Digital predistortion transmitter
There is a need for effectively compensating distortion when a predistortion transmitter is subject to not only a memory effect due to nonlinearity of an amplifier, but also a modulator's DC offset, IQ unbalance, or local quadrature error. A predistortor to be used is a polynomial predistortor including a polynomial basis generation portion and an inner product calculation portion. The polynomial basis generation portion delays a real part and an imaginary part of a complex input signal Sx=Ix+jQx for up to M samples to generate 2(M+1) signals, duplicately combines these signals to generate monomials having maximum degree N, and outputs, as a basis vector, all or part of the monomials depending or needs. The inner product calculation portion performs an inner product calculation using a coefficient vector, i.e., a set of complex numbers sized equally to the basis vector to find a polynomial value and outputs the value as a complex signal.
1. A polynomial predistortor comprising a polynomial basis generation portion for generating a basis vector Xv based on a complex input signal Sx=Ix+jQx and an inner product calculation portion for outputting an inner product Cv·Xv as a polynomial value multiplied by a coefficient vector Cv,
wherein the polynomial predistortor delays a real part and an imaginary part of a complex input signal Sx=Ix+jQx for up to M samples to generate 2(M+1) signals, duplicately combines these signals to generate monomials having maximum degree N, outputs, as the basis vector Xv, all or part of the monomials depending or needs, and uses, as the coefficient vector Cv, a set of complex numbers sized equally to the basis vector Xv; and
wherein the polynomial predistortor expands Volterra series suitable for description of a nonlinear time-invariant system so that an input signal becomes a complex number Sx=Ix+jQx and an output signal becomes a complex number Sy=IY+jQy.
2. The polynomial predistortor according to claim 1 ,
wherein a monomial having the maximum order N is a polynomial having the maximum order N.
3. The polynomial predistortor according to claim 1 ,
wherein output Sy is provided with a feedthrough term for adding input Sx to yield Sy=Sx+Cv·Xv.
4. A digital predistortion transmitter comprising:
a polynomial predistortor comprising a polynomial basis generation portion for generating a basis vector Xv based on a complex input signal Sx=Ix+jQx and an inner product calculation portion for outputting an inner product Cv·Xv as a polynomial value multiplied by a coefficient vector Cv, wherein the polynomial predistortor delays a real part and an imaginary part of a complex input signal Sx=Ix+jQx for up to M samples to generate 2(M+1) signals, duplicately combines these signals to generate monomials having maximum degree N, outputs, as the basis vector Xv, all or part of the monomials depending or needs, and uses, as the coefficient vector Cv, a set of complex numbers sized equally to the basis vector Xv; and wherein the polynomial predistortor expands Volterra series suitable for description of a nonlinear time-invariant system so that an input signal becomes a complex number Sx=Ix+jQx and an output signal becomes a complex number Sy=Iy+jQy, wherein a monomial having the maximum order N is a polynomial having the maximum order N and wherein output Sy is provided with a feedthrough term for adding input Sx to yield Sy=Sx+Cv·Xv;
a quadrature modulation DA converter including a DA converter and a quadrature modulator;
a power amplifier for amplifying a quadrature modulator output;
a quadrature demodulation AD converter for quadrature-demodulating part of an output signal to output a complex demodulation signal Sz=Iz+jQz; and
an adaptation algorithm for sequentially updating elements of the coefficient vector Cv so as to minimize an electric power for residual signal Se=Sz−Sx=Ie+jQe between complex signals Sx and Sz.
5. The polynomial predistortor according to claim 2 , wherein output Sy is provided with a feedthrough term for adding input Sx to yield Sy=Sx+Cv·Xv.