# A Review of SATis5: Perspectives on Commercial and Defense 5G SATCOM Integration
## 3 Survey of Existing 5G Satellite Integration (SATis5) and Associated Challenges
### SATis5 Technical Challenges  
This section addresses the SATis5 technical challenges associated with the integration of NTN. The NTN consists of GEO/LEO satellites and 5G-NT networks. For integrating GEO satellites with a 5G terrestrial network, the large propagation delay associated with GEO satellites will pose a significant technical challenge in PHY/MAC layers design. The long propagation delay challenge associated with GEO satellites can be mitigated using LEO satellites with a large satellite constellation, i.e., an LEO network with hundreds or  
Figure 6: Strawman SATis5 Architecture for Short-Term Solution. Figure 7: Strawman SATis5 Architecture for Mid-Term Solution. thousands of small satellites providing global coverage, e.g., Starlink being launched by SpaceX [12]. However, LEO satellites pose a new technical challenge associated with large Doppler shifts. The propagation delay associated with LEO satellites is smaller than GEO satellites, but it is still larger than the delay for typical cellular networks. Thus, it still requires some modifications to PHY/MAC protocols for the 5G NR air interface. Reference [9] has assessed the impacts of channel impairments, including propagation delay and Doppler shift, on PHY/MAC layers associated with SATis5 architectures for A1 and A2 discussed above. The SATis5 system architecture for A1 is assumed for mMTC-NB-IoT services with LEO satellites, and A2 is for eMBB services with GEO satellites. For the eMBB scenario, SATis5 A2 system architecture integrates GEO satellites with 5G networks, and the key technical challenges identified in [9] are:  
* Low Signal-to-Noise Ratio (SNR): The low SNR problem inherent with GEO satellites can be mitigated by selecting a power-efficient modulation technique along with high coding gain--The selection of the combined power-efficient modulation-and-coding (ComPEMaC) technique is constrained by the reliability requirements of terrestrial NR systems. * Forward/Return Link Budget: The link budgets for the GEO satellite feeder and user links must be calculated and optimized to determine the feasibility of the proposed satellite communications links. * Large Propagation Delay: In practice, a set of ComPEMaC schemes associated with various channel qualities will be available at gNB. A typical gNB selects the most appropriate scheme based on the channel quality indicator reported by the UE. Due to propagation delay associated with the GEO satellite, the channel information at gNB is not updated as required by the 3 GPP specification. This delay problem leads to a non-optimum use of the channel resources leading to lower spectral efficiency. To mitigate this long-delay problem, a modification to the 5G \"Radio Resource Control\" (RRC) procedures is required. For non-direct access, a possible solution is to allow RRC making the RN play an active role between gNB and UE by optimizing both the UE access and the satellite user link air interfaces. This proposed mitigation technique requires an alternative technique for updating channel information between the UE and RN nodes. Regarding the mMTC-NB-IoT scenario, SATis5 A1 system architecture integrates with large LEO-constellation satellites, and the corresponding key technical challenges are [9]:  
* Propagation Delay: Like the eMBB case, RRC procedures are incompatible with SATCOM RTT delays, including: \(\bigcirc\) RRC timer procedure; \(\bigcirc\) Random-Access Response\" (RAR) time window size; \(\bigcirc\) Contention resolution window size; \(\bigcirc\) Timing Advance\" (TA); \(\bigcirc\) Hybrid automatic repeat request\" (HARQ). * Large Doppler Effect and Phase Shift: The large Doppler effects and phase impairments associated with LEO satellites can cause interruption to successful transmission. This is because of the 3 GPP standardized mMTC-NB-IoT frame structure associated with narrow-band and close OFDM subcarriers. The residual Doppler and Carrier Frequency Offset come from the Doppler compensation and frequency tracking circuitries can cause potential performance degradation.