# Low-Frequency Communication Based on Rydberg-Atom Receiver
## 4 Results and Discussion
When the SNR falls below the threshold of 4 dB, demodulation of all three signal types becomes unsuccessful. The modulation quality and signal integrity were evaluated using the IQ constellation diagram, EVM, and eye diagram. These metrics are essential for assessing the experimental results. As shown in Figure 7a-c, as the symbol rate increases, the signal point density on the BPSK constellation diagram becomes more uneven, with more points deviating from the ideal position, indicating an increase in the SNR and BER of the received signal. The eye diagram opening gradually increases with the symbol rate, as shown in Figure 7d-f. The EVM is measured at 8.8% for a symbol rate of 2 kbps, 9.4% for 4 kbps, and 13.7% for 8 kbps. Figure 5: Demodulated signal waveforms of BPSK, OOK, and 2 FSK at emission voltages of 8 V, 4 V, and 1 V. (**a**) BPSK; (**b**) OOK; (**c**) 2 FSK. As shown in Figure 8, the EVM of the received BPSK signal is compared as a function of the symbol rate. At high symbol rates (16 to 20 kbps), the EVM decreases rapidly as the symbol rate decreases. When the symbol rate is reduced below 16 kbps, the rate of EVM decrease slows, eventually stabilizing around 8.8%. This trend indicates that as the symbol rate increases, the communication quality of low-frequency communication systems using BPSK deteriorates. One of the main characteristics of low-frequency communication is its low data rate, which is influenced by the communication bandwidth and environmental factors. Consequently, it is typically unsuitable for audio and video transmission. However, in a controlled laboratory setting, we successfully received both black-and-white and color images modulated using the BPSK technique by employing Rydberg atoms, achieving a data rate of 10 Kbps. The color image transmission process, as depicted in Figure 9, consists of six primary components: decomposing the original image into its constituent R, G, and B color spaces; mapping each pixel's intensity value to an 8-bit sequence; encoding these  
Figure 6: Comparison of the SNR of the three signals as a function of the emission voltage. The SNR of the 2 PSK signal received by the Rydberg atom is higher than that of OOK and BPSK. When the SNR falls below the threshold of 4 dB, demodulation of all three signal types becomes unsuccessful. Figure 7: IQ constellation diagrams and eye diagrams of the Rydberg atom reception at different symbol rates using BPSK modulation. (**a**) The error vector magnitude (EVM) is measured at 8.8% for a symbol rate of 2 kbps; (**b**) the EVM is measured at 9.4% for a symbol rate of 4 kbps; (**c**) the EVM is measured at 13.7% for a symbol rate of 8 kbps; (**d**) eye diagrams for a symbol rate of 2 kbps; (**e**) eye diagrams for a symbol rate of 4 kbps; (**f**) eye diagrams for a symbol rate of 8 kbps. sequences into baseband signals represented by high and low voltage levels; modulating these baseband signals onto a carrier using BPSK; demodulating the received signals to retrieve the original 8-bit sequences; and reconstructing the pixel data to recover the final color image. The image dimensions are 120 pixels in width and 80 pixels in height. The reception results are shown in Figure 10. From the reception results, both black-and-white and  
Figure 8: Variation in the EVM with symbol rate for the BPSK signal received by Rydberg atoms. As the symbol rate of the BPSK signal increases, the EVM also increases, indicating a degradation of communication quality. Figure 9: Process of color image transmission using Rydberg atoms.