Carrier recovery for QPSK

Abstract: A circuit for regenerating a carrier, which is used to detect through coherent detection and demodulate a four-phase modulated wave, controlled by an improved AFC control to prevent otherwise possible false pull-in. The carrier regeneration circuit comprising detector 1, first multi-level code discriminator 2, phase deviation detection circuit 3 and voltage control oscillator 5, further comprises newly second multi-level code discriminator 6 for multi-level code discriminating two series of output signals of four-phase detector 1 at the time of data conversion, correlation circuit 7 for taking the correlation between the output signals of first multi-level code discriminator 2 and the outputs signals of second multi-level code discriminator 6, determination circuit 8 for determining that the two series of demodulated main signals of four-phase detector 1 vary at the same time and that the output levels of second multi-level code discriminator 6 become higher than a particular threshold value, and switch circuit 9 for adding the output of correlation circuit 7 to the output of phase deviation detection circuit 3 in accordance with the output of determination circuit 8. In addition to using the outputs (Dp,Dq,Ep,Eq) of a first discriminator (2), the carrier recovery circuit uses the output of an additional second discriminator (6), and a determination circuit (8), which compares the outputs of the two discriminators (2,6), in order to prevent a false lock.

Description: This invention relates to a carrier regeneration circuit for a quadrature demodulator and to a method of carrier regeneration. A previously proposed quadrature demodulator (referred to herein at times as a four-phase demodulator) is shown in Fig. 1. Referring to Fig. 1, the quadrature demodulator includes a quadrature detector 1 for performing coherent detection, a multi-level code discriminator 2 for discriminating two pairs of output signals of quadrature detector 1, phase deviation detection circuit 3 for logically processing the output of multi-level code discriminator 2 and controlling voltage control oscillator 5, low-pass filter 4 for filtering unnecessary spectral components of a control signal outputted from phase deviation detection circuit 3, and voltage control oscillator 5 for regenerating a carrier with which coherent detection is to be performed at quadrature detector 1. Based on the previously proposed construction, a quadrature modulated wave p is detected through coherent detection with regenerated carrier u outputted from voltage control oscillator 5 by phase detector 1 and is outputted as base band signal q. Base band signal q is inputted to multi-level code discriminator 2, from which demodulated main signals D P and D Q and deviation signals E P and E Q are outputted and inputted to phase deviation detection circuit 3. Phase deviation detection circuit 3 logically processes the signals inputted thereto and outputs control signal r for controlling voltage control oscillator 5. Control signal r passes through low-pass filter 4, which is provided for removing unnecessary waves, to make control signal s, which controls voltage control oscillator 5. A carrier regeneration circuit for a quadrature demodulator is disclosed in JP Patent Publication No. 57-131151 dated August 13 1982. The arrangement provides a carrier regeneration circuit wherein two-phase main signals obtained by quadrature amplitude modulation are detected through quadrature phase detection with the outputs of a voltage control oscillator (VCO) to make two base band signals in the form of digital signal consisting of predetermined number of bits, and the phase difference between the received carrier signal and a standard carrier signal outputted from the voltage control oscillator are correlated in accordance with a value obtained through the exclusive OR gate between the most significant digit (MSD) of one of the main signals and the least significant digit (LSD) of the other main signal, whereby the output frequency of the voltage control oscillator is controlled. In the previously proposed carrier regeneration circuit employed in the quadrature demodulator described above, while phase deviation information is extracted and controlled to bring the phase deviation to zero, an AFC (auto frequency control) control signal cannot be extracted, and it is very difficult to eliminate false pull-in of a regenerated carrier, that is, the phenomenon that the frequency of the voltage control oscillator is stabilized at a frequency different from its correct lock frequency. Features of a carrier regeneration circuit to be described below, by way of example in illustration of the invention are that it regenerates a carrier at the correct lock frequency and permits demodulation of a quadrature modulated wave, that it is capable of performing AFC control which resists non-linear distortion of a transmission line, and that it permits the regeneration of a carrier at a correct lock frequency of a carrier regeneration circuit of a quadrature demodulator which performs coherent detection and demodulates a quadrature modulated input signal with a reference carrier output from a voltage control oscillator. A carrier regeneration circuit, to be described below, by way of example in illustration of the invention includes a quadrature detector for detecting a quadrature modulated input signal through coherent detection with a reference carrier outputted from a voltage control oscillator, a first multi-level code discriminator for multi-level code discriminating two series of output signals of the quadrature detector, a phase deviation detection circuit for logically processing the outputs of the first multi-level code discriminator to output a phase deviation signal for controlling the voltage control oscillator, a second multi-level code discriminator for multi-level code discriminating the two series of output signals of the quadrature detector at the time of conversion of data, a correlation circuit for correlating the output signals of the first multi-level code discriminator and the output signals of the second multi-level code discriminator, a determination circuit for determining that the two series of output signals of the four-phase detector vary at the same time and that the output levels of the second multi-level code discriminator become higher than a particular threshold value, and a switch circuit for adding the output of the correlation circuit to the output of the phase deviation detection circuit in accordance with the output of the determination circuit. Preferably, a sampling point of the first multi-level code discriminator is a mostly opened signal point of an eye pattern of a base band demodulated from the input signal. Preferably, the time of conversion of data at the time multi-level code discrimination of the second multi-level code discriminator is executed is precisely between each two adjacent sampling points of the first multi-level code discriminator. Preferably, the correlation circuit calculates two exclusive ORS one calculation being operated between the value of one signal of the two series of output signals of the quadrature detector sampled after one half of a sampling period from a signal point and the value of the other signal of the two series of output signals of the quadrature detector sampled after one sampling period from the same signal point, the other calculation being operated between the two values of the two series of output signals of the quadrature detector with sampling times reversed, and output the result of the logical OR of the two calculation results of the exclusive OR. A method of carrier regeneration to be described below, by way of example in illustration of the invention includes the steps of detecting quadrature modulated input signal through coherent detection with a reference carrier outputted from a voltage control oscillator, executing a first multi-level code discrimination from the two series of resultant signals obtained by the coherent detection, detecting phase deviation of the two series of resultant signals through the logical processing of the output signals obtained by the first multi-level code discrimination, controlling the voltage control oscillation in accordance with a value based on the phase deviation, performing second multi-level code discrimination from the two series of resultant signals obtained by coherent detection at each time of data conversion, correlating the resultant signals of the first multi-level code discrimination and the resultant signals of the second multi-level code discrimination, determining that the two series of resultant signals vary at the same time and that the resultant signal level of the second multi-level code discrimination becomes higher than a predetermined threshold value, and composing, when the result of the determination is positive, the correlation value of the results of the first and second multi-level code discriminations into a control signal of coherent detection based on the phase deviation. Further, the step of correlating between the two output signals of the result of the first multi-level code discrimination and the output signals of the result of the second multi-level code discrimination may include the steps of calculating the exclusive OR between the value of one signal of the two series of resultant signals of the first multi-level code discrimination sampled after one half of a sampling period from a signal point and the value of the other signal of the two series of resultant signals sampled after one sampling period from the same signal point, and the other exclusive OR between two values of the two series of resultant signals of the first discrimination with their sampling times reversed, and outputting the result of the logical OR of the two results of the exclusive OR described above. The following description and drawings disclose, a previously proposed arrangement, and, by means of an example, the invention which is characterised in the appended claims, whose terms determine the extent of the protection conferred hereby. In the drawings:- Fig. 1 is a block diagram of a previously proposed carrier regeneration circuit; Fig. 2 is a block diagram of a carrier regeneration circuit showing a preferred embodiment of the present invention; Fig. 3 is a diagram showing discrimination levels of the output of the quadrature demodulator of the carrier regeneration circuit shown in Fig. 2, wherein curve (a) shows an eye pattern, curve (b) shows a clock for sampling main signal discrimination, and curve (c) shows a clock for sampling AFC signal discrimination; and Fig. 4 is a diagrammatic view showing demodulation loci, wherein the diagrams in column (a) show loci when the frequency is stabilized at the correct lock frequency, the diagrams in column (b) show loci in the case of false pull-in (A), and the diagrams in column (c) show loci in the case of false pull-in (B). Referring to the drawings, first a demodulation locus which is commonly known in the art will be described with reference to Fig. 4. Locus diagram (1) in Fig. 4 shows movements of a signal as considered from an arrangement of four signals of a quadrature modulation system, and particularly shows loci when signal point a changes to a, b, c and d after time t. Meanwhile, locus diagrams (2) and (3) show loci of the signal until time t after the regenerated carrier enters the false pull-in condition. The most serious matter in the false pull-in phenomenon is false pull-in in the direction of -90° (clockwise) and in the direction of +90° (counterclockwise) when the frequency is closest to the correct lock frequency. Thus, false pull-in in the direction of -90° is hereinafter referred to as false pull-in (A), and false pull-in in the direction of +90° as false pull-in (B). When the signal changes, for example, from a to c, if synchronous pull-in takes place regularly, then the signal changes from a to c passing the origin (the center in the signal point arrangement) (locus diagram (1) in Fig. 4). However, in false pull-in (A), the signal rotates in the direction of -90° in the process of changing from a to c; consequently, the signal which is to change to c changes to d (locus diagram (2) in Fig. 4). On the other hand, in false pull-in (B), since the signal rotates in the direction of +90° in the process of changing from a to c, the signal which is to change to c changes to b (locus diagram (3) in Fig. 4). Similar changes take place upon movement from a to a, from a to b and from a to d. Locus diagram (4) shows the change in logic levels H and L of a signal seen from direction  1 until time t with regard to locus diagram (1). In short, when the signal changes from a to a or from a to b, it changes from logic level H to logic level H, but when the signal changes from a to c or from a to d, it changes from logic level H to logic level L, and those loci are shown in locus diagram (4). Further, loci of a signal when it changes in logic levels H and L seen from direction  2 until time t are shown in locus diagram (5). Similarly, loci of a signal when it changes in logic levels H and L seen from direction  3 are shown in locus diagram (6), and those when a signal changes in logical levels H and L seen from direction  4 are shown in locus diagram (7). Locus diagram (8) shows the changes in logic levels H and L of a signal seen from direction  1 until time t in the case of false pull-in (A). First, when the signal changes from a to a, it changes from logic level H to logic level H, but in the process of the change, it passes the logic level L side with respect to the ordinary locus. On the other hand, when the signal changes from a to b, it changes from logic level H to logic level H, but in the process of the change, it passes the logic level H side with respect to the ordinary locus. In the meantime, when the signal changes from a to c, it does not pass the origin but passes, after time t/2, the logic level H side with respect to the origin. On the other hand, when the signal changes from a to d, it passes the logic level H side with respect to the origin until time t/2 and then passes the logic level L side with respect to the origin after time t/2 until time t. Similarly, loci of the signal changes seen from direction  2 are shown in locus diagram (9); those of the signal changes seen from direction  3 are shown in locus diagram (10); and those of the a signal changes seen from direction  4 are shown in locus diagram (11). The foregoing description can be applied similarly to false pull-in B, and demodulation loci of a signal are shown in locus diagrams (12) to (15). Here, if notice is taken only of changes of the signal from a to c, when the carrier is synchronized to pull-in with the correct frequency, the signal passes the origin without fail after t/2, but in the false pull-in condition, the signal presents either logic level H or L after t/2. Accordingly, AFC control can be realized by taking the correlation of the logic levels. Description is given below in the case of a signal chaning from a to c. In case a signal changes from a to c in false pull-in condition (A), when demodulated main signal DP changes from H to L, as seen from direction  1 , logic level H is outputted without fail if the signal is sampled after time t/2. When demodulated main signal DP changes from L to H, as seen from direction  3 , logic level L is outputted without fail if the signal is sampled after time t/2. Similarly, when demodulated main signal D P changes from H to L, as seen from direction  4 , logic level L is outputted without fail after the signal is sampled after time t/2. On the other hand, when demodulated main signal D Q changes from L to H, as seen from direction  2 , logic level H is outputted without fail if the signal is sampled after time t/2. Subsequently, discrimination levels and sampling points are described with reference to Fig. 3. Top curve (a) in Fig. 3 shows a four-phase demodulated eye pattern, and curves (b) and (c) respectively show a clock signal for sampling main signal discrimination which is used in a first multi-level code discriminator and another clock signal for sampling AFC signal discrimination which is used in a second multi-level code discriminator. Here, if sampling is performed with a clock signal of twice the clock frequency, then the clock signal can be used commonly for the first and second multi-level code discriminators. In short, sampling for discrimination of a main signal is performed at the timing of each opening of the eye pattern, and sampling for AFC discrimination is performed precisely between each adjacent opening after time t/2 (0.5 bits) described above. As regards discrimination levels, two discrimination levels are provided including discrimination level 1 and discrimination level 2 as shown in Fig. 3. In the correct synchronous pull-in condition, when the signal changes from a to c, it passes zone β without fail, but in the false pull-in condition, the signal passes zone α or γ. Accordingly, AFC control can be realized readily in accordance with the result of the discrimination. Further, since the signal is not discriminated in error. as long as it passes neither zone α or γ, it is also possible to perform AFC control which resists non-linear distortion of a transmission line. Next, the embodiment of the present invention shown in Fig. 2 will be described. The carrier regeneration circuit shown includes quadrature detector 1, first multi-level code discriminator 2, phase deviation detection circuit 3, low-pass filter 4, voltage control oscillator 5, second multi-level code discriminator 6, correlation circuit 7, determination circuit 8, switch 9, low-pass filter 10 and composite circuit 11. Four-phase modulated wave p is inputted in quadrature detector 1, where it is detected through coherent detection in accordance with regenerated carrier v outputted from voltage control oscillator 5 and is outputted as base band signals q. Base band signals q are supplied to both first and second multi-level code discriminators 2 and 6. Here, first multi-level code discriminator 2 is used to discriminate the main signal of Fig. 3, and second multi-level code discriminator 6 is used to discriminate AFC. Demodulated main signals D P and D Q and deviation signals E P and E Q are outputted from first multi-level code discriminator 2 and supplied to phase deviation detection circuit 3, correlation circuit 7 and determination circuit 8. Meanwhile, second multi-level code discriminator 6 samples base band signals q inputted thereto using clocks for sampling AFC signal discrimination, discriminates the thus sampled signals among zones α, β and γ, and supplies the result of the discrimination to correlation circuit 7 and determination circuit 8. Phase deviation detection circuit 3 produces, from demodulated main signals D P and D Q and deviation signals E P and E Q inputted thereto, and outputs automatic phase control signal (APC) r for controlling voltage control oscillator 5. Control signal r is supplied to low-pass filter 4, at which unnecessary waves are removed from control signal r to make signal s. Signal s is changed by composite circuit 11 into control signal j, which is inputted to voltage control oscillator 5. Correlation circuit 7 takes the correlation be tween signals D P and D Q inputted thereto and outputs a correction as an AFC control signal f. In particular, in the present embodiment, where (t) designates a value sampled after time t and (t/2) designates a value sampled after time t/2, correlation circuit 7 outputs the result of logical calculus OR between the result of exclusive OR between D P (t/2) and D Q (t) and another result of exclusive OR between D P (t) and D Q (t/2) as AFC signal f. Determination circuit 8 discriminates the variation of each particular signal described above from the output signals of first multi-level code discriminator 2 and the output signals of second multi-level code discriminator 6 inputted thereto, and outputs control signal g to control switch 9 when the level of each particular signal exceeds its threshold value. Switch 9 passes AFC control signal f as signal h in accordance with control signal q outputted from determination circuit 8 as a result of the determination. Unnecessary waves are removed from signal h by low-pass filter 10 to make AFC control signal i. AFC control signal i is added to APC control signal s, which is the filtered output of phase deviation detecting circuit 3, through low-path filter 4 by composite circuit 11 to make control signal J for voltage control oscillator 5. In other words, composite circuit 11 is an adding circuit. Control signal j is inputted to voltage control oscillator 5. It will be understood that, although particular arrangements have been described by way of example in illustration of the invention, variations and modifications thereof, as well as other arrangements may be made within the scope of the protection sought by the appended claims.

Claims: A carrier regeneration circuit, including a four-phase detector (1) for detecting a four-phase modulated input signal through coherent detection with a reference carrier output from a voltage control oscillator (5), a first multi-level code discriminator (2) for a multi-level code discriminating two series of output signals of the four-phase detector, and a phase deviation detection circuit (3) for logically processing the outputs of the first multi-level code discriminator (2) to output a phase deviation signal for controlling the voltage control oscillator (5) through a low pass filter (4) characterised in that there are provided a second multi-level code discriminator (6) for multi-level code discriminating the two series of output signals of the four-phase detector (1) at the time of data conversion, a correlation circuit (7) for carrying out a correlation between the output signals of the first multi-level code discriminator (2) and the output signals of the second multi-level code discrimination circuit (3) for determining that the two series of output signals of the four-phase detector (1) vary at the same time and that the output levels of the multi-level code discriminator (2) become higher than a particular threshold value, and a switch circuit (9) for adding the output of the correlation circuit (7) to the output of the phase deviation detection circuit (6), in accordance with the output of the determination circuit (6) through a composite circuit (11). A carrier regeneration circuit as claimed in claim 1, wherein the sampling point of the first multi-level code discriminator is the mostly opened signal point of the eye pattern of a base band demodulated from the input signal. A carrier regeneration circuit as claimed in claim 1 or claim 2, wherein the time of conversion of data, when the multi-level code discrimination of the second multi-level code discrimination is executed, is between each of two adjacent sampling points of the first multi-level code discriminator. A carrier regeneration circuit as claimed in any one of claims 1 to 3, wherein the correlation circuit (7) calculates two exclusive OR'S, one calculation being carried out between the value of one signal of the two series of output signals of the four-phase detector (1) which is sampled after one half of a sampling period from a signal point and the value of the other signal of the two series of output signals of the four-phase detector which is sampled after one sampling period from the same point, and the other calculation being carried out between the two values of the two series of output signals of the four-phase detector with sampling times reversed, and outputs the result of the logical OR. A method of carrier regeneration including the steps of detecting a four-phase modulated input signal through coherent detection with a reference carrier output from a voltage control oscillator (5), executing a first multi-level code discrimination from the two series of resultant signals obtained by the coherent detection, detecting phase deviation of the two series of resultant signals through the logical processing of the output signals obtained by the first multi-level code discrimination, and controlling the voltage control oscillation in accordance with a value based on the phase deviation, characterised in that it includes the steps of performing second multi-level code discrimination from the two series of resultant signals obtained by the coherent detection at the time of data conversion, taking the correlation between the resultant signals of the first multi-level code discrimination and the resultant signals of the second multi-level code discrimination, determining that the two series of resultant signals vary at the same time and that the resultant signal levels of the second multi-level code discrimination become higher than a predetermined threshold value, deriving, when the result of the determination is positive, from the correlation value of the results of the first and second multi-level code discriminations, a control signal of the coherent detection based on the phase deviation. A method as claimed in claim 5, wherein the step of carrying out the correlation between the two output signals of the result of the first multi-level code discrimination and the resultant signals of the result of the second multi-level code discrimination includes calculating the exclusive OR between the value of one signal of the two series of resultant signals of the first discrimination sampled after one half of a sampling period from a signal point and the value of the two series of resultant signals of the first discrimination sampled after one sampling period from the same signal point, and another exclusive OR between two values of the two series of resultant signals of the detection with sampling times reversed, and outputting the result from the logical OR of the two results of the exclusive OR described above.

Inventors: SATOU KAZUMASA, SATOU, KAZUMASA, Satou, Kazumasa, c/o NEC Fukushima Ltd.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H04L27
IPC codes: H04L27, H03L7