Method to create hierarchical modulation in OFDM
The present invention involves a method for transmitting data using an orthogonal frequency division multiplex (OFDM) transmission system, such as over a terrestrial network used in satellite digital audio radio. A OFDM signal is generated based on first input data. A variation is introduced in the OFDM signal based on second input data to generate a modified OFDM signal which is transmitted. The modified OFDM signal is decoded by performing a demodulation of the modified OFDM signal then a first and second detection to obtain the first input data and the second input data. The introduction of a variance may include introducing an additional phase offset across adjacent OFDM sub-carriers, across adjacent OFDM symbols, an additional phase offset in the OFDM signal, an additional amplitude offset in the OFDM signal, an additional amplitude augmentation in the OFDM signal, and/or a second variation in the first OFDM signal based on third input data.
1. A method for transmitting data using an orthogonal frequency division multiplex (OFDM) transmission system comprising the steps of:
generating OFDM sub-carriers using differential modulation based on first input data;
hierarchically modulating the OFDM sub-carriers based on second input data to generate a hierarchically modulated differential OFDM signal;
transmitting the hierarchically modulated differential OFDM signal;
hierarchically demodulating and decoding the hierarchically modulated differential OFDM signal by performing a demodulation of the hierarchically modulated differential OFDM signal then a first and second detection to obtain the first input data and the second input data.
2. The method of claim 1 wherein the transmitting step includes transmitting the hierarchically modulated differential OFDM signal with a terrestrial repeater in a satellite based system.
3. The method of claim 1 wherein the hierarchically modulating step includes introducing an additional phase offset based on the second input data across adjacent OFDM sub-carriers.
4. The method of claim 1 wherein the hierarchically modulating step includes introducing an additional phase offset based on the second input data across adjacent OFDM symbols.
5. The method of claim 1 wherein the hierarchically modulating step includes introducing an additional amplitude offset based on the second data across one of (1) adjacent OFDM sub-carriers and (2) adjacent OFDM symbols.
6. The method of claim 1 wherein the decoding step includes using an iterative decoder to obtain the second input data.
7. The method of claim 6 wherein the decoding step further includes the iterative decoder using an a posterior algorithm.
8. The method of claim 1 wherein the decoding step includes using a legacy decoder to obtain the first input data.
9. A receiver for receiving primary and secondary data from a hierarchically modulated differential orthogonal frequency division multiplex (OFDM) signal comprising:
an antenna for receiving RF signals;
a demodulator coupled to said antenna for hierarchically demodulating received RF signals comprising hierarchically modulated differential OFDM signals into a downconverted differential OFDM signal;
a first detector coupled to said demodulator, said first detector having a first output, and said first detector being adapted to detect primary data on the downconverted differential OFDM signal; and
a second detector coupled to said demodulator, said second detector having a second output, and said second detector being adapted to detect secondary data as variations on the downconverted differential OFDM signal.
10. The receiver of claim 9 wherein said antenna is adapted to receive terrestrially retransmitted satellite broadcast signals.
11. The receiver of claim 9 wherein said second detector is adapted to detect an additional phase offset across adjacent OFDM sub-carriers and derive therefrom the secondary data.
12. The receiver of claim 9 wherein said second detector is adapted to detect an additional phase offset across adjacent OFDM symbols and derive therefrom the secondary data.
13. The receiver of claim 9 wherein said second detector is adapted to detect an additional amplitude offset across one of adjacent OFDM sub-carriers and adjacent OFDM symbols as the variation in the first OFDM signal and derive therefrom the secondary data.
14. The receiver of claim 9 wherein said second detector includes an iterative decoder.
15. The receiver of claim 14 wherein said iterative decoder uses an a posterior algorithm with an interleaver to detect the primary and secondary data.
16. The method of claim 1 wherein the hierarchical modulation step includes a second differential modulation based on the second input data.
17. The method of claim 1 wherein the hierarchical modulation step includes a coherent modulation based on the second input data.