Qihao Peng

Efficient sparse codebook design is a fundamental research problem in sparse code multiple access (SCMA) systems. This paper proposes an advanced nonlinear SCMA (NL-SCMA) framework for downlink channels, which subsumes conventional (linear) SCMA as a special case. Specifically, the proposed NL-SCMA enables a direct mapping of user messages to a superimposed codeword through a nonlinear mapping mechanism, eliminating the need of per-user based codebooks. The design problem therefore shifts from the conventional user codebook design to the nonlinear mapping optimization and superimposed constellation design. We first propose a Lattice constellation-based superimposed constellation by leveraging its advantages in terms of large minimum Euclidean distance (MED), compact constellation volume, design flexibility, low peak-to-average power ratio (PAPR) and favorable shape gain. By analyzing the error patterns of the lattice-based superimposed constellation using pairwise error probability, we prove that the MED of the proposed nonlinear codebook is lower bounded by the so-called "single error pattern". Motivated by this, we propose an error pattern-inspired nonlinear mapping strategy to maximize the MED. Simulation results demonstrate that the proposed nonlinear codebooks significantly outperform state-of-the-art linear codebooks in terms of PAPR, MED, and uncoded and coded error rate performance over Rician fading channels.

Ground-satellite links for 6G networks face critical challenges, including severe path loss, tight size-weight-power limits, and congested spectrum, all of which significantly hinder the performance of traditional radio frequency (RF) front ends. This article introduces the Rydberg Atomic Quantum Receiver (RAQR) for onboard satellite systems, a millimetre-scale front end that converts radio fields to optical signals through atomic electromagnetically induced transparency. RAQR's high sensitivity and high frequency selectivity have the potential to address link-budget, payload, and interference challenges while fitting within space constraints. Theoretically, a hybrid atomic–electronic design that is supported by a consistent signal model achieves spectral efficiency exceeding 6 bit/s/Hz, extends coverage by up to 1000 km, and improves sensing accuracy by two orders of magnitude relative to conventional RF receivers. The paper concludes with integration strategies, distributed-satellite concepts, and open research challenges for bringing RAQR-enabled satellite payloads into service.