Repeated-and-Offset QPSK for Low-PAPR DFT-s-OFDM in Satellite Communications

Abstract

Motivated by the convergence of terrestrial cellular networks and satellite communications, this article considers an adaptation of offset quadrature phase shift keying (OQPSK), traditionally used with single-carrier waveforms in satellite systems, to discrete Fourier transform spread orthogonal frequency-division multiplexed (DFT-s-OFDM), as employed in the uplink of terrestrial systems. First, analytical signal-to-interference-plus-noise (SINR) expressions are derived for DFT-s-OFDM with frequency-domain spectral shaping (FDSS) carrying independently distributed pi/2-BPSK or QAM symbols and received with single-tap equalization, as in 5G. Next, a correlation-induced spectral shaping technique, termed repeated-and-offset QPSK (RO-QPSK), is introduced, relying solely on bit-level processing prior to conventional QPSK modulation. Specifically, the input bits are Manchester encoded (repeated and flipped) with an offset between the in-phase and quadrature branches, resulting in order-one OQPSK-like modulation. The induced correlation between consecutive QPSK symbols produces a Hann-shaped transmit spectrum yielding a peak-to-average power ratio (PAPR) on the order of 2 dB without explicit FDSS. At the receiver, the repetition structure is exploited through post-DFT-despreading symbol combining, and the corresponding end-to-end SINR with this transmitter-receiver pair is derived in closed form. Theoretical analysis and simulation results show that RO-QPSK provides performance gains in narrowband and moderately frequency-selective channels, as encountered in satellite communications, while remaining on par with conventional 5G uplink schemes in other scenarios. The combination of RO-QPSK with FDSS is also investigated, enabling further PAPR reduction while maintaining comparable link-level performance.