Doppler-Resilient Rydberg Atomic Receiver for High-Dynamic Communication Networks via Adaptive Local Oscillator Tracking

Abstract: Rydberg atomic receiver has emerged as promising candidate for next-generation wireless communication, due to the exceptional sensitivity and ability to overcome the physical limitations of traditional radio frequency antennas. Utilizing the resonant response of atomic energy levels for signal detection, Rydberg atomic receiver is inherently confined to a narrow instantaneous bandwidth. However, in high-mobility scenarios such as satellite communications, the severe Doppler effect induces carrier frequency offsets, which drive the signal beyond the instantaneous bandwidth and result in severe distortion. In this paper, we propose an adaptive local oscillator (LO) tracking Rydberg atomic receiver architecture designed to lock high-dynamic signals within the effective atomic response bandwidth. By employing a cross-product automatic frequency control (CPAFC) algorithm, the system dynamically estimates the instantaneous frequency offset, generates a corresponding error control signal, and adjusts the LO frequency through a feedback loop. Consequently, the intermediate frequency signal can always be locked close to the center of the atomic response bandwidth regardless of dynamics. Simulation results show that the proposed architecture significantly outperforms existing Rydberg atomic receiver, effectively alleviating performance degradation in high-dynamic environments.

• The paper proposes a CPAFC-driven adaptive LO tracking loop that physically compensates Doppler shifts and keeps the intermediate frequency centered within the narrow atomic response band.
• Simulations at 309.18 GHz with a Doppler rate of 816 kHz/s show that adaptive tracking holds the IF near 1 MHz with an error within ±50 Hz, while a fixed LO drifts beyond the atomic bandwidth in under 2 seconds.
• The tracked receiver preserves tightly clustered QPSK constellations and improves EVM and symbol-error performance compared with fixed-LO operation, although experimental validation and robustness to nonlinear Doppler remain open questions.

Rydberg atomic receivers offer exceptional sensitivity and direct RF-to-optical transduction, but their usable instantaneous bandwidth is confined to the narrow linewidth of the Rydberg transition—typically a few MHz or less. This constraint is benign for static channels but becomes critical in high-mobility links, where Doppler shifts drive the down-converted intermediate frequency (IF) outside the atomic response band. The paper under review addresses this gap by proposing an adaptive local oscillator (LO) tracking architecture, driven by a cross-product automatic frequency control (CPAFC) algorithm, that keeps the effective IF locked at the center of the atomic response regardless of channel dynamics (2607.08145).

System model and the bandwidth bottleneck

The receiver follows a heterodyne-like structure in which the classical RF front end is replaced by a quantum sensor: a vapor cell of alkali atoms interrogated by counter-propagating probe and coupling lasers. A four-level ladder system ($|g\rangle$, $|e\rangle$, $|r_1\rangle$, $|r_2\rangle$) establishes electromagnetically induced transparency (EIT), while a strong LO field and the weak incident RF field jointly couple $|r_1\rangle \rightarrow |r_2\rangle$. In the regime $A_{\rm LO} \gg A_{\rm RF}$, the atomic system acts as a quantum mixer: the LO induces dominant Autler-Townes splitting that biases the response, and the RF signal amplitude-modulates the probe transmission at the beat frequency $f_{\rm IF} = |f_c - f_{\rm LO}|$. The photodetector output reduces to $$I_{\rm PD}(t) \propto \alpha\kappa\cos(2\pi f_{\rm IF}t + \Delta\phi)$$, where $\kappa$ is the intrinsic atomic response coefficient.

The critical physical requirement is that $f_{\rm IF}$ remain within the flat region of an atomic equivalent channel response $|e\rangle$0, approximated as Lorentzian with instantaneous bandwidth $|e\rangle$1 on the order of MHz or hundreds of kHz. Once the IF drifts beyond $|e\rangle$2 of the nominal value, signal power suffers severe attenuation from the roll-off of $|e\rangle$3.

Distinctive impact of Doppler shifts

The paper's analysis makes explicit why high-dynamic scenarios are qualitatively different for atomic receivers than for conventional ones. With a fixed LO, a time-varying Doppler shift $|e\rangle$4 shifts the received carrier to $|e\rangle$5, so the IF drifts as $|e\rangle$6. Two degradation mechanisms follow: first, accumulated phase error $|e\rangle$7 rotates the demodulated constellation—a problem shared with electronic receivers; second, and distinctively, the drifting IF exits the atomic response band entirely, causing distortion that no amount of digital post-processing can recover.

The severity is quantified using 3GPP non-terrestrial network parameters: LEO relative velocities near 7 km/s and accelerations up to 81.6 m/s² imply Doppler rates of several MHz per second at carrier frequencies in the hundred-GHz range—well beyond any plausible atomic bandwidth. This motivates compensating the offset physically, at the front end, rather than digitally.

Adaptive LO tracking architecture

The proposed architecture embeds a frequency-locked loop around the standard LO-based receiver. Digitized baseband samples are fed to a CPAFC frequency discriminator, whose error estimate passes through a second-order proportional-integral loop filter characterized by natural frequency $|e\rangle$8 and damping factor $|e\rangle$9. The filtered control signal drives two parallel paths: a numerically controlled oscillator performs per-sample digital de-rotation for fine phase correction, while an integrated cumulative correction is converted via DAC to a tuning voltage for the VCO, shifting the physical LO to $|r_1\rangle$0. At lock, $|r_1\rangle$1 and the effective IF satisfies $|r_1\rangle$2.

The CPAFC discriminator itself proceeds in three steps. Modulation wiping raises the normalized baseband sample to the $|r_1\rangle$3-th power, removing unknown data modulation and amplifying the residual offset by $|r_1\rangle$4: $|r_1\rangle$5. Frequency discrimination forms the cross-product $|r_1\rangle$6, which linearizes to $|r_1\rangle$7 for small arguments; dividing by $|r_1\rangle$8 yields the raw error estimate. Loop filtering applies the PI controller recursively to produce a smoothed control signal. This design inherits the well-known robustness of cross-product AFC against data modulation, adapted here to the atomic front-end context.

Simulation results

Simulations use $|r_1\rangle$9Rb with levels $|r_2\rangle$0, a 309.18 GHz carrier, 1 MHz nominal IF, QPSK modulation at 100 kBaud, and a linear Doppler rate $|r_2\rangle$1 kHz/s derived from non-terrestrial network profiles. Atomic parameters (dipole moments, decay rates) are drawn from the ARC library.

Three results stand out:

• IF stabilization: under the fixed-LO scheme, the IF drifts over 2.5 MHz and exits the atomic bandwidth in under 2 seconds; the adaptive scheme locks the IF near 1 MHz indefinitely.
• Tracking precision: the CPAFC discrimination error oscillates within ±50 Hz of zero, indicating high-precision tracking under sustained dynamics.
• Link-level performance: fixed-LO constellations degrade into ring-shaped arcs from combined phase rotation and nonlinear atomic distortion, whereas the tracked receiver maintains tightly clustered QPSK constellations both early (1–2 ms) and late (3–3.001 s) in the transmission. EVM and SER decrease markedly with increasing RF field strength $|r_2\rangle$2 at noise power $|r_2\rangle$3, while the fixed-LO baseline remains flat at poor values.

These results support the central claim that shifting frequency compensation from the digital demodulation stage to the physical front end is both feasible and necessary for atomic receivers in high-dynamic environments.

Limitations and open questions

Several caveats temper the reported gains. All results are simulation-only; no experimental validation on a physical vapor cell is provided, and real systems would face laser noise, atom number fluctuations, and VCO tuning latency not modeled here. The Doppler profile is restricted to a linear ramp $|r_2\rangle$4; jerk-dominated or multi-path-induced dynamics may stress the second-order loop differently, and the choice $|r_2\rangle$5 rad/s is not optimized against a tracking-versus-noise trade-off analysis. The CPAFC discriminator assumes small per-sample phase increments ($|r_2\rangle$6), so acquisition behavior under large initial offsets—loop pull-in range—is not characterized. Finally, the analysis presumes $|r_2\rangle$7 and a single-path narrowband channel; whether the architecture extends to wideband or frequency-selective atomic channels remains open.

Conclusion

This paper identifies the narrow atomic instantaneous bandwidth as the binding constraint on Rydberg atomic receivers in high-mobility links and proposes a closed-loop adaptive LO tracking solution based on CPAFC. By physically nullifying Doppler-induced IF drift, the architecture maintains the signal within the optimal atomic response region, with simulations showing sub-100 Hz tracking error and substantially improved constellation quality, EVM, and SER relative to fixed-LO baselines. The main open questions concern experimental validation, robustness to non-linear Doppler profiles, and acquisition under large initial frequency offsets.