Apparatus and method for symbol alignment in a multi-point OFDM/DMT digital communications system

Abstract

A multi-point communications system is set forth herein. The communications system comprises a head end unit disposed at a primary site and a plurality of receivers disposed at remote sites. The head end unit includes a transmitter for transmitting OFDM / DMT symbols over a predetermined number of bins across a transmission medium. The OFDM / DMT symbols are transmitted in periodically occurring formatted symbol frames. The cyclic prefix includes a predetermined periodic signal superimposed thereon. The receivers receive the OFDM / DMT symbols over a subset of the predetermined number of bins from the transmission medium and use the superimposed signals to attain symbol alignment. Preferably, the superimposed signal is an impulse signal that varies in polarity throughout the transmission cycle and which is superimposed on one or more symbols occurring during a cyclic prefix of the formatted symbol frames. In accordance with a further aspect of the present invention, the receivers apply a predetermined incremental phase shift to received samples corresponding to the received OFDM / DMT symbols to thereby compensate for phase shifts resulting from the cyclic prefix. The multi-point communications system may include a similar system for aligning symbols transmissions from a remote service unit having a transmitter. Such a system includes a plurality of remote service units each including a transmitter for transmitting OFDM / DMT symbols over a predetermined number of bins across a transmission medium. Each of the plurality of remote service units is operable in a symbol alignment mode in which the transmitter transmits a broad band periodic signal. The head end unit includes a receiver for receiving the OFDM / DMT symbols, including the broad band periodic signal, from the transmission medium. The head end unit uses the time position of the broad band periodic signal to align the symbol transmissions of the remote service unit with other ones of the remote service units.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]The present application is a continuation application of U.S. application Ser. No. 11 / 201,853, filed Aug. 11, 2005, which is a continuation of U.S. application Ser. No. 09 / 312,990, filed Jul. 25, 2001, now U.S. Pat. No. 6,950,388, which is a continuation-in-part application of U.S. application Ser. No. 08 / 700,779, filed Aug. 22, 1996, now U.S. Pat. No. 5,790,514, which is a continuation of U.S. application Ser. No. 08 / 845,544, filed Apr. 24, 1997, now U.S. Pat. No. 6,285,654.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT[0002]Not ApplicableBACKGROUND OF THE INVENTION[0003]The present invention is directed to an OFDM / DMT digital communications system. More particularly, the present invention is directed to an apparatus and method for attaining symbol alignment in a multi-point, OFDM / DMT digital communications system.[0004]Multi-point communications systems having a primary site that is coupled for communication with a plur...

AI Technical Summary

Benefits of technology

[0009]In accordance with a further aspect of the present invention, the receivers apply a predetermined incremental phase shift to received samples corresponding to the received OFDM / DMT symbols to thereby compensate for phase shifts resulting from the cyclic prefix.

Problems solved by technology

As the number of cable telephony subscribers increases over time, the increased use will strain the limited bandwidth allocated to the cable telephony system.

It is often impractical to increase the bandwidth allocated to the cable telephony system given the competition between services for the total bandwidth available to the cable service provider.

Such systems, however, present a number of technical problems.

One such problem is the determination of how a newly added receiver is to attain symbol alignment without disrupting communications to the other receivers that are already in communication with a primary transmitter at a central site.

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