Wideband multi-channel quadrature amplitude modulation of cable television signals
View Patent ↗Wideband, direct-to-RF multi channel QAM modulation is described whereby IFFT processing is employed to modulate a plurality of encoded symbol streams in parallel onto a like plurality of uniformly spaced carriers. A high-frequency master clock and high-performance, high-frequency D/A converters are employed. The IFFT modulation process yields a multi-channel multiplex at an intermediate “baseband” frequency. The multi-channel multiplex is then synchronously converted to analog form and synchronously frequency shifted upward by one-half the master clock frequency using a quadrature doubly-balanced mixing scheme. The resultant multi-channel encoded multiplex occupies a frequency band centered around one half the master clock frequency. Multi-rate techniques can be employed to yield high-speed digital processing from digital logic incapable of operating directly at the master clock frequency.
1. A wideband multi-channel direct-to-RF QAM modulator for modulating a plurality of digital data streams onto a single multi-RF output, comprising:
a high-frequency master clock source operating at a master clock frequency;
encoding means for encoding each of the digital data streams into a set of QAM encoded-streams;
processing means for simultaneously converting the plurality of QAM-encoded streams into a single digital multi-channel IF stream encoding the multiple QAM encoded streams onto a set of uniformly spaced carrier frequencies in an intermediate frequency band, said processing means including an inverse FFT (IFFT) and a post-IFFT combined filtering means, said combined filtering means performing a combined equivalent of baseband and anti-imaging filtering;
digital-to-analog conversion means for converting the single digital multi-channel IF stream into an analog multi-channel IF stream;
synchronously-clocked quadrature modulation means to frequency-shift the analog multi-channel IF stream upward by one-half of the master clock frequency onto said single multi-RF output; and
digital quadrature correction means between the combined filtering means and the digital-to-analog conversion means for compensating quadrature error in the synchronously clocked quadrature modulation means.
2. The modulator according to claim 1 , wherein:
at least a portion of the encoding means and IFFT processing means are implemented as a set of parallel-execution sections operating at a binary submultiple of the master clock frequency to produce outputs at the submultiple rate.
3. The modulator according to claim 2 , further comprising:
multiplexing means for switching between outputs of the parallel-execution sections at the master clock rate to produce outputs at the master clock rate.
4. The modulator according to claim 1 , wherein:
the digital data streams are QAM-256 encoded.
5. The modulator according to claim 1 , further comprising:
interpolation means for compensating for a difference between a QAM symbol rate and a channel spacing.
6. A multi-channel direct-to-RF QAM modulator for modulating a plurality of digital data streams onto a multi-RF output, comprising:
QAM encoding means for encoding the digital data streams into a like plurality of QAM-encoded symbol streams;
IFFT processing means having each QAM-encoded symbol stream applied to a specific complex frequency input thereof, said IFFT processing means producing a time-domain signal representative of the plurality of QAM-encoded symbol streams modulated onto a set of uniformly spaced carrier frequencies in an intermediate frequency IF band;
post-IFFT filtering means, producing a filtered time domain signal, for performing the combined equivalent of baseband filtering, anti-imaging filtering and rate interpolation to compensate for a difference between a QAM symbol rate and a channel spacing;
digital-to-analog conversion means for converting the filtered time-domain signal from complex digital to complex analog form to produce an analog multi-channel IF stream;
synchronously-clocked quadrature modulation means to frequency-shift the analog multi-channel IF stream upward by one-half of the master clock frequency onto said single multi-RF output; and
digital quadrature correction means between the post-IFFT filtering means and the digital-to-analog conversion means for compensating quadrature error in the synchronously clocked quadrature modulation means.
7. The modulator according to claim 6 , wherein:
at least a portion of the encoding means and IFFT processing means are implemented as a set of parallel-execution sections operating at a binary submultiple of the master clock frequency to produce outputs at the submultiple rate.
8. The modulator according to claim 7 , further comprising:
multiplexing means for switching between outputs of the parallel-execution sections at the master clock rate to produce outputs at the master clock rate.
9. A method for direct-to-RF multi-channel QAM modulation of a plurality of digital data streams onto a multi-RF output, comprising:
providing a high-frequency master clock source at a master clock frequency;
providing a plurality of digital data input streams;
encoding each of the digital data streams into a set of QAM encoded streams;
processing the QAM-encoded streams at the master clock frequency to modulate the plurality of QAM-encoded streams into a single complex digital multi-channel IF stream encoding the multiple QAM encoded streams onto a set of uniformly spaced carrier frequencies in an intermediate frequency band, said processing comprising inverse FFT (IFFT) processing and post-IFFT filtering, said filtering comprising a combined baseband and anti-imaging filtering;
converting the digital multi-channel IF stream to analog form;
synchronously frequency-shifting the analog multi-channel IF stream upward by one-half the master-clock frequency onto said multi-RF output; and providing digital quadrature correction between the post-IFFT filtering and the analog converting for compensating quadrature error in the synchronously frequency-shifted analog multi-channel IF stream.
10. A method according to claim 9 , further comprising: QAM-256 encoding the digital data streams.
11. The method according to claim 9 , further comprising:
interpolating the digital multi-channel IF stream to compensate for a difference between a QAM symbol rate and a channel spacing.
12. A wideband multi-channel direct-to-RF QAM modulator for modulating a plurality of digital data streams onto a single multi-RF output, comprising:
a high-frequency master clock source operating at a master clock frequency;
encoding means for encoding each of the digital data streams into a set of QAM encoded-streams;
processing means for simultaneously converting the plurality of QAM-encoded streams into a single digital multi-channel IF stream encoding the multiple QAM encoded streams onto a set of uniformly spaced carrier frequencies in an intermediate frequency band, said processing means including an inverse FFT (IFFT) and a post-IFFT combined filtering means, said combined filtering means performing a combined equivalent of baseband and anti-imaging filtering;
digital-to-analog conversion means for converting the single digital multi-channel IF stream into an analog multi-channel IF stream;
synchronously-clocked quadrature modulation means to frequency-shift the analog multi-channel IF stream upward by one-half of the master clock frequency onto said single multi-RF output; and
digital quadrature correction means for compensating quadrature error to precompensate for subsequent DC offset in the synchronously clocked quadrature modulation means.
13. The modulator according to claim 12 wherein:
at least a portion of the encoding means and IFFT processing means are implemented as a set of parallel-execution sections operating at a binary submultiple of the master clock frequency to produce outputs at the submultiple rate.
14. The modulator according to claim 13 , further comprising:
multiplexing means for switching between outputs of the parallel-execution sections at the master clock rate to produce outputs at the master clock rate.
15. The modulator according to claim 12 , further comprising:
interpolation means for compensating for a difference between a QAM symbol rate and a channel spacing.
16. A multi-channel direct-to-RF QAM modulator for modulating a plurality of digital data streams onto a multi-RF output, comprising:
QAM encoding means for encoding the digital data streams into a like plurality of QAM- encoded symbol streams;
IFFT processing means having each QAM-encoded symbol stream applied to a specific complex frequency input thereof, said IFFT processing means producing a time-domain signal representative of the plurality of QAM-encoded symbol streams modulated onto a set of uniformly spaced carrier frequencies in an intermediate frequency IF band;
post-IFFT filtering means, producing a filtered time domain signal, for performing the combined equivalent of baseband filtering, anti-imaging filtering and rate interpolation to compensate for a difference between a QAM symbol rate and a channel spacing;
digital-to-analog conversion means for converting the filtered time-domain signal from complex digital to complex analog form to produce an analog multi-channel IF stream;
synchronously-clocked quadrature modulation means to frequency-shift the analog multi-channel IF stream upward by one-half of the master clock frequency onto said single multi-RF output; and
digital quadrature correction means for compensating quadrature error to precompensate for subsequent DC offset in the synchronously clocked quadrature modulation means.
17. The modulator according to claim 16 , wherein:
at least a portion of the encoding means and IFFT processing means are implemented as a set of parallel-execution sections operating at a binary submultiple of the master clock frequency to produce outputs at the submultiple rate.
18. The modulator according to claim 17 , further comprising:
multiplexing means for switching between outputs of the parallel-execution sections at the master clock rate to produce outputs at the master clock rate.
19. A method for direct-to-RF multi-channel QAM modulation of a plurality of digital data streams onto a multi-RF output, comprising:
providing a high-frequency master clock source at a master clock frequency;
providing a plurality of digital data input streams;
encoding each of the digital data streams into a set of QAM encoded streams;
processing the QAM-encoded streams at the master clock frequency to modulate the plurality of QAM-encoded streams into a single complex digital multi-channel IF stream encoding the multiple QAM encoded streams onto a set of uniformly spaced carrier frequencies in an intermediate frequency band, said processing comprising inverse FFT (IFFT) processing and post-IFFT filtering, said filtering comprising a combined baseband and anti-imaging filtering;
converting the digital multi-channel IF stream to analog form;
synchronously frequency-shifting the analog multi-channel IF stream upward by one-half the master-clock frequency onto said multi-RF output; and
providing digital quadrature correction for compensating quadrature error to precompensate for subsequent DC offset during the synchronously frequency-shifting step.
20. The method according to claim 19 , further comprising:
interpolating the digital multi-channel IF stream to compensate for a difference between a QAM symbol rate and a channel spacing.