Equalizer for time domain signal processing

A digital equalizer comprises a matched filter that, in conjunction with an FIR filter, assures a single peak with substantially greater energy than other peaks caused by ghosts, thereby permitting synchronization even with multiple, arbitrarily strong ghosts caused by strong multipathing, multiple transmitters, or both.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to signal processing and, more particularly, to an equalizer for time domain signal processing.

BACKGROUND OF THE INVENTION

Equalizers are an important element in many diverse digital information applications, such as voice, data, and video communications. These applications employ a variety of transmission media. Although the various media have differing transmission characteristics, none of them is perfect. That is, every medium induces variation into the transmitted signal, such as frequency-dependent phase and amplitude distortion, multipath reception, other kinds of ghosting, such as voice echoes, and Rayleigh fading. In addition to channel distortion, virtually every sort of transmission also suffers from noise, such as additive white gausian noise (“AWGN”). Equalizers are therefore used as acoustic echo cancelers (for example in full-duplex speakerphones), video deghosters (for example in digital television or digital cable transmissions), signal conditioners for wireless modems and telephony, and other such applications.

Those skilled in the art will recognize that prior art equalizers have difficulty coping with ghosts having a signal strength close to that of the main signal. Typically ghosts are caused by multipathing—that is, portions of the transmitted signal that are reflected by one or more terrain features to arrive at the receiver by less direct paths. Consequently, ghosts are typically weaker, and arrive after, the main signal. However, in certain environments, especially downtown areas, which have numerous large buildings that can completely mask a signal, signal strength can be highly directional. A receiver positioned in the shadow of a tall building, for example, might not receive any direct signal, but still receive strong signals that are reflected off of one or more other buildings. Thus, in this environment, ghosts that are as strong as the “main” signal are possible. Furthermore, since the strength of the signal may be controlled as much by the albedo, size, or shape of the reflective surface as by the number of reflections in the path, ghosts that arrive before the strongest signal are far more likely.

Similar problems occur in systems that use multiple transmitters in order to provide the widest possible coverage for a digital transmission. Multiple transmitters would permit a wider area to be covered using less total broadcast power, and could 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.

Those skilled in the art will appreciate that existing receiver technology handles ghosts by filtering them out in order to interpret the “main” signal. But in a multi-transmitter environment, or an area which generates multiple reflections and highly directional signals, 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. Near the margins the best this subtractive strategy can ever provide is a signal strength equal to the strongest single echo—the energy from the secondary signals, whether from reflections or additional transmitters, is wasted.

In short, in a multi-transmitter or downtown environments the “main” signal becomes a meaningless concept. In order to operate efficiently in such a multi-signal 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.

FIG. 1illustrates a block diagram of a typical digital communication receiver, including channel coding and equalization, indicated generally at100. The receiver100comprises a demodulation and sync component110, which converts the received analog signal back into a digital format. The receiver100further comprises an equalizer120, an inner decoder130, a de-interleaver140, and an outer decoder150. The inner coding is typically convolutional coding, while the outer coding is typically block coding, most often Reed-Solomon coding. The convolutional and block coding are generally combined in order to exploit the complementary advantages of each.

FIG. 2is a diagram of an equalizer120such as is commonly used in the digital receiver100shown inFIG. 1. Typically, the equalizer120includes a controller228, a finite impulse response (“FIR”) filter222, a decision device226, and a decision feedback equalizer (“DFE”)224. The FIR filter222receives the input signal221. The FIR filter222is used to cancel pre-ghosts—that is, ghost signals that arrive before the main transmission signal. The decision device226examines its inputs and makes a decision as to which one of the received signals at its input is the signal to be transmitted to the output229. The input to the decision device226is modified by a decision feedback equalizer224, which is used to cancel post-ghosts—that is, ghost signals that arrive after the main transmission signal—and the residual signal generated by the FIR filter222.

The decision device226is typically a hard decision device, such as a slicer. For example, in an 8VSB system, the slicer can be a decision device based upon the received signal magnitude, with decision values of 0, ±2, ±4, and ±6, in order to sort the input into symbols corresponding to the normalized signal values of ±1, ±3, ±5, and ±7. For another example, the slicer can be multi-dimensional, such as those used in quadrature amplitude modulation (“QAM”) systems.

The controller228receives the input data and the output data and generates filter coefficients for both the FIR filter222and the decision feedback filter224. Those skilled in the art will appreciate that there are numerous methods suitable for generating these coefficients, including LMS and RLS algorithms.

FIG. 4is a graph of signal magnitude versus time illustrating a post ghost having a magnitude 100% of the “main” signal. The main transmission signal M is illustrated at a relative magnitude of 1 (0 dB). After a time delay of D, a 100% post-ghost signal G arrives. In this situation, the prior art equalizer120ofFIG. 2has difficulty selecting the “main” signal for the proper output229—since the very concept of a “main” signal is meaningless with a 100% ghost. If the ghost signal G is treated as a pre-ghost, then the FIR filter222will have tap values equal to 1 and will go infinite. If the ghost signal G is treated as a post-ghost, then the error of the feedback filter224will be magnified and the filter becomes unstable. Further, if the ghost G changes magnitude due to its phase variation or channel variation, the main signal M and the ghost signal G can exchange roles (based upon maximum magnitude).

Therefore, what is needed is an equalizer that is better adapted to cope with ghosts having an arbitrarily large magnitude relative to the main signal, including the possibility of a “ghost” having a magnitude that can temporarily exceed the magnitude of the “main” signal. The present invention is directed towards meeting these needs, as well as providing other advantages over prior equalizers.

SUMMARY OF THE INVENTION

A first embodiment digital equalizer according to the present invention comprises: a matched filter; an FIR filter connected to the matched filter; a decision device connected to the FIR filter; a feedback filter connected to decision device; and a controller connected to the matched filter, the FIR filter, and the feedback filter.

A second embodiment digital equalizer according to the present invention comprises: a matched filter; an FIR filter connected to the matched filter; a decision device connected to the FIR filter; a feedback filter connected to decision device; and a controller connected to the matched filter, the FIR filter, and the feedback filter. The matched filter has a response equal to a channel response of a channel used to transmit a received signal. The feedback filter is substantially shorter than the FIR filter. The channel response is determined using a test sequence contained in the transmitted signal. The equalizer synchronizes with a synthesized signal constructed by the composite function of the channel response of a channel used to transmit a received signal and itself.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment digital equalizer according to the present invention deals effectively with ghosts of any magnitude. Furthermore, it uses an additive paradigm which permits energy from all ghosts to contribute to the signal used by the equalizer. Thus, the preferred embodiment equalizer can synchronize even with multiple, arbitrarily strong ghosts caused by severe multipathing, multiple transmitters, or both.

FIG. 3is a block diagram showing certain elements of a preferred embodiment time-domain digital equalizer according to the present invention, indicated generally at300. The preferred embodiment equalizer300is similar to the prior art equalizer120, and includes the same components shown inFIG. 2. However, in the equalizer300an extra linear filter321, called a “matched” filter, is inserted at the input221. The controller228generates the tap coefficients for the matched filter321, in addition to those for the FIR filter222and the decision feedback filter224. The matched filter321acts as a pre-channel filter. Pre-channel filters have a response that is the complex conjugate of the channel response, which is a representation of the transmission medium characteristic between the transmitter and the receiver. Those skilled in the art will appreciate that the transmission medium can be space (satellite broadcasts), air (terrestrial broadcast), or via a transmission cable (e.g. cable TV). The channel response is defined as the impulse response of the transmission medium. Ideally, the channel response should be flat across the frequency band of interest; however, the channel response can be distorted due to mutipath flat fading, reflections, or both.

The coefficients of the pre-channel filter321can be obtained from analysis of the transmission channel, such as the result of the correlation of a known training sequence. As is known in the art, in some transmission systems, such as the one used for terrestrial digital television broadcasting, training sequences are inserted into the data stream to assist receivers in synchronization, equalization, and initialization. A commonly used sequence is a maximum length pseudo random PN sequence. For example, in a terrestrial digital television broadcasting system (e.g. ATSC, 8-VSB), a 511 bit binary PN sequence is inserted every 313 segments (24 ms). An autocorrelation between this known sequence and the received sequence will generate the channel impulse response, which is used to construct the filter taps for the matched filter321.

Under the 0 dB ghost situation shown inFIG. 4, the main signal M and a delayed version of this signal (Ghost G) with the same magnitude are added together with a delay of D. The pre-channel filter321should have a response that is the complex conjugate of the transmission channel and, therefore, it has tap values as shown inFIG. 5B. However, because in an 8VSB system the signal is simple in the time domain, the pre-channel filter321can have a response identical to the transmission channel, and the result will be correct except for a delay. Consequently, the pre-channel filter321may also have tap values as shown inFIG. 5A.

The output from a matched filter321having tap values shown inFIG. 5Bis shown inFIG. 6.FIG. 6shows that the 0 dB ghost signal G has been turned into a pair of ghosts A and B whose signal strength is much less than 0 dB. It will also be noted that the peak signal value Mn is now at sample point6instead of sample point0, as inFIG. 4. This offset is the consequence of using filter taps corresponding to the channel response, rather than its complex conjugate. But regardless of the linear offset, this peak location is where the system will now lock onto. The system will not attempt to lock onto either the main signal M or the ghost signal G. In the equalizer300, the largest peak is treated as the main signal. With the pre-channel filter321present in the equalizer300a single peak Mn, and only one peak, is guaranteed.

It will be appreciated that, using the inventive equalizer ofFIG. 3, the peak signal Mn contains contributions from the original main signal M and all of the ghost signals—the equalizer uses an additive paradigm for synchronization. Thus, using such a newly generated main signal will significantly increase the system equalization performance when there are strong ghosts, since all useful signal power from those ghosts is used. Furthermore, the equalizer300will not switch its main tap if a ghost temporarily exceeds the magnitude of the strongest signal, as is the case with the prior art equalizer120, because it uses the newly generated peak as the main signal. The strongest peak in the newly generated signal will remain the strongest signal regardless of fluctuations in the magnitudes of the ghosts that contribute energy to it.

Another benefit of the inventive equalizer300is that, so long as there are fewer than 3 signals near 100% of the strongest signal, no single ghost greater than 50% (˜3 dB) will exist and, as a result, the convergence speed, stability, and accuracy of the equalizer300are all greatly improved. As the system300locks onto the newly generated peak signal Mn, the peak location will not change regardless of the magnitude variation in the main signal or the ghost signals.

It will be appreciated that, in order to simplify the hardware design, the pre-channel filter321and the FIR filter222may be combined.

It is desirable that the FIR filter222be longer than the feedback filter224. As shown inFIG. 6, the new main signal Mn is at sample point6, which is the central position between the original main signal M and the ghosted signal B. In other words, the new generated main signal Mn has moved toward the middle of the entire channel response, and therefore, there should be enough taps for the FIR filter222to cancel the newly generated pre-ghost A. The center tap (the location of the newly generated main signal Mn) should be near the center so that ghosts on both sides can be effectively removed. It should be pointed out that the number of taps used for the FIR filter222and the feedback filter224do not need to be identical, but the FIR filter222should be longer than the feedback filter224. This is different than most prior art equalizer designs, since in those equalizers the FIR filter222is significantly shorter than the feedback filter224, usually by a ratio of between 4:1 and 10:1.

The FIR filter222is used to cancel any interference before the main peak Mn after the pre-channel filter321. The ideal solution which totally eliminates inter-symbol interference (“ISI”) caused by mulitpathing requires the FIR filter222to be infinitely long. In this case, only noise enhancement is present after the FIR filtering and no ISI remains after the FIR filtering. However, an approximation can be done so that the FIR filter222has shortened length and, as a result, there will be some amount of ISI left after the FIR filtering, but a trade off between ISI and noise enhancement can be done for the optimum results. In other words, in a practical system, a certain amount of ISI can be allowed in exchange for smaller noise enhancement. Therefore an optimum system has limited length of an FIR filter222and trades off between ISI residual and noise enhancement.

The feedback filter224is used to cancel any interference after the main peak Mn. The interference signal after the main peak Mn comes after the FIR filter222has filtered the signal coming out from the pre-channel filter321.

FIG. 7is a graph of the envelope of the equalizer taps vs. time showing the theoretical output after ideal FIR filtering. There is only noise enhancement and no ISI present.

FIG. 8is a graph showing the envelope of the taps of the FIR filter222under both ideal and approximated conditions. The ideal solution goes to infinite, while the approximated solution is much shorter. The small arrows indicate the residual ISI; this is tolerable since a small amount of ISI can simply be treated as noise. Also, the ideal solution has a linearly decreasing slope while the optimum approximated solution has an exponentially decreasing slope.

FIG. 9is a graph showing the output after the approximated FIR filtering. There are both ISI and noise enhancement components present.

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. Only the preferred embodiment, and certain alternative embodiments deemed useful for further illuminating the preferred embodiment, has been shown and described. All changes and modifications that come within the spirit of the invention are desired to be protected.