## 3 Design of Isl Network for IoT
### Medium Access Control  
The Media Access Control (MAC) layer is the lower sublayer of the data link layer (layer 2) in the Open Systems Interconnection Model (OSI) model. Technical feasibility of hardware (antenna), application requirements, network topology, available bandwidth, and the number of nodes are the main criteria for selecting a suitable channel access method for a satellite network. This section provides a short analysis of common multiple access protocols, such as TDMA, FDMA, CDMA, and SDMA, with respect to their suitability for nanosatellite networking. Additionally, a short analysis on packet switching methods--namely, random access and token passing procedures--is provided. Space Division Multiple Access (SDMA) is not analyzed, due to large and complex antennas (spot beam and phased array antennas) required for a nano-spacecraft and the strict requirements for attitude control. A good overview of the requirements for antennas and corresponding concepts has been given in [7], which is adapted in Table 7. The current technology allows the use of reconfigurable phased antenna arrays even for Cubesats, as proposed in [18]. FDMA and CDMA have the advantage that multiple crosslinks can be established simultaneously. In the case of FDMA, the most problematic issue is that, with an increasing number of nodes, the required bandwidth increases as well, as one center frequency is required for each cross-link. S Band frequencies are extensively used, at present, for TM/TC services and thus may cause regulatory conflicts. Furthermore, the use of FDMA leads to a significant increase of complexity and cost of the communication system, due to the necessity of extensive band-pass filtering. On the other hand, CDMA requires quick and precise RF power control to limit interference, as well as complex signal \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Modulation**} & \multicolumn{3}{c}{**Distance**} \\ \cline{2-4}  & **100 km** & **200 km** & **400 km** \\ \hline DBPSK, \(r=1/2\) & 27.5 & 24.85 & 19.5 \\ DBPSK, \(r=3/4\) & 41.65 & 37.675 & 29.65 \\ DQPSK, \(r=1/2\) & 55.8 & 50.5 & 39.8 \\ DQPSK, \(r=3/4\) & 84.1 & 76.15 & 60.1 \\
8-ADPSK, \(r=1/2\) & 84.1 & 76.15 & 60.1 \\
8-ADPSK, \(r=3/4\) & 126.55 & 114.625 & 90.55 \\
16-ADPSK, \(r=1/2\) & 112.4 & 101.8 & 80.4 \\
16-ADPSK, \(r=3/4\) & 169.0 & 153.1 & 121.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Modulation types and data rates (in kbit/s) for TDD (from caller to responder). \begin{table}
\begin{tabular}{l c c c} \hline \hline \multirow{2}{*}{**Feature**} & \multicolumn{3}{c}{**Single Antenna**} & \multirow{2}{*}{**Antenna Array**} \\ \cline{2-3} \cline{2-3}  & **Without Beam Forming** & & **With Beam Forming** \\ \hline Directivity & low & medium & high \\ Beam control & no & yes & yes \\ Attitude control & low & medium & high \\ Angular coverage & high & high & low \\ Occupied area & medium & medium & large \\ ISL range & low & medium & high \\ Complexity, cost & low & high & high \\ Medium access & TDMA, FDMA, CDMA & TDMA, FDMA, CDMA & TDMA, CDMA, SDMA \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of antenna concepts for ISL missions. processing. Satellites, particularly those within different orbits, can have large relative velocities, which may result in a strong variation of signal strength in the decoder and, thus, make safe signal separation difficult. FDMA and especially CDMA would be good choices for PFF missions with a small number of S/Cs and very strict time latency requirements, as proposed in [19]. For small formations, synchronous TDD can be used (classic TDMA). The TDD method assigns time slots to each node; therefore, a single frequency is sufficient for all cross-links, which allows for the replication of each node, in terms of frequency and bandwidth. The constraints of TDD are that time synchronization among the nodes is needed and, for a large number of S/C, the duty interval for transmission increases, thus resulting in lower data throughput.