically results in a doubling of communication channels, as long as the polarized signals preserve their orthogonality (or a sufficient degree of orthogonality). The lack of "pure" orthogonal polarization (i.e. depolarization) will result in a coupling between the two communication channels which has been called "polarization crosstalk".
Polarization crosstalk between orthogonally polarized signals is introduced by imperfections in either or both of the transmitting and the receiving antennas, and the medium; the dominant contributor being the medium. One of the major elements of the medium to cause polarization crosstalk is rain. Orthogonal polarization components experience differential phase shift and attentuation due to the oblate nature of raindrops. This problem is well recognized in the art and solutions have been proposed to solve it. The articles "Adaptive Polarization Control for Satellite Frequency Reuse Systems"

- 1 - ~

11~9Z55 by D.F. DiFonzo, W.S. Trachtman, and A.E. Williams in COMSAT Review pages 253-283, Vol. 6, No. 2 Fall 1976 and "Spectrum Reuse by Adaptive Polarization Separation" by B.D. Cullen, A. Giantasio, G. Pelchat and L.R. Young in 1975 National Telecommunications Conference, pages 43-18 to 43-25 address themselves to solving this problem.
Both of the foregoing articles employ the use of pilot signals to achieve orthogonality correction and thereby reduce the polarization crosstalk. The article by B.D. Cullen et al also mentions alternatives to the pilot signal technique (i.e. Carrier Offset Frequency, Known Pattern Injection, and Decision Voltage Signal-to-Noise Ratio, all on page 43-22).
Stated in rather simplistic terms, the prior art method of detecting polarization crosstalk employing pilot signals works as follows. A pilot signal having frequency A is transmitted on polarization 1, and a pilot signal having frequency B is transmitted on polarization 2, which is orthogonal to polarization 1. At the receive end, if perfect orthogonality and separation is maintained throughout the transmission path, the pilot signal with a frequency A
should be detected on polarization 1 and be absent from polarization 2;
similarly, the pilot signal having a frequency B should be detected on polarization 2 and be absent from polarization 1. If one pilot signal (e.g. frequency A~ is detected on both polarizations 1 and 2, then this is an indication of polarization crosstalk.
In the prior art, orthogonality correction is applied after reception according to the following general method. A small portion of the signal appearing on polarization 2 is diverted and is passed through a variable attenuator and through a variable phase shifter and is then added to the signal appearing on polarization 1, thereby cancelling (at least partially) that component of ll~9ZS5 polarization 2 appearing on polarization 1. The variable attenuator and the variable phase shifter are controlled by signals derived from monitoring the output of polarization 1 (after correction) for the presence of the pilot signal with frequency B (i.e. the performance of the correction is monitored). This method can be used in the RF (radio frequency~ portion of the receiver or in the IF (intermediate frequency) portion of the receiver. Correction for polarization 2 is accomplished in an analogous manner. This method assumes that the undesired s;gnal on one polarization is completely correlated with the desired signal on the other polarization. More details and formulae regarding this corrective technique can be found in the aforementioned articles by D.F. DiFonzo et al and by B.D. Cullen et al, and attention is directed to them.
An article entitled "Phase of Crosspolarized Signals on Microwave Satellite Links" by N.J. McEwan appearing in Electonics Letters, August 4, 1977, Vol. 13, No. 16 describes a system differing from that of the aforementioned prior art, in that it only controls the amplitude of the "cancellation" signal, and not its phase (i.e. a one parameter corrective system). Such a system is possible since N.J. McEwan is primarily concerned with polarization crosstalk caused by ice crystals. N.J. McEwan states that when the polarization crosstalk is caused by ice crystals, a one-parameter adaptive cancellation system injecting a pure quadrature cancellation signal produces acceptable results. The article is silent on the method for controlling the operation of this one-parameter cancellation system.
The subject matter of the present invention is similar to that of the prior art discussed above, in that the general object is the same (i.e. to reduce the effects of polarization crosstalk).
The differences between the present invention and the prior art reside in how the corrective apparatus (i.e. the variable attenuators and variable phase shifters~ are interconnected and are controlled to produce the desired results. The corrective apparatus of the present invention is applied not to the receiver, but rather, is applied to the transmitter. In other words, the corrective apparatus of the present invention is interconnected so as to "pre-distort" the signals of two transmitters, by introducing a controlled amount of coupling between the two signals to thereby simulate the effects of polarization crosstalk. The amount of coupling (or pre-distortion) so introduced is controlled such that the polarization crosstalk introduced by the medium cancels most of the pre-distortion and the result, at the receive antenna, is a signal with reduced polarization crosstalk.
Considering orthogonally polarized RF signals, the phase and amplitude of the cross-polarized unwanted signal, relative to the main (desired) signal, is directly related, during a rainfall, to the amount of fade in the main signal. Depolarization studies carried out by BNR (Bell-Northern Research) and CRC (Communications Research Centre) of the Department of Communications have shown that this relationship between the various parameters is, by and large, valid for most rainfall situations. Thus, on a statistical basis, one can predict the amount of interference cancellation required during a rainfall, by measuring the amount of fade of the main signal.
The present invention takes advantage of this fact and applies it to transmitter circuitry to "pre-distort" the transmitted signals and to perform the correction for polarization crosstalk at the transmit end of a communications link, rather than at the receive end as is done in the prior art. According to the preferred embodiment of the present invention the received magnitude of an RF signal, received at the transmit end and originating at the receive end, is employed to determine the amount of polarization crosstalk being introduced by the transmission medium. Suitable logic circuitry, responsive to the ~1~9255 magnitude of this received signal, is employed to control the variable attenuators and variable phase shifters to provide a predetermined amount of pre-distortion in the two transmitted signals dependent upon the magnitude of the aforementioned received signal.
It should be noted that such a technique requires a relatively constant power output from the transmitter producing the signal referred to as the received signal. It should also be noted that a correction scheme of this type is not expected to be 100% perfect;
other factors, besides rain, may cause a change in the magnitude of the main signal. Additionally, as previously noted, this scheme is based upon statistical methods, and as such, may seldom (or never) produce a perfect result, but it will usually produce an improved result. It should be noted also that this scheme does not require any special pilot signals.
Briefly stated, one embodiment of the present invention is a circuit, for use in transmitter circuitry at a transmit location, for reducing the effects of polarization crosstalk occurring on a first RF signal at a remote location due to a second RF signal, wherein the first and second RF signals, after transmission by suitable antenna means at the transmit location, are two approximately orthogonally polarized RF signals having essentially the same frequency, the circuit characterized by the introduction, in response to a control signal, of a selective amount of the second RF signal into the first RF signal prior to transmission in order to at least partially cancel the effects of the polarization crosstalk introduced on the first RF signal, by the second RF signal, during their passage between the antenna means and the remote location.
Stated in other terms, the present invention is a circuit, for reducing the effects of polarization crosstalk at a remote location due to the interaction of two RF signals wherein, after transmission by suitable antenna means at a transmit location, lll~ZSS

the two RF signals are two approximately orthogonally polarized RF
signals having essentially the same frequency and travelling via essentially the same route to the remote location, the circuit characterized by the introduction of a selective amount of coupling between the two signals prior to transmission so as to approximately cancel the effects of the polarization crosstalk introduced on both the RF signals during their passage between the antenna means and the remote locat;on.
Stated in yet other terms, the present invention is a circuit for use in transmitter circuitry at a transmit location, for reducing the effects of polarization crosstalk occurring on a first RF signal at a remote location due to a second RF signal, wherein the first and second RF signals, after transmission by suitable antenna means, are two approximately orthogonally polarized RF signals having essentially the same frequency, the circuit operating on the signals before they are transmitted by the suitable antenna means, the circuit comprising: a first signal path for the first RF signal prior to transmission, the first path comprising both RF and IF sections; a second signal path for the second RF signal prior to transmission, the second path comprising both RF and IF sections; a first coupler means for diverting a portion of the signal on the first signal path into a flrst network comprising the series combination of a variable attenuator and a variable phase shifterj a first summing means for summing both the signal on the second signal path and the signal output from the first network; a second coupler means for diverting a portion of the signal on the second signal path into a second network comprising the series combination of a variable attenuator and a variable phase shifter; a second summing means for summing both the signal on the first signal path and the signal output from the second networki a receiver means, at the transmit location, both for receiving lll9Z55 a third RF signal originating at the remote location and transmitted through the same medium as are the first and second signals, and for producing an output signal indîcative of the received magnitude of the third RF signal; logic means, responsive to the output signal, for controlling both the variable attenuators and both the variable phase shifters in a predetermined manner.
Stated in still other terms, the present invention is a method, for use in transmitter circuitry at a transmit location, for reducing the effects of polarization crosstalk occurring on a first RF signal at a remote location due to a second RF signal, wherein the first and second RF signals, after transmission by suitable antenna means at the transmit location, are two approximately orthogonally polarized RF signals having essentially the same frequency, the method characterized by the introduction of a predetermined amount of the second RF signal into the first RF signal prior to transmission, in order to cancel the effects of the polarization crosstalk introduced on the first RF signal, by the second RF signal, during the passage of the two RF signals between the antenna means and the remote location.
The invention will now be described in more detail with reference to the accompanylng single figure which is a simplified block diagram of the preferred embodiment of the present invention.
Figure 1 depicts the final stages of a first transmitter referenced generally by the numeral 10 and the final stages of a second transmitter referenced generally by the numeral 11. Additionally, there is depicted a portion of a receiver circuit referenced generally by the numeral 12, and coupling circuitry 13 for introducing a predetermined amount of coupling between transmitters 10 and 11 to simulate the effects of polarization crosstalk. A duplexer 14 permits a single antenna 15 to be used for both transmitter 10 and receiver 12; a duplexer 16 is employed to permit a single antenna 17 to be used for both transmitter 11 and a receiver (not shown~.
In more detail, the final stages of transmitter 10 (the earlier stages are not germane to this discussion and are being omitted in the interest of conciseness) comprise an IF (intermediate frequency) amplifier 20, a coupler 21, a summing circuit 22, an amplifier 23, a local oscillator 24, a frequency mixer 25 otherwise known as an "up-converter", and an amplifier 26. The RF (radio frequency) output signal 27 of transmitter 10 is applied to one terminal of duplexer 14.
The final stage of transmitter 11 (the earlier stages are not germane to this discussion and are being omitted in the interest of conciseness) comprise an IF amplifier 30, a coupler 31, a summing circuit 32, an amplifier 33, a local oscillator 34, a frequency mixer 35 otherwise known as an "up-converter", and an amplifier 36.
The RF output signal 37 of transmitter 11 is applied to one terminal of duplexer 16, signal 37 has the same frequency as does signal 27 from transmitter 10 (e.g. 4 Gigahertz).
As transmitters 10 and 11 are conventional in their operation they will not be discussed in any further detail. Attention will now be directed to coupling circuitry 13 and its interconnection, via couplers 21 and 31 and summing circuit 22 and 32, to the transmitters 10 and 11.
Coupler 21 allows most of the signal energy from amplifier 20 to pass to summing circuit 22, a small portion of the signal energy from amplifier 20 is diverted by coupler 21 to attenuator 40. Attenuator 40 is connected in series with phase shifter 41, and the output of phase shifter 41 is connected to one input of summing circuit 32. The purpose of the series circuit of attenuator 40 and phase shifter 41 is to introduce a signal from transmitter 10 (via coupler 21), of controlled magnitude and phase, into the signal being carried by lll9Z5~ii transmitter 11 tVia summing circuit 32~. Logic circuitry 42 provides control signals on lines 43 and 44 for controlling the operation of attenuator 40 and phase shifter 41~ respectively. Similarly, phase shifter 45 and attenuator 46 are connected in a series circuit relationship between coupler 31 and summing circuit 22 as shown in the Figure. The purpose of this is to introduce a signal from transmitter 11 (via coupler 31), of controlled magnitude and phase, into the signal being carried by transmitter 10 (via summing circuit 22).
Phase shifter 45 and attenuator 46 are responsive to the control signals on lines 44 and 43, respectively, as shown in the Figure.
Logic circuitry 42 is itself responsive to the magnitude of the received RF signal from antenna 15, via duplexer 14. Duplexer 14 couples antenna 15 to low noise amplifier 50 via line 51. The output of amplifier 50 is applied to one input of mixer 52. The other input of mixer 52 is fed from local oscillator 53 as is well known in the art. The output of mixer 52, on line 54, is an intermediate frequency (IF) signal that is applied to IF amplifier 55. The output of amplifier 55 is fed to coupler 56 which allows most of the signal energy from amplifier 55 to pass to the remainder of the receiver circuitry which is not germane to this discussion and hence is not depicted in the drawings in the interest of clarity. Coupler 56 allows a small portion of the signal energy from amplifier 55 to be applied to line 57. Crystal diode detector 58 rectifies the IF voltage it receives on line 57 from coupler 56 and produces on line 59 a signal the magnitude of which is indicative of the magnitude of the received RF
signal from antenna 15. Logic circuitry 42 is responsive to the signal on line 59, according to a predetermined relationship, to provide on lines 43 and 44 signals for controlling the amount of attenuation and the amount of phase shift respectively, applied to the coupling circuitry 13 for producing a predetermined amount of coupling between the signals ~119~55 of transmitters 10 and 11 and thereby simulate the effects of polarization crosstalk. The amount and phase of the coupling is controlled so that in theory it will be completely eliminated by the time the RF signals reach the receive location due to the polarization crosstalk introduced by the med;um through which the RF waves pass. In short~ rather than cancel the polarization crosstalk at the receive location after it has been introduced by the transmission medium, the "correction" is added in the form of coupling between the two RF signals, prior to transmission, and the polarization crosstalk introduced by the transmission medium cancels out (theoretically) the coupling introduced in the transmitters 10 and 11.
Logic circuitry 42 controls the amount of coupling introduced by coupling circuitry 13 in response to the magnitude of the signal received by receiver circuit 12. As stated previously, when considering orthogonally polarized RF signals, the phase and amplitude of the cross-polarized unwanted signal is related, during a rainfall, to the amount of fade in the main signal. Consequently, by measuring the magnitude of a received signal one can obtain an approximate measure of the amount of polarization crosstalk that will be produced between two orthogonal RF signals traversing the same path as the received signal. In Figure 1, the magnitude of the signal received by receiver circuitry 12 is applied to logic circuitry 42 (by line 59) which in turn controls the degree and phase of coupling provided by circuitry 13 to correspond to the degree of polarization crosstalk that statistical studies have shown will be present due to rainfall and that magnitude of received signal. As this method of polarization crosstalk is based upon the magnitude nf the received signal other factors besides rainfall that affect the magnitude of the received signal can produce erroneous results (such as uneven 1 1 1 9 ~ 5 power output from the transmitter producing the RF signal that is used as the reference "received signal").
While Figure l depicts the preferred embodiment of the present invention with the control of circuitry 13 depending upon the magnitude of the RF signal received by receiver circuitry l2, an alternative embodiment dispenses with receiver circuitry 12. This alternative embodiment allows the input on line 59 to be set manually from a variable voltage source. In short, if there is no rainfall known to be in the transmission path, an operator sets the voltage applied to line 59 at a predetermined reference value; if there is a light rainfall he sets the voltage level applied to line 59 to a lower value. As the rainfall volume increases he keeps lowering the reference voltage applied to line 59, thereby increasing the amount of coupling between the two transmitted signals. This is of course a very rough and ready approach, but it is intended to be covered by the appended claims.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A circuit, for use in transmitter circuitry at a transmit location, for reducing the effects of polarization crosstalk occurring on a first RF signal at a remote location due to a second RF signal, wherein said first and second RF signals, after transmission by suitable antenna means at said transmit location, are two approximately orthogonally polarized RF signals having essentially the same frequency, said circuit characterized by the introduction, in response to a control signal, of a selective amount of said second RF signal into said first RF signal prior to transmission in order to at least partially cancel the effects of the polarization crosstalk introduced on said first RF signal, by said second RF signal, during their passage between said antenna means and said remote location, wherein said control signal is representative of the received magnitude of an RF signal originating at said remote location and received at said transmit location.

2. A circuit for reducing the effects of polarization crosstalk at a remote location due to the interaction of two RF signals wherein, after transmission by suitable antenna means at a transmit location, said two RF signals are two approximately orthogonally polarized RF signals having essentially the same frequency and travelling via essentially the same route to said remote location, said circuit characterized by the introduction of a selective amount of coupling between the two signals prior to transmission so as to approximately cancel the effects of the polarization crosstalk introduced on both said RF signals during their passage between said antenna means and said remote location, wherein the magnitude and phase of said selective amount of coupling is based upon the magnitude of a received RF signal originating at said remote location, received at said transmit location and traversing approximately the same route as do said two RF signals from said transmit location.

3. A circuit, for use in transmitter circuitry at a transmit location, for reducing the effects of polarization crosstalk occurring on a first RF signal at a remote location due to a second RF signal, wherein said first and second RF signals are, after transmission by suitable antenna means, two approximately orthogonally polarized RF signals having essentially the same frequency, and wherein, before transmission by said suitable antenna means said first RF signal follows a first signal path including both RF and IF sections, and said second RF signal follows a second signal path including both RF and IF sections, a portion of the signal in said first signal path is passed through the series combination of a first variable attenuator and a first variable phase shifter, and the resultant signal is added to said second signal path, a portion of the signal in said second signal path is passed through the series combination of a second variable attenuator and a second variable phase shifter, and the resultant signal is added to said first signal path, said circuit characterized by:
a receiver means for receiving, at said transmit location, a third RF signal originating at said remote location and transmitted through the same medium as are said first and second RF signals;
said receiver means producing an output signal indicative of the received magnitude of said third RF signal, logic means responsive to said output signal, for controlling both said variable attenuators and both said variable phase shifters in a predetermined manner.

4. A circuit for use in transmitter circuitry at a transmit location, for reducing the effects of polarization crosstalk occurring on a first RF signal at a remote location due to a second RF signal, wherein said first and second RF signals, after transmission by suitable antenna means, are two approximately orthogonally polarized RF signals having essentially the same frequency, said circuit operating on said signals before they are transmitted by said suitable antenna means, said circuit comprising:
a first signal path for said first RF signal prior to transmission, said first path comprising both RF and IF sections, a second signal path for said second RF signal prior to transmission, said second path comprising both RF and IF sections a first coupler means for diverting a portion of the signal on said first signal path into a first network comprising the series combination of a variable attenuator and a variable phase shifter;
a first summing means for summing both said signal on said second signal path and the signal output from said first network;
a second coupler means for diverting a portion of the signal on said second signal path into a second network comprising the series combination of a variable attenuator and a variable phase shifter;
a second summing means for summing both said signal on said first signal path and the signal output from said second network;
a receiver means, at said transmit location, both for receiving a third RF signal originating at said remote location and transmitted through the same medium as are said first and second signals, and for producing an output signal indicative of the received magnitude of said third RF signal;
logic means, responsive to said output signal, for controlling both said variable attenuators and both said variable phase shifters in a predetermined manner.

5. The circuit of claim 4 wherein said polarization crosstalk is caused by rain occurring along the route which said first and second RF signals traverse.

6. The circuit of claim 4 wherein said transmit location is a ground station situated on the earth's surface and said remote location is a satellite in earth orbit.

7. The circuit of claim 4, 5 or 6 wherein the frequency of said first and second RF signals is in the order of 4 Gigahertz.

8. A method, for use in transmitter circuitry at a transmit location, for reducing the effects of polarization crosstalk occurring on a first RF signal at a remote location due to a second RF signal, wherein said first and second RF signals, after transmission by suitable antenna means at said transmit location, are two approximately orthogonally polarized RF signals having essentially the same frequency, said method characterized by the introduction of a predetermined amount of said second RF signal into said first RF signal prior to transmission, in order to cancel the effects of the polarization crosstalk introduced on said first RF signal, by said second RF signal, during the passage of said two RF signals between said antenna means and said remote location, wherein said predetermined amount of said second RF signal is controlled in response to the received magnitude of a third RF signal originating at said remote location and received at said transmit location.