Precise frequency estimation of short data bursts

A method of estimating a frequency offset between a transmitter and a receiver based on the received spectrum at the receiver, the transmitter sending the receiver bursts having a preamble and data, the method comprising:

• a) providing processed samples derived from the received spectrum, including a current sample;
• b) providing a typical sample variance;
• c) generating a time average of the processed samples;
• and
• d) during the burst data, excluding the current sample in the time average when the difference between the current sample and the time average exceeds the typical sample variance.

A burst demodulator for a receiver, the receiver receiving bursts from a transmitter, the burst demodulator comprising:

• a) a stage generating pre-detection signal samples;
• b) a detector stage generating detected symbols;
• and
• c) a subsystem for estimating the frequency offset between the transmitter and the receiver, the subsystem including
• i. a first processing stage, coupled to the pre-detection signal samples and the detected symbols, the first processing stage generating first processed samples;
• ii. a time averaging stage, having a samples input and a first estimate indication output, the first estimate indication being a time average of the input samples;
• and
• iii. an atypical samples stage, coupled to receive the first processed samples and the first estimate indication, the atypical samples stage having a sample status indication selectively characterizing particular first processed samples as typical.

A cable modem termination system (CMTS) for a cable system, the cable system including at least one cable modem having a transmitter, the CMTS comprising:

• a) a modulator;
• b) a network interface including control, media-access-control, and framing functions;
• and
• c) a demodulator coupled to receive bursts from the cable modem transmitter, the demodulator including a stage generating pre-detection signal samples, a detector stage generating detected symbols, and a subsystem for estimating the frequency offset between the cable modem transmitter and the demodulator, the subsystem including
• i. a first processing stage, coupled to the pre-detection signal samples and the detected symbols, the first processing stage generating first processed samples;
• ii. a time averaging stage, having a samples input and a first estimate indication output, the first estimate indication being a time average of the input samples;
• and
• iii. an atypical samples stage, coupled to receive the first processed samples and the first estimate indication, the atypical samples stage having a sample status indication selectively characterizing particular first processed samples as typical.

A method of adjusting the frequency of a cable modem, the cable modem having a transmitter that sends bursts to the demodulator of a cable modem termination system, the bursts having a preamble and data, the demodulator having a received spectrum at its input, the method comprising:

• a) commanding the cable modem to operate in ranging mode;
• b) providing processed samples derived from the received spectrum, the particular processed sample at the current time being referred to as the current sample;
• c) defining a typical sample variance and an acceptable frequency offset limit;
• d) generating a running time average of the processed samples;
• e) during the burst preamble, including the current sample in the time average;
• f) during the burst data, excluding the current sample in the time average when the difference between the current sample and the time average exceeds the typical sample variance;
• g) deriving a frequency offset estimate from the time average;
• and
• h) commanding the cable modem to adjust its transmit frequency until the frequency offset estimate is reduced to within the frequency offset limit.