Imported: 10 Mar '17 | Published: 27 Nov '08

USPTO - Utility Patents

A receiver comprises a multiple source phase estimator. The latter comprises a pilot-phase estimator, a data-driven average phase estimator, a selector and a common interpolation controller. The selector selects either the pilot-phase estimator or the data-driven average phase estimator as the source of determined phase estimates at particular times. At other times, the common interpolation controller provides interpolated phase estimates as a function of a linear interpolation based on a respective determined phase estimate.

The present invention generally relates to communications systems and, more particularly, to carrier recovery.

A carrier recovery loop, or carrier tracking loop, is a typical component of a communications system. The carrier recovery loop is a form of phase locked loop (PLL) and, in general, takes the form of a Costas Loop. The latter typically uses a decision-directed phase error estimator to drive the PLL. In a decision-directed phase error estimator, the loop is driven by phase errors between received signal points and respective sliced symbols (nearest symbols) taken from a symbol constellation. In other words, for each received signal point a hard decision is made as to which is the closest (and presumably correct) symbol (also referred to as the sliced symbol) of the symbol constellation. From this hard decision, the phase error between the received signal point and the associated sliced symbol is then used to drive the PLL. When the carrier frequency offset, i.e., the frequency difference between the carrier of the received signal and the recovered carrier, is outside the lock range of the loop, the so-called pull-in process occurs, in which, under proper operating conditions, the loop operates to reduce the carrier frequency offset until the carrier frequency offset falls inside the lock range of the loop and phase lock follows.

However, as the signal-to-noise ratio (SNR) drops the above-mentioned phase error estimate approach of the Costas loop becomes increasingly unreliable because the hard decision process begins to make more and more wrong decisions as to the received symbols. As such, other methods of estimating the phase are preferable. For example, in a system with known pilot symbols, a corresponding receiver includes a pilot-based phase interpolator so that the phase may be reliably determined at the pilot times and linearly interpolated in between the pilot times. Conversely, in a system lacking pilot symbols, a receiver includes a data-driven interpolator such that the phase estimate may also be determined periodically by using a data-driven average, such as represented by the Viterbi and Viterbi algorithm (A. J. Viterbi and A. M. Viterbi, Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission, IEEE Transactions on Information Theory, vol. IT-29, pp. 543-551, July, 1983). Again, in this data-driven process linear interpolation is used to estimate the phase at other times.

I have observed that it is beneficial for a receiver to be able to incorporate both a pilot-based phase estimator and a non-pilot-based phase estimator. For example, this provides the ability to select between a pilot-based interpolating process with the non-pilot-based phase interpolating process. Therefore, and in accordance with the principles of the invention, a receiver includes a pilot-based phase estimator, a non-pilot-based phase estimator and a selector for selecting between the pilot-based phase estimator and the non-pilot-based phase estimator for use in performing carrier recovery on a received signal.

In an embodiment of the invention, a receiver comprises a multiple source phase estimator. The latter comprises a pilot-phase estimator, a data-driven average phase estimator, a selector and a common interpolation controller. The selector selects either the pilot-phase estimator or the data-driven average phase estimator as the source of determined phase estimates at particular times. At other times, the common interpolation controller provides interpolated phase estimates as a function of a linear interpolation based on a respective determined phase estimate.

In accordance with a feature of the invention, the use of a common interpolation controller minimizes any additional circuitry and/or processing in the receiver.

In another embodiment of the invention, a receiver comprises a multiple source phase estimator. The latter comprises a pilot-phase estimator, a data-driven average phase estimator, a selector, a Costas loop and a common interpolation controller. The selector selects either the pilot-phase estimator or the data-driven average phase estimator as the source of determined phase estimates at particular times. At other times, the common interpolation controller provides interpolated phase estimates as a function of a linear interpolation based on a respective determined phase estimate and at least one decision-directed phase error estimate from the Costas loop.

Other than the inventive concept, the elements shown in the figures are well known and will not be described in detail. Also, familiarity with satellite-based systems is assumed and is not described in detail herein. For example, other than the inventive concept, satellite transponders, downlink signals, symbol constellations, carrier recovery, interpolation, phase-locked loops (PLLs), a radio-frequency (rf) front-end, or receiver section, such as a low noise block downconverter, formatting and encoding methods (such as Moving Picture Expert Group (MPEG)- 2 Systems Standard (ISO/IEC 13818-1)) for generating transport bit streams and decoding methods such as log-likelihood ratios, soft-input-soft-output (SISO) decoders, Viterbi decoders are well-known and not described herein. In addition, the inventive concept may be implemented using conventional programming techniques, which, as such, will not be described herein. Finally, like-numbers on the figures represent similar elements and some of the figures simplify the processing representation. For example, those skilled in the art appreciate that carrier recovery involves processing in the real and the complex domains.

An illustrative portion of a communications system in accordance with the principles of the invention is shown in FIG. 1. As can be observed from FIG. 1, a signal **104** is received by a receiver **105**. Signal **104** conveys information representative of control signaling, content (e.g., video), etc. In the context of this example, it is assumed that signal **104** represents a downlink satellite signal after reception by an antenna (not shown). Receiver **105** processes signal **104** in accordance with the principles of the invention (described below) and provides a signal **106** for conveying particular content to a multi-media endpoint as represented by television (TV) **10** for display thereon.

A prior art signal format for signal **104** is shown in FIG. 2. For the purposes of this example, signal **104** comprises a sequence of frames **20**, each frame **20** comprising at least a pilot portion **26** and a data portion **27**. Pilot portion **26** comprises one, or more, pilot symbols, which are predefined symbols known a priori to receiver **105**. If there is more than one pilot symbol in pilot portion **26**, it is assumed that at least one of the pilot symbols is predesignated as a reference symbol **25** (described below). It should be noted that the picture of FIG. 2 is not to scale and is merely representative of a signal comprising one or more pilot symbols interspersed with data symbols, which convey other information such as the above-mentioned control signaling and content, as well as, e.g., header and error correction/detection information, etc.

An illustrative portion of receiver **105** in accordance with the principles of the invention is shown in FIG. 3. Receiver **105** includes front end filter **110**, analog-to-digital (A/D) converter **115**, demodulator **120** and decoder **125**. Demodulator **120**, in accordance with the principles of the invention, includes at least one multiple source phase estimator (a circuit and/or process) (described below). Front end filter **110** down-converts (e.g., from the satellite transmission bands) and filters received signal **104** to provide a near baseband signal to A/D converter **115**, which samples the down converted signal to convert the signal to the digital domain and provide signal **116**, which is a sequence of samples, to demodulator **120**. The latter performs demodulation of signal **116** (including carrier recovery) and provides a demodulated signal **121** to decoder **125**, which decodes the demodulated signal point stream **121** to provide signal **126**, which is a bit stream of N bits per symbol interval T. Signal **126** represents the recovered data conveyed on signal **104** of FIG. 1. Data from output signal **126** is eventually provided to TV **10** via signal **106**. (In this regard, receiver **105** may additionally process the data before application to TV **10** and/or directly provide the data to TV **10**.)

Turning now to FIG. 4, an illustrative block diagram of demodulator **120** in accordance with the principles of the invention is shown. Demodulator **120** includes digital resampler **150**, filter **155**, carrier recovery element **200**, and timing recovery element **165**. Signal **116** is applied to digital resampler **150**, which resamples signal **116** using timing signal **166**, which is provided by timing recovery element **165**, to provide resampled signal **151**. Resampled signal **151** is applied to filter **155**. The latter is a band-pass filter for filtering resampled signal **151** about the carrier frequency to provide a filtered signal **156** to both carrier recovery element **200** and the above-mentioned timing recovery element **165**, which generates therefrom timing signal **166**. In accordance with the principles of the invention, carrier recovery element **200** includes a multiple source phase estimator for use in derotating, i.e., removing the carrier from, filtered signal **156** to provide a demodulated signal point stream, as represented by signal **121**, to decoder **125** of FIG. 3.

An illustrative embodiment of carrier recovery element **200** is shown in FIG. 5. The elements illustrated in FIG. 5 represent one form of a carrier recovery element that includes a multiple source phase estimator that can be implemented in either hardware and/or software. Carrier recovery element **200** comprises pilot phase estimator **205**, a pilot synchronization (sync) block **230**, a non-pilot-based phase estimator as illustrated by data-driven estimator **250**, multiplexer (mux) **255**, interpolator/controller **210**, sine/cosine (sin/cos) lookup table **215**, symbol buffer **220** and derotator **225** (which is a complex multiplier). Filtered signal **156** is applied to pilot phase estimator **205**, pilot sync block **230**, symbol buffer **220** and data-driven estimator **250**.

Turning first to symbol buffer **220**, this buffer collect symbols over a time period (described below), thus providing a time delay to enable calculation of a phase estimate by interpolator/controller **210** before application of a received symbol to derotator **225**. In particular, interpolator/controller **210** controls symbol buffer **220**, via signal **212**, to both synchronize the writing of symbols represented by filtered signal **156** to buffer **220**, and the reading of stored symbols from buffer **220** for application to derotator **225** (via signal **221**) along with application of the appropriate phase estimate via sin/cos lookup table **215** (via signal **216**). It should be noted that other mechanisms can be used to provide the appropriate delay, e.g., a delay line, a first-in-first-out (FIFO) buffer, etc.

Turning next to pilot sync block **230**, this block provides a timing signal **231** for use by other elements of FIG. 5 as required. Timing signal **231** provides a time reference with respect to the detection of pilot symbols in filtered signal **156**.

Next up is pilot phase estimator **205**, this element provides determined phase estimates to mux **255**. In particular, upon detection of the one, or more, pilot symbols in filtered signal **156**, pilot phase estimator **205** provides a determined phase estimate to mux **255**. As noted above, each pilot portion **26** of FIG. 2, or pilot interval, comprises one or more known symbols transmitted at known times. Pilot phase estimator **205** averages the symbols in the pilot intervals to determine an average phase estimate during the pilot interval. For example, if the pilot portion comprises a number of different pilot symbols, an average phase may be determined as illustrated by the equation below:

where R_{i }are the received pilot symbols, P_{i}* is the complex conjugate of the known pilot symbols, and the index, i, is over the all the pilot symbols.

This determined phase estimate may be referenced, e.g., to the center symbol (reference symbol) of the pilot interval (as represented by reference symbol **25** of FIG. 2). In other words, the determined phase estimate over the pilot interval is assumed to be the phase **20** at the middle of the pilot interval. Thus, pilot phase estimator **205** provides determined phase estimates at particular times, e.g., every pilot interval, to mux **255**.

Likewise, the non-pilot-based phase estimator provides determined phase estimates at particular times, e.g., periodically, to mux **255**. In this example, one illustration of a non-pilot-based estimator is provided by data-driven estimator **250**. The latter illustratively determines a phase estimate by using a data-driven average, such as represented by the Viterbi and Viterbi algorithm (A. J. Viterbi and A. M. Viterbi, Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission, IEEE Transactions on Information Theory, vol. IT-29, pp. 543-551, July, 1983). For example, in a quadrature phase-shift keying (QPSK) system, an estimate is made over M symbols of an average phase by adding modified symbols z_{mod }as

and where the power p is, e.g., equal to 2. It should be noted that, here, the estimate, due to the factor 0.25, is ambiguous beyond plus or minus /4, rather than plus or minus .

In view of the above, both pilot phase estimator **205** and data-driven phase estimator **250** provide a sequence of determined phase estimates to mux **255** (also referred to herein as a selector). The latter selects the particular source of determined phase estimates for application to interpolator/controller **210**. It should be noted that although in this example only two sources of determined phase estimates are shown, the invention is not so limited and is applicable to any number of sources. Selection of a particular source is performed by signal **254**. The latter can either be under software control (e.g., a mode setting, system parameter, etc.) or done via hardware (e.g., a switch). Once a particular source is selected, that sequence of determined phase estimates is provided by mux **255** to interpolator/controller **210**. For example, if no pilot is detected in a predetermined amount of time, carrier recovery element **200** defaults to using a non-pilot-based phase estimator source.

Illustratively, the time between determined phase estimates, whether from pilot phase estimator **205** or data-driven estimator **250**, is referred to herein as an EPOCH. This is illustrated in FIG. 6 for an illustrative EPOCH **54** spanning a portion of time along time axis **51**. The beginning of an EPOCH is marked by the generation of a determined phase estimate, as represented by _{start }in FIG. 6. Likewise, the end of an EPOCH is marked by the generation of a subsequent determined phase estimate, as represented by _{end }in FIG. 6. (It should be noted that the end of one EPOCH is the start of another EPOCH, i.e., _{end }of one EPOCH is the _{start }for the following EPOCH.) During an EPOCH, N symbols are received and buffered in symbol buffer **220**, i.e., the period of time covered by the EPOCH is equal to NT, where T is the symbol interval. (It should be noted that the inventive concept does not require that all EPOCHs have the same time duration.)

Interpolator/controller **210** operates on the sequence of determined phase estimates to provide signal **211** to sin/cos lookup table **215**. In accordance with a feature of the invention, it should be noted that interpolator/controller **210** is used whatever the source of determined phase estimates, i.e., interpolator/controller **210** is common, thus minimizing any additional circuitry and/or processing in the receiver. Signal **211** represents a value for the estimated amount of phase needed to derotate a corresponding symbol, i.e., the amount of phase derotation to remove any phase offset. Sin/cos lookup table **215** provides the corresponding sine and cosine values of this phase estimate to complex multiplier **225** for de-rotation of signal **221** to provide down-converted received signal **121**.

The estimated phase value represented by signal **211** is referred to herein as _{derot}. At the start of an EPOCH, the amount of phase needed to derotate a symbol is _{start}, which is equal to:

_{start}=_{start},(3)

where all angles are expressed in radians. As defined herein, _{start }is also referred to herein as the inverse of _{start}. At the end of an EPOCH, the amount of phase needed to derotate a symbol is equal to:

_{start}+diff_{lin}.(4)

In this particular example, values for diff_{lin }differ depending on the selected source of determined phase estimates. When pilot phase estimator **205** is selected, diff_{lin }is defined as:

and where _{end }is the inverse of _{end}, i.e.,

_{end}=_{end},(6)

However, when data-driven estimator **250** is selected, diff_{lin }is defined as:

Equation (7) takes account of the fact that when no pilot symbols are available, and if the Viterbi and Viterbi algorithm is used, the phase estimates of the start phase and end phase may each vary from /4 to +/4. Since, in this example, values for diff_{lin }may vary as a function of the source of determined phase estimates, signal **254** is also applied to interpolator/controller **210** as an indicator of which source is currently selected.

In between the start and end of an EPOCH, the phase required for derotating a received symbol is not known. In order to provide a phase estimate, interpolator/controller **210** performs linear interpolation to generate a value for _{derot}. In particular, the above noted value for diff_{lin }is assumed to be linearly distributed over the N symbols of the EPOCH, i.e., for the k^{th }symbol of the EPOCH, the phase estimate, _{derot,k }is:

where k represents the symbol index in the EPOCH and N is the total number of symbols within the EPOCH.

Turning now to FIG. 7, another embodiment in accordance with the principles of the invention is shown. The embodiment of FIG. 7 is similar to the embodiment of FIG. 5 except that signal **254** is provided by pilot detector **260**. The latter automatically controls the selection of the source of determined phase estimates. For example, upon detection of the pilot signal, pilot detector **260** controls mux **255**, via signal **254**, to select pilot phase estimator **205**. However, if no pilot signal is detected, e.g., upon expiration of a predetermined amount of time, pilot detector **260** controls mux **255** to select a non-pilot-based phase estimator source (such as represented by data-driven estimator **250**). Thus, receiver **105** uses pilot intervals for phase estimates if they exist, uses data-based estimates otherwise, or supplements the pilot-based phase estimates with additional data-based estimates in between.

Attention should now be directed to FIG. 8, which shows an illustrative flow chart in accordance with the principles of the invention for use in receiver **105** of FIG. 1. In step **505**, receiver **105** selects a source of determined phase estimates at particular times from a number of possible sources. In step **510**, receiver **105** provides an estimate of a phase value at other times as a function of the determined phase estimates from the selected source (e.g., using linear interpolation as illustrated by equation (8)). Illustratively, the provided phase estimates are used for derotation of received symbols.

Unfortunately, without knowing how many radians the incoming carrier traversed between the pilot times, the above-described linear interpolation estimate may yield the wrong value for _{derot,k}. This is further illustrated in FIGS. 9 and 10. FIG. 9 shows respective values for _{start }and _{end }for an illustrative EPOCH. However, as demonstrated by arrows **1** and **2**, the starting and ending determined phase estimates do not provide information as to whether the incoming carrier traversed the path represented by arrow **1** or the path represented by arrow **2**. Likewise, a similar situation is shown in FIG. 10, which illustrates by the path associated with arrow **3** that the number of radians traversed by the incoming carrier can even be greater than 2. Therefore, and in accordance with a feature of the invention, decision-directed carrier recovery is used to resolve this ambiguity. This is illustrated in the embodiment of FIG. 11 by the application of filtered signal **156** to decision-directed carrier recovery circuit **300**.

Turning briefly to FIG. 12, an illustrative block diagram for decision-directed carrier recovery circuit **300** is shown. Decision-directed carrier recovery circuit **300** comprises complex multiplier **310**, sine/cosine (sin/cos) lookup table **340**, phase detector **315**, loop filter **330** and phase integrator **335**. It is assumed that the processing illustrated by FIG. 12 is in the digital domain (although this is not required), i.e., the carrier recovery circuit **300** includes a digital phase-locked loop (DPLL) driven by hard decisions. Signal **156** is a complex sample stream comprising in-phase (I) and quadrature (Q) components. It should be noted that complex signal paths are not specifically shown in FIG. 12. Complex multiplier **310** receives the complex sample stream of signal **156** and performs de-rotation of the complex sample stream by recovered carrier signal **341**. In particular, the in-phase and quadrature components of signal **156** are derotated by a phase of recovered carrier signal **341**, which represents particular sine and cosine values provided by sin/cos table **340** (described below). The output signal from complex multiplier **310** is a down-converted received signal **311**, e.g., at baseband, and represents a de-rotated complex sample stream of received signal points. The down-converted received signal **311** is applied to phase detector **315**, which computes any phase offset still present in the down-converted signal **311** and provides a phase error estimate signal **326** indicative thereof.

As can be observed from FIG. 12, phase detector **315** includes two elements: phase error estimator **325** and slicer **320**. As known in the art, the latter makes a hard decision as to the possible symbol (target symbol) represented by the in-phase and quadrature components of each received signal point of down-converted signal **311**. In particular, for each received signal point of down-converted signal **311**, slicer **320** selects the closest symbol (target symbol) from a predefined constellation of symbols. As such, the phase error estimate signal **326** provided by phase error estimator **325** represents the phase difference between each received signal point and the corresponding target symbol. In particular, phase error estimate signal **326** represents a sequence of phase error estimates, _{error}_{}_{estimate}, where each particular _{error}_{}_{estimate }is determined by calculating the imaginary part of the received signal point times the conjugate of the associated sliced symbol, i.e.,

_{error}_{}_{estimate}=imag(*zz*_{sliced}*)=|*z||z*_{sliced}| sin (*zz*_{sliced})|*z|*^{2}(_{error}).(9)

In the above equation, Z represents the complex vector of the received signal point, Z_{sliced }represents the complex vector of the associated sliced signal point and Z_{sliced}* represents the conjugate of the complex vector of the associated sliced signal point.

The phase error estimate signal **326** is applied to loop filter **330**, which further filters the phase error estimate signal **326** to provide a filtered signal **331**. Typically loop filter **330** is a second-order filter comprising proportional and integral paths. Filtered signal **331** is applied to phase integrator **335**, which further integrates filtered signal **331** and provides an output phase angle signal **336** to sin/cos lookup table **340**. The latter provides the associated sine and cosine values to complex multiplier **310** for de-rotation of signal **156** to provide down-converted received signal **311**. Although not shown for simplicity, a frequency offset, F_{OFFSET}, may be fed to loop filter **330**, or phase integrator **335**, to increase acquisition speed. Also, it should be noted that carrier recovery circuit **300** may operate at multiples of (e.g., twice) the symbol rate of signal **156**. As such, phase integrator **335** continues to integrate at all sample times. The output phase angle signal **336** is also applied to interpolator/controller **210** of FIG. 11 to assist in generating a phase estimate. (It should be noted that the output phase angle **336** is already in the form of a derotating phase value and, as such, is the inverse of the signal phase to be corrected.)

Returning now to FIG. 11, the phase of the decision-directed carrier recovery is monitored by interpolator/controller **210** via phase angle signal **336**. In particular, interpolator/controller **210** monitors phase angle signal **336** between the start and end of each EPOCH to determine the total phase excursion, diff_{cr}, from beginning to end of an EPOCH, which may exceed or be less than . This total phase excursion, diff_{cr}, is used by interpolator/controller **210** as additional information for use in estimating a value for _{derot }for a respective symbol. Although the decision-directed carrier recovery may slightly slip, or be noisywhich is the reason for using an interpolation scheme in the first placedecision-directed carrier recovery should be robust enough for use as an aid to interpolated carrier recovery.

Referring now to FIG. 13, an illustrative phase excursion calculator **400** for use in interpolator/controller **210** for monitoring the total phase excursion diff_{cr }is shown. The elements illustrated in FIG. 13 represent one form of phase excursion calculator that can be implemented in either hardware and/or software. Phase excursion calculator **400** comprises sample delay **405**, phase register **435**, difference elements **410** and **440**, comparators **415** and **420**, a counter **425**, a multiplier **430** and an adder **445**. At the start of an EPOCH (conveyed by signal **434**) the value represented by phase angle signal **336** is stored in phase register **435** and counter **425** is reset to a value of zero. Difference element **440** provides a phase difference value **441** between the starting phase value stored in phase register **435** and subsequent phase values during the EPOCH. This phase difference value **441** is also referred to herein as the uncorrected phase difference. The remaining elements of phase excursion calculator **400** track how many times, and in what direction, the value of phase angle signal **336** crosses the / radial (this radial is represented in FIGS. 9 and 10, described earlier). In particular, during an EPOCH, difference element **410** provides a phase difference signal **411**, representing sample-to-sample phase difference values by subtracting a previous phase value provided by sample delay element **405** from a current phase value provided by phase angle signal **336**. This phase difference value signal is applied to the A input leads of comparators **415** and **420**. Comparator **415** compares the value of phase difference signal **411** to (applied to the B input lead of comparator **415**); while comparator **420** compares the value of phase difference signal **411** to (applied to the B input lead of comparator **420**). If the phase difference value is greater , then comparator **415** provides a signal from the AB lead of comparator **415** to counter **425**. However, if the phase difference value is less than , then comparator **420** provides a signal from the AB lead of comparator **420** to counter **425**. Counter **425** is, in effect, a 2 counter, i.e., counter **425** counts the number of times and in what direction the / radial is crossed. If the phase difference value is greater than , then counter **425** is decremented (DN input of counter **425**), while if the phase difference value is less than , counter **425** is incremented (UP input of counter **425**). The output signal **426** from counter **425** is applied to multiplier **430** which multiplies the value represented therein by 2 for addition to the uncorrected phase difference (signal **441**) via adder **445** to provide the total phase excursion diff_{cr }(signal **446**) for use by interpolator/controller **210**. In other words, every time the / radial is crossed in the clockwise direction, the total phase excursion during the EPOCH needs to be decremented by 2 relative to the uncorrected phase difference (signal **441**) during the EPOCH. Similarly, every time the / radial is crossed in the counterclockwise direction, the total phase excursion during the EPOCH needs to be incremented by 2 relative to the uncorrected phase difference (signal **441**).

As noted above, the beginning and end phases, _{start }and _{end}, of the linear interpolation are assumed to be robust from pilot phase estimator **205**, and are the inverses of the detected pilot interval phases at the start and end of an EPOCH, respectively. However, the unassisted difference from beginning to end, i.e.,

diff_{lin}=_{end}_{start},(10)

is assumed, in the absence of additional information, to be off by an integer number, m, of rotations of 2. The information from the decision-directed carrier recovery is used to select a value for the number m such that the difference interpolated over is within plus or minus radians of the corrected decision-directed carrier recovery estimate. In particular, the following equations are defined:

diff_{lin,assist}=_{end}_{start}+2*m;*(11)

diff_{cr}diff_{lin,assist}diff_{cr}+; and(12)

diff_{cr}_{end}_{start}+2*mdiff*_{cr}+,(13)

where diff_{lin,assist }is the difference to be used in the linear interpolator (instead of equation (8)), as assisted by decision-directed carrier recovery; and diff_{cr }is the phase difference from beginning to end of an EPOCH as calculated by the decision-directed carrier recovery, corrected for 2 wraps.

From equation (13), the value for m can be found by noting the following:

2*m*diff_{cr}+(_{end}_{start}), or(14)

*m*diff_{cf}/(2)+0.5(_{end}_{start})/( 2), or(15)

*m*=floor[diff_{cr}/(2)+0.5(_{end}_{start})/( 2)],(16)

where floor(x) is the largest integer that is less than or equal to x. It should be noted that this floor calculation is easy to perform in the digital domain, as it involves a truncation of bits.

Once m is determined thusly, this value of m is used to determine the value for diff_{lin,assist }from equation (11), above. As such, interpolator/controller **210** provides phase estimates with carrier assist in accordance with the following equation:

Attention should now be directed to FIG. 14, which shows an illustrative flow chart in accordance with the principles of the invention for use in receiver **105** of FIG. 1. In step **605**, receiver **105** selects a source of determined phase estimates at particular times from a number of possible sources. In step **610**, receiver **105** forms a decision-directed phase estimate (e.g., using the above-described Costas loop). In step **615**, receiver **105** provides an estimate of a phase value at other times as a function of the determined estimate and the decision-directed phase estimate (e.g., using linear interpolation as modified by equation (17)).

Another illustrative embodiment of the inventive concept is shown in FIG. 15. In this illustrative embodiment an integrated circuit (IC) **705** for use in a receiver (not shown) includes a carrier recovery loop (CRL) **720** and at least one register **710**, which is coupled to bus **751**. Illustratively, IC **705** is an integrated analog/digital television demodulator/decoder. However, only those portions of IC **705** relevant to the inventive concept are shown. For example, analog-digital converters, filters, decoders, etc., are not shown for simplicity. Bus **751** provides communication to, and from, other components of the receiver as represented by processor **750**. Register **710** is representative of one, or more, registers, of IC **705**, where each register comprises one, or more, bits as represented by bit **709**. The registers, or portions thereof, of IC **705** may be read-only, write-only or read/write. In accordance with the principles of the invention, CRL **720** includes the above-described multiple source phase estimator feature, or operating mode, and at least one bit, e.g., bit **709** of register **710**, is a programmable bit that can be set by, e.g., processor **750**, for enabling or disabling this operating mode (e.g., to turn-on or turn-off multiple source selection). Likewise, a bit of register **710** may be used to select a particular one of a number of sources of determined phase estimates. In the context of FIG. 3, IC **705** receives an IF signal **701** (e.g., signal **116** of FIG. 3) for processing via an input pin, or lead, of IC **705**. A derivative of this signal, **702**, is applied to CRL **720** for carrier recovery as described above. CRL **720** provides signal **721**, which is a derotated version of signal **702**. CRL **720** is coupled to register **710** via internal bus **711**, which is representative of other signal paths and/or components of IC **705** for interfacing CRL **720** to register **710** as known in the art. IC **705** provides one, or more, recovered signals, e.g., a composite video signal, as represented by signal **706**.

In view of the above, it should be noted that although described in the context of a satellite communications system, the inventive concept is not so limited. For example, the elements of FIG. 1 may represent other types of systems and other forms of multi-media endpoints. For example, satellite radio, terrestrial broadcast, cable TV, etc. Also, although described herein in the context of a single demodulator, it should be realized that the inventive concept is applicable to multi-modulation receivers, where information may be conveyed on different signal layers. For example, layered modulation receivers, hierarchical modulation receivers, or combinations thereof. Indeed, the invention is applicable to any type of receiver in which carrier recovery is performed. Finally, it should be noted that the embodiments described above may operate at the symbol rate or some other rate, for example, samples at twice the symbol rate. This is so other processing, e.g., a fractionally-spaced equalizer, may be also be used in the receiver.

As such, the foregoing merely illustrates the principles of the invention and it will thus be appreciated that those skilled in the art will be able to devise numerous alternative arrangements which, although not explicitly described herein, embody the principles of the invention and are within its spirit and scope. For example, although illustrated in the context of separate functional elements, these functional elements may be embodied on one or more integrated circuits (ICs). Similarly, although shown as separate elements, any or all of the elements may be implemented in a stored-program-controlled processor, e.g., a digital signal processor (DSP) or microprocessor that executes associated software, e.g., corresponding to one or more of the elements shown in FIG. 5, etc. Further, although shown as separate elements, the elements therein may be distributed in different units in any combination thereof. For example, receiver **105** may be a part of TV **10** or receiver **105** may be located further upstream in a distribution system, e.g., at a head-end, which then retransmits the content to other nodes and/or receivers of a network. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

a pilot-based phase estimator;

a non-pilot-based phase estimator; and

a selector for selecting between the pilot-based phase estimator and the non-pilot-based phase estimator as a source of determined phase estimates for use in performing carrier recovery on a received signal.

a pilot-based phase estimator;

a non-pilot-based phase estimator; and

a selector for selecting between the pilot-based phase estimator and the non-pilot-based phase estimator as a source of determined phase estimates for use in performing carrier recovery on a received signal.

a demodulator for demodulating a received signal; and

a decoder for decoding the demodulated received signal to provide a decoded signal;

wherein the demodulator includes a multiple source phase estimator for demodulating the received signal comprising

a pilot-based phase estimator;

a non-pilot-based phase estimator;

a selector for selecting between the pilot-based phase estimator and the non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;

an interpolator for providing interpolated phase estimates at other times, wherein the interpolator performs linear interpolation based on a respective determined phase estimate; and

a derotator for providing the demodulated received signal, wherein the derotator derotates symbols of the received signal in accordance with the interpolated phase estimates.

a demodulator for demodulating a received signal; and

a decoder for decoding the demodulated received signal to provide a decoded signal;

wherein the demodulator includes a multiple source phase estimator for demodulating the received signal comprising

a pilot-based phase estimator;

a non-pilot-based phase estimator;

a selector for selecting between the pilot-based phase estimator and the non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;

an interpolator for providing interpolated phase estimates at other times, wherein the interpolator performs linear interpolation based on a respective determined phase estimate; and

a derotator for providing the demodulated received signal, wherein the derotator derotates symbols of the received signal in accordance with the interpolated phase estimates.

a pilot-based phase estimator;

a non-pilot-based phase estimator;

a selector for selecting between the pilot-based phase estimator and the non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;

an interpolator for providing interpolated phase estimates at other times, wherein the interpolator performs linear interpolation based on a respective determined phase estimate; and

a derotator for providing the demodulated received signal, wherein the derotator derotates symbols of the received signal in accordance with the interpolated phase estimates.

a demodulator for demodulating a received signal; and

a decoder for decoding the demodulated received signal to provide a decoded signal;

wherein the demodulator includes a multiple source phase estimator for demodulating the received signal comprising
a selector for selecting between the pilot-based phase estimator and the non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;

a pilot-based phase estimator;

a non-pilot-based phase estimator;

a decision-directed phase estimator;

an interpolator for providing interpolated phase estimates at other times, wherein the interpolator performs linear interpolation based on a respective determined phase estimate and at least one decision-directed phase error estimate from the decision-directed phase estimator; and

a derotator for providing the demodulated received signal, wherein the derotator derotates symbols of the received signal in accordance with the interpolated-phase estimates.

a demodulator for demodulating a received signal; and

a decoder for decoding the demodulated received signal to provide a decoded signal;

wherein the demodulator includes a multiple source phase estimator for demodulating the received signal comprising
a selector for selecting between the pilot-based phase estimator and the non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;

a pilot-based phase estimator;

a non-pilot-based phase estimator;

a decision-directed phase estimator;

an interpolator for providing interpolated phase estimates at other times, wherein the interpolator performs linear interpolation based on a respective determined phase estimate and at least one decision-directed phase error estimate from the decision-directed phase estimator; and

a derotator for providing the demodulated received signal, wherein the derotator derotates symbols of the received signal in accordance with the interpolated-phase estimates.

a pilot-based phase estimator;

a non-pilot-based phase estimator;

a decision-directed phase estimator;

an interpolator for providing interpolated phase estimates at other times, wherein the interpolator performs linear interpolation based on a respective determined phase estimate and at least one decision-directed phase error estimate from the decision-directed phase estimator; and

a derotator for providing the demodulated received signal, wherein the derotator derotates symbols of the received signal in accordance with the interpolated-phase estimates.

a carrier recovery element for use in demodulating a received signal; and

a register, wherein the register sets one of a number of modes for the carrier recovery element, and wherein one of the number of modes is a pilot-based phase estimator mode and another of the number of modes is a non-pilot-based phase estimator mode.

a carrier recovery element for use in demodulating a received signal; and

a register, wherein the register sets one of a number of modes for the carrier recovery element, and wherein one of the number of modes is a pilot-based phase estimator mode and another of the number of modes is a non-pilot-based phase estimator mode.

selecting one of a number of sources for provided determined phase estimates of a received signal; and

providing interpolated phase estimates of the received signal using determined phase estimates from the selected source.

selecting one of a number of sources for provided determined phase estimates of a received signal; and

providing interpolated phase estimates of the received signal using determined phase estimates from the selected source.

providing decision-directed phase estimates of the received signal; and

interpolating the phase estimates of the received signal using the determined phase estimates from the selected source and at least one of the provided decision-directed phase estimates.

providing decision-directed phase estimates of the received signal; and

interpolating the phase estimates of the received signal using the determined phase estimates from the selected source and at least one of the provided decision-directed phase estimates.

demodulating a received signal using a multiple source phase estimator; and

decoding the demodulated received signal to provide a decoded signal;

and wherein the demodulating step includes the steps of:

selecting between a pilot-based phase estimator and a non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;

providing interpolated phase estimates at other times, wherein the interpolation is a form of linear interpolation based on a respective determined phase estimate; and

derotating symbols of the received signal in accordance with the interpolated phase estimates to provide the demodulated received signal.

demodulating a received signal using a multiple source phase estimator; and

decoding the demodulated received signal to provide a decoded signal;

and wherein the demodulating step includes the steps of:

selecting between a pilot-based phase estimator and a non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;

providing interpolated phase estimates at other times, wherein the interpolation is a form of linear interpolation based on a respective determined phase estimate; and

derotating symbols of the received signal in accordance with the interpolated phase estimates to provide the demodulated received signal.

selecting between a pilot-based phase estimator and a non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;

providing interpolated phase estimates at other times, wherein the interpolation is a form of linear interpolation based on a respective determined phase estimate; and

derotating symbols of the received signal in accordance with the interpolated phase estimates to provide the demodulated received signal.

demodulating a received signal using a multiple source phase estimator; and

decoding the demodulated received signal to provide a decoded signal;

and wherein the demodulating step includes the steps of:
selecting between a pilot-based phase estimator and a non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;
derotating symbols of the received signal in accordance with the interpolated phase estimates to provide the demodulated received signal.

providing decision-directed phase estimates of the received signal;

providing interpolated phase estimates at other times, wherein the interpolation is a form of linear interpolation based on a respective determined phase estimate and at least one of the decision-directed phase error estimates; and

demodulating a received signal using a multiple source phase estimator; and

decoding the demodulated received signal to provide a decoded signal;

and wherein the demodulating step includes the steps of:
selecting between a pilot-based phase estimator and a non-pilot-based phase estimator as a source of determined phase estimates for the received signal at particular times;
derotating symbols of the received signal in accordance with the interpolated phase estimates to provide the demodulated received signal.

providing decision-directed phase estimates of the received signal;

providing interpolated phase estimates at other times, wherein the interpolation is a form of linear interpolation based on a respective determined phase estimate and at least one of the decision-directed phase error estimates; and

providing decision-directed phase estimates of the received signal;

providing interpolated phase estimates at other times, wherein the interpolation is a form of linear interpolation based on a respective determined phase estimate and at least one of the decision-directed phase error estimates; and

using a register to set one of a number of modes for a carrier recovery element; and

using the carrier recovery element in the set mode for demodulating a received signal;

wherein one of the number of modes is a pilot-based phase estimator mode and another of the number of modes is a non-pilot-based phase estimator mode.

using a register to set one of a number of modes for a carrier recovery element; and

using the carrier recovery element in the set mode for demodulating a received signal;

wherein one of the number of modes is a pilot-based phase estimator mode and another of the number of modes is a non-pilot-based phase estimator mode.