# ICARUS--Very Low Power Satellite-Based IoT
## 3 Physical Layer and Doppler Compensation
### ICARUS Channel Properties
#### 3.3.2 Radio Channel Properties and Propagation Conditions  
Much effort have been made to properly understand the ICARUS propagation channel at the selected transmission frequencies [7] before designing the physical layer of our transmission system. Figure 13 depicts the typical ICARUS communication signal propagation scenario which is modelled as a classical 2-ray channel model. Both signal components dominate the entire propagation setup: the direct path (LOS--line of sight) and the reflection paths (NLOS--non-line of sight). A dedicated additional path attenuation of the reflection component is introduced by means of the ground reflection coefficient, R.  
As a reference for multiple ground types, Table 4 lists reflection coefficients, as outlined in [13].  
Figure 14 shows measured normalized receive signal power of the ICARUS downlink with vertical polarized antenna versus height of the receiver over ground. We compare the measurement results with the simulation result for the two-ray model with a ground reflection on wet ground.  
Any variation of the antenna height causes an alternation between constructive and destructive superposition of the line-of-sight signal and the ground reflection signal. The simulation result from the two-ray model coincides well with the measured data with respect to the deep fades of the signal power. There might however be propagation scenarios which are not dominated by a non-resolvable ground reflection. In such cases, we need to model the radio channel as a complex base-band impulse response which exhibits resolvable LOS and NLOS tap components, as depicted in Figure 15.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Surface** & **Dielectric Constant \(\epsilon_{\mathbf{r}}\)** & **Reflection Coefficient R** \\ \hline Dry poor ground & 4–7 & \(-9.7\) dB,..., -7.0 dB \\ Average ground & 15 & \(-4.7\) dB \\ Wet ground & 25–30 & \(-3.6\) dB,..., \(-3.3\) dB \\ Water & 81 & \(-1.97\) dB \\ \hline \hline \end{tabular}
\end{table}
Table 4: Reflection Coefficients for Typical Ground Types.  
Figure 13: ICARUS Multipath Propagation Scenario.  
variables. The ISS velocity causes a time-varying radio channel which is characterized by receive power fluctuations during UL or DL packet reception. The downlink signal power deviations between consecutive pilot blocks is exemplarily and statistically evaluated in Figure 16 by means of its CCDF (complementary cumulative distribution function).  
Based on the channel measurement campaigns and the ICARUS receiver implementations, we can derive the following implications on the ICARUS system design:  
1. UL: The appliance of a bit interleaver prior to FEC encoding [14] shows negligible impact on system performance. This is because the UL frame is QPSK-modulated and FEC-encoded by a strong LDPC code.
2. The ICARUS UL performance is dominated by long-term fades due to ground reflection; fast fading does not show any considerable performance impact since it can be tracked easily by the inserted pilot signal sequences.
3. The current UL and DL pilot distance setting appears to be chosen right.
4. The current receiver implementation can cope with strongly faded pilot blocks if the fading duration is no longer than two pilot blocks (no corruption of timing and CFO tracking).