Signal modulation for radiofrequency communications
A signal modulation device comprising: an input for receiving a complex input signal ( 106 ) comprising an in-phase component signal and a quadrature-phase component signal, a sigma-delta modulator ( 110 ) for modulating the complex input signal at an oversampling clock rate (F1) into an intermediary signal ( 112 ), a numerical oscillator ( 60 ) for generating a phase signal ( 61 ) oscillating at a selected carrier frequency (FC), wherein the phase signal takes a finite number of quantized states, and a symbol mapping table ( 114 ) comprising a predefined quantized symbol for each quantized complex state of the intermediary signal and each quantized state of the phase signal, and operates at each oversampling clock period (F1) to select a quantized symbol ( 116 ) as a function of a current quantized complex state of the intermediary signal ( 112 ) and a current quantized state of the phase signal ( 61 ).
1. A signal modulation device for providing an output signal to a radio frequency front-end, the device comprising:
an input for receiving a complex input signal comprising an in-phase component signal and a quadrature-phase component signal,
a sigma-delta modulator for modulating the complex input signal at an oversampling clock rate into an intermediary signal, wherein the intermediary signal includes a first finite number of quantized complex states,
a symbol mapping table for mapping the quantized complex states of the intermediary signal to predefined quantized symbols each encoding a pulse train segment of the output signal,
a numerical oscillator for generating a phase signal oscillating at a selected carrier frequency, wherein the phase signal includes a second finite number of quantized states,
wherein the symbol mapping table comprises said predefined quantized symbols, at least one predefined quantized symbol for each quantized complex state of the intermediary signal and each quantized state of the phase signal, and operates at each of a plurality of oversampling clock periods to select a predefined quantized symbol as a function of a current quantized complex state of the intermediary signal and a current quantized state of the phase signal, and
a pulse generator arranged to receive the predefined quantized symbol selected at each of the plurality of oversampling clock periods, wherein the pulse generator is adapted to generate the pulse train segment encoded by the received predefined quantized symbol, wherein the generated pulse train segment encodes an information content of the complex input signal in the positions and/or widths and amplitudes of the pulses of the pulse train segment, wherein the pulse generator operates at a second oversampling clock rate.
2. The device in accordance with claim 1 , further comprising:
a digital predistortion table for mapping the quantized complex states of the intermediary signal to predefined predistorsion symbols, and
a feedback loop for providing the predefined predistorsion symbols to the sigma-delta modulator.
3. The device in accordance with claim 2 , wherein the digital predistortion table comprises a predefined predistortion symbol for each quantized complex state of the intermediary signal and each quantized state of the phase signal, and operates at each of the plurality of oversampling clock periods to select a predistortion symbol as a function of the current quantized complex state of the intermediary signal and the current quantized state of the phase signal.
4. The device in accordance with claim 2 , wherein the digital predistortion table comprises a predefined predistortion symbol for each quantized complex state of the intermediary signal and each quantized state of the phase signal and wherein the each predefined predistortion symbol associated to a corresponding complex quantized state of the intermediary signal and a corresponding quantized state of the phase signal is a function of a frequency-domain complex energy coefficient of the pulse train segment encoded by the corresponding predefined quantized symbol at the selected carrier frequency and a complex phase coefficient corresponding to the quantized state of the complex phase signal.
5. The device in accordance with claim 1 , wherein the quantized symbol comprises a parallel binary sequence and wherein the pulse generator comprises a serializer adapted to serialize the parallel binary sequence at the second oversampling clock rate.
6. The device in accordance with claim 1 , wherein the phase signal is coded over a predefined number of bits.
7. The device in accordance with claim 6 , wherein the numerical oscillator comprises a phase accumulator operating as a function of a control signal defining the selected carrier frequency coded over the predefined number of bits and a clock signal at the first oversampling clock rate, the device further comprising a multi-stage low pass noise shaping sigma delta filter arranged to provide the control signal coded over the predefined number of bits by filtering a refined control signal coded over a higher number of bits.
8. The device in accordance with claim 1 , further comprising a rate-adjustable clock arranged to provide a clock signal at the first oversampling clock rate, wherein the first oversampling clock rate is adjustable over a small adjustment range.
9. The device in accordance with claim 1 , wherein the symbol mapping table is obtained as follows:
selecting an initial set of pulse train segments having a time-granularity corresponding to the second oversampling clock rate,
determining a frequency-domain complex energy coefficient of the pulse train segments of the initial set at the carrier frequency, and
for each quantized state of the phase signal,
computing a complex phase coefficient corresponding to the quantized state of the complex phase signal, and
selecting a subset of the pulse train segments of which the frequency-domain complex energy coefficients at the carrier frequency closely approximate the quantized complex states of the intermediary signal multiplied by the complex phase coefficient, so that each quantized complex state of the intermediary signal is uniquely mapped to a pulse train segment of the subset,
for each pulse train segment to which a quantized complex state is mapped, recording a quantized symbol encoding the pulse train segment at a memory address associated to a combination of the quantized complex state of the intermediary signal combined with the quantized state of the phase signal.
10. A signal modulation method for providing an output signal to a radio frequency front-end, the method comprising:
receiving a complex input signal comprising an in-phase component signal and a quadrature-phase component signal,
sigma-delta modulating the complex input signal at an oversampling clock rate into an intermediary signal, wherein the intermediary signal includes a first finite number of quantized complex states,
generating a phase signal oscillating at a selected carrier frequency, wherein the phase signal includes a second finite number of quantized states,
accessing a symbol mapping table at each oversampling clock period to select a quantized symbol encoding a pulse train segment of the output signal as a function of a current quantized complex state of the intermediary signal and a current quantized state of the phase signal,
wherein the symbol mapping table comprises a predefined quantized symbol for each quantized complex state of the intermediary signal and each quantized state of the phase signal,
generating the pulse train segment encoded by the selected quantized symbol with a pulse generator operating at a second oversampling clock rate, wherein the generated pulse train segment encodes an information content of the complex input signal in the positions and/or widths and amplitudes of the pulses of the pulse train segment.