OFDM cognitive radio with zero overhead signalling of deleted subcarriers frequencies
View Patent ↗A system, device and method are described for suppressing certain sub-carrier frequencies within a multi-path transmission system and efficiently notifying a receiver ( 400 ) of the suppressed sub-carriers. In various embodiments of the invention, an OFDM system uses the preamble ( 210 ) of an OFDM burst for indicating which sub-carrier frequencies are being suppressed by a transmitter ( 300 ).
1. A method for suppressing at least one sub-carrier frequency within a channel frequency profile, the method comprising:
identifying the at least one sub-carrier frequency, within the wireless channel frequency profile, having an interference level above a first threshold;
notching the at least one sub-carrier frequency in a preamble of a burst to generate a notched preamble;
transmitting the burst, comprising the notched preamble and a payload, into a wireless channel;
receiving the burst from the wireless channel and analyzing the notched preamble to identify the at least one sub-carrier frequency;
inserting at least one erasure in the payload of the burst at the at least one sub-carrier frequency identified by analyzing the notched preamble; and
decoding the payload of the burst.
2. The method of claim 1 wherein the wireless channel is a frequency-selective fading OFDM channel.
3. The method of claim 1 further comprising: providing a first guard sub-carrier, adjacent to the at least one notched sub-carrier frequency, that reduces energy from spilling into the at least one notched sub-carrier frequency; and providing a second guard sub-carrier, adjacent to the at least one notched sub-carrier frequency, that reduces energy from spilling into the at least one notched sub-carrier frequency.
4. The method of claim 3 wherein the first and second guard sub-carriers use active interference cancellation to reduce energy from spilling into the at least one notched sub-carrier frequency.
5. The method of claim 1 wherein the at least one notched sub-carrier frequency in the burst preamble is identical to the at least one suppressed sub-carrier frequency in the burst payload.
6. The method of claim 1 wherein the at least one sub-carrier frequency is notched by applying a frequency domain nulling mask to channel estimation symbols associated within the at least one sub-carrier frequency.
7. The method of claim 6 wherein sub-carrier frequencies within the received burst are weighted by complex conjugates of channel estimation symbols that implicitly contain the frequency domain nulling mask.
8. The method of claim 1 wherein sub-carrier frequencies within the received burst are compared to a threshold level to determine whether a frequency notch is present.
9. A transmitter module apparatus located within a multi-frequency wireless transmitter, the module comprising:
an interference detector, coupled to an antenna, that analyzes frequency characteristics within a frequency spectrum and identifies undesirable sub-carrier frequencies within the frequency spectrum;
a preamble generator, communicatively coupled to the interference detector, that generates a notched preamble having at least one sub-carrier frequency notch corresponding to the identified undesirable frequencies, wherein the notched preamble is configured to instruct a receiver to insert at least one erasure in a payload of a burst at least one of the undesirable sub-carrier frequencies; and
an active interference cancellation module, communicatively coupled to the interference detector, that generates sub-carrier guards that are adjacent to the at least one sub-carrier frequency notch to reduce energy from spilling into the at least one sub-carrier frequency notch.
10. The transmitter module apparatus of claim 9 wherein the at least one sub-carrier frequency notch and the sub-carrier guards are combined within the preamble of a burst and transmitted into a wireless channel.
11. The transmitter module apparatus of claim 9 wherein the at least one sub-carrier frequency notch in the preamble of a burst is identical to at least one suppressed frequency in a payload of the burst.
12. The transmitter module apparatus of claim 9 wherein the sub-carrier guards reduce energy spillover by active interference cancellation.
13. The transmitter module apparatus of claim 9 wherein the interference detector identifies the undesirable sub-carrier frequencies based on a signal-to-noise ratio analysis of frequencies within the frequency spectrum.
14. A receiver module apparatus located within a multi-frequency wireless transmitter, the module comprising:
a channel estimation module, coupled to receive a burst from a wireless channel, that analyzes a notched preamble to identify at least one notched sub-carrier frequency within the notched preamble of the burst;
a nulled sub-carrier identification and list module, coupled to the channel estimation module, that stores the at least one notched sub-carrier frequency identified by the channel estimation module; and
a channel decoder, coupled to the channel estimation module and the nulled sub-carrier identification and list module, that associates at least one suppressed sub-carrier frequency within a payload of the burst with the at least one notched sub-carrier frequency in the notched preamble, and decodes the notched preamble.
15. The receiver module apparatus of claim 14 wherein the channel estimation module inserts at least one erasure in the at least one suppressed sub-carrier frequency within the payload.
16. The receiver module apparatus of claim 14 wherein a threshold is applied to symbol frequencies within the preamble to identify the at least one notched sub-carrier frequency.
17. The receiver module apparatus of claim 14 wherein a priori knowledge of guard sub-carriers is used to identify the at least one notched sub-carrier frequency within the preamble.
18. The receiver module apparatus of claim 14 wherein the nulled sub-carrier identification and list module stores the at least one notched sub-carrier frequency in a table that associates the at least one notched sub-carrier frequency in the preamble to the at least one suppressed sub-carrier in the payload.
19. The receiver module apparatus of claim 14 wherein the channel decoder decodes the payload using forward error correction processes.
20. The receiver module apparatus of claim 14 wherein the at least one sub-carrier frequency is weighted by a complex conjugate of channel estimation symbols containing a frequency domain nulling mask that was applied at a transmitter.