## 2 Principles  
Figure 2 is the architecture of HARQ-aided adaptive diversity combining for hybrid MMW and FSO links system based on the maximum ratio combining (MRC) algorithm. The data to be transmitted is first CRC and FEC encoded and then modulated to achieve a higher transmission rate. The modulated signal sequence that is denoted by \(S_{FSO}\) is then n times downsampled at different positions to obtain signal sequences \(S_{RF,1}\), \(S_{RF,2}\), to \(S_{RF,M}\), where \(M\) stands for the number of retransmission rounds. The FSO and RF signals are then modulated, respectively, on the FSO laser source and RF source to send corresponding signals. At the receiving side, the signals go through an analog-to-digital convert (ADC) process and carry out traditional communication digital signal processing to obtain two signal sequences, \(S^{\prime}_{FSO}\) and \(S^{\prime}_{RF,M}\), corresponding to FSO link and RF link, respectively. The processed RF sequence \(S^{\prime}_{RF,M}\) and its corresponding position of the FSO sequence \(S^{\prime}_{FSO}\) are then processed by the MRC algorithm. Due to the n times downsampling, \(S^{\prime}_{FSO}\) corresponds to \(S^{\prime}_{RF,M}\) with n symbols at every interval, and the corresponding sampling sequence is denoted as \(S_{FSO-down}\). The signal sequence after MRC is denoted as \(S_{MRC}\). The rules of MRC are as follows [11]: Assume that the normalized combining gain coefficients of RF and FSO links are set as \(\alpha\) (alpha) and \(1-\alpha\), respectively, and then the combined sequence can be expressed as  
\[S_{MRC}=\alpha S^{\prime}_{RF,M}+(1-\alpha)S_{FSO-down} \tag{1}\]  
According to MRC, by selecting the optimal combining gain coefficient of each channel, the signal-to-noise ratio (SNR) of the combined output signal should be the sum of the SNRs of each sub-link. The normalized SNR of each signal sequence is denoted as \(\gamma_{RF}\) and \(\gamma_{FSO}\). Hence, we chose \(\alpha\), satisfying [8]  
\[\alpha=\sqrt{\gamma_{RF}}/(\sqrt{\gamma_{RF}}+\sqrt{\gamma_{FSO}}) \tag{2}\]  
We used the combined sequence \(S_{MRC}\) to replace the corresponding positions in \(S^{\prime}_{FSO}\); therefore, the downsampled part of the sequence \(S^{\prime}_{FSO}\) will obtain the gain provided by signals transmitted through the RF link and enhance the overall SNR of the received signal. After MRC, \(S^{\prime}_{FSO}\) is demodulated and then decoded by FEC to obtain stronger encoding gain. The power gain of MRC and the coding gain of FEC are used to improve the SNR of the FSO link and reduce the outage threshold of the FSO link, thus improving transmission reliability and ensuring high-speed transmission. After the FEC decoding, if the CRC check fails, the hybrid automatic repeat request (HARQ) technique intervenes, and the HARQ controller implements RF or RF/FSO hybrid retransmission. Hybrid automatic repeat request (HARQ) transmission is an error control method that combines forward error correction and automatic request retransmission [16]. HARQ can not only detect error data in the transmission process, but also has certain error correction capability, and thus it can greatly improve system transmission efficiency and reliability. The principle of HARQ technology is shown in Figure 3. We employed low-density parity-check (LDPC) coding in the HARQ protocol [17]. After the sender performs CRC and FEC  
Figure 2: Architecture of HARQ-aided adaptive diversity combining for hybrid MMW and FSO link system based on MRC. encoding, the receiver performs error correction and decoding after receiving the data and then verifies the corrected data. If the data verification of CRC check passes, the decoding is judged to be successful and it sends an ACK signal back to the sender. The sender will continue to send the next set of data. Otherwise, if the data verification fails, the decoding fails.