## 4 Experiment Setup and Performance Analysis
### Experiment Platform  
The macrodiversity reception scheme is evaluated by four experiments. In our experiments, we use a maritime intelligent observation platform as our testbed. The maritime intelligent observation platform is a communication buoy, which is designed by our researched group. The maritime intelligent observation platform includes hardware and software system of sensor node and buoy structure.  
The hardware of sensor node is mainly composed of an information collection unit, an ARM processor and an information transmission unit. Figure 5a shows the hardware structure of sensor node. The information collection unit is composed of a GPS module and a multi-parameter water quality sensor, collecting the location information of sensor node and water quality parameters. The information transmission unit is composed of a MCU and a RF module. The MCU configures the RF module to send and receive package through the SPI port. The workflow of sensor node mainly consists of three steps. First, the ARM processor sends the request frame to the information collection unit and checks the received package by the CRC value. Second, the ARM processor assembles the received sensor parameters into a frame and performs channel encoding and interleaving. Finally, the coded frame is divided into four packages, which are sent to the information transmission unit via serial port and transmitted in a fixed time interval by the RF module sequentially.  
The maritime communication buoy designed in this paper is composed of a 433 MHz RF antenna, an antenna feeder tube, a fixed flange, a GPS antenna, an equipment bay and a counterweight ring. The actual picture is shown in Figure 5b. The 433 MHz antenna feeder is connected to the RF module in the equipment bay through the antenna feeder tube. The bottom of the antenna is designed as a structure of screw nut, which can be directly fixed to the feeder tube through threads during installation. We add a rubber pad between the antenna and the feeder tube to guarantee the watertightness of the buoy. The feeder tube is fixed on the cover of equipment bay by flange, and the position where the feeder tube is connected to the cover of the equipment bay need to be sealed. The GPS antenna interface is also designed on the cover of the equipment bay, and it needs to be sealed during installation. The bottom of the buoy is equipped with a counterweight ring, which can provide optical the drifting monitoring with a light load or the fixed-point monitoring with the heavy load.  
The structure of the sink node has already been explained in the Section 2. Based on proposed macrodiversity reception structure, we design the printed circuit board (PCB) of local convergence module and convergence center. In the local convergence module, the RF module and the ARM processor are detachable to facilitate the maintenance and update of the equipment. The local convergence module connects with the convergence center via ethernet. The local convergence module and convergence center are shown in Figure 6.  
The RF module of sensor node and sink node is based on wireless RF module CC1101 (Texas Instruments) for transmission, which is configured to send and receive package by the C8051F120 MCU. The parameters of CC1101 is shown in Table 1. The data processing unit of sensor node and sink node is based on tiny6410 microprocessor for signal processing uniformly.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Radio Module & CC1101 \\ Base Frequency & 433 MHz \\ TX Power & 10 dBm \\ Modulation Mode & GPSK \\ Preamble Size & 8 Bytes \\ Sync word Size & 4 Bytes \\ Baud Rate & 10 kbps \\ Bandwidth & 116 kHz \\ \hline \hline \end{tabular}
\end{table}
Table 1: RF module parameters.  
Figure 5: The sensor node in buoy. (**a**) The hardware structure of sensor node. (**b**) The maritime communication buoy.