Demodulators – Phase shift keying or quadrature amplitude demodulator – Input signal combined with local oscillator or carrier...
Reexamination Certificate
1997-10-31
2001-05-22
Kinkead, Arnold (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
Input signal combined with local oscillator or carrier...
C455S295000, C455S296000, C342S361000, C375S349000, C375S235000, C329S304000, C329S306000, C329S310000
Reexamination Certificate
active
06236263
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a demodulator having a cross polarization interference canceling function for canceling the interference between a main polarization and a cross polarization. More particularly, the invention relates to a demodulator capable of canceling cross polarization interference even if there is a phase difference between a demodulated signal on the side of main polarization and a demodulated signal (interference signal) on the side of cross polarization.
A method making joint use of orthogonally polarized waves in which the planes of polarization of two carrier waves having the same frequency are made orthogonal to each other to suppress interference and form two co-channels is advantageous in that radio frequencies can be utilized effectively. Such a method is employed in digital multiplexed wireless apparatus and in other transmission apparatus. A deviation in the planes of polarization occurs in such a transmission apparatus owing to distortion of propagation path caused by falling rain or some other phenomena and it results in that one channel is acted upon by interference from the other channel. Accordingly, the apparatus is provided with a cross polarization interference canceller (XPIC) which suppresses such interference on the receiving end.
FIG. 15
is a diagram illustrating an example of the construction of the receiving section in a transmission apparatus which employs orthogonally polarized waves. As shown in
FIG. 15
, a receiving antenna
101
is connected to the input of an orthogonal polarizer
102
. One output (a V-polarized output) of the polarizer
102
is connected to the input of a QAM demodulating unit
104
a
via a frequency converter
103
a
. A first output of the QAM demodulating unit
104
a
provides a first demodulated signal (Qch, Ich) to a subsequent stage via a cross polarization interference canceller
105
a
. The other output (an H-polarized output) of the orthogonal polarizer
102
is connected to the input of a QAM demodulating unit
104
b
via a frequency converter
103
b
. A first output of the QAM demodulating unit
104
b
provides a second demodulated signal (Qch, Ich) to a subsequent stage via a cross polarization interference canceller
105
b.
A second output of the QAM demodulator
104
a
enters the cross polarization interference canceller
105
b
and is used in order to cancel the interference between the cross polarized waves. The second output of the QAM demodulating unit
104
b
enters the cross polarization interference canceller
105
a
and is likewise used in order to cancel the interference between the cross polarized waves.
FIG. 16
is a block diagram of a demodulator provided on the side of main polarization and equipped with the cross polarization interference canceling function. Here the V-polarized waves are the main polarized waves and the H-polarized waves represent the cross polarization. The demodulating unit
104
a
is on the side of the V-polarized waves and so is the cross polarization interference canceller (XPIC)
105
a
The construction of the demodulator on the cross polarization output side has a similar construction.
An intermediate-frequency signal resulting from the frequency conversion performed by the frequency converter
103
a
is applied to one input of each of two mixer circuits
111
a
,
111
b
which construct a quadrature demodulator. A local oscillator
112
which oscillates at a carrier frequency fc applies its output to a hybrid device
113
. The latter separates this signal into two signals that are 90° out of phase and applies these signals to respective ones of the mixer circuits
111
a
,
111
b
at the other input terminals thereof. The output signal of the frequency converter
103
a
is orthogonally detected by being mixed with the two orthogonal signals in the mixer circuits
111
a
,
111
b
and is separated into a baseband in-phase signal and quadrature signal. The baseband in-phase signal (I-channel signal) and quadrature signal (Q-channel signal) have higher harmonics eliminated by low-pass filters
114
a
,
114
b
, respectively.
Next, using a sampling clock f output by a voltage-controlled oscillator (VCO)
115
of an internal PLL synchronized to the modulation frequency f of the received signal, A/D converters
116
a
,
116
b
convert the in-phase signal and quadrature signal to digital I- and Q-channel signals, respectively, each consisting of, e.g. eight bits. The digital data output by the A/D converters
116
a
,
116
b
is such that, in case of 16 QAM, the two higher order bits of eight bits represent digital information allocated to the I- and Q-channel signals. The six lower order bits serve as a signal which represents, in the form of a digital value, error produced by waveform distortion or the like.
The digital I- and Q-channel signals enter a transversal equalizer (TVEQ)
117
, which subjects the signals to waveform equalization processing and eliminates transmission path distortion and quadrature distortion. More specifically, the transversal equalizer
117
(1) compensates for transmission path distortion components of the I- and Q-channel signals by digital signal equalization processing, (2) compensates for quadrature components by canceling the Q-channel signal component contained in the I-channel signal and the I-channel signal component contained in the Q-channel signal, and (3) enters the compensated I- and Q-channel signals into subtractors
121
a
and
121
b
, respectively, of the cross polarization interference canceller (XPIC)
105
a.
FIG. 17
is a diagram showing the construction of a well-known two-dimensional transversal equalizer capable of being used as the transversal equalizer
117
. The equalizer includes transversal filters
201
a
,
202
a
for eliminating transmission path distortion of the I-channel signal, transversal filters
201
b
,
202
b
for eliminating transmission path distortion of the Q-channel signal, and subtractors
203
,
204
. The subtractor
203
subtracts the Q-channel signal from the I-channel signal to cancel the quadrature component (Q-channel component) contained in the I-channel signal. The subtractor
204
subtracts the I-channel signal from the Q-channel signal to cancel the quadrature component (I-channel component) contained in the Q-channel signal.
Each of the transversal filters
201
a
~
202
b
is constituted by an N-tap FIR filter (not shown) in which the coefficients can be changed. The filters decide coefficients so as to compensate for transmission path distortion. As mentioned above, when the I- and Q-channel signals are each expressed by eight bits in 16 QAM, the two higher order bits represent data and the six lower order bits represent the error due to waveform distortion, etc. The relationship between identification threshold values of two higher order bits and digital data is illustrated in FIG.
18
. (1) When a third bit E is “1”, the digital data is greater than an intermediate value (the dashed line) of the identification threshold values. (2) When E is “0”, the digital data is less than the intermediate value.
In order to eliminate the effects of transmission path distortion, it will suffice to perform control in such a manner that the value of the six lower order bits become equal to the intermediate value (the ideal value) of the identification threshold values. The transversal filters
201
a
~
202
b
eliminate the influence of transmission path distortion by causing the coefficients of the FIR digital filters to converge toward predetermined values in accordance with the above-described logic.
With reference again to
FIG. 16
, A/D converters
122
a
,
122
b
of the cross polarization interference canceller (XPIC)
105
a
use the sampling clock output by the voltage-controlled oscillator
115
to convert the I- and Q-channel signals that enter from the QAM demodulating unit
104
b
(see
FIG. 15
) on the side of the H-polarized waves to 8-bit digital signals. The resulting digital I- and Q-channel signals enter a transversal equalizer (TVEQ)
123
, which applies w
Fujitsu Limited
Helfgott & Karas P.C.
Kinkead Arnold
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