## 3 Implementation Method of Proposed Macrodiversity System for Maritime Wireless Sensor Networks
### Cascaded Combining Scheme  
Diversity combining is the core of diversity technology, which determines the combining gain and complexity of diversity system. The traditional diversity combining scheme mainly includes selection combining (SC) [36], equal gain combining (EGC) [37] and MRC [38]. The MRC needs to estimate channel state information (CSI) of each diversity branch strictly and calculate weights based on current CSI. The SC only selects one diversity branch with the largest signal-to-noise ratio for the output of combining. When the number of antennas is large, the combining gain of SC is low. The EGC adds the signals of all the diversity branches with the same coefficient without the need to estimate the CSI of each diversity branch, which can be implemented based on hard decision information and is suitable for wireless sensor networks with lower computational complexity. The proposed macrodiversity reception scheme provides a cascaded combing scheme to distribute the computing complexity of the convergence center to each local convergence module, which can effectively reduce the demand for computing resources of sink node. The scheme consists of two diversity combinings. The first-level diversity combining is completed on the local convergence module, and its output is sent to the convergence center based on the user data protocol (UDP) for the second-level diversity combining. The specific process of the cascaded combining scheme is shown in Figure 4.  
The receiving and combining of multi-channel packages are finished in the first-level diversity combining. To guarantee the robustness of combining, we need to set two buffers size of \(M\times N\), where \(M\), \(N\) represent the number of antennas in the local convergence module and packages in a coded frame respectively. The first buffer is used to cache the packages received by the RF module, and the second buffer is used to cache the packages with the same coded frame ID for grouping and combining. The specific process of the first-level diversity combining includes receiving stage of packages, and combination stage of hard-decision results. Among them, the receiving stage of packages completes receiving and buffering of packages. The grouping stage of packages achieves grouping and transforming of packages. Finally, in the combining stage of hard-decision results, the processor completes EGC scheme of packages and assembles combined packages into a frame. The specific process of each stage is as follows. 1. Receiving packages. In this stage, the processor receives the hard decision results output by the four RF modules and stores them in the first buffer cyclically. When the first buffer is saturated, the processor identifies coded frame ID of each package in the first buffer and determines the current coded frame ID using majority-voting decision algorithm, firstly. Then, the processor  
Figure 4: Proposed cascaded combining scheme. stores the package with the current coded frame ID into the second buffer for combining. Finally, these packages with current coded frame ID in the first buffer are cleared. 2. Grouping packages. In this stage, the processor divides the buffered packages into four groups according to the transmission package ID in the second buffer and stores the packages into the corresponding group. The packages within each group are then converted into binary sequences. 3. Combination of hard-decision results. In this stage, processor adopts the EGC strategy to complete the combination of the information sequences in each group. First, the processor maps the information sequence in the group into a bipolar sequence using the mapping rule {0\(\rightarrow-\), 1\(\rightarrow\)1}. Second, the corresponding positions of the mapped sequences in the group are summed to generate a combined information sequence. Thirdly, the information sequence generated by each group is reassembled into an information frame in the order of the transmission package ID.