Abstract:
A quadrature demodulator for demodulating an input signal which includes respective data signals modulating in-phase and quadrature carriers. The demodulator includes a voltage controlled oscillator responsive to a control signal for generating an oscillatory signal. A demodulator, coupled to receive the oscillatory signal from the voltage controlled oscillator and the input signal, provides the in-phase and quadrature components of the input signal. Phase comparison circuitry, responsive to the in-phase and quadrature components of the input signal generates a phase error signal. The phase error signal represents the difference, in phase and magnitude, between a vector defined by the in-phase and quadrature components of the input signal and reference vectors. Filter circuitry, responsive to the phase error signal, generates a control signal for the voltage controlled oscillator. Phase error correction circuitry selectively applies the error signal to the filter circuitry when the magnitude of a vector defined by the in-phase and quadrature components of the input signal exceeds a first threshold. Another embodiment provides the phase error signal to the filter circuitry when the magnitude of a vector defined by the in-phase and quadrature components of the input signal either exceeds a first threshold or is less than a second threshold.

Description:
FIELD OF THE INVENTION 
     The invention relates to a quadrature demodulator which demodulates input signals having in-phase and quadrature carriers. In particular, the invention selectively adjusts the phase and frequency of the output of a voltage controlled oscillator (VCO) depending upon the magnitude and phase of a vector defined by the demodulated in-phase and quadrature components of the input signal. 
     BACKGROUND OF THE INVENTION 
     Quadrature amplitude modulation (QAM) is a type of multi-phase modulation that uses two carriers, an in-phase carrier and a quadrature carrier. Each carrier is modulated respectively to e.g. 2, 4 or 8 modulation states or levels. Thus a multi-amplitude modulation offers, e.g. 4, 16 or 64 states corresponding to the initials 4-QAM, 16-QAM and 64-QAM. The grouping of these states is known as a constellation. In order to demodulate a QAM signal it is desirable to recover the carrier signal from the modulated signal. 
     There are several known methods of carrier recovery in QAM systems. One type is the selective type digital Costas loop. In contrast to a conventional Costas loop, selective type Costas loops process the baseband phase error signal, generate phase compensation control signals, and then selectively feed these signals to a voltage controlled oscillator (VCO). Specifically, a logical control circuit in the loop passes the phase error signal to the VCO only when the concurrent in-phase and quadrature components of the phase error signal both belong to a predefined rectangular region in the IQ plane. 
     FIG. 4 shows the in-phase and quadrature coordinate space for 16-QAM where the horizontal axis represents the in-phase component and vertical axis represents the quadrature component. The predefined rectangular regions are shown by dashed lines. Problems occur when phase errors in the recovered carrier signal cause the constellation to be rotated such that a received signal falls in a detection region corresponding to a different signal. This occurs in shaded regions 1 and 2 of FIG. 4. When this happens the reference carrier output signal of the VCO is driven to the wrong phase. For example, in a 16-QAM system as shown in FIG. 4, a signal in the middle ring produces a false error signal (self noise) and appears to be a signal from the outer ring. This causes the Costas loop to try to lock the carrier at a phase which results in a tilted 15 constellation. FIG. 5a illustrates the constellation in the correct position. FIG. 5b illustrates a constellation which has become tilted due to the Costas loop locking the carrier on point A. 
     SUMMARY OF THE INVENTION 
     In this invention, polar observation regions, (ring shaped) are used for carrier recovery instead of the rectangular observation regions of the conventional selective type digital Costas loops. These ring shaped detection regions correspond to a constant magnitude vector from the origin for each of the three detection levels in a 16-QAM system, for example. The purpose of the ring-shaped detection regions is to suppress the self-noise effects which occur during carrier acquisition and phase tracking. The overall effect is an improvement in the performance of the Costas loop in being able to acquire and track the carrier component of multilevel QAM modulated signals. 
     Accordingly there is provided a quadrature demodulator for demodulating an input signal which includes respective data signals modulating in-phase and quadrature carriers. The demodulator includes a voltage controlled oscillator responsive to a control signal for generating an oscillatory signal. A demodulating means, coupled to receive the oscillatory signal from the voltage controlled oscillator and the input signal, provides the in-phase and quadrature components of the input signal. Phase comparison circuitry, responsive to the in-phase and quadrature components of the input signal generates a phase error signal. Filter circuitry, responsive to the phase error signal, generates a control signal for the voltage controlled oscillator. Phase error correction circuitry selectively applies the phase error signal to the filter circuitry when the magnitude of a vector defined by the in-phase and quadrature components of the input signal exceeds a first threshold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a quadrature demodulator according to the present invention. 
     FIG. 2 illustrates the details of the processing unit of FIG. 1. 
     FIG. 3 illustrates a portion of the phase detector. 
     FIG. 4 illustrates a 16-QAM constellation and the rectangular decision regions conventionally used. 
     FIGS. 5a and 5b illustrate 16-QAM constellations and the effect of rotation on the constellation. 
     FIG. 6 illustrates the details of the processing unit including circular mapping. 
     FIGS. 7a-7g illustrate the signal ranges at various locations in the processing unit. 
     FIGS. 8a and 8b illustrate the use of circular mapping to define regions in a 16-QAM constellation. FIG. 9 illustrates one quadrant stored in a read only memory for implementing the circular mapping. 
     FIG. 10 illustrates the rectangular mapping performed in the processing unit. 
     FIG. 11 illustrates the phase error signal ranges in one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates the overall configuration of the demodulator. In FIG. 1, phase detector 10 receives the modulated input signal and produces the respective I and Q components of the input signal. FIG. 3 is a block diagram of a conventional synchronous demodulator which converts the modulated input signal into the respective I and Q components of the input signal, Ic and Qc, respectively. 
     In FIG. 3, multipliers 11 and 12 are used to multiply the received signal by in-phase and quadrature reference carriers. The quadrature reference carrier is provided from the voltage controlled oscillator 60. The in-phase reference carrier is derived by phase shifting the quadrature carrier by negative 90 degrees in device 13. The output signal of multiplier 11 is the in-phase signal component Ic and the output signal of multiplier 12 is the quadrature signal component Qc. 
     Referring to FIG. 1, the in-phase and quadrature signal components are low pass filtered by filters 20 and 21. The low pass filtering is performed in order to remove second harmonic components. The low pass filtered in-phase and quadrature components are then converted to digital form by analog to digital convertors 30 and 31. 
     The processing unit 40 derives the phase error signal J and selectively provides the phase error signal to low pass filter 50. Low pass filter 50 is the loop filter of the phase locked loop (PLL) that generates the carrier signals for the phase detector 10. The output signal of the low pass filter 50 controls the voltage controlled oscillator 60 and alters the phase and frequency of the oscillatory signal generated by the voltage controlled oscillator 60 in a sense which tends to reduce the amplitude of the phase error. 
     FIG. 2 shows the details of a processing unit 40 which uses rectangular detection regions for both carrier recovery and signal detection. The signal ranges at various locations in the processing unit 40 are illustrated in FIGS. 7a-7g. In FIG. 2, the I and Q signals provided by the analog to digital convertors 30 31 are applied to two full adders 41 and 43. The adder 41 forms the sum of I and Q while the adder 43 forms the difference between these two signals by subtracting Q from I through use of two&#39;s complement circuit 42. The overflow bits of the adders 41 and 43 are their respective output signals. The output signal of full adder 41 (location C in FIG. 2) is illustrated in FIG. 7a. The shaded region in FIG. 7a represents values of the I and Q signals for which I plus Q is greater than 255. If the I and Q values applied to the processing unit 40 (shown in FIG. 2) lie within the shaded region of FIG. 7a, then full adder 41 provides a one. Outside of the shaded region of FIG. 7a, the output signal of the full adder 41 is zero. FIG. 7b illustrates the output signal of full adder 43 (location D in FIG. 2). The shaded region in FIG. 7b represents values of the I and Q signals for which I plus the two&#39;s complement of Q is greater than 255. If the I and Q signals applied to the processing unit 40 (shown in FIG. 2) lie within the shaded region of FIG. 7b, then full adder 43 provides a one. Outside of the shaded region of FIG. 7b, the output signal of the full adder 43 is zero. The difference between the output signals of the full adders 41 and 43 is created through the use of device 42, which generates the two&#39;s complement of the Q signal component. 
     Referring to FIG. 2, signal C from adder 41 and signal D from adder 43 are applied to an exclusive 0R gate 44. The shaded region in FIG. 7c represents values of the I and Q signals for which the exclusive 0R of signals C and D is one. As shown in FIG. 7c, the output signal of the exclusive OR gate 44 is one when the I and Q signal components applied to the processing unit 40 (shown in FIG. 2) lie within the shaded region. The output signal of exclusive OR gate 44 is zero when the I and Q signal components applied to the processing unit 40 do not lie within the shaded region in FIG. 7c. 
     Referring to FIG. 2, programmable read only memory (PROM) 48 receives the I and Q signal components applied to the processing unit 40 and maps the I and Q signal components into four single bit values i1, q1, i2, and q2. As illustrated in FIG. 10, the i1 and q1 values indicate which quadrant within the I and Q coordinate space the applied I and Q signal components appear. The values i1 and q1 also define thresholds which are used for establishing the rectangular detection regions. Values i2 and q2 define signal locations within each of the four quadrants of the I and Q coordinate space. The use of values i2 and q2 is discussed below with respect to exclusive OR gate 49. 
     Referring to FIG. 2, exclusive OR gate 45 receives i1 and q1 as input signals. The output signal of exclusive OR gate 45 is illustrated in FIG. 7d. The shaded regions in FIG. 7d represents values of the I and Q signals for which i1 and q1 do not have the same value. If the I and Q signal components applied to the processing unit 40 (shown in FIG. 2) lie in a quadrant shaded in FIG. 7d, the output signal of the exclusive OR gate 45 is one. If I and Q signal components applied to the processing unit 40 lie in a quadrant that is not shaded in FIG. 7d, then the output signal of exclusive OR gate 45 is zero. 
     The output signals of exclusive OR gate 44 and exclusive OR gate 45 are applied to exclusive OR gate 46 (shown in FIG. 2). FIG. 7e shows the output signal of exclusive OR gate 46 relative to the I and Q signal components applied to the processing unit 40 (shown in FIG. 2). The shaded region in FIG. 7e represents values of the I and Q signals for which the exclusive 0R of signal E from gate 44 and signal F from gate 45 is one. As shown in FIG. 7e, the output signal of the exclusive OR gate 46 is one when the I and Q signal components applied to the processing unit 40 lie within the shaded region. The output signal of exclusive 0R gate 46 is zero when the I and Q signal components applied to the processing unit 40 do not lie within the shaded region in FIG. 7e. 
     Referring to FIG. 2, the single bit values i2 and q2 are applied to exclusive OR gate 49. FIG. 7f shows the output signal of exclusive OR gate 49 relative to the I and Q signal components applied to the processing unit 40 (shown in FIG. 2). The shaded region of FIG. 7f represents values of the I and Q signals for which i2 and q2 do not have the same value. As shown in FIG. 7f, the output signal of the exclusive OR gate 49 is one when the I and Q signal components applied to the processing unit 40 lie within the shaded region. The output signal of exclusive OR gate 49 is zero when the I and Q signal components applied to the processing unit 40 do not lie within the shaded region in FIG. 7f. When the output signal of exclusive OR gate 49 is zero, the sample and hold 47 samples and holds the input signal received from exclusive OR gate 46. 
     The output signal of sample and hold 47 (shown in FIG. 2) is illustrated in FIG. 7g. As shown in FIG. 7g, the output signal of the sample and hold 47 is one when the I and Q signal components applied to the processing unit 40 (shown in FIG. 2) lie within the shaded region, designated D J . The output signal of the sample and hold 47 is zero when the I and Q signal components applied to the processing unit 40 lie within the region designated D J  &#39;. The output signal of the sample and hold 47 remains the same, i.e. in the hold state, when the I and Q signal components applied to the processing unit 40 do not lie in either region D J  or region D J  &#39;. This state is illustrated by the word HOLD in certain regions of FIG. 7g. 
     FIG. 7g illustrates how the processing unit 40 (shown in FIG. 2) generates a phase error signal. For example, if the I and Q signal components applied to the processing unit 40 lie in the shaded region D J , this indicates that the 16-QAM constellation has become rotated from the desired position. The processing unit 40 provides a value of one to low pass filter 50 which controls the voltage controlled oscillator 60. This alters the phase of the oscillatory signal provided by the voltage controlled oscillator and will rotate the 16-QAM constellation in a clockwise direction to the proper position. Conversely, when the I and Q sisal components applied to the processing unit 40 (shown in FIG. 2) lie in the region labeled D J  &#39;, then the processing unit 40 provides a zero which has the opposite effect on the rotation of 16-QAM constellation (i.e. the constellation is rotated in a counterclockwise direction). 
     FIG. 6 illustrates the details of processing unit 40 (shown in FIG. 2) when circular mapping is used to control the sample and hold 47. FIG. 6 is similar to FIG. 2 and components which are functionally identical are assigned similar reference numerals. The processing unit 40 illustrated in FIG. 6 includes a circular mapping programmable read only memory (PROM) 410. The PROM 410 substantially reduces the problem of self noise, associated with rectangular detection regions, by making circular detection regions for carrier recovery. In this way, only sample points belonging to the proper vector magnitude contribute to the voltage controlled oscillator 60 (shown in FIG. 1). In addition, the full range of signal rotation can exist and a valid error signal can still be produced. 
     FIG. 8a illustrates an example of a circular detection region used for carrier recovery. A phase error signal is supplied from the sample and hold 47 (shown in FIG. 6) when a vector defined by the I and Q signal components has a magnitude larger than the value d shown in FIG. 8a. If the circular detection region is set to the value d, then the signals in the outer orbit of the constellation can be used for carrier recovery and there will not be any self-noise contributed by other orbits. 
     FIG. 8b illustrates the use of two circular detection regions. In general, any number of circular detection regions can be used, only the details of the implementation are more complex. For example, in the case of 16-QAM, detection regions for the inner 4 signals and the outer four signal could be implemented with a slightly larger ROM than the single detection region illustrated in FIG. 8a. In FIG. 8b, if the vector defined by the I and Q components of the input signal has a magnitude that is either less than d1 or greater than d2, then the sample and hold 47 (shown in FIG. 6) will provide a phase error signal. 
     FIG. 9 illustrates the upper quadrant of the circularly mapped points stored within PROM 410 (shown in FIG. 6) using 256×256 points. In the example shown the vector magnitude d is selected to be 77 units long. If the magnitude of the vector defined by the I and Q components of the input signal exceeds 77 units, the output signal H from PROM 410 (shown in FIG. 6) will be zero. This will enable the sample and hold 47 (shown in FIG. 6) to provide a phase error signal. 
     FIG. 11 illustrates the output signal from sample and hold 47 (shown in FIG. 6) in the embodiment which compares the magnitude of the vector defined by the I and Q components of the input signal to a single threshold. The output signal of the sample and hold 47 is one when the I and Q signal components applied to the processing unit 40 (shown in FIG. 2) lie within the shaded region, designated D K . The output signal of the sample and hold 47 is zero when the I and Q signal components applied to the processing unit 40 lie within the region designated D K  &#39;. The output signal of the sample and hold 47 remains the same, i.e. in the hold state when the I and Q signal components applied to the processing unit 40 do not lie in either region D J  or region D K  &#39;. This state is illustrated by the word HOLD in certain regions of FIG. 11. 
     FIG. 11 illustrates how the processing unit 40 (shown in FIG. 2) generates a phase error signal. For example, if the I and Q signal components applied to the processing unit 40 lie in the shaded region D K , this indicates that the 16-QAM constellation has become rotated from the desired position. The processing unit 40 provides a value of one to low pass filter 50 which controls the voltage controlled oscillator 60. This alters the phase of the oscillatory signal provided by the voltage controlled oscillator and will tend to rotate the 16-QAM constellation in a clockwise direction to the proper position. Conversely, when the I and Q signal components applied to the processing unit 40 (shown in FIG. 2) lie in the region labeled D K  &#39;, then the processing unit 40 provides a zero which has the opposite effect on the rotation of 16-QAM constellation (i.e. the constellation is rotated in a counterclockwise direction). The various regions in FIG. 11 effectively define a plurality of reference vectors coincident with diagonal boundaries B1, B2, B3 and B4 in FIG. 11. The vector defined by the in-phase and-quadrature components of the input signal is compared, in phase and magnitude, to these reference vectors in order to determine the appropriate phase error signal. 
     The use of polar observation regions for carrier recovery is a distinct improvement over the conventional use of rectangular detection regions. The ring shaped detection regions suppress the effects of self-noise which occur during carrier acquisition and phase tracking. The overall effect is an improvement in the performance of the Costas loop in being able to acquire and track the carrier component of multilevel QAM modulated signals. 
     While the invention has been described with reference to exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the true spirit and scope of the present invention.