Overlay modulation technique of COFDM signals based on amplitude offsets
Systems and methods are presented for transmitting additional data over preexisting differential COFDM signals by changing the amplitude of the legacy data symbols. In exemplary embodiments of the present invention, additional data capacity can be achieved for a COFDM signal which is completely backwards compatible with existing legacy satellite broadcast communications systems. In exemplary embodiments of the present invention, additional information can be overlaid on a legacy COFDM signal by applying an amplitude offset to the legacy symbols. In exemplary embodiments of the present invention, special receiver processing can be implemented to extract this additional information, which can include performing channel equalization across frequency bins to isolate the amplitude modulated overlay signal. For example, at each FFT symbol time, average power across neighboring active data bins can be used to determine the localized power at the corresponding FFT bins, and a channel inversion can then, for example, be performed on the data bins to restore, as best as possible, the original transmitted symbol amplitude.
1. A method of transmitting a plurality of data streams over a single signal, the method comprising:
applying a differential phase encoding process to a first data stream to generate a base signal including a plurality of active frequency bins;
generating an output signal including modulating an amplitude of the base signal to overlay a second data stream on the first data stream, the output signal being modulated to have an averaged unity power across each of the active frequency bins such that a receiver can perform channel amplitude equalization on the output signal and extract the second data stream; and
transmitting the output signal.
2. A method according to claim 1 , wherein the modulating comprises:
decreasing a nominal amplitude value of the base signal by a first offset for each logical “0” of the second data stream; and
increasing the nominal amplitude value of the base signal by a first offset for each logical “1” of the second data stream.
3. A method according to claim 2 , wherein the first offset and the second offset are substantially equal.
4. A method according to claim 2 , wherein the first offset and the second offset are substantially different.
5. A method according to claim 2 , wherein the base signal is a differential COFDM signal.
6. A method according to claim 2 , wherein the first and second offsets satisfy the equation [(1−C) 2 +(1+D) 2 ]/(2*2)=1; where C represents the first offset and D represents the second offset.
7. A system for transmitting a plurality of data streams over a single signal, the system comprising:
a processor, a memory coupled to the processor, and a program stored in the memory, the program including instructions for:
applying a differential phase encoding process to a first data stream to generate a base signal including a plurality of active frequency bins;
generating an output signal including modulating an amplitude of the base signal to overlay a second data stream on the first data stream, the output signal being modulated to have an averaged unity power across each of the active frequency bins such that a receiver can perform channel amplitude equalization on the output signal and extract the second data stream; and
transmitting the output signal.
8. A system according to claim 7 , wherein the modulating comprises:
decreasing a nominal amplitude value of the base signal by a first offset for each logical “0” of the second data stream; and
increasing the nominal amplitude value of the base signal by a first offset for each logical “1” of the second data stream.
9. A system according to claim 8 , wherein the first offset and the second offset are substantially equal.
10. A system according to claim 8 , wherein the first offset and the second offset are substantially different.
11. A system according to claim 8 , wherein the base signal is a differential COFDM signal.
12. A system according to claim 8 , wherein the first and second offsets satisfy the equation [(1−C) 2 +(1+D) 2 ]/(2*2)=1; where C represents the first offset and D represents the second offset.
13. A method of decoding a plurality of data streams from a single signal, the method comprising:
receiving a transmitted signal that was generated by applying a differential phase encoding process to a first data stream to generate a base signal and modulating the amplitude of the base signal to overlay a second data stream on the first data stream;
applying an inverse differential phase decoding process to the transmitted signal to obtain the first data stream; and
performing channel amplitude equalization on the transmitted signal and extracting the second data stream from the transmitted signal.
14. A method according to claim 13 , wherein the inverse differential phase decoding process is performed by a trellis decoder.
15. A method according to claim 13 , wherein the transmitted signal includes a plurality of frequency bins, and the channel amplitude equalization is performed across the frequency bins.
16. A method according to claim 13 , wherein the modulating comprises:
decreasing a nominal amplitude value of the base signal by a first offset for each logical “0” of the second data stream; and
increasing the nominal amplitude value of the base signal by a first offset for each logical “1” of the second data stream.
17. A method according to claim 16 , wherein the first offset and the second offset are substantially equal.
18. A method according to claim 16 , wherein the first offset and the second offset are substantially different.
19. A method according to claim 16 , wherein the base signal is a differential COFDM signal.
20. A method according to claim 16 , wherein the first and second offsets satisfy the equation [(1−C) 2 +(1+D) 2 ]/(2*2)=1; where C represents the first offset and D represents the second offset.