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Repeated-and-Offset QPSK for DFT-s-OFDM in Satellite Access

Renaud-Alexandre Pitaval

TL;DR

This work introduces Repeated-and-Offset QPSK (RO-QPSK) for DFT-s-OFDM uplinks in satellite-access scenarios, aiming to maintain low PAPR while staying compatible with 3GPP frameworks. It provides a complete modulation/demodulation design, spectrum normalization, and exact SINR expressions for RO-QPSK, and compares performance against $ rac{ ext{pi}}{2}$-BPSK with FDSS and standard QPSK under NTN/TN channel models. The analysis shows RO-QPSK achieves a Hann-window-like spectrum, with PAPR about $2~\text{dB}$, and offers SINR gains in narrowband and moderately frequency-selective channels, withFDSS providing further PAPR reductions at small BER/SINR costs. End-to-end BER and capacity results, across LOS and NLOS satellite channels, indicate RO-QPSK can outperform baseline low-PAPR schemes while reducing envelope fluctuations, suggesting strong practical potential for NTN access in 6G+.

Abstract

Motivated by the convergence of terrestrial cellular networks with satellite networks, we consider an adaptation of offset quadrature phase shift keying (OQPSK), used with single-carrier waveform in traditional satellite systems, to discrete Fourier transform spread (DFT-s-) orthogonal frequency-division multiplexed (OFDM) waveform employed in the uplink of terrestrial systems. We introduce a new order-one constellation modulation, termed repeated-and-offset QPSK (RO-QPSK), derive its basic properties, and compare it with pi/2-BPSK with frequency-domain spectral shaping (FDSS), as supported in 5G. RO-QPSK naturally produces a Hann-window-shaped spectrum, resulting in a very low maximum peak-to-average power ratio (PAPR) on the order of 2 dB. Moreover, with single-tap equalization and symbol combining at the receiver, RO-QSPK can improve the signal-to-interference-plus-noise (SINR) compared to pi/2-BPSK with FDSS, in narrowband and/or moderately frequency-selective channels, as encountered in satellite communications. A moderate FDSS can also be combined with RO-QSPK to further reduce the PAPR while providing similar performance. Of independent interest, general SINR expressions for DFT-s-OFDM are also provided.

Repeated-and-Offset QPSK for DFT-s-OFDM in Satellite Access

TL;DR

This work introduces Repeated-and-Offset QPSK (RO-QPSK) for DFT-s-OFDM uplinks in satellite-access scenarios, aiming to maintain low PAPR while staying compatible with 3GPP frameworks. It provides a complete modulation/demodulation design, spectrum normalization, and exact SINR expressions for RO-QPSK, and compares performance against -BPSK with FDSS and standard QPSK under NTN/TN channel models. The analysis shows RO-QPSK achieves a Hann-window-like spectrum, with PAPR about , and offers SINR gains in narrowband and moderately frequency-selective channels, withFDSS providing further PAPR reductions at small BER/SINR costs. End-to-end BER and capacity results, across LOS and NLOS satellite channels, indicate RO-QPSK can outperform baseline low-PAPR schemes while reducing envelope fluctuations, suggesting strong practical potential for NTN access in 6G+.

Abstract

Motivated by the convergence of terrestrial cellular networks with satellite networks, we consider an adaptation of offset quadrature phase shift keying (OQPSK), used with single-carrier waveform in traditional satellite systems, to discrete Fourier transform spread (DFT-s-) orthogonal frequency-division multiplexed (OFDM) waveform employed in the uplink of terrestrial systems. We introduce a new order-one constellation modulation, termed repeated-and-offset QPSK (RO-QPSK), derive its basic properties, and compare it with pi/2-BPSK with frequency-domain spectral shaping (FDSS), as supported in 5G. RO-QPSK naturally produces a Hann-window-shaped spectrum, resulting in a very low maximum peak-to-average power ratio (PAPR) on the order of 2 dB. Moreover, with single-tap equalization and symbol combining at the receiver, RO-QSPK can improve the signal-to-interference-plus-noise (SINR) compared to pi/2-BPSK with FDSS, in narrowband and/or moderately frequency-selective channels, as encountered in satellite communications. A moderate FDSS can also be combined with RO-QSPK to further reduce the PAPR while providing similar performance. Of independent interest, general SINR expressions for DFT-s-OFDM are also provided.

Paper Structure

This paper contains 56 sections, 7 theorems, 95 equations, 13 figures, 1 table.

Table of Contents

  1. Introduction
  2. System Model, Background and Preliminaries
  3. DFT-s-OFDM Transmission
  4. DFT precoding
  5. OFDM with FDSS
  6. Legacy QPSK-based Constellations
  7. Gray-mapped QPSK
  8. BPSK
  9. $\pi/2$-BPSK
  10. DFT-s-OFDM Demodulation
  11. Channel
  12. OFDM Demodulation
  13. One-tap Equalization
  14. DFT De-spreading
  15. Effective SINR With i.i.d. Complex Symbols
  16. ...and 41 more sections

Key Result

Proposition 1

The effective SINR of eq:r[m] with i.i.d. complex symbols is where the noise power is and the interference power is

Figures (13)

  • Figure 1: Illustration of the differential encoding aspect of RO-QPSK.
  • Figure 2: Average subcarrier power for RO-QPSK and i.i.d. QAM, with and without FDSS.
  • Figure 3: RO-QPSK modulation and demodulation process.
  • Figure 4: PAPR comparison.
  • Figure 5: Average OOB emission for the considered constellations.
  • ...and 8 more figures

Theorems & Definitions (8)

  • Proposition 1
  • Corollary 1: Nisar2007
  • Proposition 2
  • Remark 1
  • Lemma 1
  • Lemma 2
  • Lemma 3
  • Proposition 3