Polar transmitter with zero crossing avoidance

A polar transmitter comprising:

• a symbol mapper circuit configured to receive an input bit stream and generate a stream of in-phase (I) and quadrature-phase (Q) (“IQ”) symbol pairs each representing a portion of the bit stream, to obtain an IQ waveform;
• a zero crossing avoidance signal generator circuit configured to generate a zero crossing avoidance signal that is applied during a window encompassing a π phase shift transition, and having a characteristic defined by a primary peak at a zero crossing time coincident with a zero crossing of the IQ waveform, a first valley at a time between a start time of the window and the zero crossing time, and a second valley at a time between the zero crossing time and an end time of the window;
• a zero crossing region avoidance circuit configured to generate a modified IQ signal from the IQ waveform by applying the zero crossing avoidance signal to avoid a zero crossing region;
• an IQ to polar conversion circuit configured to convert the modified IQ signal into a polar signal;
• and
• a polar modulator circuit configured to modulate a radio frequency (RF) carrier using the polar signal.

A non-transitory computer-readable recording medium storing instructions that, when executed by at least one processor, implement a method for modulating a bit stream, the method comprising:

• generating, based on a stream of in-phase (I) and quadrature-phase (Q) (“IQ”) symbol pairs each representing a portion of the bit stream, a IQ waveform in I-Q space;
• generating, using the at least one processor, a modified IQ waveform from the IQ waveform to avoid a zero crossing region in the I-Q space by intermittently adding thereto a zero crossing avoidance signal with a frequency spectrum comprising at least first and second tones defining first and second peaks on opposite sides of a center-frequency valley;
• generating a digital polar signal comprising a digital polar amplitude and phase, based on the modified IQ waveform;
• and
• outputting the digital polar signal to a digital to analog converter for subsequent modulation of a radio frequency (RF) carrier using the polar signal.