Distributed receiving system with local digitization and combination for SNR enhancement

We demonstrate an X-band distributed receiving system with 4 remote ends for signal-to-noise ratio (SNR) enhancement. The X-band analog signal received by 4 remote ends is first transmitted to the local end through optical fiber links and is then down-converted with a photonic method for digitization and further coherent combination. Finally, a combined signal with a higher SNR can be obtained. In the proposed system, a frequency-tunable single-tone signal is stably transmitted to the remote end for both down-converting the received signal and for generating a dithered sample clock to eliminate the transmission delay jitter with an unlimited compensation range. Experimentally, X-band binary phase shift keying signals are used for system performance evaluation. After 20 to 25 km transmission, the relative timing drifts between different links are at the order of picoseconds, and a near-theoretical SNR enhancement is achieved. The proposed scheme has a simple remote structure with no need for time synchronization, increasing its signal combining precision, flexibility, and scalability, making it an ideal candidate for long-distance weak signal detection.