## 4 Experiments and Results
### The Verification of Communication Distance  
To verify the communication performance at different distances for the alignment of the transceiver coils, we set the underwater MI-MIMO communication performance in both horizontal and vertical deployment modes. The transmitter sent the address code and data code, and the receiver received and demodulated the address code and data code. The receiving coil was placed in different ways at the same distance. The number of turns of the coil was 20, and its diameter was 0.25 m. We tested the results of control signal reception and data signal reception. The relevant experimental results are shown in Table 3.  
The analysis of the underwater experimental data showed that when the two coils were at 3.5 m, and the receiving coil was parallel to the transmitting coil, the receiving data could be demodulated. At the same time, the vertical condition resulted in data code errors. The address and data codes failed to decode at 6.5 m between the transmit and receive coils. This shows that distance influences magnetic induction MIMO communication. The perpendicular orientation between the transceiver coils led to high data errors. Therefore, developing high-performance omnidirectional MIMO antennas will be necessary to ensure a strict magnetic field. Experiments showed that if the transmitting and receiving antennas could not be aligned, the coupling would be lost, and the sender and receiver would not be able to maintain proper communication. We compared the end-to-end node communication distances for different frequencies, antenna radii, and maximum coil turns. The results are shown in Figure 6a. The lower the frequency, the greater the relative transmission distance. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Channel type & Rayleigh & Noise type & Gauss noise \\ Coding method & V-BLAST & Modulation method & QPSK,16-QAM \\ Carrier frequency & 125 kHz & Resonant frequency & 10-30 kHz \\ Number of frames & 1000 & Population & 100 \\ Transmitting power & 3.3–5.0 w & Receiving sensitivity & 500 \(\mathrm{\SIUnitSymbolMicro V}\) \\ Depth of nodes in water & 1 m & Transmission distance & 0.5–6.5 m \\ Coil radius & 0.25 m & Coil diameter & 0.59 mm \\ Coil turns & 30 & Coil resistance & 1.4456 \(\mathrm{\SIUnitSymbolOhm}\) \\ Number of symmetrical & & Number of asymmetric & \\ antennas & & antennas & 2–4,–4,6–8 \\ Tuned capacitance & 390 nF & Inductance & 534.518 \(\mathrm{\SIUnitSymbolMicro m}\) \\ Propagation speed & 3.0 \(\times\) 10\({}^{8}\) m/s & Node mobility & 1.5 m/s \\ Magnetic permeability & 4\(\pi\times\) 10\({}^{-7}\) H/m & The relative speed of nodes & 0 m/s \\ Type of water body & Seawater & Conductivity & 25,000 \(\mathrm{\SIUnitSymbolMicro s}\)/cm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The main \(\mathrm{e}\times\)perimental parameters. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Distance (m)** & **Alignment Mode** & \
\begin{tabular}{c} **Control Signal** \\ **Reception** \\ \end{tabular} &
\begin{tabular}{c} **Data signal** \\ **Reception** \\ \end{tabular} \\ \hline
3.5 &
\begin{tabular}{c} Horizontal \\ Vertical \\ \end{tabular} & \(\surd\) & \(\surd\) \\
4.5 &
\begin{tabular}{c} Horizontal \\ Vertical \\ \end{tabular} & \(\times\) & \(\times\) \\
5.5 &
\begin{tabular}{c} Horizontal \\ Vertical \\ \end{tabular} & \(\times\) & \(\times\) \\
6.5 &
\begin{tabular}{c} Horizontal \\ Vertical \\ \end{tabular} & \(\times\) & \(\times\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Decoding results in different deployment modes and distances. As the coil radius and the number of turns increased, the communication distance tended to increase for all three magnetic induction communication frequencies. It should be noted that if the coil radius is too large and the number of turns too high, this can dramatically increase node deployment and development costs, which is unacceptable for underwater networks with many node deployments.