# Is Satellite Ahead of Terrestrial in Deploying NOMA for Massive Machine-Type Communications?
## 5 Ongoing R&D
### The GEMMA Project  
The ESA ARTES AT GEMMA project (MBI S.r.L. and AIRBUS Italia S.p.A. (ADSR)). S-MIM and F-SIM rely on DVB-SH and DVB-S2 standards, respectively, on the forward link (FL). Fulfilling the need for an adaptation of the FL air specifications to also operate mobile IoT terminals in different scenarios is the goal of this project. In particular, the main requirements are as follows:  
1. The support of both GEO and LEO scenarios, keeping the user terminal inexpensive and easy to operate;
2. The support of both fixed and mobile terminal applications;
3. The support of different types of applications (point-to-point, multicast and broadcast) and data rates also enabling those services relying on data transfer to terminals (e.g., firmware upgrade);
4. The possibility to implement different Tx/Rx activity modes that could help in reducing the power consumption of the terminal; the possibility to implement loop functionalities (e.g., ARQ, congestion control, power randomization and/or variable ModCods/SFs) for network management to increase the system capacity. The new air interface was designed capitalizing on the most suitable technology solutions adopted in satellite standards such as DVB-S2, DVB-SH and ETSI-SDR. The channel coding is based on the 3 GPP LTE Turbo codes as they provide a good trade-off between performance and complexity. They perform well at low coding rates compared to LDPC codes which perform better at higher coding rates but have a higher memory requirement. A channel-programmable length time interleaver is employed to counteract outages due to shadowing or short blockages in mobile scenarios. It is based on convolutional interleaving, as used in DVB-SH, since, compared to block interleavers, it provides a reduction in the memory occupation by a factor 2. The transmission is organized in equal length frames with a constant pilot symbol spacing to ease the acquisition process at the terminal. In order to support different quality of services (Q.o.S. ), several physical layer pipes (PLP) are defined, each of them identified by a combination of physical layer parameters (modulation order, coding rate, convolutional interleaver parameters) and mapped into a frame. A spreading up to factor 4, common for all the frames, can be applied in order to improve the minimum SNR demodulation threshold. This can be useful, for instance, in a LEO scenario where, towards the poles, a terminal may see more than one LEO satellite belonging to the same constellation. In this case, the second satellite in view creates co-channel interference, decreasing the received signal over noise plus interference ratio. Another case could be the one where lower gain antennas are employed at the terminal so that such gain loss is compensated by the processing gain. The synergy between MASSIVE and GEMMA projects focuses on different performance requirements and scenarios. The main research activity is aimed at adapting the RA E-SSA protocol to the LEO constellation with a smaller available bandwidth (on the order of hundreds of kHz). Within this context, the main challenges consist in dealing with a higher Doppler shift range and a non-negligible Doppler rate typical of LEO scenarios. This has an impact on the PS, which shall be robust to these effects, and calls for the need for a Doppler rate estimator at the receiver. Furthermore, the LEO scenario requires low-power consumption demodulation algorithms, as demodulation takes place on board and is typically based on low-cost COTS hardware. The synergy between these two projects also led to the definition of an air interface to be adopted in GEO Ku and Ka band scenarios, with terminals equipped with low-gain flat (e.g., patch array) antennas.