Spectral shaping in multiband OFDM transmitter with clipping

1. A method of shaping an orthogonal frequency division multiplexing (OFDM) signal spectrum, comprising:

receiving an input signal at an interface; and

inserting a modified synchronization sequence in the input signal by a processor to generate an amplitude limited signal that has a substantially even frequency spectrum; wherein:

the modified synchronization sequence includes one or more reduced spectral peaks in a preamble waveform; and

the modified synchronization sequence was previously derived, including by:

applying a Fourier transform to a synchronization sequence to obtain a plurality of spectral components;

selecting a spectral component from the plurality of spectral components;

determining the clip level relative to the amplitude of the selected spectral component; and

clipping the plurality of spectral components to the clip level.

2. A method as recited in claim 1 , wherein the modified synchronization sequence corresponds to a preamble.

3. A method as recited in claim 1 , wherein selecting a spectral component from the plurality of spectral components includes selecting a spectral component with a maximum amplitude.

4. A method as recited in claim 1 ,

wherein the clip level is selected to be approximately 3 dB below the amplitude of the selected spectral component.

5. A method as recited in claim 1 , wherein selecting a spectral component from the plurality of spectral components includes selecting a spectral component with a minimum amplitude.

6. A method as recited in claim 1 , wherein selecting a spectral component from the plurality of spectral components includes selecting a spectral component with a minimum amplitude and the clip level is determined to be less than or equal to the minimum amplitude.

7. A method as recited in claim 1 , the modified synchronization sequence is selected from a plurality of pre-computed, modified synchronization sequences that correspond to a plurality of preamble sequences.

8. An orthogonal frequency division multiplexing (OFDM) transmitter comprising:

an input interface configured to receive an input signal; and

a processor coupled to the input interface, configured to insert in the input signal a modified synchronization sequence to generate a clipped signal that has a substantially flat frequency spectrum, wherein:

the modified synchronization sequence includes one or more reduced spectral peaks in a preamble waveform; and

the modified synchronization sequence was previously computed, including by:

applying a Fourier transform to the input signal to obtain a plurality of spectral components;

selecting a spectral component from the plurality of spectral components;

determining the clip level relative to the amplitude of the selected spectral component; and

clipping the plurality of spectral components to the clip level.

9. An OFDM transmitter as recited in claim 8 , wherein the modified synchronization sequence corresponds to a preamble.

10. An OFDM transmitter as recited in claim 8 , wherein selecting a spectral component from the plurality of spectral components includes selecting a spectral component with a maximum amplitude.

11. An OFDM transmitter as recited in claim 8 ,

wherein the clip level is selected to be approximately 3 dB below the amplitude of the selected spectral component.

12. An OFDM transmitter as recited in claim 8 , wherein selecting a spectral component from the plurality of spectral components includes selecting a spectral component with a minimum amplitude.

13. An OFDM transmitter as recited in claim 8 , wherein selecting a spectral component from the plurality of spectral components includes selecting a spectral component with a minimum amplitude and the clip level is determined to be less than or equal to the minimum amplitude.

14. An OFDM transmitter as recited in claim 8 , wherein the modified synchronization sequence is selected from a plurality of pre-computed, modified synchronization sequences that correspond to a plurality of preamble sequences.