Communication system with QAM modulation

1. A transmitter for generating a composite modulated signal from first and second independent information-containing signals for transmission over a communications link to a remote receiver where the first and second information-containing signals are to be reconstructed, the transmitter comprising:

(a) a first information-containing signal source characterized by a first data rate and encoding onto a first signal constellation symbols of the first information-containing signal according to a first modulation scheme, the first modulation scheme mapping each one of the symbols of the first signal onto a corresponding one of a plurality of constellation points of the first signal constellation, each one of the constellation points of the first signal constellation representing a corresponding one of a first plurality of symbols employed in the first signal;

(b) a second information-containing signal source characterized by a second data rate, lower than the first data rate, and encoding onto a second signal constellation symbols of the second information-containing signal according to a second modulation scheme, the second modulation scheme mapping each one of the symbols of the second signal onto a corresponding one of a plurality of constellation points of the second signal constellation, each one of the constellation points of the second signal constellation representing a corresponding one of a second plurality of symbols employed in the second signal; and

(c) a processor operatively coupled to the first and second signal sources and arranged to encode onto a composite signal constellation the symbols of the first and second signals according to a composite modulation scheme,

wherein the processor is arranged so that:

(d) the composite signal constellation includes all constellation points of the first signal constellation except for one omitted constellation point, the omitted constellation point corresponding to a designated carrier symbol among the first plurality of symbols;

(e) the composite signal constellation includes, at a location of the omitted constellation point, a scaled replica of the second signal constellation;

(f) the composite modulation scheme (i) maps each one of the symbols of the first signal, except for the designated carrier symbol, onto a corresponding one of the constellation points of the first signal constellation included in the composite signal constellation, (ii) maps the designated carrier symbol of the first signal onto one of the constellation points of the replica of the second signal constellation included in the composite signal constellation, and (iii) maps each one of the symbols of the second signal onto a corresponding one of the constellation points of the replica of the second signal constellation included in the composite signal constellation;

(g) each one of the constellation points of the first signal constellation included in the composite signal constellation represents a corresponding one of the first plurality of symbols, except for the designated carrier symbol; and

(h) each symbol of the second signal is mapped onto the corresponding constellation point of the replica of the second signal constellation only when the designated carrier symbol of the first signal is encoded onto the composite signal constellation, so that each one of the constellation points of the replica of the second signal constellation represents both (i) a corresponding one of the second plurality of symbols and (ii) the designated carrier symbol of the first plurality of symbols.

2. The transmitter of claim 1 wherein (i) each one of the constellation points of the first signal constellation included in the composite signal constellation represents a corresponding one of the first plurality of symbols, except for the designated carrier symbol, in accordance with the first modulation scheme, and (ii) each one of the constellation points of the replica of the second signal constellation included in the composite signal constellation represents one of the symbols of the second signal in accordance with the second modulation scheme.

3. The transmitter of claim 2 wherein the first signal constellation is a Quadrature Amplitude Modulation (QAM) constellation.

4. The transmitter of claim 2 wherein the second signal constellation is a Phase Shift Keying (PSK) modulation or a Quadrature Amplitude Modulation (QAM) modulation.

5. The transmitter of claim 2 wherein a metric distance within the composite signal constellation between any one of the constellation points of the replica of the second signal constellation and the location of the omitted constellation point is equal to or greater in magnitude than a noise magnitude at the remote receiver of the signal associated with the omitted constellation point of the first signal constellation.

6. The transmitter of claim 5 wherein the metric distance is a Hamming or Euclidean distance.

7. The transmitter of claim 2 wherein the first signal source is arranged so as to apply to the processor a first information-containing radio frequency signal representative of the first signal for modulating a carrier wave with the first information-containing radio frequency signal input to form the constellation points of the first signal constellation, and a second information-containing radio frequency signal representative of the second signal for modulating the carrier wave associated with the first signal constellation with the second information-containing signal so as to form constellation points of the replica of the second signal constellation at the location of the omitted constellation point.

8. The transmitter of claim 7 wherein the first signal constellation is a Quadrature Amplitude Modulation (QAM) constellation, and the second signal constellation is either a Phase Shift Keying (PSK) modulation or a Quadrature Amplitude Modulation (QAM) modulation.

9. The transmitter of claim 2 further comprising a digital signal processor coupled to the first and said second signal sources to produce the first and second signal constellations in a radio frequency signal for transmission over a wired, radio frequency wireless, or optical fiber communications link.

10. The transmitter of claim 9 , further comprising a receiver coupled to the communications link, the receiver including a digital signal processor for demodulating the radio frequency signal.

11. The transmitter of claim 1 wherein the communications link is a wired, radio frequency wireless, or optical fiber link.

12. The transmitter of claim 1 wherein the communications link is a wired or wireless radio frequency communications link.

13. A method for data communication by generating a composite modulated signal from first and second independent information-containing signals for transmission over a communications link to a remote receiver, the method comprising:

(a) encoding onto a first signal constellation symbols of the first information-containing signal according to a first modulation scheme, the first modulation scheme mapping each one of the symbols of the first signal onto a corresponding one of a plurality of constellation points of the first signal constellation, each one of the constellation points of the first signal constellation representing a corresponding one of a first plurality of symbols employed in the first signal, the first signal being characterized by a first data rate;

(b) encoding onto a second signal constellation symbols of the second information-containing signal according to a second modulation scheme, the second modulation scheme mapping each one of the symbols of the second signal onto a corresponding one of a plurality of constellation points of the second signal constellation, each one of the constellation points of the second signal constellation representing a corresponding one of a second plurality of symbols employed in the second signal, the second signal being characterized by a second data rate that is lower than the first data rate; and

(c) using a processor operatively coupled to respective sources of the first and second signal, encoding onto a composite signal constellation the symbols of the first and second signals according to a composite modulation scheme,

wherein:

(d) the composite signal constellation includes all constellation points of the first signal constellation except for one omitted constellation point, the omitted constellation point corresponding to a designated carrier symbol among the first plurality of symbols;

(e) the composite signal constellation includes, at a location of the omitted constellation point, a scaled replica of the second signal constellation;

(f) the composite modulation scheme (i) maps each one of the symbols of the first signal, except for the designated carrier symbol, onto a corresponding one of the constellation points of the first signal constellation included in the composite signal constellation, (ii) maps the designated carrier symbol of the first signal onto one of the constellation points of the replica of the second signal constellation included in the composite signal constellation, and (iii) maps each one of the symbols of the second signal onto a corresponding one of the constellation points of the replica of the second signal constellation included in the composite signal constellation;

(g) each one of the constellation points of the first signal constellation included in the composite signal constellation represents a corresponding one of the first plurality of symbols, except for the designated carrier symbol; and

(h) each symbol of the second signal is mapped onto the corresponding constellation point of the replica of the second signal constellation only when the designated carrier symbol of the first signal is encoded onto the composite signal constellation, so that each one of the constellation points of the replica of the second signal constellation represents both (i) a corresponding one of the second plurality of symbols and (ii) the designated carrier of the first plurality of symbols.

14. The method of claim 13 wherein the first signal constellation is a Quadrature Amplitude Modulation (QAM) constellation.

15. The method of claim 13 wherein the second signal constellation is an Amplitude Shift Keying (ASK) or Phase Shift Keying (PSK) modulation.

16. The method of claim 13 wherein a metric distance within the composite signal constellation between any one of the constellation points of the replica of the second signal constellation and the location of the omitted constellation point is equal to or greater in magnitude than a noise magnitude at the remote receiver of the signal associated with the omitted constellation point of the first signal constellation.

17. The method of claim 13 wherein the encoding of parts (a) and (b) is performed by a digital signal processor.

18. The method of claim 13 further comprising transmitting the composite modulated signal over the communications link, wherein the communications link is a wired or wireless radio frequency communications link.