Method and apparatus for time and frequency synchronization of OFDM communication systems
First Claim
1. An orthogonal frequency division multiplexing (OFDM) communication device, comprising:
- an OFDM receiver for receiving an OFDM signal containing a multitone synchronization signal;
a synchronization interval sampler coupled to said receiver;
an initial time and frequency offset estimator connected to said sampler and said receiver; and
a frequency offset estimate refinement unit connected to said receiver, said sampler and said estimator, wherein a reference multitone synchronization signal is used by said estimator and said refinement device in calculating a time offset and a frequency offset of said multitone synchronization signal, said receiver utilizing said time offset and said frequency offset to synchronize with said received OFDM signal.
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Abstract
A method and apparatus for joint time and frequency synchronization for orthogonal frequency division multiplexing (OFDM) systems. A multitone pilot signal is sent in a designated OFDM symbol period. The receiver synchronizes to the pilot signal in a two-stage procedure. The first stage estimates the frequency offset coarsely with a frequency-domain correlation method and estimates the time offset with smoothed time-domain correlation. In a multipath channel, the smoothed time offset estimate is used to locate a cyclic prefix interval which captures the maximum total signal energy. The second stage improves the frequency estimate with a computationally efficient numerical optimization method.
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Citations
36 Claims
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1. An orthogonal frequency division multiplexing (OFDM) communication device, comprising:
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an OFDM receiver for receiving an OFDM signal containing a multitone synchronization signal;
a synchronization interval sampler coupled to said receiver;
an initial time and frequency offset estimator connected to said sampler and said receiver; and
a frequency offset estimate refinement unit connected to said receiver, said sampler and said estimator, wherein a reference multitone synchronization signal is used by said estimator and said refinement device in calculating a time offset and a frequency offset of said multitone synchronization signal, said receiver utilizing said time offset and said frequency offset to synchronize with said received OFDM signal. - View Dependent Claims (2, 3, 4, 6)
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5. The system of claim 5, wherein said new frequency offset candidate and a time offset associated with said new frequency offset candidate are used by said receiver if said new frequency offset candidate is a candidate that yields a maximum correlation output.
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7. A time domain correlation estimator for receiving said received signal transform and said reference signal transform and said initial frequency offset estimate and outputting a time offset estimate.
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8. A method of synchronizing an orthogonal frequency division multiplexing (OFDM) receiver with a received OFDM signal comprising a multitone synchronization signal, comprising the steps of:
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obtaining a coarse time offset estimate of said received signal;
sampling said received signal in a selected time interval to derive samples of said multitone synchronization signal;
analyzing said samples with respect to a reference multitone synchronization signal to obtain, for each sample analyzed, a time offset, a frequency offset, and a signal energy;
selecting a one of said analyzed samples with the greatest signal energy to yield a selected time offset estimate and a selected frequency offset estimate for use by said receiver in synchronizing with said received OFDM signal. - View Dependent Claims (9)
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10. A method of carrying out OFDM communications comprising:
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receiving an OFDM signal including within it a multitone synchronization signal;
locating said synchronization signal within said OFDM signal;
determining a time offset value of said synchronization signal;
determining an initial frequency offset value of said synchronization signal; and
recursively refining said frequency offset estimate to yield a selected pair of time and frequency offset values to be used by said OFDM receiver. - View Dependent Claims (11, 12, 15, 16, 17, 18, 19, 20)
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13. A method of carrying out OFDM communications comprising:
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receiving, in an OFDM receiver, an OFDM signal including within it a multitone synchronization signal;
obtaining an FFT transform of said received signal;
obtaining an FFT transform of said reference multitone synchronization signal;
correlating said received signal transform and said reference signal transform and outputting an initial frequency offset estimate when said aforementioned transforms are maximally correlated; and
correlating said received signal transform and said reference signal transform and said initial frequency offset estimate and outputting a time offset estimate when said aforementioned transforms are maximally correlated.
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14. An orthogonal frequency division multiplexing (OFDM) communication device, comprising:
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means for receiving an OFDM signal containing a multitone synchronization signal;
means, coupled to said receiving means, for sampling a synchronization interval of said OFDM signal;
means, connected to said sampling means and said receiving means, for obtaining an initial time estimate and an initial frequency offset estimate of said OFDM signal;
means, connected to said receiving means, said sampling means and said estimating means, for obtaining a frequency offset estimate refinement; and
storage means, connected to said estimating means and said refinement means, for storing a reference multitone synchronization signal for use by said estimating means and said refinement means in calculating a time offset and a frequency offset of said multitone synchronization signal, said receiving means utilizing said time offset and said frequency offset to synchronize with said received OFDM signal.
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21. A device for synchronizing an orthogonal frequency division multiplexing (OFDM) receiver with a received OFDM signal comprising a multitone synchronization signal, comprising:
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means for obtaining a coarse time offset estimate of said received signal;
means for sampling said received signal in a selected time interval to derive samples of said multitone synchronization signal;
means for analyzing said samples with respect to a reference multitone synchronization signal to obtain, for each sample analyzed, a time offset, a frequency offset, and a signal energy; and
means for selecting one of said analyzed samples with the greatest signal energy to yield a selected time offset estimate and a selected frequency offset estimate, wherein said selected time offset estimate and said selected frequency offset estimate are used by said receiver in synchronizing with said received OFDM signal. - View Dependent Claims (22, 24, 25, 28, 29, 30, 31, 32, 33, 34)
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23. A device for carrying out OFDM communications comprising:
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means for receiving an OFDM signal including within it a multitone synchronization signal;
means for locating said synchronization signal within said OFDM signal;
means for determining a time offset value of said synchronization signal;
means for determining an initial frequency offset value of said synchronization signal; and
means for recursively refining said frequency offset estimate to yield a selected pair of time and frequency offset values to be used by said OFDM receiver.
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26. A system for carrying out OFDM communications comprising:
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means for receiving an OFDM signal including within it a multitone synchronization signal means for obtaining an FFT transform of said received signal;
means for obtaining an FFT transform of said reference multitone synchronization signal;
means for correlating said received signal transform and said reference signal transform and outputting an initial frequency offset estimate when said aforementioned transforms are maximally correlated; and
means for correlating said received signal transform and said reference signal transform and said initial frequency offset estimate and supplying as an output a time offset estimate when said aforementioned transforms are maximally correlated.
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27. An OFDM signal processor comprising:
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an OFDM receiver for receiving an OFDM signal containing a multitone synchronization signal;
a synchronization interval sampler connected to said input and said receiver;
an initial time and frequency offset estimator connected to said sampler and said receiver; and
a frequency offset estimate refinement device connected to said receiver, said sampler and said estimator, wherein a reference multitone synchronization signal is used by said estimator and said refinement device in calculating a time offset and a frequency offset of said multitone synchronization signal, said receiver utilizing said time offset and said frequency offset to synchronize with said received OFDM signal.
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35. An OFDM transmitter comprising:
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means for transmitting an OFDM signal comprising a first time interval and a second time interval;
means for transmitting data at one or more data frequencies during said first time interval; and
means for transmitting, during said second time interval, a synchronization tone, at one or more synchronization frequencies, for a predetermined time period, the frequencies of said synchronization tone being distinct from said data frequencies.
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36. A method for transmitting an OFDM signal comprising the steps of:
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transmitting an OFDM signal comprising a first time interval and a second time interval;
means for transmitting data at one or more data frequencies during said first time interval; and
means for transmitting, during said second time interval, a synchronization tone, at one or more synchronization frequencies, for a predetermined time period, the frequencies of said synchronization tone being distinct from said data frequencies.
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Specification