## 3 Design of Isl Network for IoT
### Channel Coding and Modulation  
Basically, two principles of channel coding are used. If the transmission errors are corrected in the decoder, it is referred to as Forward Error Correction (FEC). This approach is suitable for non-optimal channels; the error rate depends on the channel quality, while the data throughput remains constant. Thus, FEC is a better choice for simplex channels (e.g., for payload data downlinks). Error correction is especially recommended for tight link budgets [17]. The second principle is based on Automatic Repeat Request (ARQ), where acknowledgments and timeouts are used to achieve reliable communication. In contrast to FEC, the reliability is constant and the data throughput can vary, depending on the channel quality. For bi-directional data transfer, ARQ can increase the reliability for very low Signal-to-Noise Ratio (SNR). A hybrid scheme of FEC and ARQ seems to be the best channel coding method for nanosatellite networks, for the following reasons: The communication range can vary from some hundred meters up to several hundred kilometers; a truly isotropic antenna pattern is not possible; and RF power is limited and directional antennas rely on the position information of nodes and attitude control to point towards the target. Thus, even though the channel itself has a low noise level, loss-free data transmission is difficult to realize. Modulation is a trade off between spectral bandwidth efficiency and power efficiency. BPSK requires the least power for a given transmission rate and Bit Error Rate (BER) and is, thus, often the baseline for small satellite TM/TC. QPSK is more bandwidth-efficient; however, it is sensitive to phase distortion on the receiver side. This can be compensated for by DQPSK. Higher order PSK \begin{table}
\begin{tabular}{c c c} \hline \hline
**Band** & **Frequency** & **Allocation Status** \\ \hline UHF & \begin{tabular}{c} 410–420 MHz \\ 430–450 MHz \\ \end{tabular} & \begin{tabular}{c} Space research (space-to-space) \\ Amateur radio \\ \end{tabular} \\ \hline L & \begin{tabular}{c} 1645.5–1656.5 MHz \\ 2200–2290 MHz \\ \end{tabular} & \begin{tabular}{c} Distress and safety communications \\ \end{tabular} \\ \hline S & \begin{tabular}{c} 2025–2110 MHz \\ 2200–2290 MHz \\ \end{tabular} & \begin{tabular}{c} Space operations, Earth exploration, and space research services \\ \end{tabular} \\ \hline Ku & \begin{tabular}{c} 13.4–13.65 GHz \\ 14.5–15.35 GHz \\ \end{tabular} & \begin{tabular}{c} Space research services \\ \end{tabular} \\ \hline Ka & \
\begin{tabular}{c} 22.55–23.55 GHz \\ 25.25–27.5 GHz \\ \end{tabular} &
\begin{tabular}{c} Intersatellite \\ Space research services and Earth exploration \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Frequency bands for space-to-space communication. techniques allow for faster data rates at the cost of higher sensitivity, as shown in Figure 3, where \(E_{b}\) is the amount of energy received per information bit while \(N_{0}\) is the power spectral density of the noise and the BER probability \(P_{b}\). The maximum data throughput of an ARQ protocol with respect to the channel robustness (BER) can be simplified when assuming constant signal runtime and protocol overhead, as represented by  
\[r_{max}=\frac{n\cdot(1-P_{b})^{n}}{n+\alpha}, \tag{1}\]  
where \(n\) is the packet size in bits and \(\alpha\) is the symbol overhead, determined by the signal runtime and ACK time. The following Equation (2) provides the optimal packet size for given \(P_{b}\), in order to maximize the data rate \(r\). A respective simulation of \(n\) for QPSK and different distances is given in Figure 4. The theoretical data rates at BER = 0 for selected modulation TDD distances are shown in Table 6. The data transfer defined by the caller is asymmetrical, as described in Section 3.5. \[\frac{ln(1-P_{b})}{(1-P_{b})^{n}}=\frac{\alpha}{n^{2}}. \tag{2}\]  
Figure 4: Optimum packet size for QPSK with symbol rate \(f_{s}=80\,k\), convolutional coding with coding rate \(r=3/4\), \(k=9\), and BER \(P_{b}=10^{-4}\).