###### Abstract  
We propose and demonstrate a hybrid communication architecture that combines millimeter-wave (MMW) in the radio frequency (RF) domain and free-space-optics (FSO) technologies using adaptive combining and hybrid automatic repeat request (HARQ) techniques. At the receiving end, we employed joint signal processing with an adaptive diversity combining technique (ADCT) based on a maximum ratio combining (MRC) algorithm. We derived closed-form expressions for the outage probability and throughput of the hybrid RF and FSO (RF/FSO) system, considering various characteristics of atmospheric turbulence in the FSO link. Experimental testing with 10-Gbaud quadrature phase shift keying (QPSK) data was conducted under different simulated atmospheric turbulence intensities, FSO and MMW speed-ratios, and forward error correction (FEC) overheads. Additionally, we validated improvements in terms of bit error ratio (BER), outage probability, and throughput performance. Key words and phrases:free-space-optics (FSO) communications; millimeter-wave (MMW) communications; maximum ratio combining (MRC); hybrid automatic repeat request (HARQ) 2023 10 1  
one link to transmit data, thus wasting the bandwidth of the other link, reducing the transmission efficiency and the throughput of the system in engineering applications. The other is the soft switching depending on channel hybrid coding and modulation with the simultaneous transmission of the two channels and combined using the maximal ratio combining (MRC) algorithm [6]. The main disadvantage lies in that it does not improve the overall performance of the system when both links are unavailable [7]. Hybrid switching is proposed in [8], where the FSO link is the primary information transmission link. When the received SNR of the FSO link falls below a threshold, the RF link is activated to transmit the same data at the same rate as the FSO link. At the receiver, the transmitted data are obtained using the maximum ratio combining (MRC) algorithm. Both the hard and soft switch system require channel state information (CSI), which brings time delay and data rate loss [9], as well as complicated practical devices [10]. The simultaneous transmission system, however, addresses the over-switching issue and uses adaptive diversity combining technique (ADCT) at the receiver [11]. The CSI is no longer needed, resulting in a more simpler hybrid system compared to switchover systems. The hybrid automatic repeat request (HARQ) protocols has also been implemented to improve communication quality in satellite-terrestrial transmissions [12]. Works have been done concerning HARQ-aided power optimization for low-earth orbit (LEO) satellite-based FSO systems [13] and incremental redundancy (IR) HARQ-aided LEO satellite-based FSO systems [14; 15]. We aimed to find a way to combine the advantages of HARQ technology with the hybrid RF/FSO simultaneous transmission system to further enhance the reliability of satellite-to-ground communication. Therefore, in this paper, we present a novel communication system that leverages the hybrid RF and FSO architecture, incorporating adaptive combining and hybrid automatic repeat request (HARQ) techniques. Figure 1 shows the proposed hybrid RF/FSO architecture for communication between the Earth and satellites. We used FSO as the primary link. When the feedback link conveys CSI, the FSO link continues to transmit the next frame of data, while the RF link retransmits data that failed data validation. This approach addresses the latency introduced by retransmissions and leverages the high stability of the RF link while maintaining high data transmission speed and throughput with FSO. The outage probability is also reduced. We provide theoretical analysis and experimental demonstrations to validate the effectiveness of our approach. We derive closed-form expressions for the outage probability and throughput of the RF/FSO system, considering the characteristics of atmospheric turbulence in the FSO link.