METHOD AND APPARATUS FOR IMPLEMENTING HIGH-ORDER MODULATION SCHEMES USING LOW-ORDER MODULATORS

1 . A processing device comprising:

a plurality of modulators, the plurality of modulators each performing modulation according to a same first modulation scheme;

a combiner configured to combine outputs from the plurality of modulators and to produce a modulated output based on the combined outputs of the plurality of modulators; and

a signal processor configured to receive a bit stream, convert the bit stream into a plurality of input signals for the plurality of modulators, and to provide the plurality of input signals to the plurality of modulators in such a manner that the combiner generates the modulated output according to a second modulation scheme.

2 . The processing device of claim 1 , wherein:

the first modulation scheme is a phase shift keying (QPSK) scheme and the second scheme is a rotated QPSK scheme, and

the plurality of modulators includes at least first and second modulators.

3 . The processing device of claim 2 , wherein

the signal processor is configured to provide a first input signal from among the plurality of input signals to a Q branch of the first modulator and to provide a fixed signal to an I branch of the first modulator such that the first modulator generates a first output,

the signal processor is configured to provide a second input signal from among the plurality of input signals to an I branch of the second modulator and to provide a fixed signal to a Q branch of the second modulator such that the second modulator generates a second output, and

the combiner is configured to generate the modulated output by combining the first and second outputs.

4 . The processing device of claim 2 , wherein the signal processor is configured to provide first and second input signals to the first and second modulators and to control the first and second modulators to generate the first and second outputs having first and second amplitudes, respectively, the first and second amplitudes being selected to provide a desired amount of rotation for a rotated constellation (X,Y).

5 . The processing device of claim 4 , wherein the rotated constellation (X,Y) is defined as

(

X

Y

)

=

(

cos





Θ

sin





Θ

-

sin





Θ

cos





Θ

)



(

A

B

)

,

where A is the first amplitude, B is the second amplitude, Θ=π/4−α, and α=arctan(A/B).

6 . The processing device of claim 2 wherein signal processor is configured such that the fixed signal provided to the first modulator prevents the I branch of the first modulator from causing variation in the output of the first modulator, and the fixed signal provided to the second modulator prevents the Q branch of the second modulator from causing variation in the output of the second modulator.

7 . The processing device of claim 1 , wherein

the first modulation scheme is a quadrature phase shift keying (QPSK) scheme and the second scheme is a pulse amplitude modulation (PAM) scheme, and

the plurality of modulators includes at least first and second modulators.

8 . The processing device of claim 7 , wherein

the signal processor is configured to provide a first input signal from among the plurality of input signals to an I branch of the first modulator and to provide a fixed signal to a Q branch of the first modulator such that the first modulator generates a first output,

the signal processor is configured to provide a second input signal from among the plurality of input signals to an I branch of the second modulator and to provide a fixed signal to a Q branch of the second modulator such that the second modulator generates a second output, and

the combiner is configured to generate the modulated output by combining the first and second outputs.

9 . The processing device of claim 7 wherein the first and second modulators are configured such that an amplitude of the first output is twice an amplitude of the second output.

10 . The processing device of claim 7 wherein the signal processor is configured such that the fixed signal provided to the first modulator prevents the Q branch of the first modulator from causing variation in the output of the first modulator, and the fixed signal provided to the second modulator prevents the Q branch of the first modulator from causing variation in the output of the second modulator.

11 . The processing device of claim 1 , wherein

the first modulation scheme is a quadrature phase shift keying (QPSK) scheme and the second scheme is a pulse amplitude modulation (QAM) scheme, and

the plurality of modulators includes at least first and second modulators.

12 . The processing device of claim 11 , wherein

the signal processor is configured to provide a first input signal from among the plurality of input signals to an I branch of the first modulator and to provide a second input signal from among the plurality of input signals to a Q branch of the first modulator such that the first modulator generates a first output,

the signal processor is configured to provide a third input signal from among the plurality of input signals to an I branch of the second modulator and to provide a fourth input signal from among the plurality of input signals to a Q branch of the second modulator such that the second modulator generates a second output, and

the combiner is configured to generate the modulated output by combining the first and second outputs.

13 . The processing device of claim 11 wherein the first and second modulators are configured such that an amplitude of the first output is twice an amplitude of the second output.

14 . The processing device of claim 11 wherein

the plurality of modulators includes a third modulator,

the signal processor is configured to provide a fifth input signal from among the plurality of input signals to an I branch of the third modulator and to provide a sixth input signal from among the plurality of input signals to a Q branch of the third modulator such that the third modulator generates a third output, and

the combiner is configured to generate the modulated output by combining the first, second and third outputs.

15 . The processing device of claim 14 wherein the first, second and third modulators are configured such that an amplitude of the first output is twice an amplitude of the second output, and an amplitude of the second output is twice an amplitude of the third output.

16 . A method of modulating a bit stream, the method comprising:

converting the bit stream into a plurality of input signals;

providing the plurality of input signals to a plurality of modulators, each of the plurality of modulators performing modulation according to a same first modulation scheme,

generating outputs from the plurality of modulators, and

combining the outputs from the plurality of modulators to generate a modulated signal, the plurality of input signals being provided to the plurality of modulators in such a manner that the combining of the outputs generates the modulated signal according to a second modulation scheme.

17 . The method of claim 16 , wherein

the first modulation scheme is a quadrature phase shift keying (QPSK) scheme and the second modulation scheme is a rotated QPSK scheme, and

the plurality of modulators includes at least first and second modulators.

18 . The method of claim 17 , wherein

the generating step includes

generating a first output from the first modulator by providing a first input signal from among the plurality of input signals to a Q branch of the first modulator and providing a fixed signal to an I branch of the first modulator, and

generating a second output from the second modulator by providing a second input signal from among the plurality of input signals to an I branch of the second modulator and providing a fixed signal to a Q branch of the second modulator.

19 . The method claim 17 , wherein the first and second outputs have first and second amplitudes, respectively, the first and second amplitudes being selected to provide a desired amount of rotation for a rotated constellation (X,Y).

20 . The method of claim 19 , wherein the rotated constellation (X,Y) is defined as

(

X

Y

)

=

(

cos





Θ

sin





Θ

-

sin





Θ

cos





Θ

)



(

A

B

)

,

where A is the first amplitude, B is the second amplitude, Θ=π/4−α, and α=arctan(A/B).

21 . The method of claim 17 , wherein the fixed signal provided to the first modulator prevents the I branch of the first modulator from causing variation in the output of the first modulator, and the fixed signal provided to the second modulator prevents the Q branch of the second modulator from causing variation in the output of the second modulator.

22 . The method of claim 16 , wherein

the first modulation scheme is a quadrature phase shift keying (QPSK) scheme and the second modulation scheme is a pulse amplitude modulated (PAM) scheme,

the plurality of modulators includes at least first and second modulators.

23 . The method of claim 22 , wherein

the generating step includes

generating a first output from the first modulator by providing a first input signal from among the plurality of input signals to an I branch of the first modulator and providing a fixed signal to a Q branch of the first modulator, and

generating a second output from the second modulator by providing a second input signal from among the plurality of input signals to an I branch of a second modulator and providing a fixed signal to a Q branch of the second modulator.

24 . The method of claim 22 wherein an amplitude of the first output is twice an amplitude of the second output.

25 . The method of claim 22 wherein the fixed signal provided to the first modulator prevents the Q branch of the first modulator from causing variation in the output of the first modulator, and the fixed signal provided to the second modulator prevents the Q branch of the second modulator from causing variation in the output of the second modulator.

26 . The method of claim 16 , wherein

the first modulation scheme is a quadrature phase shift keying (QPSK) scheme and the second modulation scheme is a quadrature amplitude modulated (QAM) scheme, and

the plurality of modulators includes at least first and second modulators.

27 . The method of claim 26 , wherein

the generating step includes

generating a first output from the first modulator by providing a first input signal from among the plurality of input signals to a Q branch of a first modulator and providing a second input signal from among the plurality of input signals to an I branch of the first modulator, and

generating a second output from the second modulator by providing a third input signal from among the plurality of input signals to an I branch of the second modulator and providing a fourth input signal from among the plurality of input signals to a Q branch of the second modulator.

28 . The method claim 26 , wherein an amplitude of the first output is twice an amplitude of the second output.

29 . The method of claim 26 , wherein

the plurality of modulators includes a third modulator, and

the generating step further includes

generating a third output from the third modulator by providing a fifth input signal from among the plurality of input signals to a Q branch of a third modulator and providing a sixth input signal from among the plurality of input signals to an I branch of the third modulator.

30 . The processing device of claim 29 , wherein the first, second and third modulators are configured such that an amplitude of the first output is twice an amplitude of the second output, and an amplitude of the second output is twice an amplitude of the third output.

31 . The method of claim 26 further comprising:

determining a desired spacing for a constellation corresponding to the modulated signal;

determining amplitudes of the outputs of the first and second modulators based on the desired spacing.