Free-Space Optical Integrated Sensing and Communication Based on DCO-OFDM: Performance Metrics and Resource Allocation

As one of the six usage scenarios of the sixth-generation (6G) mobile communication system, integrated sensing and communication (ISAC) is regarded as a key enabler for the future Internet of Everything (IoE). While numerous studies have been conducted in radio-frequency (RF)-ISAC, free-space optical (FSO)-ISAC is also laying the foundation for the era of connection and intelligence. In this article, a direct-current (DC)-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) scheme is proposed for FSO-ISAC. To derive the performance metrics for communication and sensing, we model the clipping noise of DCO-OFDM as additive colored Gaussian noise and establish an equivalent frequency-selective channel for FSO-ISAC. In addition, joint power allocation problems are formulated for both communication-centric and sensing-centric scenarios based on the derived performance metrics. Subsequently, these nonconvex joint optimization problems are decomposed into subproblems for DC bias and subcarriers, which can be solved by block coordinate descent algorithms. Furthermore, numerical simulations demonstrate the effectiveness of proposed methods and reveal the tradeoff between communication and sensing functionalities of FSO-ISAC based on DCO-OFDM.