High SNR at a Cost: The Information-Theoretic Dilemma of Magnitude-Only Atomic Receivers

Atomic wireless receivers (AWRs) have emerged as promising components for next-generation wireless systems due to their inherent immunity to thermal noise, which can enable substantial signal-to-noise ratio (SNR) gains. However, higher SNR for an AWR does not necessarily mean higher capacity. Specifically, under the electrometer configurations, AWRs can only detect the magnitude of the radio-frequency electric field, losing phase information and reducing channel capacity. This paper explores whether the benefits of thermal noise immunity can offset the capacity loss from magnitude-only detection. To be specific, we derive the channel capacity for AWRs in both reference (transmitted by a local oscillator)-free and reference-aided scenarios. According to the numerical results, even with halved mutual information, AWRs can still outperform traditional RF receivers in capacity, due to their extremely high SNR brought by immunity to thermal noise. Moreover, we demonstrate that reference signals enable a spectral efficiency improvement of up to 0.688 bps/Hz for an AWR, providing an information theoretic interpretation for the benefits of using reference signals.