Seamless rate adaptation for wide SNR range in FSO systems based on rate compatible modulation.

We propose and experimentally demonstrate a large-range and seamless rate-adaptive free-space optical (FSO) scheme based on rate compatible modulation (RCM). It utilizes a mapping method through weighted summation, enabling an even increase of bit energy. This allows the system to seamlessly adjust the throughput. In addition, the handover mechanism based on rate check and acknowledgment signals ensures a wide coverage of signal-to-noise ratio (SNR) fluctuation. It is worth noting that this scheme does not require any changes to the mapping matrix and decoding algorithm, making it easier to implement in real-world systems. Simulation proves the superiority of the proposed approach in terms of coding rate coverage by applying a fixed mapping matrix compared to a traditional adaptive modulation and coding scheme. Experimental demonstration over a 50 m FSO link verifies that the SNR dynamic range of this scheme is >15 dB with a seamless rate adjustment between 6.7 Gb/s and 53.6 Gb/s and the operability of RCM is not limited by the time scale of turbulence, revealing huge potential for the massive connections in future smart cities.

Precise pointing angle deviation measurement for beaconless laser communication.

How to measure the pointing angle precisely without the beacon light is crucial for beaconless laser communication. The conventional intensity method directly measures the intensity of a part of the communication signal beam, which has low sensitivity. We propose the characteristic signal method by superimposing a low-frequency sinusoidal signal on the communication signal to promote the measuring sensitivity. Simultaneously, a fast cyclic cross-correlation algorithm is used to reduce operational complexity. Compared with the experimental results of the direct intensity method, the proposed method can improve the measuring sensitivity about 9.17 dB and increase the power budget for communication about 1.96 dB.