Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
2000-02-04
2001-09-04
Tse, Young T. (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S281000, C375S326000, C329S304000, C329S307000
Reexamination Certificate
active
06285721
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a method for synchronizing a received carrier to a transmitted carrier for a dispersed-energy QPSK signal.
The invention relates in general to the field of digital modulation methods, such as the digital video broadcast via satellite (DVB-S) method. In particular, the present invention relates to carrier recovery on reception of a transmitted digital signal, in particular of a dispersed-energy quadrature phase shift keying (QPSK) signal. A QPSK signal is the mixed product of two orthogonal signals I and Q (referred to as the I signal and the Q signal, respectively, in the following text), which are phase-shifted through 90° with respect to one another. The I and Q signals are also completely independent of one another at the receiving end, provided the received carrier and the transmitted carrier are at the same frequency, and their phases are coupled.
At the receiving end, the QPSK signal is mixed in one path with a TI carrier signal in order to obtain the I signal, and is mixed in a further path with a TQ carrier signal in order to obtain the Q signal, which is phase-shifted through 90° with respect to the carrier signal TI. The frequency of the carrier signal at the reception end must correspond precisely to the frequency of the carrier signal at the transmitter end, in order to ensure synchronous demodulation of the dispersed-energy QPSK signal. Non-matching carrier frequencies lead to rotation of the constellation containing the I signal and the Q signal. This rotation is brought to rest by suitable control of the received carrier, or, in modern methods, is compensated for by computational methods (for example the CORDIC algorithm). In this case, the method according to the invention can be used directly in analog QPSK demodulators by use of a comparatively simple circuit, with the minimum theoretically possible number of measured values or, using numerical QPSK processors, can determine the control value for the rotation compensation.
A known procedure for carrier recovery in the context of digital modulation methods is known, for example, from the reference titled “Digitale Modulationsverfahren” [Digital Modulation Methods], Rudolf Mäusl, 1985/1991, ISBN 3-7785-2085-X. In chapter 3.4.2, the author describes the so-called COSTAS loop, under “carrier recovery”. Based on the circuit shown in FIG. 3.12, the COSTAS loop for carrier recovery with two-phase keying contains the addition of a second multiplier. In the variant of the so-called “hard” COSTAS loop, the demodulation product of the in-phase and quadrature demodulator is supplied, after low-pass filtering, to one input of the two multipliers, and the demodulation product limited to the mathematical sign by a comparator is supplied, crossed over, to the other input. The difference between the two multiplication output signals is used to provide a correction voltage for controlling the voltage-controlled oscillator. The known circuit is balanced, that is to say it is based not only on SI-controlled detection of the complete SQ signal, but also on SQ-controlled detection of the complete SI signal.
The following reference should also be cited from the prior art: “Pulse Code Modulation Techniques”, Bill Waggener, 1995, ISBN 0-442-01436-8. The chapter titled “Symbol Synchronization”, pages 291 to 306, describes various complicated concepts for pulse code modulation, which are likewise balanced in the sense described above, and are based on a measurement of the overall signals.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for assisting simple synchronization to the carrier of a dispersed-energy QPSK signal that overcomes the above-mentioned disadvantages of the prior art methods of this general type, which can be carried out without major complexity, including from the component point of view and to providing an apparatus for carrying out the method, which can be produced without any major component complexity.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for synchronization of a received carrier to a transmitted carrier of a dispersed-energy quadrature phase shift keying (QPSK) signal transmitted as a mixed product containing an I signal and a Q-signal phase-shifted through 90° with respect to the I signal with the transmitted carrier, at a receiving end an SI signal being demodulated by mixing with a TI carrier signal and an SQ signal being demodulated by mixing with a TQ carrier signal that is phase-shifted through 90° with respect to the TI carrier signal, which includes:
measuring a first mean value of an amplitude of the SQ signal at a time of a zero crossing of a rising edge of the SI signal;
measuring a second mean value of an amplitude of the SI signal at a time of a zero crossing of a falling edge of the SQ signal, the first mean value and second mean value reflecting a discrepancy from synchronicity between the received carrier and the transmitted carrier; and
varying a frequency of the received carrier until the amplitude of the SQ signal is zero.
The invention is based on the following knowledge: in the event of a frequency shift or frequency offset between the carrier signal at the transmitter end and the carrier signal at the reception end, the orthogonal I and Q signals of the QPSK signal rotate. That is to say the I signal and the Q signal have components in the SI and SQ signals at the reception end. A component of the SI signal, phase-shifted through 90°, is thus superimposed on the pseudo-random signal (which is based on dispersed energy) in the SQ signal and this component has its inversion point, that is to say its positive or negative maximum, at the time of the zero crossing of the rising flank of the SI signal. The magnitude of this value increases as the frequency offset increases. The same is true for the SI signal. Accordingly, a component of the SQ signal, shifted through 90°, is superimposed on the pseudo-random signal in the SI signal, the SQ mean value of which component is not equal to zero at the time of the zero crossing of the rising flank of the SI signal, and which is formed as a positive or negative maximum of the mean value (in comparison with mean values formed at other times). The magnitude of the mean value formed from these values increases as the relative frequency offset increases. The relative frequency offset DF is the absolute frequency offset divided by the symbol rate. The magnitude of the mean value is calculated from the error function erf(DF).
The invention makes use of this knowledge in that the mean value of the SQ signal at the time of the zero crossing of the rising flank of the SI signal, and the amplitude of the SI signal at the time of the zero crossing of the falling flank of the SQ signal, are measured or detected as a measure of the discrepancy of the synchronicity between the received carrier and the transmitted carrier, with the frequency of the received carrier being varied until this amplitude is zero, by which the received carrier is synchronized to the transmitted carrier of the dispersed-energy QPSK signal.
On the other hand, the method according to the invention requires only the theoretical minimum number of measured values of a component of the signal pair, whose detection is controlled by the other component of the signal pair. The solution proposal according to the invention does not require detection of SI, which is required at the same time and is controlled by SQ, or detection of SQ, controlled by SI, with a symmetrical synchronizer configuration, since it is sufficient just to detect SQ controlled by SI (or SI controlled by SQ). The more complex balanced configuration is prior art both for digital and analog methods.
For demonstration, the method according to the invention for matching the transmitter to the receiver can be carried out without any problems by using an oscilloscope to which the received signal is applied, and from whose screen the z
Greenberg Laurence A.
Infineon - Technologies AG
Lerner Herbert L.
Stemer Werner H.
Tse Young T.
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