CARRIER FREQUENCY ESTIMATOR FOR A SIGNAL RECEIVER

Abstract: A method and apparatus for recovering a carrier frequency of an incoming signal. The incoming signal includes a carrier signal modulated by modulation symbols having at least two symbol magnitudes and N approximately evenly spaced symbol phases at a particular one of said symbol magnitudes. A carrier frequency recovery system includes a phase multiplier (12) for multiplying the symbol phases by N, a magnitude selector (14) for selecting modulation symbols within a certain magnitude range about the particular magnitude, and a phase subtractor (16) coupled to the phase multiplier (12) and the magnitude selector (14) for determining phase differences between the N multiplied symbol phases for the selected modulation symbols within the certain magnitude range. The phase diferences are then used by a local oscillator system for recovering carrier frequency.

Description: CARRIER FREQUENCY ESTIMATOR FOR A SIGNAL RECEIVER BACKGROUND OF THE INVENTION Field of the Invention The invention relates generally to carrier recovery systems and more particularly to a carrier recovery system for recovering the carrier frequency for a quadrature amplitude modulation (QAM) signal. Description of the Prior Art Modern communication signals carry information in the form of modulation symbols, where each of the modulation symbols represents a state for one or more bits of data. Each modulation symbol has a particular in-phase (I) amplitude and a particular quadrature phase (Q) amplitude. When observed in an IQ plane, the I amplitudes and the Q amplitudes of the modulation symbols form a pattern or format. Many IQ formats have been used or proposed including formats termed 16 QAM, 64 QAM, 256 QAM, and the like. Such formats have the benefits of being relatively easy to generate at the transmitting end of a communication signal link and relatively easy to decode into data bits at the receiving end. However, before the modulation symbols can be decoded into data bits, the incoming communication signal must be demodulated using a local signal having a frequency that corresponds to the carrier frequency of the incoming signal. The process of determining the correct frequency for the local signal, termed carrier recovery, is relatively difficult because the communications signals for such formats do not have significant energy at the carrier frequency. Therefore traditional frequency lock and phase lock loops are of no use. Many workers have developed and proposed carrier recovery systems in order to overcome this difficulty. However, all the known systems have limitations and so there continues to be a need for improved carrier recovery systems. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved method and apparatus for a carrier recovery system. Briefly, in a preferred embodiment a signal receiver includes a carrier recovery system of the present invention for receiving an incoming signal having modulation symbols having at least two symbol magnitudes and N approximately evenly spaced symbol phases at a particular one of the symbol magnitudes. An example of a pattern of such modulation symbols is illustrated in Fig. 1 and described below. The signal receiver provides local signals for converting the incoming signal to a representative input signal having an in-phase (I) signal component corresponding to the I amplitude level of the modulation symbols and a quadrature phase (Q) signal component corresponding to the Q amplitude level of the modulation symbols. The carrier recover system includes a phase multiplier for multiplying the symbol phases by N, a magnitude selector for selecting modulation symbols within a certain magnitude range about the particular magnitude, and a phase subtractor coupled to the phase multiplier and the magnitude selector for determining phase differences between the N multiplied symbol phases for the selected modulation symbols. The phase differences correspond to a rotation of the modulation pattern due to a frequency offset between the carrier frequency of the incoming signal and a local frequency or frequencies in the local signals. The signal receiver uses the phase differences for tuning the local frequency or frequencies to track (recovery) the carrier frequency. An advantage of the present invention is that the frequency of the carrier of an incoming signal is recovered in a simple and inexpensive manner that may be integrated into digital circuitry. These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an IQ diagram showing exemplary modulation symbols for an incoming signal received by the signal receivers of the present invention of Figs. 3 and 4; Fig. 2 is a block diagram of a carrier recovery system of the present invention; Fig. 3 is a block diagram of a signal receiver of the present invention having the carrier recovery system of Fig. 2; and Fig. 4 is a block diagram of an alternative signal receiver having a carrier recovery system of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A signal receiver 100 (Fig. 3a) or 200 (Fig. 3b) of the present invention receives an incoming signal having modulation symbols in a quadrature amplitude modulation (QAM) format and decodes the data bits that were carried by the incoming signal. The signal receivers 100 and 200 use a local signal to convert the frequency of the incoming signal to a convenient frequency for further processing. Fig. 1 is an IQ diagram of the modulation symbols of a 16 QAM format. The IQ diagram has an in-phase (I) axis showing amplitude levels for an I component of the modulation symbols at I amplitudes of -3, -1, 1, and 3; and a quadrature phase (Q) axis showing amplitude levels for a Q component of the modulation symbols at Q amplitudes of 3, 1, -1, and -3. The sixteen modulation symbols of the 16 QAM format are shown as dots. Each modulation symbol has a total amplitude or magnitude that is a square root of the sum of the squares of its I amplitude and its Q amplitude; and a phase that is an arc tangent of its I amplitude divided by its Q amplitude. In the 16 QAM format, four modulation symbols having the largest magnitude are within a magnitude range D, eight modulation symbols having an intermediate magnitude are within a magnitude range C, and four modulation symbols having the smallest magnitude are within a magnitude range B. A magnitude range A is smaller than the magnitude of any of the modulation symbols for the 16 QAM format. A similar IQ diagram may be drawn for any format using modulation symbols that can be described in terms of an I amplitude level and a Q amplitude level. The IQ diagram of Fig. 1 rotates when there is a frequency offset between the carrier frequency of the incoming signal and a frequency or frequencies of the local signal that corresponds to the carrier frequency of the incoming signal. The rotation causes the modulation symbols to change phase but not magnitude. The rate of change of phase is proportional to the frequency offset. An object of the present invention is to determine the rate of change of phase and then use the rate of change of phase for determining the frequency offset, thereby recovering the frequency of the carrier signal. In a preferred embodiment, the present invention multiplies the phases of the modulation symbols by N where N is the number of modulation symbols in the format that are within the magnitude range D (N is four for Fig. 1) . The effect of the phase multiplication is to superimpose the modulation symbols within the magnitude range D upon each other so that all have the same phase. For example, four times the phase of the modulation symbol at 45° is a phase of 180°. Four times phase of the modulation symbol at 135° is a multiplied phase of 540°, however, because a phase repeats after each circle of 360°, the phase of 540° is also a phase of 180°. Four times the phase of the modulation symbol at 225° is a multiplied phase of 900°, however, because two circles are 720°, the phase of 900° is also a phase of 180°. Four times the phase of the modulation symbol at 315° is a multiplied phase of 1260°, however, because three circles are 1080°, the phase of 1260° is also a phase of 180°. Therefore, all the multiplied phase are the same unless there is a rotation in the pattern. The rotation, if there is any, results in a phase difference between the multiplied phases for the modulation symbols is due to rotation of the pattern due to the frequency offset. The present invention determines the phase difference between the multiplied phases for two successive modulation symbols when the two successive modulation symbols are both within the magnitude range D. The phase difference is then divided by the time between the two successive modulation symbols for determining the frequency offset for recovering the carrier frequency of the incoming signal. Fig. 2 is a block diagram of a carrier recovery system of the present invention referred to by the reference ' number 10. The carrier recovery system 10 is embedded in a signal receiver for receiving an incoming signal having modulation symbols having at least two symbol magnitudes and N approximately evenly spaced symbol phases at a particular one of the symbol magnitudes. An example of such modulation symbols is illustrated in Fig. 1 and described above. In a preferred embodiment the signal receiver 100 or 200 uses a local signal for converting the incoming signal to an input signal having an in-phase (I) signal component corresponding to the I amplitude level of the modulation symbols and a quadrature phase (Q) signal component corresponding to the Q amplitude level of the modulation symbols. The carrier recover system 10 provides phase differences due to the rotation of the modulation symbols for determining a frequency offset between the carrier frequency of the incoming signal and a frequency of the local signal in order to recover the carrier frequency. Preferably, the elements of the carrier recovery system 10 are implemented in digital hardware and clocked by a symbol clock signal at a rate that is approximately proportional to the symbol rate of the modulation symbols. The digital hardware can be designed a several ways including CMOS circuitry using a gate array or standard cell design and manufactured in any one of several commercial foundries such as Lucent Technologies, Inc. of Murray Hill, New Jersey, or VLSI Technology, Inc. of San Jose, California. The carrier recover system 10 includes a digital phase multiplier 12 and a magnitude selector 14 for receiving the input signal; and a phase subtractor 16 for providing an output signal for the phase differences for use in recovering the carrier frequency of the incoming signal. The digital phase multiplier 12 converts the I and Q component signals into a phase signal having a level proportional to N times the arc tangent of the amplitude of the I signal component divided by the Q signal component where N is the number of modulation symbols having a magnitude with a preferred magnitude range. In a preferred embodiment illustrated in Fig. 1, the number of modulation symbols having a magnitude in the preferred magnitude range D is four and so the preferred N is four. Such digital phase multiplier 12 may be implemented with trigonometric lookup tables and a scaling multiplier. The magnitude selector 14 includes a magnitude detector 18 for determining the magnitude level issuing a magnitude range signal when the magnitude of a present modulation symbol is within threshold boundaries for the magnitude range D to a delay circuit 22 a first input of an AND gate 24. The delay circuit 22 delays the magnitude range signal by one modulation symbol time period and passes the delayed magnitude range signal to a second input of the AND gate 24. The AND gate 24 issues a magnitude detection signal to the phase subtractor 16 when both the magnitude range signal and the delayed magnitude range signal indicate that the two successive modulation symbols each have the magnitude range D. The phase subtractor 16 receives the phase signal from the digital phase multiplier 12 in a delay circuit 26 and a first input of a summer 28. The delay circuit 26 delays the phase signal by one modulation symbol time period and passes the delayed phase signal to a second input of the summer 28. The summer 28 takes a difference between the phase signal and the delayed phase signal and issues a phase difference signal to a phase limit detector 32 and a first input of a first phase gate 34. The phase limit detector 32 issues a phase detection signal to a first input of an AND gate 36 when the phase difference signal is less than a threshold boundary. In a preferred embodiment where N equals four, the threshold boundary is equivalent to +45° about the multiplied phase. For proper operation of the carrier recovery system 10, the frequency offset of the carrier frequency cannot be so great that the modulation symbols rotate by ±315° within one symbol period. A second input of the AND gate 36 is connected for receiving the magnitude detection signal from the magnitude selector 14. The AND gate 36 issues a first gate signal to a counter 38 and to a second input of the first phase gate 34 when both the phase detection signal and the magnitude detection signal are simultaneously received. The first phase gate 34 uses the first gate signal for passing the phase difference signal to an accumulator 42. The counter 38 counts the first gate signals and issues a second gate signal to the accumulator 42 and a second phase gate 44 when a pre-determined count threshold is reached. The accumulator 42 sequentially adds the level of each phase difference signal to the level of the previous phase difference signals and passes an accumulated phase difference signal to a second phase gate 44. The second gate signal resets the accumulator 42 and causes the second phase gate 44 to issue the accumulated phase difference signal ad the output signal. Information for the level of the accumulated phase difference signal and the count threshold is used for recovering the carrier frequency of the incoming signal. Fig. 3a is a block diagram of a signal receiver of the present invention referred to by the reference number 100 for receiving a radio frequency (RF) incoming signal having data bits encoded in the modulation symbols, exemplified in Fig. 1 described above, and modulated onto an RF carrier signal. The signal receiver 100 includes the carrier recovery system 10, a local oscillator system 102 for providing a local signal, a frequency converter 104 for using the local signal for providing an intermediate signal, an IQ converter 106 for converting the intermediate signal to the input signal described above having I and Q signal components, and a data bit converter 108 for converting the I and Q signal components into the data bits. The carrier recovery system 10 issues the accumulated phase difference signal described above to the local oscillator system 102. The local oscillator system 102 uses either a state machine or an executable program for directing a microprocessor for adjusting a local frequency of the local signal in order to reduce or substantially eliminate phase rotation of the modulation symbols in the input signal received by the carrier recovery system 10, thereby tracking and recovering the carrier frequency of the RF incoming signal. The IQ converter 106 uses the intermediate signal for providing the symbol clock signal and the input signal to the carrier recovery system 10. Fig. 4 is a block diagram of a signal receiver of the present invention referred to by the reference number 200 for receiving a radio frequency (RF) incoming signal having data bits encoded in the modulation symbols, exemplified in Fig. 1 described above, and modulated onto an RF carrier signal. The signal receiver 200 includes embodiments of the magnitude selector 14 and the phase subtractor 16 illustrated in Fig. 2 and embodiments of the local oscillator system 102, the frequency converter 104, the IQ converter 106, and the data bit detector 108 illustrated in Fig. 3b and described above in the accompanying detailed descriptions, respectively. The signal receiver 200 further includes an intermediate frequency (IF) phase multiplier 202 for receiving a first intermediate signal at an IF frequency and multiplying the frequency and phase of the first IF signal by N, where N is preferably four, for the RF incoming signal having modulation symbols exemplified in Fig. 1 and described above. The IF phase multiplier 202 issues a second IF signal having a frequency of N times the first IF signal and passes the second IF signal to the IQ converter 106. Referring back to Fig. 1, it should be understood that the present invention is not limited to use of the magnitude range D. For example, the magnitude range A could be used in place of or in addition to the magnitude range D while still retaining N equal to four for the phase multiplication of the phase multipliers 12 and 202. Further, the present invention is not limited to a 16 QAM modulation format. For example, equivalent magnitude threshold boundaries for a 64 QAM format, a 256 QAM format, and the like may be constructed and then detected in an embodiment of the magnitude selector 14 described above. Further, the modulation format need not be square but may have any arbitrary shape having two modulation symbols having an equal phase spacing within a selected magnitude range where N is the number of modulation symbols within that magnitude range. The phase limit detector 32 (Fig. 2) preferably uses a threshold boundary of 180° divided by N for providing the phase detection signal. Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.

Claims: What is claimed is: 1. A method of recovering a carrier frequency from an incoming modulated signal having modulation symbols having at least two symbol magnitudes and N approximately evenly spaced symbol phases at a particular one of said symbol magnitudes, comprising steps of: multiplying said symbol phases by said N; selecting modulation symbols within a certain magnitude range about said particular symbol magnitude; and determining phase differences between said N multiplied symbol phases for said modulation symbols having said certain range, said phase differences for use in recovering said carrier frequency. 2. The method of claim 1, further including a step of: detecting when said incoming modulated signal includes two successive modulation symbols are within said certain magnitude range. 3. The method of claim 2, wherein: the step of determining phase differences includes determining said phase differences only when two successive modulation symbols are within said certain magnitude range. 4. The method of claim 1, wherein: the step of determining said phase differences further includes accumulating said phase differences for two or more sets of said two successive modulation symbols. 5. The method of claim 1, wherein: the step of multiplying said symbol phases includes steps of receiving an IQ signal representative of said incoming modulated signal, said IQ signal having an in-phase (I) component signal and a quadrature phase (Q) component signal; and converting said IQ signal to a phase signal having a level proportional to said N times said symbol phases. 6. The method of claim 1, wherein: the step of multiplying said symbol phases includes steps of receiving an intermediate signal representative of said incoming modulated signal; frequency multiplying said intermediate signal by said N; and converting said N multiplied signal to a phase signal having a level proportional to said N times said symbol phases. 7. The method of claim 1, wherein: said particular symbol magnitude is a magnitude of said modulation symbols having a largest magnitude of said modulation symbols. 8. The method of claim 7, wherein: said incoming modulated signal is a quadrature amplitude modulation (QAM) signal having at least 16 states of said modulation symbols. 9. The method of claim 1, wherein: said N is four. 10. The method of claim 1, further comprising steps of: frequency converting said incoming modulated signal with a local signal; adjusting a frequency of said local signal according to said phase differences; converting said frequency converted signal into an IQ signal representative of said incoming modulated signal, said IQ signal having an in-phase (I) component signal and a quadrature phase (Q) component signal; using said IQ signal in the step of determining said phase differences; and determining incoming data carried by said incoming signal from said IQ signal. 11. A receiver having carrier frequency recovery for an incoming modulated signal having modulation symbols having at least two symbol magnitudes and N approximately evenly spaced symbol phases at a particular one of said symbol magnitudes, comprising: a phase multiplier for multiplying said modulation symbol phases by N; a magnitude selector for selecting modulation symbols within a certain magnitude range about said particular magnitude; and a phase subtractor coupled to the phase multiplier and the magnitude selector for determining phase differences between said N multiplied symbol phases for said selected modulation symbols within said certain magnitude range, said phase differences for use in recovering said carrier frequency. 12. The receiver of claim 11, further comprising: a magnitude selector coupled to the phase gate for detecting when said incoming modulated signal includes two successive modulation symbols are within said certain magnitude range. 13. The receiver of claim 12, wherein: the phase subtractor includes a symbol delay circuit for receiving a phase signal having a level proportional to said N times said symbol phases and issuing a representative delayed phase signal; a subtracting circuit for issuing a phase difference signal for a difference between said phase signal and said delayed phase signal; and a phase gate for passing said phase difference signal only when two successive modulation symbols are within said certain magnitude range . 14. The receiver of claim 11, wherein: the phase subtractor further includes an accumulator for accumulating said phase differences passed by said phase gate for two or more sets of said two successive modulation symbols and issuing an accumulated phase difference signal for said use in recovering said carrier frequency. 15. The receiver of claim 11, wherein: the phase multiplier is for receiving an IQ signal representative of said incoming modulated signal, said IQ signal having an in-phase (I) component signal and a quadrature phase (Q) component signal and converting said IQ signal to a phase signal having a level proportional to said N times said symbol phases. 16. The receiver of claim 11, wherein: the phase multiplier is for receiving an intermediate signal representative of said incoming modulated signal and frequency multiplying said intermediate signal by said N. 17. The receiver of claim 11, wherein: said particular symbol amplitude is a magnitude of said modulation symbols having a largest magnitude of said modulation symbols. 18. The receiver of claim 17, wherein: said incoming modulated signal is a quadrature amplitude modulation (QAM) signal having at least 16 states of said modulation symbols. 19. The receiver of claim 11, wherein: said N is four. 20. The receiver of claim 11, further comprising: a frequency converter for using a local signal for frequency converting said incoming modulated signal; a local oscillator for issuing said local signal at a frequency that is adjusted according to said phase differences; an IQ converter for converting said frequency converted signal into an IQ signal representative of said incoming modulated signal, said IQ signal having an in-phase (I) component signal and a quadrature phase (Q) component signal; and a data bit detector for determining incoming data bits from said IQ signal, said incoming data bits carried in said modulation symbols; and wherein: the phase subtractor is for using said IQ signal for determining said phase differences.

Inventors: HUANG HERBERT W, SAID AHMED M, HUANG, HERBERT, W., SAID, AHMED, M.
Applicants: STANFORD TELECOMM INC, STANFORD TELECOMMUNICATIONS, INC.
Application year: 1999
Main classification: H04L5
IPC codes: H04L27