Ghost eliminating equalizer

Details Ghost eliminating equalizer Ghost eliminating equalizer

1998-09-22

2002-08-27

Corrielus, Jean (Department: 2631)

Pulse or digital communications

Receivers

Interference or noise reduction

C348S614000

Reexamination Certificate

active

06442221

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention is directed to an equalizer that substantially eliminates ghosts in signals processed by a receiver.
BACKGROUND OF THE INVENTION
Ghosts are produced in a receiver usually because a signal arrives at the receiver through different transmission paths. For example, in a system having a single transmitter, the multipath transmission of a signal may occur because of signal reflection. That is, the receiver receives a transmitted signal and one or more reflections of the transmitted signal. As another example, the multipath transmission of a signal may occur in a system having plural transmitters that transmit signals to the same receiver using the same carrier frequency. A network which supports this type of transmission is typically referred to as a single frequency network.
When a signal reaches a receiver through two or more different transmission paths, an interference pattern results. In the frequency domain, this interference pattern is manifested by a variable signal amplitude along the frequency axis. The worst case interference pattern results when the ghost is 100% and is shown in FIG.
1
. This interference pattern has amplitude nulls or near amplitude nulls at certain frequencies. Therefore, any information contained in the received signal at these frequencies is likely lost because the signal to noise ratio near these frequencies is below a usable threshold.
A variety of systems have been devised to deal with the problems caused by ghosts. For example, spread spectrum systems deal very adequately with the problem of a 100% ghost by spreading the transmitted data over substantial bandwidth. Accordingly, even though a 100% ghost means that some information may be lost at the frequencies corresponding to amplitude nulls, a data element can still be recovered because of the high probability that it was spread over frequencies which do not correspond to amplitude nulls. Unfortunately, the data rate R associated with spread spectrum systems is typically too low for many applications. (The data rate R is defined as the number of data bits per Hertz of channel bandwidth.)
It is also known to use a matched filter in a receiver in order to deal with the problem of a ghost. In this approach, data is transmitted as a data vector. The matched filter correlates the received data with reference vectors corresponding to the possible data vectors that can be transmitted. Correlation of the received signal to the reference vector corresponding to the transmitted data vector produces a large peak, and correlation of the received signal to the other possible reference vectors produces small peaks. Accordingly, the transmitted data vector can be easily determined in the receiver. Unfortunately, the data rate R typically associated with the use of matched filters is still too low for many applications.
When high data rates, such as R≧
1
, are required, equalizers are often used in a receiver in order to reduce ghosts. A classic example of a time domain equalizer is an FIR filter. An FIR filter convolves its response h(t), shown generally in
FIG. 2
, with the received signal and produces a large peak representative of the main received signal. Ghosts have small components in the output of the FIR filter. However, as shown in
FIG. 2
, the values a
1
, a
2
, a
3
, . . . of the taps of an FIR filter depend on the value of a and, in order to perfectly cancel a 100% ghost using an FIR filter, the value a of the FIR filter response must approach 1. As the value a approaches 1, the values of the taps of the FIR filter do not asymptotically decrease toward zero. Therefore, the FIR filter becomes infinitely long if a 100% ghost is to be eliminated, making the FIR filter impractical to eliminate a 100% ghost.
Also, another problem with the use of an FIR filter is noise enhancement. If the transmitted signal picks up noise N
c
in the channel, this noise is enhanced by the FIR filter so that the noise N
0
at the output of the FIR filter is greater than the channel noise N
c
. Also, if the channel noise N
c
is white, the noise N
0
at the output of the FIR filter is non-white, i.e., bursty.
An example of a frequency domain equalizer
10
is shown in FIG.
3
. The frequency domain equalizer
10
includes a Fast Fourier Transform (FFT) module
12
which performs a Fast Fourier Transform on the received signal in order to transform the received signal to the frequency domain. A multiplier
14
multiplies the frequency domain output of the FFT module
12
by a compensation vector which includes a row of coefficients b
i
. An inverse FFT module
16
performs an inverse FFT on the multiplication results from the multiplier
14
in order to transform the multiplication results to the time domain.
It should be noted that, when the frequency domain equalizer
10
is used to eliminate ghosts, the frequency domain equalizer
10
must be included in every receiver. In order to reduce receiver cost, therefore, it is known to incorporate the inverse FFT module
16
into the transmitter so that the receivers require only the FFT module
12
and the multiplier
14
. A consequence of moving the inverse FFT
16
to the transmitter is that data is transmitted in many discrete frequency channels. Accordingly, in the presence of a 100% ghost, the transmitted data is not recoverable around the null frequencies of FIG.
1
.
FIG. 4
illustrates an exemplary set of coefficients b
i
which may be used by the frequency domain equalizer
10
. In order to derive the coefficients b
i
, an estimator may be used at the output of the Fast Fourier Transform (FFT) module
12
. This estimator models FIG.
1
and inverts this model in order to produce the coefficients b
i
of FIG.
4
. Accordingly, the coefficients b
i
are chosen so that, when they and the FFT of the received signal are multiplied by the multiplier
14
, the coefficients b
i
cancel the ghost. It should be noted that the coefficients b
i
should have infinite amplitudes at the frequencies where the interference pattern has a zero amplitude. However, the coefficients b
i
cannot be made infinite as a practical matter. Accordingly, the coefficients b
i
are cut off at these frequencies. An advantage of cutting off the coefficients b
i
is that noise enhancement at the frequencies where the coefficients b
i
are cut off is materially reduced. Thus, noise enhancement is lower at the output of the frequency domain equalizer
10
than would otherwise be the case. However, a disadvantage of cutting off the coefficients b
i
is that information in the received signal is lost at the cut off frequencies so that the output of the inverse FFT module
16
becomes only an approximation of the transmitted data.
Moreover, it is known to use empty guard intervals between the vectors employed in the frequency domain equalizer
10
of FIG.
3
. The guard intervals are shown in FIG.
5
and are provided so that received vectors and ghosts of the received vectors do not overlap because such an overlap could otherwise cause intersymbol interference. Thus, the guard intervals should be at least as long as the expected ghosts. It is also known to use cyclic extensions of the vectors in order to give the received signal an appearance of periodicity. Accordingly, a Fast Fourier Transform of the received signal and a Fourier Transform of the received signal appear identical.
The present invention is directed to an equalizer which overcomes one or more of the above noted problems.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a receiver receives a signal containing data distributed in both time and frequency. The receiver comprises a vector transform and a vector adjuster. The vector transform is arranged to perform a transform on the received signal using a plurality of transform vectors. The vector adjuster is responsive to the transform of the received signal in order to adjust the transform vectors so that the data can be recovered even in the presence of a strong ghost.
In accordance with another a

Affiliated with

Also associated with

Rating

Ghost eliminating equalizer does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Ghost eliminating equalizer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ghost eliminating equalizer will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2965814