CA2164018C - Adaptive cross-polarization equalizer - Google Patents
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- CA2164018C CA2164018C CA002164018A CA2164018A CA2164018C CA 2164018 C CA2164018 C CA 2164018C CA 002164018 A CA002164018 A CA 002164018A CA 2164018 A CA2164018 A CA 2164018A CA 2164018 C CA2164018 C CA 2164018C
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/06—Receivers
- H04B1/16—Circuits
- H04B1/18—Input circuits, e.g. for coupling to an antenna or a transmission line
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/01—Equalisers
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- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
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- Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
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Abstract
An equalizer employed with a receiver for cancelling an interfering signal due to low cross-polarization isolation. The present equalizer provides a simple and inexpensive cross-polarization interference cancellation system. The equalizer uses an error signal generated in an adaptive baseband equalizer to determine the magnitude of an interfering signal and inputs a cancelling signal to the co-polarized channel until power in the error signal derived from the adaptive baseband equalizer is minimized. In the equalizer, the determination of the magnitude of the interfering signal is performed at baseband (after demodulation but prior to data detection) and the cancellation is performed at RF, prior to dispersive microwave elements. The equalizer enhances the capability for transmitting two unique signals in the same frequency allocation on orthogonal polarizations. The equalizer operates even if the modulations and data rates on the two channels are different. In the receiver, co- and cross-polarized signals are received and amplified. A copy of the cross channel input signal is supplied to each receiver channel. An RF vector modulator is used to control the phase and amplitude of the cross-polarized signal prior to summation with a desired co-polarized signal. When the amplitude and the phase of the vector modulator are correctly set the interfering signal is cancelled. Control for the vector modulator is generated by minimizing the magnitude of the error signal in the adaptive baseband equalizer. The error signal is the difference between the received baseband data and an estimate of the ideal transmitted data. A control logic circuit sets the phase and amplitude for the vector modulator to minimize the magnitude of the error signal by stepping the setting of the vectormodulator by one unit and sampling the power in the error signal. If the power is reduced the vector modulator is stepped again in the same direction. If the power is increased the vector modulator is stepped in the opposite direction.
Description
216401~
ADAPTIVE CROSS-POLARIZATION EQUALIZER
BACKGROUND
The present invention relates to adaptive equali~rs, and more particularly, to an adaptive cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-polarization isolation caused by rain on high data rate 5 co,,,mullicdtion links.
With the increasing utilization of radio frequency co~ nunications~ the allocatable radio frequency ~Jecllull~ is rapidly becoming limited. Con~ ;ial allocations are encroaching on frequencies which to date have been available forbroadband co-l-l"ullications. In the future, it will be difficult to provide contiguous RF
10 allocations of several gigahertz for multi-gigahertz communications systems.
Therefore, the radio frequency spectrum must be more efficiently managed and utilized.
r~uellcy reuse, using orthogonal polarizations, is one approach to greater efficiency.
Adaptive baseband cancella~ion architectures have h~l~torole been developed in order to increase the available radio frequency spectrum utilization, but without 15 complete success. These types of architectures typically use "four rail" adaptive basebd-ld transversal equalizers (ABBE). These architectures require co,l",ulation of the correlation of ~he interference on the I and Q signals of each channel with the I and Q signals of the other channel as well as the I and Q signals within each channel. These are very complex architectures and are tied to one modul~tion and data rate.
The present invention addresses the problem of depolarization in co.~ ications systems. Depolarization introduces an interfering signal from the cross-polarized signal into the co-polarized signal. The primary source of interfering signals that cause depolarization is rain. To oplh~ize system p~-ro.,-~ance, the effects 5 of the i~lt~lr~ g signals must be minimi7ed Available field test data and analysis to date indicdle that cross-polarization inle.r~le,lce due to rain has a n~;lu~ g amplitude and phase shift but that the depolarizing phenomena is nondispersive~ The rate of fluctuation is e~ ed to be less than 1 Hz. Furthermore, due to the dispersive nature of filters in downconverters and demodulators used in adaptive equali~rs and to avoid 10 having to match delays between tl-e two receivers, it is desirable to perform cross-polarization cancellation as close to the front end of the microwave chain as possible.
For the pu,~oses of reference, U.S. Patent No. 5,157,697 issued to Anvaru describes a system that sup~ sses crosstalk between orthogonal channels by ~ubLI~-ing a portion of the signal of one channel from the other as controlled by correlation factors. U.S. Patent No. 4,466,132 issued to Namiki describes a system that eliminates crosstalk between two mutually or~hogonal cross-polarized channels. The following references describe systems that are generally similar to the Namiki patent:
U.S. Patent No. 4,112,370 issued to Monson; U.S. Patent No. 4,438,530 issued to Steinberger, U.S. Patent No. 4,479,258 issued to Namiki; U.S. Patent No. 4,637,067 20 issued to Steinberger; and U.S. Patent No. 4,688,235 issued to Tahara et al. U.S.
Patent No. 3,735,266 issued to Amitay and U.S. Patent No. 4,090,137 issued to Soma et al describe systems that reduce or minimi7e crosstalk between cross-polarized channels utilizing pilot signals to indicate the level of crosstalk.
Therefore, it is an objective Or the present invention to provide for an adaptive 25 cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-polarization isolation caused by rain on high data rate co~ u,lication links.
SUMMARY OF THE INVENTION
In order to meet the above and other objectives, the present invention comprises30 an adaptive cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-polarization isolation caused by rain on high data rate commu-nication links. The adaptive cross-polarization equalizer is employed with a receiver that comprises co-polarization and cross-polarization receiver channels for processing CQ- and cross-polarized input signals. Each channel comprises a low noise amplifier, a 35 su~ ing device, a downconverter, a demodulator, a least mean-square estimate adaptive basebdnd equ~li7çr, and a bit sync data detection circuit.
2l64nls The adaptive cross-polarization equalizer comprises a baseband error signal detector, control logic, and a vector modulator. The baseband error signal detector includes first and second squaring circuits that process error signals geneMted by the least mean-square estim~te adaptive baseband equalizer. Squared outputs of the first 5 and second squaring circuits are st~mmed in a summing device and filtered by a low pass filter. The filtered output of the low pass filter is applied to the control logic.
The control logic comprises an analog to digital converter which pr~cesses the output of the low pass filter to produce l1igiti7~d signal for ~rocessing. The output of the analog to digital converter is processed by a logic circuit which gene.~t~s control 10 signals that control the vector modulator. The output of the logic circuit is demulti-plexed by a demultiplexer, and outputs of the demultiplexer are pl~cessed by the vector modulator.
The vector modulator comprises first and second multipliers that col-lbille output signals derived from the respective demultiplexer with output signals gene-aled 15 by a zero-degree hybrid. The zero-degree hybrid generates its output signals from the output of the power divider of the cross-polarization receiver channel. Outputs of the ~,;,yeclive multipliers are applied to a ninety-degree hybrid which combines thelG~peeli~e output signals lheferlunl and applies the summed signal to the ~ g device in the co-polari~d receiver channel which sums this signal with the amplified 20 co-polarized input signal.
The present equalizer uses an error signal generated in the adaptive baseband equali~r to determine the m~gnitllde of the in~e.r~.ing signal and then inputs ac lce-lling signal at the front end of the co-polarized channel until power in the error signal derived from the adaptive baseband equali~r is minimized. The del~-..lillalioll of the magnitude of the interfering signal is pt;.ro.llled at baseband (after demodulation but prior to data detection) and the cancellation is performed at RF, prior to dispersive microwave elements. The present equalizer enhances the capabilit,v for transmitting two unique signals in the same frequency allocation on orthogonal polarizations, andope-~t~s even if the modulations and data rates on the two channels are different.
The present equalizer provides a simple and inexpensive cross-polarization inte-r~l~nce cancellation system. The equalizer may be used with any digital modulation format and any data rate above 10 Mbps on either channel. The cancellation system can track variations in cross-polarization inl~-rerellce amplitude and phase of 10 Hz, ll~inilnulll. If the cross-polarization phenomena is dispersive, additional vector mod~ tors may be added at RF in a transversal filter arrangement and controlled using the same a,.;}~ile~;lulc,.
. 2164018 Control for the vector modulator is generated by minimizing the magnitude of the error signal in the adaptive baseband equalizer. The error signal is the dirre~ ce between the received baseband data and an idealized estim~te of the tl~ns.~ d data.
This signal is made up of data noise due to non-ideal channel, thermal noise, and the i"t~ l~t;,ing signal. Although the present equalizer is designed for receivers that employ an adaptive b~ceb~ncl transversal equalizer, the design is also applicable to receivers that do not have an adaptive baseband transversal equali~r. In this case, the error signal inputs to the equalizer are replaced by I and Q analog inputs and the error signal circuitry provided by the adap~ivc baseband transversal equalizer is hlcol~ ed into the equali~r.
The control logic circuit steps the se~ting of the vector modulator by one unit and sampling the power in the error signal. If the power is reduced the vector modulator is stepped again in the same direction. If the power is increased the vector modulator is stepped in the opposite direction. The vector modulator has two controls cul--,sponding to an I-Q coordinate system. The controls are dithered one at a time, four consecutive times each.
The present equalizer implementation does not require symmetry between the orthogonal receiver channels. They may differ in both data rate and modulation. If the two cross-polarization phenomena are not symmetrical, the equali~r is not adversely err~cled. If the cross-polarization pheno"~ella is dispersive, multiple vector modulators may be added in a transversal filter arrangement and controlled in the same manner.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken inconjullclion with the accompanying drawings, wherein like reference numerals desig~l~te like structural elements, and in which:
Fig 1 is a block diagram of an adaptive cross-polarization equalizer in accor~ance with the principles of the present invenlion; and Fig. 2 shows error signal generation circuit that may be employed with the equalizer of Fig. 1.
DETAILEI) DESCRIPTION
Referring to the drawing ~Igures, Fig 1 is a block diagram of a receiver 50 employing an adaptive cross-polarization equalizer l () in accordance with the principles of the present invention. The receiver 50 comprises co-polarization and cross-polarization receiver channels 51, 52 for .Gspecli~ely p,~cessh~g co-polarized and 2164()1~
cross-polarized signals. The co-polarized signals are amplified in a low noise amplifier 11 and applied to a summin~ device 12 which sums the ampli~1ed co-polarized signals with an output derived from the equalizer 10. The output of the su-l--nit-g device 12 is downconverted in a downconverter 13 and demodulated in a demodulator 15. I and Q5 output signals from the demodulator 15 are processed by a least mean-square e .li-llale adaptive baseband equalizer 16 whose I and ~ output signals are prucessed by a bit sync data detection circuit (BSDD) 17 to produce baseband output I and Q and clock (CLK) signals from the receiver 50. The cross-polarized signals are also ~ cessed by a substantially identical receiver channel 52 (allhough only a portion there is shown).
10 An output of a power divider 18 of the co-polarization receiver channel 51 is applied as an input signal to the adaptive cross-polarization equalizer 10. The circuitry shown in Fig. 1 illustrates only the adaptive cross polarization equalizer 10. A second equalizer i-lentir~l 10 is provided to equalize the cross-polarization channel 52.
The adaptive cross-polarization equalizer 10 comprises a baseband error signal 15 detector 40, control logic 30, and a vector modulator 20, .e~.~,ecli~ely encircled by dashed lines in Fig. 1. The baseband error signal detector 40 includes first and second squaring circuits (X2) 41, 42 that process error signals (Ic. Qc) generated by the least mean-square estim~te adaptive baseband equalizer 16. Squared outputs of the first and second squaring circuits 41, 42 are sl-mmed in a summing device 44 and filtered by a 20 low pass filter (LP~;) 43. The filtered output of the low pass filter 43 is applied to the control logic 30.
The control logic 30 compriscs an analog to digital converter (A/D) 36 which ~cesses the output of the low pass filter 43 to produce digitized signal for proces~.ing.
The output of the analog to digital converter 36 is processed by a logic circuit 35 25 (LOGIC) which generates control signals that control the vector modulator 20. The output of the logic circuit 35 is demultiplexed by a demultiplexer 33, and outputs of the demultiplexer 33 control the vector modulator 20.
The vector modulator 20 comprises first and second multipliers 22, 23 that combine output signals derived from the respective demultiplexer 33 with output 30 sig,nals generated by a zero-degree hybrid 24. The zero-degree hybrid 24 generates its output signals from the output of the power divider 18 of the cross-polarization receiver channel 52. Outputs of the ~.~ective multipliers 22, 23 are applied to a ninety-degree hybrid 21 which combines the respeclive output signals therefrom and applies thes~lmmed signal to the summing device 12 in the co-polarized receiver channel 51 which 35 sums this signal with the amplified co-polarized input signal.
The present equalizer 10 uses the error signal generated by the adaptive baseband equalizer 16 to determine the magnitude of the interfering signal and then 216~nls inputs a cancelling signal until power in the error signal from the adaptive baseband eqU~li7er 16 is minimi7~ In the present equalizer 10, the delelminalion of the m~gninlde of the interfering signal is performed at baseband (after demodulation but prior to data detection) and cancellation is performed at RF, prior to dispersive S microwave elements. The equalizer 10 enhances the capability for l-~"smilling two unique signals in the same frequency allocation on orthogonal polarizations. Theequ~li7~ 10 operates even if the modulations and data rates on the two channels 51, 52 are dirr~lent.
The present equalizer 10 provides a simple and in~n~ e cross-polarization 10 inl~lrelellce cancellation system. The equalizer 10 may be used with any digital modulation format and any data rate above ten Mbps on ei~her channel. The equali~r 10 tracks variations in cross-polarization interference amplitude and phase of 10 Hz, IllinilllUIII. If Ihe cross-polarization phenomena is dispersive, additional taps may be added at RF and controlled using the same architecture.
The present equalizer 10 provides a solution to the problem of cancelling an interfering signal due to low cross-polarization isolation caused by rain, for example, on high data rate communication links. In the receiver 50, both transmitted polarizations are received and amplified. A signal from the cross channel is supplied to the other receiver channel 51, 52. The RF vector modulator 20 is used to control the 20 phase and amplitude of the cross-polarized signal prior to ~u.~",~tion with the co-polari~d signal. When the amplitude and the phase of the vector modulator 20 areCOI~ ly set, the i~ re~ g signal is cancelled.
Control for the vector modulator 20 is generated by ",h~i,.,;,.;ng the m~gnitudeof the error signal in the least mean-square e~l;"~e adaptive baseband equalizer 16.
25 The error signal is the di~rer~"ce between the received baseband data and an esli--.~le of the ideal tr~nsmitted data. This signal is made up of data noise due to non-ideal channel, thermal noise, and the inte~re,ing signal. The ratio of the interfering signal power to noise power determines the effectiveness of the equalizer lO.
The control logic circuit 35 tllat sets ~he phase and amplitude for the vector 30 modulator 20 to minimize the magnitude of the error signal does so by stepping the setting of the vector modulator 20 by one unit and sampling the power in the error signal. If the power is reduced the vector modulator 20 is stepped again in the same direction. If the power is increased the vector modulator 20 is stepped in the opposite direction. The vector modulator 20 has two controls corresponding to an I-Q
35 coordinate system. The controls are dithered one at a time, four consecu~i~/e times each using the demultiplexer 33.
216~018 Although the present equalizer 10 is designed for receivers 50 that employ an adaptive baseband transversal equalizer 16, the design is also applicable to receivers 50 that do not have an adaptive baseband transversal equali~r 16. In this case, error signal inputs to the equali~r 10 are replaced by I and Q analog inputs and the error S signal circuitry normally provided by the adaptive baseband transversal equalizer is inco,yo~ted into the present equalizer 10. An error signal gen~ ion circuit 60 that implements this is shown in Fig. 2. The circuit 60 is comprised of a power divider 61 and inverter 62, a limiter 63, and a summing device 64 connecl~d as shown that ,~e;"~)ecli~ely process the I and Q signals to generate the I and Q error signals (Ic, Qe).
10 The circuit 60 of Fig. should be well understood by those skilled in the art.The present equalizer 10 does not require symmetry between the orthogonal channels 51, 52. They may differ in both data rate and modulation. If the two cross polarization phenomena are not symmetrical, the equalizer 10 is not adversely err~ ed.
If the cross-polarization phenomena is dispersive, multiple vector modulators 20 may 15 be added in a transversal filter arrangement and controlled in the same manner as described above. The present equalizer 10 has been built and tested in a labo,atc,.~-against a cross-polarization model and works very well.
Thus there has been described a new and improved adaptive cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-20 polarization isolation caused by rain on high data rate co,-,-..,lnication links. It is to be undc,~lood that the above-described embodiment is merely illustrative of some of the many specific embodiments which reyl~sent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.
ADAPTIVE CROSS-POLARIZATION EQUALIZER
BACKGROUND
The present invention relates to adaptive equali~rs, and more particularly, to an adaptive cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-polarization isolation caused by rain on high data rate 5 co,,,mullicdtion links.
With the increasing utilization of radio frequency co~ nunications~ the allocatable radio frequency ~Jecllull~ is rapidly becoming limited. Con~ ;ial allocations are encroaching on frequencies which to date have been available forbroadband co-l-l"ullications. In the future, it will be difficult to provide contiguous RF
10 allocations of several gigahertz for multi-gigahertz communications systems.
Therefore, the radio frequency spectrum must be more efficiently managed and utilized.
r~uellcy reuse, using orthogonal polarizations, is one approach to greater efficiency.
Adaptive baseband cancella~ion architectures have h~l~torole been developed in order to increase the available radio frequency spectrum utilization, but without 15 complete success. These types of architectures typically use "four rail" adaptive basebd-ld transversal equalizers (ABBE). These architectures require co,l",ulation of the correlation of ~he interference on the I and Q signals of each channel with the I and Q signals of the other channel as well as the I and Q signals within each channel. These are very complex architectures and are tied to one modul~tion and data rate.
The present invention addresses the problem of depolarization in co.~ ications systems. Depolarization introduces an interfering signal from the cross-polarized signal into the co-polarized signal. The primary source of interfering signals that cause depolarization is rain. To oplh~ize system p~-ro.,-~ance, the effects 5 of the i~lt~lr~ g signals must be minimi7ed Available field test data and analysis to date indicdle that cross-polarization inle.r~le,lce due to rain has a n~;lu~ g amplitude and phase shift but that the depolarizing phenomena is nondispersive~ The rate of fluctuation is e~ ed to be less than 1 Hz. Furthermore, due to the dispersive nature of filters in downconverters and demodulators used in adaptive equali~rs and to avoid 10 having to match delays between tl-e two receivers, it is desirable to perform cross-polarization cancellation as close to the front end of the microwave chain as possible.
For the pu,~oses of reference, U.S. Patent No. 5,157,697 issued to Anvaru describes a system that sup~ sses crosstalk between orthogonal channels by ~ubLI~-ing a portion of the signal of one channel from the other as controlled by correlation factors. U.S. Patent No. 4,466,132 issued to Namiki describes a system that eliminates crosstalk between two mutually or~hogonal cross-polarized channels. The following references describe systems that are generally similar to the Namiki patent:
U.S. Patent No. 4,112,370 issued to Monson; U.S. Patent No. 4,438,530 issued to Steinberger, U.S. Patent No. 4,479,258 issued to Namiki; U.S. Patent No. 4,637,067 20 issued to Steinberger; and U.S. Patent No. 4,688,235 issued to Tahara et al. U.S.
Patent No. 3,735,266 issued to Amitay and U.S. Patent No. 4,090,137 issued to Soma et al describe systems that reduce or minimi7e crosstalk between cross-polarized channels utilizing pilot signals to indicate the level of crosstalk.
Therefore, it is an objective Or the present invention to provide for an adaptive 25 cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-polarization isolation caused by rain on high data rate co~ u,lication links.
SUMMARY OF THE INVENTION
In order to meet the above and other objectives, the present invention comprises30 an adaptive cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-polarization isolation caused by rain on high data rate commu-nication links. The adaptive cross-polarization equalizer is employed with a receiver that comprises co-polarization and cross-polarization receiver channels for processing CQ- and cross-polarized input signals. Each channel comprises a low noise amplifier, a 35 su~ ing device, a downconverter, a demodulator, a least mean-square estimate adaptive basebdnd equ~li7çr, and a bit sync data detection circuit.
2l64nls The adaptive cross-polarization equalizer comprises a baseband error signal detector, control logic, and a vector modulator. The baseband error signal detector includes first and second squaring circuits that process error signals geneMted by the least mean-square estim~te adaptive baseband equalizer. Squared outputs of the first 5 and second squaring circuits are st~mmed in a summing device and filtered by a low pass filter. The filtered output of the low pass filter is applied to the control logic.
The control logic comprises an analog to digital converter which pr~cesses the output of the low pass filter to produce l1igiti7~d signal for ~rocessing. The output of the analog to digital converter is processed by a logic circuit which gene.~t~s control 10 signals that control the vector modulator. The output of the logic circuit is demulti-plexed by a demultiplexer, and outputs of the demultiplexer are pl~cessed by the vector modulator.
The vector modulator comprises first and second multipliers that col-lbille output signals derived from the respective demultiplexer with output signals gene-aled 15 by a zero-degree hybrid. The zero-degree hybrid generates its output signals from the output of the power divider of the cross-polarization receiver channel. Outputs of the ~,;,yeclive multipliers are applied to a ninety-degree hybrid which combines thelG~peeli~e output signals lheferlunl and applies the summed signal to the ~ g device in the co-polari~d receiver channel which sums this signal with the amplified 20 co-polarized input signal.
The present equalizer uses an error signal generated in the adaptive baseband equali~r to determine the m~gnitllde of the in~e.r~.ing signal and then inputs ac lce-lling signal at the front end of the co-polarized channel until power in the error signal derived from the adaptive baseband equali~r is minimized. The del~-..lillalioll of the magnitude of the interfering signal is pt;.ro.llled at baseband (after demodulation but prior to data detection) and the cancellation is performed at RF, prior to dispersive microwave elements. The present equalizer enhances the capabilit,v for transmitting two unique signals in the same frequency allocation on orthogonal polarizations, andope-~t~s even if the modulations and data rates on the two channels are different.
The present equalizer provides a simple and inexpensive cross-polarization inte-r~l~nce cancellation system. The equalizer may be used with any digital modulation format and any data rate above 10 Mbps on either channel. The cancellation system can track variations in cross-polarization inl~-rerellce amplitude and phase of 10 Hz, ll~inilnulll. If the cross-polarization phenomena is dispersive, additional vector mod~ tors may be added at RF in a transversal filter arrangement and controlled using the same a,.;}~ile~;lulc,.
. 2164018 Control for the vector modulator is generated by minimizing the magnitude of the error signal in the adaptive baseband equalizer. The error signal is the dirre~ ce between the received baseband data and an idealized estim~te of the tl~ns.~ d data.
This signal is made up of data noise due to non-ideal channel, thermal noise, and the i"t~ l~t;,ing signal. Although the present equalizer is designed for receivers that employ an adaptive b~ceb~ncl transversal equalizer, the design is also applicable to receivers that do not have an adaptive baseband transversal equali~r. In this case, the error signal inputs to the equalizer are replaced by I and Q analog inputs and the error signal circuitry provided by the adap~ivc baseband transversal equalizer is hlcol~ ed into the equali~r.
The control logic circuit steps the se~ting of the vector modulator by one unit and sampling the power in the error signal. If the power is reduced the vector modulator is stepped again in the same direction. If the power is increased the vector modulator is stepped in the opposite direction. The vector modulator has two controls cul--,sponding to an I-Q coordinate system. The controls are dithered one at a time, four consecutive times each.
The present equalizer implementation does not require symmetry between the orthogonal receiver channels. They may differ in both data rate and modulation. If the two cross-polarization phenomena are not symmetrical, the equali~r is not adversely err~cled. If the cross-polarization pheno"~ella is dispersive, multiple vector modulators may be added in a transversal filter arrangement and controlled in the same manner.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken inconjullclion with the accompanying drawings, wherein like reference numerals desig~l~te like structural elements, and in which:
Fig 1 is a block diagram of an adaptive cross-polarization equalizer in accor~ance with the principles of the present invenlion; and Fig. 2 shows error signal generation circuit that may be employed with the equalizer of Fig. 1.
DETAILEI) DESCRIPTION
Referring to the drawing ~Igures, Fig 1 is a block diagram of a receiver 50 employing an adaptive cross-polarization equalizer l () in accordance with the principles of the present invention. The receiver 50 comprises co-polarization and cross-polarization receiver channels 51, 52 for .Gspecli~ely p,~cessh~g co-polarized and 2164()1~
cross-polarized signals. The co-polarized signals are amplified in a low noise amplifier 11 and applied to a summin~ device 12 which sums the ampli~1ed co-polarized signals with an output derived from the equalizer 10. The output of the su-l--nit-g device 12 is downconverted in a downconverter 13 and demodulated in a demodulator 15. I and Q5 output signals from the demodulator 15 are processed by a least mean-square e .li-llale adaptive baseband equalizer 16 whose I and ~ output signals are prucessed by a bit sync data detection circuit (BSDD) 17 to produce baseband output I and Q and clock (CLK) signals from the receiver 50. The cross-polarized signals are also ~ cessed by a substantially identical receiver channel 52 (allhough only a portion there is shown).
10 An output of a power divider 18 of the co-polarization receiver channel 51 is applied as an input signal to the adaptive cross-polarization equalizer 10. The circuitry shown in Fig. 1 illustrates only the adaptive cross polarization equalizer 10. A second equalizer i-lentir~l 10 is provided to equalize the cross-polarization channel 52.
The adaptive cross-polarization equalizer 10 comprises a baseband error signal 15 detector 40, control logic 30, and a vector modulator 20, .e~.~,ecli~ely encircled by dashed lines in Fig. 1. The baseband error signal detector 40 includes first and second squaring circuits (X2) 41, 42 that process error signals (Ic. Qc) generated by the least mean-square estim~te adaptive baseband equalizer 16. Squared outputs of the first and second squaring circuits 41, 42 are sl-mmed in a summing device 44 and filtered by a 20 low pass filter (LP~;) 43. The filtered output of the low pass filter 43 is applied to the control logic 30.
The control logic 30 compriscs an analog to digital converter (A/D) 36 which ~cesses the output of the low pass filter 43 to produce digitized signal for proces~.ing.
The output of the analog to digital converter 36 is processed by a logic circuit 35 25 (LOGIC) which generates control signals that control the vector modulator 20. The output of the logic circuit 35 is demultiplexed by a demultiplexer 33, and outputs of the demultiplexer 33 control the vector modulator 20.
The vector modulator 20 comprises first and second multipliers 22, 23 that combine output signals derived from the respective demultiplexer 33 with output 30 sig,nals generated by a zero-degree hybrid 24. The zero-degree hybrid 24 generates its output signals from the output of the power divider 18 of the cross-polarization receiver channel 52. Outputs of the ~.~ective multipliers 22, 23 are applied to a ninety-degree hybrid 21 which combines the respeclive output signals therefrom and applies thes~lmmed signal to the summing device 12 in the co-polarized receiver channel 51 which 35 sums this signal with the amplified co-polarized input signal.
The present equalizer 10 uses the error signal generated by the adaptive baseband equalizer 16 to determine the magnitude of the interfering signal and then 216~nls inputs a cancelling signal until power in the error signal from the adaptive baseband eqU~li7er 16 is minimi7~ In the present equalizer 10, the delelminalion of the m~gninlde of the interfering signal is performed at baseband (after demodulation but prior to data detection) and cancellation is performed at RF, prior to dispersive S microwave elements. The equalizer 10 enhances the capability for l-~"smilling two unique signals in the same frequency allocation on orthogonal polarizations. Theequ~li7~ 10 operates even if the modulations and data rates on the two channels 51, 52 are dirr~lent.
The present equalizer 10 provides a simple and in~n~ e cross-polarization 10 inl~lrelellce cancellation system. The equalizer 10 may be used with any digital modulation format and any data rate above ten Mbps on ei~her channel. The equali~r 10 tracks variations in cross-polarization interference amplitude and phase of 10 Hz, IllinilllUIII. If Ihe cross-polarization phenomena is dispersive, additional taps may be added at RF and controlled using the same architecture.
The present equalizer 10 provides a solution to the problem of cancelling an interfering signal due to low cross-polarization isolation caused by rain, for example, on high data rate communication links. In the receiver 50, both transmitted polarizations are received and amplified. A signal from the cross channel is supplied to the other receiver channel 51, 52. The RF vector modulator 20 is used to control the 20 phase and amplitude of the cross-polarized signal prior to ~u.~",~tion with the co-polari~d signal. When the amplitude and the phase of the vector modulator 20 areCOI~ ly set, the i~ re~ g signal is cancelled.
Control for the vector modulator 20 is generated by ",h~i,.,;,.;ng the m~gnitudeof the error signal in the least mean-square e~l;"~e adaptive baseband equalizer 16.
25 The error signal is the di~rer~"ce between the received baseband data and an esli--.~le of the ideal tr~nsmitted data. This signal is made up of data noise due to non-ideal channel, thermal noise, and the inte~re,ing signal. The ratio of the interfering signal power to noise power determines the effectiveness of the equalizer lO.
The control logic circuit 35 tllat sets ~he phase and amplitude for the vector 30 modulator 20 to minimize the magnitude of the error signal does so by stepping the setting of the vector modulator 20 by one unit and sampling the power in the error signal. If the power is reduced the vector modulator 20 is stepped again in the same direction. If the power is increased the vector modulator 20 is stepped in the opposite direction. The vector modulator 20 has two controls corresponding to an I-Q
35 coordinate system. The controls are dithered one at a time, four consecu~i~/e times each using the demultiplexer 33.
216~018 Although the present equalizer 10 is designed for receivers 50 that employ an adaptive baseband transversal equalizer 16, the design is also applicable to receivers 50 that do not have an adaptive baseband transversal equali~r 16. In this case, error signal inputs to the equali~r 10 are replaced by I and Q analog inputs and the error S signal circuitry normally provided by the adaptive baseband transversal equalizer is inco,yo~ted into the present equalizer 10. An error signal gen~ ion circuit 60 that implements this is shown in Fig. 2. The circuit 60 is comprised of a power divider 61 and inverter 62, a limiter 63, and a summing device 64 connecl~d as shown that ,~e;"~)ecli~ely process the I and Q signals to generate the I and Q error signals (Ic, Qe).
10 The circuit 60 of Fig. should be well understood by those skilled in the art.The present equalizer 10 does not require symmetry between the orthogonal channels 51, 52. They may differ in both data rate and modulation. If the two cross polarization phenomena are not symmetrical, the equalizer 10 is not adversely err~ ed.
If the cross-polarization phenomena is dispersive, multiple vector modulators 20 may 15 be added in a transversal filter arrangement and controlled in the same manner as described above. The present equalizer 10 has been built and tested in a labo,atc,.~-against a cross-polarization model and works very well.
Thus there has been described a new and improved adaptive cross-polarization equalizer that provides for cancellation of an interfering signal due to low cross-20 polarization isolation caused by rain on high data rate co,-,-..,lnication links. It is to be undc,~lood that the above-described embodiment is merely illustrative of some of the many specific embodiments which reyl~sent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.
Claims (6)
1. An adaptive cross-polarization equalizer for use with a receiver that provides for cancellation of an interfering signal due to low cross-polarization isolation, wherein the receiver comprises co-polarization and cross-polarization receiver channels for respectively processing co-polarized and cross-polarized input signals, and wherein each channel comprises a low noise amplifier for amplifying the input signals, a power divider, a summing device, a downconverter, a demodulator, an adaptive baseband equalizer, and a bit sync data detection circuit for providing baseband output signals, and wherein said equalizer comprises:
a baseband error signal detector comprising first and second squaring circuits for squaring error signals (Ie, Qc) generated by the adaptive baseband equalizer, a summing device coupled to the squaring circuits for summing the squared error signals, and a low pass filter coupled to the summing device for filtering the squared error signals;
control logic comprising an analog to digital converter for processing the filtered squared error signals from the low pass filter to produce digitized signals for processing, a logic circuit coupled to the analog to digital converter for generating control signals, and a demultiplexer coupled to the logic circuit; and a vector modulator comprising a zero-degree hybrid, first and second multipliers coupled to the zero-degree hybrid and to the demultiplexer that combine output signals derived from the demultiplexer with output signals generated by the zero-degree hybrid, and a ninety-degree hybrid coupled to outputs of the respective multipliers for combining the output signals from the multipliers and applying it to the first summing device.
a baseband error signal detector comprising first and second squaring circuits for squaring error signals (Ie, Qc) generated by the adaptive baseband equalizer, a summing device coupled to the squaring circuits for summing the squared error signals, and a low pass filter coupled to the summing device for filtering the squared error signals;
control logic comprising an analog to digital converter for processing the filtered squared error signals from the low pass filter to produce digitized signals for processing, a logic circuit coupled to the analog to digital converter for generating control signals, and a demultiplexer coupled to the logic circuit; and a vector modulator comprising a zero-degree hybrid, first and second multipliers coupled to the zero-degree hybrid and to the demultiplexer that combine output signals derived from the demultiplexer with output signals generated by the zero-degree hybrid, and a ninety-degree hybrid coupled to outputs of the respective multipliers for combining the output signals from the multipliers and applying it to the first summing device.
2. The equalizer of Claim 1 which processes the error signal generated by the adaptive baseband equalizer to determine the magnitude of an interfering signal and then inputs a cancelling signal until power in the error signal is minimized.
3. The equalizer of Claim 1 wherein the vector modulator controls the phase and amplitude of the cross-polarized signal prior to summation with the co-polarized signal, and wherein the interfering signal is cancelled when the amplitude and the phase of the vector modulator are correctly set.
4. The equalizer of Claim 1 wherein the control circuit generates control signals for the vector modulator by minimizing the magnitude of the error signal in the adaptive baseband equalizer.
5. The equalizer of Claim 4 wherein the error signal is the difference between received baseband data and an estimate of ideal transmitted data.
6. The equalizer of Claim 1 wherein the control logic circuit sets the phase andamplitude for the vector modulator to minimize the magnitude of the error signal by stepping the setting of the vector modulator by one unit and sampling the power in the error signal, wherein if the power is reduced the vector modulator is stepped again in the same direction, and wherein if the power is increased the vector modulator is stepped in the opposite direction, and wherein the control signals are dithered one at a time, four consecutive times each.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002164018A CA2164018C (en) | 1995-11-29 | 1995-11-29 | Adaptive cross-polarization equalizer |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002164018A CA2164018C (en) | 1995-11-29 | 1995-11-29 | Adaptive cross-polarization equalizer |
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| CA2164018A1 CA2164018A1 (en) | 1997-05-30 |
| CA2164018C true CA2164018C (en) | 2003-01-21 |
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| CA002164018A Expired - Fee Related CA2164018C (en) | 1995-11-29 | 1995-11-29 | Adaptive cross-polarization equalizer |
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| CN112290917B (en) * | 2020-10-20 | 2022-10-28 | 中国科学技术大学 | Microwave pulse amplitude-phase self-adaptive control method, device, equipment and medium |
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