OFDM frame synchronization
View Patent ↗FFT demodulation of OFDM symbols is achieved using a frame synchronization pulse generated by deriving absolute values of successive complex samples of the OFDM symbol, determining the difference between such values and other values separated therefrom by a period representing the useful part of the OFDM symbol, integrating the difference values over a plurality of symbols and determining the sample position representing the point at which the integrated difference values substantially change.
1. A method of generating a synchronization pulse representing a symbol boundary in an OFDM signal comprising useful symbol periods separated by guard spaces, with data in each guard space corresponding to part of the data in a respective useful symbol period, the method comprising:
deriving the difference between absolute values of samples of the OFDM signal separated by a period corresponding to the useful symbol period,
providing the difference between the absolute values to an edge detector, and
generating the synchronization pulse in response to a substantial change in the difference between the absolute values.
2. A method as claimed in claim 1 , wherein the difference between the absolute values represents integrated values obtained over a plurality of symbol periods.
3. A method as claimed in claim 2 , wherein an infinite impulse response filter is used for producing the integrated values.
4. A method of claim 2 , wherein the edge detector comprises a filter.
5. A method of claim 4 , wherein the filter has variable coefficients.
6. A method as claimed in claim 1 , wherein the edge detector comprises a filter.
7. A method as claimed in claim 6 , wherein the filter has variable coefficients.
8. A method as claimed in claim 6 , wherein the filter is a finite impulse response filter.
9. A method of claim 1 , further comprising:
demodulating the OFDM signal to produce complex samples of transmitted OFDM samples;
applying a Fourier Transform to the complex samples; and
synchronizing a Fourier Transform window with OFDM symbols using the synchronization pulse.
10. An apparatus for generating a synchronization pulse for an OFDM signal, comprising:
a device for receiving the OFDM signal including useful symbol periods separated by guard spaces, with data in each guard space corresponding to part of the data in a respective useful symbol period;
a first circuit formed by an edge detector for determining and monitoring the difference between absolute values of samples of the OFDM signal separated by a period corresponding to the useful symbol period; and
a second circuit for generating the synchronization pulse in response to said first circuit detecting a substantial change in the difference between the absolute values.