Method and system for decoding OFDM signals subject to narrowband interference
A method estimates a wireless channel at a receiver. The signal is transmitted using narrowband orthogonal frequency division demultiplexing (OFDM) and frequency subcarriers, and the signal includes a set of data tones and a set of pilot tones. The channel and pilot tone interference are estimated based on all the pilot tones extracted from the signal and a channel model. The set of data are equalized based on the channel estimate. Data interference is detected according to the pilot interference and the equalized data tones. Subcarrier interference-to-noise ratios are determined based on the data interference. Signal strengths of the data tones are determined based on the equalized data tones, log-likelihood ratios of bits represented by the data tones are determined based on the subcarrier interference-to-noise ratios and the signal strength of the data tones.
1. A method for estimating a channel, detecting interference, and decoding a signal at a receiver, wherein the channel and receiver are wireless, and wherein the signal is transmitted using narrowband orthogonal frequency division demultiplexing (OFDM) and frequency subcarriers, and the signal includes a set of data tones and a set of pilot tones, comprising the steps of:
estimating the channel and pilot tone interference based on all the pilot tones extracted from the signal and a channel model;
equalizing the set of data tones extracted from the signal based on the channel estimate;
detecting data interference according to the pilot interference and the equalized data tones;
determining subcarrier interference-to-noise ratios based on the data interference;
determining signal strengths of the data tones based on the equalized data tones; and
determining log-likelihood ratios of bits represented by the data tones based on the subcarrier interference-to-noise ratios and the signal strength of the data tones.
2. The method of claim 1 , wherein the extracting uses a fast frequency transform.
3. The method of claim 1 wherein the tones are embedded in different frequency subcarriers.
4. The method of claim 1 , wherein the estimating uses a previous channel estimate.
5. The method of claim 1 , further comprising:
revising the channel estimate based on the data interference.
6. The method of claim 1 , wherein the log-likelihood ratios are weighted and summed.
7. The method of claim 1 , wherein the estimating further comprises:
projecting the pilot tones to a nearest point in a pilot subspace.
8. The method of claim 7 , wherein the projecting uses a least square error projection.
9. The method of claim 1 , wherein the estimating results in a plurality of channel estimates.
10. The method of claim 1 , wherein the receiver maintains probability of each of the channel estimates.
11. The method of claim 1 , wherein the estimating further comprises:
using the log-likelihood ratios in an error correction procedure.