Demodulators – Phase shift keying or quadrature amplitude demodulator – Input signal combined with local oscillator or carrier...
Reexamination Certificate
2000-11-06
2003-05-27
Kinkead, Arnold (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
Input signal combined with local oscillator or carrier...
C329S304000, C329S308000, C375S344000, C375S345000, C375S324000
Reexamination Certificate
active
06570441
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to incoherent demodulators, and more specifically to an incoherent demodulator having an improved converging performance at signal points and excellent stability against low-speed phase rotation.
2. Description of the Related Art
A prior art demodulator, as shown in
FIG. 1
, receives a QAM (quadrature amplitude modulated) IF signal from a down-converter, not shown, and includes a pair of analog mixers
21
and
22
in which the IF signal is mixed with orthogonal carriers. A local oscillator
41
produces an in-phase IF carrier that is supplied to the mixer
21
. The in-phase IF carrier is &pgr;/2 phase-shifted in a phase shifter
42
to produce a quadrature IF carrier which is fed to the mixer
22
. Mixers
21
and
22
perform multiplication between the received IF signal and the sinusoidal waveforms of the orthogonal IF carriers to detect in-phase (P) and quadrature (Q) baseband components for respective channels. Undesired high frequency components of the P- and Q-channel baseband signals are eliminated by analog low-pass filters
31
and
32
, respectively.
Since the frequency of the local carriers is very close to, but not synchronized to, the frequency of the received IF signal, there exists a small phase rotation. In addition, variability inherent in the operating characteristics of these analog circuits causes the recovered baseband signals to vary in amplitude independently of each other with varying ambient temperature. As a result, there is a small power difference between the P- and Q-channel baseband signals.
The P and Q analog baseband signals from the low-pass filters are converted to digital baseband signals by A/D converters
61
and
62
. The output signals of the A/D converters
61
,
62
are supplied to a digital endless phase shifter (EPS)
80
. Phase shifter
80
is comprised of a numerical controlled oscillator (NCO) and digital circuits. Using the digital orthogonal carriers, the endless phase shifter
80
removes the demodulator's inter-channel rotating phase difference according to the following equations and produces output signals P′ and Q′:
P′=P*
cos &thgr;−
Q*
sin &thgr;
Q′=P*
sin &thgr;−
Q*
cos &thgr;
where &thgr; represents the phase angle which is controlled by the phase shifter
80
so that its output signals P′ and Q′ would be exactly what is recovered if the locally generated orthogonal IF carriers were precisely synchronized to the received orthogonal IF carriers. More specifically, if the angle of phase rotation between the recovered P- and Q-channel baseband signals is equal to &thgr;(t), the phase shifter
80
introduces a time-varying phase shift of −&thgr;(t) to the input P- and Q-channel signals.
Similar to the power difference between the recovered P- and Q-channel baseband signals, there is a power difference between the baseband signals which is produced by the transmitter's modulator as a result of the variability of its analog components. This modulator's baseband power difference is eliminated by automatic gain control (AGC) circuits
91
and
92
connected to the outputs of phase shifter
80
. Additionally, the AGC circuits
91
and
92
monitor their outputs to detect their offset values from prescribed levels and control their output levels so that they confirm to the prescribed levels.
The baseband power difference caused by the analog demodulator is reduced by a closed-loop AGC control circuit
70
formed by a difference detector
71
, an integrator
72
and a multiplier
73
. Difference detector
71
is connected to the outputs of AGC circuits
91
and
92
to detect a difference which exists between the output levels of these AGC circuits according to the phase rotation angle &thgr; detected by the phase shifter
80
. The output of the difference detector
71
is integrated by the integrator
72
to produce a gain control signal, which is applied to the multiplier
73
. The output of the integrator
72
is used in the multiplier
73
to control the amplitude of the P-channel input signal of the digital phase shifter
80
from the A/D converter
61
. By the feedback operation of the closed loop
70
, the baseband power difference between the recovered baseband signals reduces towards zero.
However, the inter-channel power difference of the demodulator cannot be completely eliminated by the closed-loop AGC control circuit
70
. As a result, the demodulator suffers degradation in converging performance at signal points in the 16-QAM signal constellation. As shown in
FIG. 2
, due to the degraded convergence performance, the size of signal points in the 16-QAM signal constellation increases with distance from the center of the constellation. Hence, the bit error rate performance of the demodulator suffers degradation. In addition, if the inter-channel phase rotation varies at a considerably low speed (or near zero), the control signals of all AGC circuits
70
,
91
and
92
will assume an equal value, causing the demodulator to enter an unstable state if these circuits have substantially the same time constant value. On the other hand, if the AGC circuit
70
has a greater time constant value than the AGC circuits
91
,
92
, the former ceases to function properly and the latter takes control to absorb the inter-channel power difference. When the phase rotation begins increasing its speed under such conditions, an error would result until the demodulator is stabilized. Similar problems occur in an interference canceller for cancelling interference between cross-polarizations.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an incoherent demodulator that is improved in terms of convergence performance and stability against low speed phase rotation.
According to a first aspect of the present invention, there is provided an incoherent demodulator comprising local oscillator circuitry for producing a pair of orthogonal carriers, a pair of analog mixers for incoherently demodulating a pair of modulated orthogonal signals with the orthogonal carriers to produce a pair of analog orthogonal baseband signals, there being a phase rotation in the analog orthogonal baseband signals resulting from the incoherent demodulation of the modulated signals. A pair of analog-to-digital converters process the analog orthogonal baseband signals to produce first and second digital signals. Gain controlled circuitry is provided for scaling the first digital signal so that a difference which exists between average power of the scaled first digital signal and average power of the second digital signal reduces to zero. A digital phase shifter processes the first and second output signals of the gain controlled circuitry so that the processed first and second output signals no longer contain the phase rotation.
In a preferred embodiment, the gain controlled circuitry includes a digital multiplier for multiplying the first digital signal with a control signal to produce a scaled first digital signal, averaging circuitry for producing a first average value representing the average power of the scaled first digital signal and a second average value representing the average power of the second digital signal, and control circuitry for deriving a signal from the first and second average values and supplying the signal to the digital multiplier as the control signal so that a difference which exists between the first and second average values reduces to zero.
According to a second aspect of the present invention, there is provided an incoherent demodulator system for a cross-polarization communication system, comprising a common local oscillator for producing a pair of orthogonal carriers, a pair of first and second demodulators for receiving first and second cross-polarized signals, respectively, and an interference canceller for cancelling interference between the first and second cross-polarized signals. Each of the first and second demodulat
Kinkead Arnold
McGinn & Gibb PLLC
NEC Corporation
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