Demodulation using a time domain guard interval with an overlapped transform

• US 5,886,749 A
• Filed: 01/28/1997
• Issued: 03/23/1999
• Est. Priority Date: 12/13/1996
• Status: Expired due to Term

• Current Assignee: Cable Television Laboratories Incorporated, Nippon Hoso Kyokai
• Original Assignee: Cable Television Laboratories Incorporated, Nippon Hoso Kyokai
• Inventors: Ito, Yasuhiro, Williams, Thomas H., Chelehmal, Majid
• Primary Examiner(s): HSIA, SHERRIE Y
• Application Number:
  US08/787,342
• Time in Patent Office: 784 Days
• Field of Search: 348/607, 348/614, 348/611, 348/914, 348/726, 382/280, 364/726.01, 364/726.02
• US Class Current: 348/614
• CPC Class Codes: H04N 5/211 Ghost signal cancellation H04N23/81 takes precedence
• CPC Class Codes: Y10S 348/914 Delay for equalization

Abstract

A system and method for demodulation of an RF signal on a transmission channel is provided. The RF signal is demodulated to baseband as an in-phase (I) data signal and a quadrature (Q) data signal. A first block of I data is captured and a first block of Q data is captured. A time domain guard interval is provided in the captured first blocks of I and Q data. A complex discrete Fourier transform is performed on the captured first I and Q data blocks. An inverse frequency response for the transmission channel is determined. The inverse frequency response is multiplied by the complex discrete Fourier transform of the guard-interval protected first I and Q data blocks to generate a frequency domain product signal. An inverse Fourier transform on the product of the multiplying step is performed to generate a first equalized time domain signal. In a preferred embodiment, the method also includes using an overlapped Fourier transform and discarding a first portion of each equalized time domain signal.

Claims

1. A method for signal demodulation comprising the steps of:

providing a radio frequency (RF) time domain signal on a transmission channel, the RF signal comprising a band-limited data signal modulated on an RF carrier, the data signal comprising a plurality of transmission blocks of data;

providing a time domain guard interval attached to each transmission block in the transmitted signal;

demodulating the RF signal to baseband as an in-phase (I) data signal and a quadrature (Q) data signal;

capturing a first block of I data;

capturing a first block of Q data;

performing a complex discrete Fourier transform on the captured first I and Q data blocks;

determining an inverse channel response for the transmission channel;

multiplying the inverse channel response by the complex discrete Fourier transform of the guard-interval protected first I and Q data blocks to generate a frequency domain product signal;

performing an inverse Fourier transform on the product of the multiplying step to generate a first equalized time domain signal; and

discarding a first portion of the first equalized time domain signal while retaining a second portion of the first equalized signal.

2. The method of claim 1 wherein the RF signal is a vestigial side band (VSB) signal and the method further comprises performing band filtering before performing the inverse Fourier transform.

3. The method of claim 1 wherein the method further comprises

determining from an impulse response of the channel whether any echoes in the first block of I data and the first block of Q data are longer than the guard interval;

and

performing the step of discarding the first portion only when it is determined that at least one echo is longer than the guard interval.

4. The method of claim 1 further comprising the steps of digitizing the demodulated I and Q signals before performing the complex discrete Fourier transform.

5. The method of claim 1 wherein the step of providing a guard interval comprises:

copying a portion of the data signal from the back of each of the transmission blocks; and

appending the copied portion in front of each of the transmission blocks.

6. The method of claim 5 wherein the copied portion comprises data from a time duration longer than a duration of an expected echo.

7. The method of claim 1 wherein the step of providing a guard interval comprises:

periodically including a substantially identical portion of data in the time domain data signal; and

selecting the first blocks of I and Q data such that the first blocks begin just after the occurrence of one of the substantially identical portions and the first blocks end just after a subsequent occurrence of the substantially identical portion.

8. The method of claim 7 wherein the RF signal comprises an NTSC signal and the substantially identical portions comprise horizontal blanking intervals.

9. The method of claim 1 wherein the captured first blocks correspond to two sequential lines of an NTSC signal and the step of providing a time domain guard interval comprises selecting the first blocks such that the first blocks begin immediately a horizontal sync interval of the first line and the first blocks end immediately after a horizontal sync interval of the second line.

10. The method of claim 1 further comprising the steps of:

capturing a second block of I data, wherein the second block of I data overlaps the first block of I data in a time interval corresponding to the retained portion of the first equalized signal;

capturing a second block of Q data, wherein the second block of Q data overlaps the first block of Q data in a time interval corresponding to the retained portion of the first equalized signal;

providing a time domain guard interval in the captured second blocks of I and Q data;

performing a complex discrete Fourier transform on the captured second I and Q data blocks;

multiplying the inverse frequency response by the complex discrete Fourier transform of the guard-interval protected second I and Q data blocks to generate a frequency domain product signal;

performing an inverse Fourier transform on the product of the multiplying step to generate a second equalized time domain signal;

discarding a first portion of the second equalized time domain signal while retaining a second portion of the second equalized signal, wherein the first portion of the second equalized signal corresponds to the time domain data in the second portion of the first equalized signal; and

combining the retained portion of the first equalized signal with the retained portion of the second equalized signal to form a composite equalized signal.

11. A method for demodulating a signal transmitted over a channel comprising the steps of:

determining an inverse frequency response of the channel;

digitizing the transmitted signal;

dividing the transmitted signal into a plurality of discrete blocks, wherein each of the blocks overlaps two adjacent blocks and each portion of the transmitted signal appears in two adjacent blocks;

transforming each block using a Fourier transform;

multiplying each transformed block by the inverse frequency response to generate a frequency domain product signal; and

performing an inverse Fourier transform on the frequency domain product signal to generate an equalized time domain signal;

discarding the first portion of the equalized time domain signal for each of the blocks; and

combining the retained portion of each of the blocks to form a composite time domain filtered, signal.

12. The method of claim 11 further comprising a step of adding a time domain guard interval to each of the discrete blocks.

13. The method of claim 11 wherein the transmitted signal is an NTSC signal and each block is two lines long and is selected to begin just after a horizontal blanking interval and to end just after a horizontal blanking interval.

14. A system for transmission of a signal comprising:

a communication channel;

a transmitter transmitting a time domain signal on the communication channel;

a receiver receiving the signal after it has passed through and been distorted by the communication channel;

means in the receiver for determining an inverse frequency response of the channel;

a capture circuit for capturing overlapping time domain blocks of the received signal;

a transform circuit performing a transformation process on each of the captured time domain blocks so as to transform each time domain block to a frequency domain block;

a multiplier circuit coupled to multiply each frequency domain block by the inverse frequency response to generate a filtered frequency domain block;

an inverse transform circuit performing an inverse transformation process on each of the filtered frequency domain blocks so as to transform each filtered frequency domain block into a filtered time domain block;

a selector coupled to discard a first portion of each filtered time domain block and retain a second portion of each time domain block; and

a combining circuit coupled to combine the retained portions of each of the filtered time domain blocks.

15. The system of claim 14 further comprising:

a guard interval circuit for adding a time domain guard interval to each of the overlapping time domain blocks.

16. The system of claim 15 wherein the guard interval circuit is located in the transmitter and adds a substantially identical signal portion to the time domain signal and the capture circuit further comprises means for selecting each of the overlapping time domain blocks based upon the location of the substantially identical signal portion.

17. A demodulating apparatus for a television signal comprising:

an input for receiving a television signal from a communication channel;

a block capture circuit coupled to sequentially capture overlapping time domain blocks of the received television signal such that each of the captured blocks includes a time domain guard interval;

a Fourier transform circuit receiving each of the overlapping blocks and outputting a frequency domain block for each of the overlapping time domain blocks;

an adaptive equalizer coupled to receive the frequency domain blocks from the Fourier transform circuit and output an equalized frequency domain block for each received frequency domain block;

an inverse Fourier transform circuit coupled to the output of the adaptive equalizer and generating a filtered time domain block for each of the equalized frequency domain blocks; and

a selector for discarding a first portion of each of the filtered time domain blocks and combining a remainder portion of each of the filtered time domain blocks into a ghost canceled demodulated signal.

18. The apparatus of claim 17 wherein the block capture circuit further comprises a detector for detecting a horizontal blanking interval in the received television signal and synchronizing the beginning and ending of each captured block with the horizontal blanking interval.