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

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 or DMT symbols comprising symbol data over a predetermined number of bins across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each formatted symbol frame having a cyclic prefix, the cyclic prefix including at least a portion of said symbol data and having an impulse signal superimposed thereon, the polarity of the impulse signal being alternated on consecutive symbol frames;
• and
• a plurality of remote service units each including a receiver for receiving the OFDM or DMT symbols over a subset of the predetermined number of bins from the transmission medium, the receiver using the locations of the superimposed impulse signals to attain symbol alignment.

A transmitter at a head end unit disposed at a primary site in a multipoint communications system, the transmitter transmitting OFDM or DMT symbols comprising symbol data over a predetermined number of bins across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each frame having a cyclic prefix including at least a portion of said symbol data, the cyclic prefix having an impulse signal superimposed thereon, the polarity of the impulse signal alternating on consecutive frames, the locations of the impulse signals being provided to facilitate symbol alignment in remote receivers of the multi-point communications system.

A communications system comprising:

• a transmitter for transmitting OFDM or DMT symbols comprising symbol data across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each formatted symbol frame having a cyclic prefix including at least a portion of said symbol data, the cyclic prefix having an impulse signal superimposed thereon, the polarity of the impulse signal being alternated on consecutive frames;
• and
• a receiver for receiving the OFDM or DMT symbols from the transmission medium, the receiver using the locations of the superimposed impulse signals to attain symbol alignment of the received OFDM or DMT symbols.

A communications system comprising:

• a first unit including a transmitter transmitting OFDM or DMT data symbols each including a cyclic prefix across a transmission medium during symbol periods and transmitting a plurality of predetermined signals during at least a plurality of said symbol periods during which said data symbols are transmitted;
• and
• a second unit including a receiver receiving the data symbols during said symbol periods and receiving the predetermined signals during said plurality of said symbol periods from the transmission medium, the receiver detecting the locations of the predetermined signals during said plurality of said symbol periods and using the locations of the predetermined signals to attain symbol alignment of the received data symbols, said receiver applying a predetermined incremental phase shift to samples, including samples of the predetermined signals, corresponding to the received data symbols to thereby compensate for phase shifts resulting from the cyclic prefix.

A communications system comprising:

• a first unit including a transmitter transmitting OFDM or DMT data symbols across a transmission medium during symbol periods and transmitting a plurality of predetermined signals during at least a plurality of said symbol periods during which said data symbols are transmitted;
• and
• a second unit including a receiver receiving the data symbols during said symbol periods and receiving the predetermined signals during said plurality of said symbol periods from the transmission medium, the receiver detecting the locations of the predetermined signals during said plurality of said symbol periods, using the locations of the predetermined signals to attain symbol alignment of the received data symbols, attaining symbol alignment by collecting digital data samples corresponding to the data symbols, applying a sliding window of fixed 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 data symbols received from the transmitter.

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 or DMT symbols over a predetermined number of bins across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each formatted symbol frame having a cyclic prefix, the cyclic prefix having an impulse signal superimposed thereon, the polarity of the impulse signal being alternated on consecutive symbol frames;
• and
• a plurality of remote service units each including a receiver for receiving the OFDM or DMT symbols over a subset of the predetermined number of bins from the transmission medium, the receiver using the locations of the superimposed impulse signals to attain symbol alignment and applying a predetermined incremental phase shift to received samples corresponding to the received OFDM or DMT symbols to thereby compensate for phase shifts resulting from the cyclic prefix.

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 or DMT symbols over a predetermined number of bins across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each formatted symbol frame having a cyclic prefix, the cyclic prefix having an impulse signal superimposed thereon and transmitted at a rate equal to the symbol rate of the transmitted OFDM or DMT symbols, the polarity of the impulse signal being alternated on consecutive symbol frames;
• and
• a plurality of remote service units each including a receiver for receiving the OFDM or DMT symbols over a subset of the predetermined number of bins from the transmission medium, the receiver using the locations of the superimposed impulse signals to attain symbol alignment and comprising a lookup table to facilitate application of the predetermined incremental phase shift.

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 or DMT symbols over a predetermined number of bins across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each formatted symbol frame having a cyclic prefix, the cyclic prefix having an impulse signal superimposed thereon, the polarity of the impulse signal being alternated on consecutive symbol frames;
• and
• a plurality of remote service units each including a receiver for receiving the OFDM or DMT symbols over a subset of the predetermined number of bins from the transmission medium, the receiver using the locations of the superimposed impulse signals to attain symbol alignment, attaining symbol alignment by collecting digital data samples corresponding to the OFDM or DMT symbols, applying a sliding window equal to the length of the cyclic prefix 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 of occurrence of the cyclic prefix used by the receiver to align symbols received from the transmitter.

A communications system comprising:

• a transmitter for transmitting OFDM or DMT symbols across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each formatted symbol frame having a cyclic prefix, the cyclic prefix having an impulse signal superimposed thereon, the polarity of the impulse signal being alternated on consecutive frames;
• and
• a receiver for receiving the OFDM or DMT symbols from the transmission medium, the receiver using the locations of the superimposed impulse signals to attain symbol alignment of the received OFDM or DMT symbols and applying a predetermined incremental phase shift to received samples corresponding to the received OFDM or DMT symbols to thereby compensate for phase shifts resulting from the cyclic prefix.

A communications system comprising:

• a transmitter for transmitting OFDM or DMT symbols across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each formatted symbol frame having a cyclic prefix, the cyclic prefix having an impulse signal superimposed thereon, the polarity of the impulse signal being alternated on consecutive frames;
• and
• a receiver for receiving the OFDM or DMT symbols from the transmission medium, the receiver using the locations of the superimposed impulse signals to attain symbol alignment of the received OFDM or DMT symbols and comprising a lookup table to facilitate application of a predetermined incremental phase shift.

A communications system comprising;

• a transmitter for transmitting OFDM or DMT symbols across a transmission medium, the OFDM or DMT symbols being transmitted in periodically occurring formatted symbol frames, each formatted symbol frame having a cyclic prefix, the cyclic prefix having an impulse signal superimposed thereon, the polarity of the impulse signal being alternated on consecutive frames;
• and
• a receiver for receiving the OFDM or DMT symbols from the transmission medium, the receiver using the locations of the superimposed impulse signals to attain symbol alignment of the received OFDM or DMT symbols by collecting digital data samples corresponding to the OFDM or DMT symbols, applying a sliding window of fixed 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.

A communications system comprising:

• a first unit including a transmitter transmitting OFDM or DMT data symbols across a transmission medium during symbol periods and transmitting a plurality of predetermined impulse signals during at least a plurality of said symbol periods during which said data symbols are transmitted, the polarities of the impulse signals being altered on consecutive ones of said impulse signals;
• and
• a second unit including a receiver receiving the data symbols during said symbol periods and receiving the predetermined impulse signals during said plurality of said symbol periods from the transmission medium, the receiver detecting the locations of the predetermined impulse signals during said plurality of said symbol periods and using the locations of the predetermined impulse signals to attain symbol alignment of the received data symbols by sampling said symbols to generate samples of said symbols having amplitudes, alternately adding and subtracting consecutive ones of said amplitudes to generate summed amplitudes, searching for summed amplitudes with peak values and using said peak values to align said data symbols.