Wideband, oversampled I/Q modulation architecture of the JHU/APL Frontier Software Defined Radio

The modulation architecture in the Frontier Software Defined Radio platform supports a wide variety of modulation formats with a single low-power hardware/firmware circuit: PSK, QAM, subcarrier BPSK and ranging. The wideband hardware supports downlink data rates from 1 sps to 150 Msps, yet the firmware circuitry can generate tightly controlled narrow-band modulation. This flexibility is achieved with oversampled waveform generation, including sine-wave subcarriers and interpolated turnaround ranging. For phase modulation, a composite modulation signal is converted to parallel I/Q signals that modulate the RF carrier. Due to imperfections in the analog I/Q modulator, harmonic upcon-verter and DACs, calibration of the digital phase-to-I/Q conversion is necessary to accurately generate the desired RF signal. The calibration procedure optimizes a discrete PSK constellation, and then interpolates the constellation for applications that require continuous phase modulation, such as sine-wave subcarrier or turnaround ranging. For S-band and Ka-band exciters, the modulation is applied at the RF frequency, and the I/Q conversion subtends 360°. In contrast, for X-band exciters, the modulation is applied at 1/4 th the RF frequency, and the I/Q conversion subtends only 90°. In summary, this architecture can generate wide-band high-rate downlink when sufficient link margin and bandwidth are available, and can also generate tightly controlled narrow-band signals when only low-rate links are available or limited bandwidth can be allocated.