Data directed frequency acquisition loop that synchronizes to a received signal by using the redundancy of the data in the frequency domain

A data-directed frequency-acquisition loop capable of generating a frequency error having a magnitude and direction from a double sideband suppressed signal comprises a first multiplier that multiplies the signal by the output of a VCO. The output of the first multiplier is convolved by a second multiplier. The I output of the second multiplier passes through a first low-pass filter. The filtered I output and the Q output are then multiplied by a third multiplier. The output of the third multiplier is filtered through a second low-pass filter, amplified, and return to the VCO to complete the feedback loop.

BACKGROUND

In order to provide the widest possible coverage for a digital transmission, such as for cell phones or a digital television broadcast, it's desirable to use multiple transmitters that are separated from each other spatially. This permits a wider area to be covered, uses less total broadcast power, and can help to fill in dark areas where the transmission from one transmitter may be blocked. Thus, using multiple transmitters can provide wider and more complete coverage for virtually any digital transmission.

However, using multiple transmitters creates a serious problem when the receiver is at a “seam” between two transmitters, because the additional signal can appear as a “ghost” that can be as large as the “main” signal. Furthermore, destructive interference creates a series of perfect or near perfect nulls.

Existing receiver technology handles ghosts by filtering them out in order to interpret the “main” signal. But in a multi-transmitter environment this strategy is unworkable. It makes little sense to design a system to filter out a ghost that can be an arbitrarily large fraction of the “main” signal's size. Furthermore, near the margins the best this subtractive strategy can ever provide is a signal strength equal to the stronger transmitter's signal—the energy from the secondary signal is wasted.

Even when the ghosts are smaller than 100% of the “main” signal, there is an equal probability of pre- and post-ghosts. In the most common situation, the strongest signal is the one following the most direct path. Ghosts are most often produced by “multipathing,” that is, by portions of the signal following paths of different lengths from the transmitter to the receiver. Thus, ghosts are typically produced by one or more strong reflections. The first signal to arrive is typically the most direct, and therefore the strongest, and so in the usual situation the ghost is a post-ghost. In a multi-transmitter environment, though, while the receiver is near a seam the stronger signal can easily arrive after the ghost. With signals arriving from two directions, it is possible that the more direct path may be the longer one. Consequently, pre-ghosts are about as likely as post-ghosts, and may be arbitrarily strong. Furthermore, if the transmitters are out of sync with each other by even a small amount, where the one lagging happens to be the closer one the receiver will likely see pre-ghosts.

Existing technology relies on the assumption that post-ghosts predominate (i.e., existing systems are not generally designed to deal with Raleigh fading). Thus, existing receivers generally will be either inefficient or incapable of dealing with a multi-transmitter environment, even if the ghosts are sufficiently small compared to the “main” signal.

In short, in a multi-transmitter environment, the “main” signal becomes a meaningless concept at the seams of the transmission. In order to operate efficiently in a multi-transmitter environment, a digital receiver must operate with a different paradigm. What is needed is a digital receiver that employs an additive strategy—that is, one in which the energy from one or more relatively large ghosts can be captured and used to aid in the synchronization process, rather than filtered out and discarded. Such a receiver could both function with ghosts 100% of the size of the “main” signal, and provides substantially superior performance whenever ghosts exceed about 70% of the size of the “main” signal.

From the receiver's perspective, most of the signal is useless for synchronization, because it is indistinguishable from white noise. The more information that is packed into a signal, the more closely it will resemble white noise, so this is both a desirable and inevitable feature of the signal. Nevertheless, some bandwidth must be “wasted” in order to provide the receiver a means to orient itself. Typically, one of two strategies is employed. In some systems, a pilot signal is included. This is a sharp peak of energy in a very narrow frequency band, which is very easy for the receiver to pick out.

A phase-lock loop, such as the one shown inFIG. 1, indicated generally at100, is a typical way to synch up a receiver using a pilot. A multiplier110multiplies the signal and the output of a voltage controlled oscillator120(“VCO”) to produce a beat note (a sine wave with a frequency equal to the difference between the frequency of the pilot signal and the VCO's output). The beat note passes through a low-pass filter130. The output of the filter130is amplified and input to the VCO120to complete the feedback loop. The low-pass filter130has competing design parameters. The more narrow the band pass of the filter130the smaller the response, so the slower the loop100is to lock up. However, a wide pass filter passes more noise and makes it harder for the loop100to capture at all.

It will be appreciated that the response of the loop100is driven by the frequency difference output of the first multiplier110. The direction of error can only be determined by observing the slope of the time rate of change of the output. The second filter130distorts the sine wave, increasing the amplitude on the closer side, and decreasing it on the further side. Convergence is driven by this asymmetry of the distorted beat note.

However, because the amplitude of the beat note drops with increasing frequency difference, that distortion output drops as well, so the response of the phase-lock loop100decreases as the frequency of the VCO120diverges from the signal frequency. Thus, unless the signal happens to be close to the initial VCO120frequency, it will converge slowly, or not at all. A typical phase lock loop can capture when the initial VCO120frequency is within a factor of about 3-10 times the bandwidth of the loop.

Another, more robust, strategy for synching is to provide a signal in which information in the data is redundant in the frequency domain. The receiver can look for a correlation in the data created by this repetition to synch up. The receiver could use this same technique to find correlations in the data from signals from multiple transmitters. In mathematical terms, the correlation between the repeated signal portion can be identified by fully complex convolution. Convolution inherently corrects for the asymmetry produced by the slope of the Nyquist band, so that the peak value occurs when the limits of integration exactly correspond to the beginning and the end of the repeated data segment (and it's negative time image).

A typical existing means for performing such a convolution is the Costas Loop, shown in FIG.2. The Costas Loop operates on a complex signal, such as a QAM signal. As with the phase-lock loop, a first multiplier210multiplies the signal with the output of a VCO220, though, as shown inFIG. 2, this is a complex multiplication, which produces both an I′ and a Q′ output. As with the phase-lock loop, the output of the first multiplier is passed through a low-pass filter230where the unwanted (frequency sum) portion of multiplied signal is removed. The in-phase and quadrature portions are then multiplied by a second multiplier240to produce a beat note (assuming the sideband isn't balanced—otherwise it's merely a DC voltage.) The beat note is passed through a second low-pass filter250, then amplified at299and returned to the VCO220to complete the feedback loop. Thus, the portion of the Costas loop following the second multiplier240, which drives the convergence of the loop, is basically a phase-lock loop. Consequently, like the phase-lock loop, the Costas loop has the disadvantage of slow convergence.

A frequency-and-phase-lock loop (“FPLL”) (shown inFIG. 3, and described in U.S. Pat. No. 4,072,909 to Citta, which is hereby incorporated by reference in its entirety) provides faster convergence. The FPLL has a first low-pass filter330and a second low-pass filter350which perform the function of the second low-pass filter250in the Costas loop, which separate the averaging and noise-elimination functions. Thus, the first low-pass filter330can have a relatively wide band pass, so that the FPLL can acquire even when the signal and initial VCO frequencies are off by as much as a factor of 1000. The second low-pass filter350can have a relatively narrow band-pass, in order to give good averaging during lock-up. The output of the second multiplier340is a rectified sine wave with a DC offset. The DC offset provides the direction information, rather than an integration of a distorted sine wave, which provides a much stronger response when the frequency difference is relatively large. The signal from the filter350is amplified at399and returned to the VCO320to complete the feedback loop.

Because of the way the FPLL uses the complex information to provide both magnitude and direction information, it locks up faster, and phase noise that is less than 90 degrees out of phase doesn't disrupt the lock. However, the FPLL does not perform a convolution of the data, and is therefore dependent upon a pilot to operate. It is therefore not suitable for use with, for example, a double sideband suppressed signal.

Because of the way the FPLL uses the complex information to provide both magnitude and direction information, it locks up faster, and phase noise that is less than 90 degrees out of phase doesn't disrupt the lock. However, the FPLL does not perform a convolution of the data, and is therefore dependent upon a pilot to operate. It is therefore not suitable for use with, for example, a double sideband suppressed signal.

Thus, what is needed is a new data-synch loop which combines the desired features of the Costas Loop—synching by finding a correlation in repeated data through convolution—with the desired faster convergence of a frequency-and-phase-lock loop.

SUMMARY OF THE INVENTION

A first embodiment data-directed frequency acquisition loop for synching to a signal comprises: a VCO, a first, second, and third multipliers, and a first and second low-pass filters. The VCO has an I and Q output. The first multiplier has as input the signal and the I and Q output, and has an I′ and Q′ output. The second multiplier has as input the I′ and Q′ output, and has an I″ and Q″ output. The first low-pass filter has as input the I″ output, and has a filtered I″ output. The third multiplier has as input the filtered I″ output and the Q″ output, and has a real output. The second low-pass filter has as input the real output, and has a feedback output that is input to the VCO.

A second embodiment data-directed frequency acquisition loop for synching to a signal comprises: a VCO, a first, second, third, fourth, fifth, and sixth multipliers, an amplifier, a summer, and first and second low-pass filters. The VCO has an I and Q output. The first multiplier receives the signal and the I output, and generates an I′ component from them. The second multiplier receives the signal and the Q VCO output and generates a Q′ component from them. The third multiplier receives the I′ and Q′ components and generates an I′Q′ signal from them. The amplifier receives the I′Q′ signal and generates a 2I′Q′ signal from it. The fourth multiplier receives the I′ component and generates an I′2signal from it. The fifth multiplier receives the Q′ component and generates a Q′2signal from it. The summer receives the I′2and Q′2signals and generates a I′2-Q′2signal. The first low pass filter receives the I′2-Q′2signal and generates a filtered I′2-Q′2signal. The sixth multiplier receives the 2I′Q′ signal and the filtered I′2-Q′2signal and generates a raw VCO driving signal from them. The second low pass filter receives the raw VCO driving signal, generates a filtered VCO driving signal from it, and sends the filtered VCO driving signal to the VCO.

A third embodiment frequency acquisition loop according to the present invention synchronizes with a signal using both a magnitude of error and a direction of error that are generated by convolving data in the signal.

In a fourth embodiment, a frequency acquisition loop according to the present invention has four points of stable equilibrium.

A fifth embodiment frequency acquisition loop according to the present invention comprises: a Costas loop, a sub-circuit, and a multiplier. The Costas loop has a VCO. The sub-circuit generates a difference of a square of an in-phase signal component and a square of a quadrature component. The multiplier changes the sign of a VCO driving voltage when the difference is less than zero.

In a sixth embodiment, a frequency acquisition loop according to the present invention comprises a frequency and phase-lock loop, and a complex multiplier that performs a fully-complex squaring operation.

In a seventh embodiment, a phase-lock loop according to the present invention provides frequency acquisition from correlation of the data.

In an eighth embodiment, a frequency acquisition and phase-lock loop according to the present invention provides frequency acquisition and phase lock derived from a signal's data.

A ninth embodiment synch loop for providing frequency acquisition and phase-lock loop for a double sideband suppressed carrier signal comprises a Costas loop and an in-phase loop. The Costas loop has a VCO. The in-phase loop generates I2-Q2, and includes a multiplier that changes the sign of a VCO driving voltage when I2-Q2is less than zero. The frequency acquisition and phase-lock loop provides frequency acquisition and phase lock derived from the signal's data by generating both magnitude of error and a direction of error that are generated by convolving data in the signal. The synch loop has four points of stable equilibrium distributed 90 degrees from one another.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A data-directed frequency-acquisition loop (“DDFL”) according to the present invention combines desirable features of a Costas loop and a frequency-and-phase-lock loop; the DDFL can synch using redundancy of the data in the frequency domain, such as in a double sideband suppressed signal, but has an output that converges like the FPLL, and that is not disrupted by noise that displaces the signal phase by 90 degrees or less.

A preferred embodiment DDFL according to the present invention is shown inFIG. 4, and indicated generally at400. The input signal and the output of a VCO420, I and Q, are multiplied by a first complex multiplier410to produce I′ and Q′. In the preferred embodiment, the I′ and Q′ output of the first complex multiplier410is then passed through an optional low-pass filter415to remove the unwanted products of the complex multiplication. The filtered I′ and Q′ output of the first complex multiplier410is then correlated by a second complex multiplier430, to produce I″ and Q″. Note that because the correlation is being performed in the frequency domain it is a convolution of the signal. The I portion of the correlation (I″) is equal to I′2-Q′2; the Q portion (Q″) is equal to 2I′Q′. The I portion of the output is then passed through a first low-pass filter440with a phase shift characteristic shown inFIGS. 7A-C. The filtered I portion (in-phase portion) is then multiplied by the Q portion (quadrature-portion) of the correlation by a third multiplier450. The output of the third multiplier450is then filtered by a second low-pass filter460, amplified, and returned to the VCO420to complete the feedback loop.

Because the filter440has the amplitude and phase characteristics shown inFIGS. 7A and 7B, which result in the discriminator characteristic shown inFIG. 7C, having a positive slope at the zero intercept, the response circuit400provides a phase lock, as well as frequency acquisition, as discussed further hereinbelow.

It will be appreciated that the circuit can function under most conditions without the optional low-pass filter415. However, in the preferred embodiment the optional low-pass filter415is included to eliminate the possibility of the multiplication of noise elements that might create a substantial error that either slows or prevents lock-up or causes a false lock.

It will be appreciated that the elements of the circuit shown inFIG. 4can be substituted, permutated, or both, to produce a number of equivalent alternative embodiment circuits. In particular, the complex multipliers shown inFIG. 4comprise a number of real multipliers organized to produce the complex product of QAM signals. The complex multipliers can be produced by a variety of sets and arrangements of subcomponents. Furthermore, the subcomponents can be reorganized or rearranged in a number of ways to produce the same mathematical result. Certain examples of such substitutions and rearrangements are described hereinbelow.

FIG. 5is a block diagram of the preferred embodiment circuit shown inFIG. 4, showing further details of the complex multipliers used in certain embodiments, and having correspondingly numbered components. As can be seen inFIG. 5, the I signal is the product of the input signal and the I component from the VCO, while the Q signal is the product of the input signal and the Q component from the VCO. Thus, in the embodiment shown inFIG. 5the first complex multiplier410comprises a pair of real multipliers511and512. Similarly, the second complex multiplier430shown inFIG. 4comprises a set of three real multipliers531,532, and533, and the third complex multiplier450comprises a real multiplier552and an amplifier551.

It will be appreciated that many of the real multipliers shown inFIGS. 4 and 5can actually be substantially simpler hardware components. For example, the VCO can simply produce a signal of oscillating 1s and −1s. In this case, the potential multiplication required by the multipliers comprising the first complex multiplier410is limited to a change of signs. Similarly, the multipliers531and532multiply the same input by itself. Thus, the range of possible outputs contains only half the possibilities of the domain of inputs. Consequently, this function can more easily be performed by a lookup table that provides the square of the input than by an actual multiplier, which requires many more gates. The multiplier533, however, must actually be a complete, real multiplier, since it has two independent and variable inputs (the I′ signal and the Q′ signal). The second complex multiplier430further comprises an amplifier534with a gain of 2 which follows the complete, real multiplier533, because the correlation requires that the I portion and Q portion be normalized.

A number of alternative embodiments can likewise be formed by rearranging or substituting the elements of the third complex multiplier430. In particular, the amplifier551can be a hard limiter, a soft limiter, or a linear amplifier. As described in further detail hereinbelow, the effect and function of the in-phase loop portion of the circuit400is to invert the signal during half of the phase cycle. Consequently, the amplifier551can simply be a hard limiter, and the real multiplier552, like real multipliers511and512, need only multiply by 1 and −1. The resulting output of the third complex multiplier450, which drives the VCO, is shown by the solid curve in FIG.6. For comparison, the response of a Costas Loop is also shown inFIG. 6by the dotted curve.

It will be appreciated that the circuit400provides both frequency acquisition and phase lock. InFIG. 5, it can be seen that multipliers511,512, and533form a Costas loop. Thus, the circuit400comprises a Costas Loop combined with an in-phase loop, including the multipliers531,532, and552, and the filter440. If the in-phase component (I′-Q′) is greater than zero, then circuit will phase lock at 0 degrees or 180 degrees, identical to a Costas loop. But if the in-phase component is negative, the additional in-phase loop inverts the output of the Costas loop, resulting in two new stable equilibrium points at plus and minus 90 degrees.

Consequently, the circuit400is a frequency acquisition loop that has four points of stable equilibrium, as shown in FIG.6. It will be appreciated that a stable equilibrium point for the feedback loop occurs at each point where the curve describing the response crosses the x-axis with a positive, finite slope, since the voltage to the VCO will be positive when the phase is slightly to one side of the zero-intercept, and negative when slightly to the other side. This contrasts with the current produced, for example, by the Costas loop, shown as the dotted graph in FIG.6. Because the Costas loop has a negative slope during half the cycle two of the finite-slope zero-intercepts are points of unstable equilibrium. When arbitrarily near one of these zeros the VCO will be driven in the wrong direction, causing a phase shift that will continue through 90 degrees, to the nearest point of stable equilibrium.

It will be appreciated that the inversion provided by the in-phase loop can be performed at other points in the circuit. For example, referring toFIG. 5, if the amplifier551is a hard limiter, the real multiplier552can be respositioned. In this case, the multiplier552is still driven by the output of the hard limiter551. In some embodiments, the inversion has already been performed by the time the signal reaches the real multiplier533, the output of the amplifier534is provided directly to the first low pass filter460. In other embodiments, the amplifier551is a soft limiter. And, in still other embodiments, amplifier511is linear amplifier. (Note that in these embodiments the real multiplier551must be a full multiplier, since the potential multiplication is not limited to a sign change.) It will be appreciated that choosing a hard limiter, a soft limiter, or a linear amplifier has implications for the capture and noise-reduction capabilities of the loop400. For example, a hard limiter is more susceptible to noise during capture, but once locked is more effective at holding the lock through noise.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment, and certain other embodiments deemed helpful in further explaining how to make or use the preferred embodiment, have been shown. All changes and modifications that come within the spirit of the invention are desired to be protected.