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Constellation Shaping under Phase Noise Impairment for Sub-THz Communications

Dileepa Marasinghe, Le Hang Nguyen, Jafar Mohammadi, Yejian Chen, Thorsten Wild, Nandana Rajatheva

Abstract

The large untapped spectrum in the sub-THz allows for ultra-high throughput communication to realize many seemingly impossible applications in 6G. One of the challenges in radio communications in sub-THz is the hardware impairments. Specifically, phase noise is one key hardware impairment, which is accentuated as we increase the frequency and bandwidth. Furthermore, the moderate output power of the sub-THz power amplifier demands limits on peak to average power ratio (PAPR) signal design. Single carrier frequency domain equalization (SC-FDE) has been identified as a suitable candidate for sub-THz, although some challenges such as phase noise and PAPR still remain to be tackled. In this work, we design a phase noise robust, modest PAPR SC waveform by geometrically shaping the constellation under practical conditions. We formulate the waveform optimization problem in its augmented Lagrangian form and use a back-propagation-inspired technique to obtain a constellation design that is numerically robust to phase noise, while maintaining a relatively low PAPR compared to the conventional waveforms.

Constellation Shaping under Phase Noise Impairment for Sub-THz Communications

Abstract

The large untapped spectrum in the sub-THz allows for ultra-high throughput communication to realize many seemingly impossible applications in 6G. One of the challenges in radio communications in sub-THz is the hardware impairments. Specifically, phase noise is one key hardware impairment, which is accentuated as we increase the frequency and bandwidth. Furthermore, the moderate output power of the sub-THz power amplifier demands limits on peak to average power ratio (PAPR) signal design. Single carrier frequency domain equalization (SC-FDE) has been identified as a suitable candidate for sub-THz, although some challenges such as phase noise and PAPR still remain to be tackled. In this work, we design a phase noise robust, modest PAPR SC waveform by geometrically shaping the constellation under practical conditions. We formulate the waveform optimization problem in its augmented Lagrangian form and use a back-propagation-inspired technique to obtain a constellation design that is numerically robust to phase noise, while maintaining a relatively low PAPR compared to the conventional waveforms.
Paper Structure (13 sections, 15 equations, 5 figures, 1 table, 1 algorithm)

This paper contains 13 sections, 15 equations, 5 figures, 1 table, 1 algorithm.

Table of Contents

  1. Introduction
  2. System model
  3. Phase noise model
  4. Phase noise compensation
  5. Problem formulation
  6. Trainable constellation
  7. Constraints
  8. Optimization Methodology
  9. System training
  10. Evaluation and Results
  11. Evaluation setup
  12. Results
  13. Conclusion

Figures (5)

  • Figure 1: PSD of the TI LMX2595 oscillator upscaled from 20GHz to 120GHz and 220GHz.
  • Figure 2: End-to-end system model for the SC-FDE transceiver
  • Figure 3: PN impacted received signal before compensation and the signal after Wiener PN compensation, which shows the residual PN. ($E_b/N_0 = 25 \:dB$)
  • Figure 4: Learned constellations under different PAPR targets and without a PAPR target
  • Figure 5: BLER performance and CCDF of normalized power of the learned schemes