Demodulators – Phase shift keying or quadrature amplitude demodulator – Input signal combined with local oscillator or carrier...
Reexamination Certificate
1999-02-12
2001-03-27
Kinkead, Arnold (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
Input signal combined with local oscillator or carrier...
C329S304000, C375S329000, C375S326000, C375S327000, C375S331000, C375S324000
Reexamination Certificate
active
06208201
ABSTRACT:
The present invention relates to the recovery of a carrier phase from a modulated input signal where the modulation is in accordance with the M-PSK modulation format and M has a value greater than 4. Such modulation is referred to herein as higher order M-PSK modulation.
In known art, the case of QPSK with M=4 is common, but it is not obvious from this simple case how to derive a more complex function correctly to extend the configuration of the demodulator in higher order modulation with M>4. A phase reference is required with which to perform the high order demodulation and process to do so includes a non-linear function which, in the known art of QPSK, is seen by inspection and illustrated in FIG.
1
. According to the present invention the general case of the complex non-linear control signal is discovered and disclosed that applies to all values of M, including that value applicable to QPSK as a special case. Thus, the invention relates directly to identifying the non-linear functions required for the high-order case and incorporating these elements as improvements to known demodulator configurations. In the following description these functions are revealed for specific cases of high-order modulation but without limitation to their general extension.
BACKGROUND OF THE INVENTION
There are two basic approaches for dealing with carrier synchronisation at a receiver. One approach is to multiplex the modulated signal with a pilot signal and extract the pilot signal at the receiver. The extracted pilot signal is used to synchronise a local oscillator at the receiver. Another approach is to synthesize a carrier signal at the receiver by means of a phase locked loop including a voltage controlled oscillator. The control of the voltage controlled oscillator is effected by a circuit which estimates the carrier phase, the estimation being made from the input modulated signal.
The first approach has the disadvantage that the transmission bandwidth has to include provision for the pilot signal. The second approach has not been applied effectively to a receiver for higher order modulation formats.
The present invention is aimed at providing an improved method and apparatus for the recovery of a carrier phase from a signal modulated in accordance with a higher order M-PSK format.
SUMMARY OF THE INVENTION
According to the present invention there is now provided a method of recovering a carrier phase from a higher order M-PSK modulated input signal, the method comprising the steps of: employing a voltage controlled oscillator to generate first and second oscillations in phase quadrature with respect to each other; mixing the modulated input signal with the first oscillation to detect a first signal component, (I); mixing the modulated input signal with the second oscillation to detect a second signal component, (Q); deriving, from the first and second signal components, (I) and (Q), a control signal, representing the phase difference between the input signal carrier and the voltage controlled oscillator according to the relationship:
a
=
M
2
2
M
-
1
∑
K
=
1
M
(
-
1
)
(
k
-
1
)
/
2
C
M
K
I
M
-
K
Q
K
where ‘a’ control signal, ‘k’ is an odd integer between 1 and M, and C
M
k
represent binomial coefficients; and controlling the frequency of the voltage controlled oscillator by means of the control signal to recover the carrier phase.
The invention also provides a receiver for receiving a higher order M-PSK modulated input signal, the receiver comprising: a voltage controlled oscillator to generate first and second oscillations in phase quadrature with respect to each other; a first mixer to mix the modulated input signal with the first oscillation to detect a first signal component, (I); a second mixer to mix the modulated input signal with the second oscillation to derive a second signal component, (Q); an estimator to derive, from the first and second signal components (I) and (Q), a control signal representing the phase difference between the input signal carrier and the voltage controlled oscillator according to the relationship:
a
=
M
2
2
M
-
1
∑
K
=
1
M
(
-
1
)
(
k
-
1
)
/
2
C
M
K
I
M
-
K
Q
K
where ‘a’ is the control signal, ‘k’ is an odd integer between 1 and M, and C
M
k
represent binomial coefficients; and a control means to control the frequency of the voltage-controlled oscillator through the control signal to recover the carrier phase.
REFERENCES:
patent: 0 369 406 A2 (1990-05-01), None
Kinkead Arnold
Ladas & Parry
Tandberg Television ASA
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