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

Abstract

A multi-point communications system that is set forth herein. The communication 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.

1 . A multi-point communications system comprising: 
 a head end unit disposed at a primary site, the head end unit including a transmitter for transmitting OFDM/DMT symbols over a predetermined number of bins across a transmission medium, the OFDM/DMT symbols being transmitted in periodically occurring symbol frames, a plurality of the symbol frames each having a predetermined signal superimposed thereon;    a plurality of remote service units each including a receiver for receiving the OFDM/DMT symbols over a subset of the predetermined number of bins from the transmission medium, the receivers using the superimposed predetermined signal to attain symbol alignment of the received OFDM/DMT symbols.    
   
   
       2 . A multi-point communications system as claimed in  claim 1  wherein the superimposed signal is transmitted at a period equal to the symbol rate.  
   
   
       3 . A multi-point communications system as claimed in  claim 1  wherein the predetermined signal is an impulse signal.  
   
   
       4 . A multi-point communications system as claimed in  claim 1  wherein the transmitter of the head end unit varies the polarity of the predetermined signal.  
   
   
       5 . A multi-point communications system as claimed in  claim 1  wherein each of the periodically occurring formatted symbol frames includes a cyclic prefix.  
   
   
       6 . A multi-point communications system as claimed in  claim 1  wherein the receivers apply a predetermined incremental phase shift to received samples, including received samples of the predetermined signal, corresponding to the received OFDM/DMT symbols to thereby compensate for phase shifts resulting from the cyclic prefix.  
   
   
       7 . A multi-point communications system as claimed in  claim 6  wherein the receiver comprises a lookup table to facilitate application of the predetermined incremental phase shift.  
   
   
       8 . A multi-point communications system as claimed in  claim 1  wherein the receivers attain symbol alignment by collecting digital data samples corresponding to the OFDM/DMT symbols, applying a sliding window of fixed the length to the digital data samples, and computing the total power in the samples of each window to determine which of the sliding window positions has samples of the greatest total power whereby the window position having the greatest total power corresponds to the time position used by the receiver to align symbols received from the transmitter.  
   
   
       9 - 32 . (canceled)