WO1997027670A1

WO1997027670A1 - Fast reacquisition catv downstream adaptive equalizer

Info

Publication number: WO1997027670A1
Application number: PCT/US1997/000943
Authority: WO
Inventors: Bruce J. Currivan
Original assignee: Stanford Telecommunications, Inc.
Priority date: 1996-01-23
Filing date: 1997-01-22
Publication date: 1997-07-31
Also published as: AU1833997A

Abstract

Training sequence pulses are periodically inserted between data packets transmitted over a cable television system from a headend unit to a subscriber. By comparing the transmitted sequence pulses with a generalized training sequence at the receiver, an adaptive filter can be periodically tuned to maintain signal acquisition and for rapid reacquisition in response to changes in channel interface.

Description

FAST REACQUISITION CATV DOWNSTREAM ADAPTIVE EQUALIZER

BACKGROUND OF THE INVENTION

This invention relates generally to digital data transmission, and more particularly the invention relates to an improvement on current technology relating to the transmission of digital information on the cable TV (CATV) medium in the downstream direction from the headend to the home.

In the near future, analog and digital TV signals and digital data signals (e.g., internet, work at home, voice, etc.) will share the same CATV cable into the home, as shown in Figure 1. Interference may occur between these two applications. For example, an instance may occur where two persons in a home are using the CATV service, one person watching TV in the bedroom and one person using the computer (PC with cable modem) in the den, both connected to the same cable. When the TV is tuned (e.g., channel surfing), the frequency response of the impedance mismatch at the input to the TV tuner changes. This causes the channel response as seen by the cable modem to change. The adaptive equalizer in the cable modem must reacquire, causing loss of data for the duration of the reacquisition process. The reacquisition time of current chips is very long (100-300 ms) , which is too long to be corrected by the receiver.

This invention reduces the reacquisition time to, for example, 1 ms, which is short enough for error correction to prevent loss of data to the PC user.

SUMMARY OF THE INVENTION In accordance with the invention, a training sequence is periodically sent downstream in the data stream. The receiver compares the known sequence to the received sequence and produces an error signal which is used to readjust the equalizer coefficients. Acquisition or reacquisition can be completed by the end of the training sequence.

The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates shared use of a CATV cable for video and data signals. Figure 2 illustrates downstream data frame format with a training sequence inserted therein.

Figure 3 illustrates a receiver for digital cable data showing frame synchronization pulse feed back to adaptive equalizer to establish time location of training sequence in received signal.

Figure 4 illustrates an adaptive equalizer structure in accordance with an embodiment of the invention with blind and training error signals feeding back to adapt filter coefficients.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT

The current art uses QAM modification with 64, 128, or 256 states in the transmitted constellation. An adaptive equalizer is used to adapt the receive filter response to take into account the response of the CATV downstream channel, including reflections (echoes) , such as occur at points of impedance mismatch in the cable system.

Current CATV downstream adaptive equalizer chips, such as the ones manufactured by Broadcom Corporation, use an adaptive technique known as "blind equalization" to adapt the equalizer coefficients. In blind equalization, the receiver is not informed of the identify of the data bits, but only of the signal's overall characteristics (64QAM, etc.). Acquisition time for this technique is from 100 to 300 ms. Reacquisition time, that is, acquisition time of the signal after lock is lost due to a change in the channel or interruption of the signal, is on the same order, over 100 ms. This results in a burst of errors which most likely cannot be corrected by the interleaver and error correction decoder which follow the equalizer. The user thus receives a burst of uncorrected errors.

A faster alternative, in accordance with the invention, is trained acquisition. Training-mode equalization has been used for decades, including early work by Bell Laboratories. It was used previously by Stanford Telecommunications, Inc. in the MLRP (Mobile Link 1/2 Receiver Program) receiver built for the U.S. Air Force. The training- mode equalization technique used therein was suggested by Mission Research Corporation, Santa Barbara, California.

In accordance with the invention, a training sequence of, for example, 100 symbols, is sent periodically, for example, every 1 ms, as shown in Fig. 2. The receiver knows the identify of the training bits and, by comparing the known signal to the receiver output, producing an error signal, and readjusting the equalizer coefficients, is able to acquire or reacquire much more rapidly than with the blind equalizer coefficients, is able to acquire or reacquire much more rapidly than with the blind technique. Acquisition or reacquisition is completed by the end of the training sequence. Acquisition or reacquisition time is thus less than 1 ms. This results in a short (1 ms or less) burst of errors ahead of the deinterleaver. A short interleaver of the order of 1 ms length, followed by an error correction decoder, is able to correct this burst of errors so that the user with many cases not experience any errors at the receiver output.

In this approach, the equalizer uses the blind technique to acquire the signal, since 100-300 ms is adequate for initial acquisition. After the signal is acquired, the frame synchronizer, which follows the equalizer, determines the location of the training sequence in the bit stream, and communicates its position back to the equalizer, using, for example, the frame sync pulse shown in Figure 3. After frame synchronization is achieved, the equalizer switches to training mode.

In training mode, the equalizer uses the known training sequence symbols each time they arrive (every 1 ms) to readapt its taps, as illustrated in Figure 4. In between training sequences, it continues to adapt using standard blind techniques, for example, a decision directed mode.

There has been described a technique for rapid reacquisition for CATV downstream adaptive equalizers. This technique alleviates the problems associated with sharing the CATV system between TV and data users in the same household.

While the invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS;

1. A method of reacquisition of data by a downstream subscriber in a shared cable transmission system comprising the steps a) transmitting packets of data in a data stream from a headend unit to a receiver, b) periodically inserting a training sequence of symbols between packets of data, and c) receiving each training sequence and in response thereto adjusting an adaptive filter for matching a receiver and the cable transmission system.

2. The method as defined by claim 1 wherein an initial frame sync pulse is generated for initial acquisition of data.

3. An adaptive equalizer for use in impedance matching a receiver and a cable transmission line comprising: an adjustable adaptive filter having an input for receiving a signal from the cable and an output, a training sequence generator for generating a training sequence of pulses, a comparator connected to said output and to said training sequence generator for comparing a generated training sequence with a transmitted training sequence, and an error signal generator responsive to said comparator for generating an error signal for application to said adjustable adaptive filter to maintain signal acquisition and for signal reacquisition.