Method for suppression of OFDM energy spectral density for minimization of out of band emission or utilization of fractured spectrum
View Patent ↗The Energy spectral density of OFDM signals inherently rolls off slowly. Slow OFDM spectral rolloff has system level implications traditionally mitigated by some combination of the following: addition of bandlimiting filtering; use of significant guard bands of zeroed tones; and, guard time shaping. Each of these techniques negatively impact system performance and/or flexibility. This application presents a methodology for active cancellation of out of band spectral energy. The technique can be used by itself or in conjunction with above traditional methods to help control out of band emission. Examples of the use of the new technique are provided. Computational cost of the new technique is also discussed.
1. A method for suppressing Orthogonal Frequency Division Multiplexing (OFDM) energy spectral density, comprising:
transmitting a plurality of data tones and at least one guard tone in a frequency band;
constructing a set of orthonormal basis vectors in at least one frequency region where the OFDM energy spectral density is to be cancelled based on the guard tone;
setting an amplitude and phase of the guard tone based on the orthonormal basis vectors; and
energizing the at least one guard tone wherein an extended spectral energy side lobe of the at least one guard tone cancels at least one extended spectral energy side lobe of the plurality of data tones in a specified region of the frequency band.
2. The method of claim 1 , wherein the specified region of the frequency band is adjacent to a band edge of the frequency band.
3. The method of claim 2 , wherein a plurality of guard tones are energized.
4. The method of claim 3 , wherein the plurality of guard tones are selected based on a characteristic of the selected guard tones.
5. The method of claim 4 , wherein the characteristic is an orthogonality of the selected guard tones.
6. A method for canceling Orthogonal Frequency Division Multiplexing (OFDM) energy spectral density, comprising:
selecting a set of cancellation tones to be used for energy spectral density cancellation;
constructing a set of orthonormal basis vectors in at least one frequency region where the OFDM energy spectral density is to be cancelled based on the selected set of cancellation tones;
computing a projection of a unity magnitude data modulated tone at each cancellation tone frequency onto the respective orthonormal basis vectors;
employing a set of data excitations to scale the unity magnitude projections to find a projection of a side lobe of symbol data onto the set of orthonormal basis vectors; and
applying the projection of the side lobe of symbol data to set an amplitude and a phase of each of the cancellation tones.
7. The method of claim 6 , wherein the set of cancellation tones are selected from a guard band of frequencies.
8. The method of claim 6 , wherein at least one of the tones in the set of cancellation tones is selected from a guard band of frequencies.
9. The method of claim 7 , wherein a frequency spectrum in the OFDM energy spectral density comprises a stay out zone.
10. The method of claim 9 , wherein tones in the set of cancellation tones are selected from a first guard band and a second guard band.
11. The method of claim 10 , wherein the first guard band is in a frequency spectrum immediately below the OFDM data band and the second guard band is in a frequency spectrum immediately above the OFDM data band.
12. The method of claim 11 , wherein the set of cancellation tones consists of fourteen cancellation tones.
13. The method of claim 7 , wherein tones in the set of cancellation tones are selected from a first guard band and a second guard band.
14. The method of claim 13 , wherein the first guard band is in a frequency spectrum immediately below the OFDM data band and the second guard band is in a frequency spectrum immediately above the OFDM data band.
15. The method of claim 14 , wherein the set of cancellation tones consists of eight cancellation tones.