Method and apparatus for time and frequency synchronization of OFDM communication systems

• US 7,027,429 B2
• Filed: 06/19/2001
• Issued: 04/11/2006
• Est. Priority Date: 06/19/2001
• Status: Expired due to Fees

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Flarion Technologies (Qualcomm, Inc.)
• Inventors: Laroia, Rajiv, Li, Junyi, Rangan, Sundeep, Sampath, Hemanth T.
• Primary Examiner(s): Ngo, Ricky
• Assistant Examiner(s): CHANG, RICHARD K
• Application Number:
  US09/884,581
• Publication Number:
  US 20020196731A1
• Time in Patent Office: 1,757 Days
• Field of Search: 370/210, 370/503, 370/350, 375/260, 375/316
• US Class Current: 370/350
• CPC Class Codes: H04L 27/2659 Coarse or integer frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/266 Fine or fractional frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/2663 Coarse synchronisation, e.g. by correlation
• CPC Class Codes: H04L 27/2665 Fine synchronisation, e.g. by positioning the FFT window
• CPC Class Codes: H04L 27/2675 Pilot or known symbols

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.

Claims

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 receiver,

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 QFDM signal, and

wherein said initial time and frequency offset estimator comprises;

a plurality of smoothed time-domain correlation estimators for outputting a series of time offset estimate and correlation estimate pairs, each pair related to a frequency offset estimate; and

a selector for selecting a selected time offset estimate and a selected initial frequency offset based in part upon the selection of the frequency offset estimate and time offset estimate that corresponds with the largest value of correlation estimate.

2. The system of claim 1, wherein the initial time and frequency offset estimator uses a coarse frequency discretization using F candidate frequency offsets.

3. The system of claim 1, wherein said reference multitone synchronization signal has a length of T, and wherein said frequency offset estimate refinement device comprises:

a T-length interval extractor for extracting a T-length sample of the output of said sampler;

a numerical oscillator for generating a complex exponential of a candidate frequency offset;

a multiplier for multiplying said T-length sample with said complex exponential to obtain a frequency shifted received signal;

a correlator for correlating said frequency shifted received signal with said reference multitone synchronization signal and producing a correlation output; and

a numerical optimizer for receiving said correlation output and outputting a new frequency offset candidate.

4. The system of claim 3, 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.

5. The system of claim 1, wherein each of said smoothed time domain correlation estimators comprises:

a time domain correlator;

a smoothing filter connected to said time domain correlator and receiving an output from said time domain correlator; and

a maximum detector connected to and receiving an output from said smoothing filter for detecting a signal energy maxima representing a time estimate at which the energy of said reference multitone synchronization signal is at a maximum.

6. 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:

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; and

using said selected time offset estimate and said selected frequency offset estimate to synchronize said receiver with said received OFDM signal.

7. A method of carrying out OFDM communications comprising:

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; and

using said selected pair of time and frequency offset values to synchronize receiver operations with said received OFDM signal.

8. A method of carrying out OFDM communications comprising:

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 synchronizatioin signal;

recursively refining said frequency offset estimate to yield a selected pair of time and frequency offset values to be used by said OFDM receiver; and

wherein said initial time offset value and said initial frequency offset value are determined by obtaining a correlation with a stored reference value of said synchronization signal.

9. The method of claim 8, wherein said correlation is performed seeking a maximum received synchronization signal energy level.

10. An orthogonal frequency division multiplexing (OFDM) communication device, comprising:

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;

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;

a plurality of means for obtaining smoothed time-domain (TDC) correlation estimates, said smoothed TDC estimate means outputting a series of time offset estimate and correlation estimate pairs, each pair related to a frequency offset estimate; and

means for selecting a selected time offset estimate and a selected initial frequency offset based in part upon the selection of the frequency offset estimate and time offset estimate that corresponds with the largest value of correlation estimate.

11. The system of claim 10, wherein each of said smoothed TDC estimate means comprises:

a time domain correlator;

a smoothing filter connected to said time domain correlator and receiving an output from said time domain correlator; and

a maximum detector connected to and receiving an output from said smooching filter for detecting a signal energy maxima representing a time estimate at which the energy of said reference multitone synchronization signal is at a maximum.

12. The system of claim 11, wherein the estimating means uses a coarse frequency discretization using F candidate frequency offsets.

13. The system of claim 10, wherein said reference multitone synchronization signal has a length of T, and wherein said refinement means comprises:

a T-length interval extractor for extracting a T-length sample of the output of said sampler;

a numerical oscillator for generating a complex exponential of a candidate frequency offset;

a multiplier for multiplying said T-length sample with said complex exponential to obtain a frequency shifted received signal;

a correlator for correlating said frequency shifted received signal with said reference multitone synchronization signal and producing a correlation output; and

a numerical optimizer for receiving said correlation output and outputting a new frequency offset candidate.

14. The system of claim 13, wherein said new frequency offset candidate and a time offset associated with said new frequency offset candidate are used by said receiving means if said new frequency offset candidate is a candidate that yields a maximum correlation output.

15. A device for synchronizing an orthogonal frequency division multiplexing (OFDM) receiver with a received OFDM signal comprising a multitone synchronization signal, comprising:

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;

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; and

means for passing said selected time offset estimate and said selected frequency offset estimate to said receiver for use by said receiver in synchronizing with said received OFDM signal.

16. A device for carrying out OFDM communications comprising:

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; and

means for passing said selected time offset estimate and said selected frequency offset estimate to a receiver circuit for use by said receiver circuit in synchronizing with said received OFDM signal.

17. A device for carrying out OFDM communications comprising:

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; and

wherein said initial time offset value and said initial frequency offset value are determined by obtaining a correlation with a stored reference value of said synchronization signal.

18. The device of claim 17, wherein said correlation is performed seeking a maximum received synchronization signal energy level.

19. An OFDM signal processor comprising:

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, and

wherein said initial time and frequency offset estimator comprises;

a plurality of smoothed time-domain correlation estimators for outputting a series of time offset estimate and correlation estimate pairs, each pair related to a frequency offset estimate; and

a selector for selecting a selected time offset estimate and a selected initial frequency offset based in part upon the selection of the frequency offset estimate and time offset estimate that corresponds with the largest value of correlation estimate.

20. The processor of claim 19, wherein each of said smoothed time domain correlation estimators comprises:

a time domain correlator;

a smoothing filter connected to said time domain correlator and receiving an output from said time domain correlator; and

a maximum detector connected to and receiving an output from said smoothing filter for detecting a signal energy maxima representing a time estimate at which the energy of said reference multitone synchronization signal is at a maximum.

21. The processor of claim 19, wherein the initial time and frequency offset estimator uses a coarse frequency discretization using F candidate frequency offsets.

22. The processor of claim 19, wherein said reference multitone synchronization signal has a length of T, and wherein said frequency offset estimate refinement device comprises:

a T-length interval extractor for extracting a T-length sample of the output of said sampler;

a numerical oscillator for generating a complex exponential of a candidate frequency offset;

a multiplier for multiplying said T-length sample with said complex exponential to obtain a frequency shifted received signal;

a correlator for correlating said frequency shifted received signal with said reference multitone synchronization signal and producing a correlation output; and

a numerical optimizer for receiving said correlation output and outputting a new frequency offset candidate.

23. The processor of claim 22, 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.

24. The processor of claim 22, wherein said initial time and frequency offset estimator comprises:

a first Fast Fourier Transformer for obtaining a transform of said received signal;

an second Fast Fourier Transformer device for obtaining a transform of said reference multitone synchronization signal;

a frequency domain correlation estimator for receiving said received signal transform and said reference signal transform and outputting an initial frequency offset estimate; and

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.

25. An OFDM transmitter comprising:

means for transmitting an OFDM signal comprising a first time interval and a second time interval, each of the first and second time intervals including at least one OFDM symbol transmission time period, the first and second time intervals recurring on a periodic basis;

means for transmitting data on a plurality frequencies during said first time interval; and

means for transmitting, during said second time interval, a plurality of synchronization tones in parallel on different non-adjacent, frequencies, said second time interval being a predetermined time period, the frequencies of said synchronization tones in said second time interval being distinct from said frequencies used to transmit data in the first time interval, the first time interval immediately preceding the second time interval.

27. The method of claim 25,

wherein the first and second time intervals are single OFDM symbol transmission time periods;

and

wherein frequencies used for synchronization tones in a symbol transmission time period are not used in any immediately consecutive symbol transmission time period for synchronization tones.

26. A method for transmitting an OFDM signal comprising the steps of:

transmitting an OFDM signal during a first time interval, the first time interval including at least one OFDM symbol transmission time period, said step of transmitting data during the first time interval including transmitting data on a plurality of data frequencies; and

transmitting an OFDM signal during a second time interval which includes at least one OFDM tone, said first and second time intervals being consecutive time intervals,

transmitting during said second time interval including transmitting a plurality of synchronization tones at a plurality of non-adjacent synchronization frequencies for a predetermined time period, the frequencies of said synchronization tones being distinct from said data frequencies used in the first time interval.

28. The method of claim 26,

wherein the first and second time intervals are single OFDM symbol transmission time periods;

and

wherein at least three frequencies are used for synchronization tones during each symbol transmission time period.