Environment-aware geometric shaping for digital FSO fronthaul networks

As one of the last-mile access network solutions, free-space optical (FSO) communication can satisfy the rapidly growing traffic demand of 6G fronthaul networks. However, outdoor FSO transmission has to confront the influence of atmospheric conditions; fog and turbulence are worth more attention. To resist the impact of fog and turbulence on the distribution of optical signal amplitudes, we propose an environment-aware geometric shaping (GS) of signal amplitudes scheme for FSO fronthaul networks with four-level pulse amplitude modulation (PAM-4). The FSO networks are aware of channel states caused by fog and turbulence through visibility and temperature sensors to avoid the need for feedback links. Based on the environment-aware channel state information, the proposed GS algorithm determines adaptively the optimal electrical signal amplitudes of PAM-4, aiming to minimize the average bit error rate (BER) under the varying channel conditions. The effects of visibility and turbulence on PAM-4 signal amplitudes are theoretically modeled and experimentally evaluated using an environmental simulation chamber. For the first time, to the best of our knowledge, we demonstrate experimentally the effectiveness of the environment-aware GS in combating the effects of visibility and turbulence on FSO transmission performance. Experimental results show that the GS algorithm can reduce the average BER by 1/3 compared to the traditional PAM-4 using uniform amplitude distribution.