SDR based CCSDS Shaped Offset QPSK Modulator for Aerospace Applications

Shaped offset QPSK (Quadrature Phase Shift Keying)-SOQPSK is a modulation scheme characterized by its constant envelope, continuous phase, excellent spectral containment, and high detection efficiency. It finds extensive usage in airborne applications such as satellite launch vehicles, missiles, and satellite data repeaters. The advantages of this scheme include enhanced spectral efficiency, reduced power requirements and compact size for realization. Digital modulation schemes like QPSK and OQPSK, which are non-constant envelope modulations require techniques like power amplifier backoff and digital pre-distortion (DPD) to mitigate distortion and interference. By employing precoding and pulse shaping techniques, shaped OQPSK (SOQPSK) achieves a constant envelope waveform, making it suitable for efficient non-linear power amplifiers. This provides the designer with flexibility in adjusting the trade-off between power efficiency and spectral efficiency using a few parameters. This research paper focuses on the implementation of a SOQPSK transmitter intellectual property (IP) using a MATLAB-Simulink model-based approach. The in-phase and quadrature signals are transmitted through ADALM-Pluto SDR platform, a direct intermediate frequency (IF) software-defined radio (SDR). The development of the SOQPSK IP involves fixed-point Simulink modeling of the SOQPSK quadrature modulator, resource optimization for the Microsemi SmartFusion II SoC FPGA, HDL code generation, and FPGA implementation. The paper presents a comprehensive analysis of the performance of Consultative Committee for Space Data Systems(CCSDS) compliant SOQPSK-B in S-band airborne telemetry data links operating at a bit rate of 1 Mbps. This includes evaluating the bit error rate (BER) and occupied bandwidth(OBW).