# Is Satellite Ahead of Terrestrial in Deploying NOMA for Massive Machine-Type Communications?
## 5 Ongoing R&D
### The GEMMA Project
Such air interface, named IURA (IoT Universal Radio Access), is based on E-SSA on the return link, which is able to work at very low carrier-to-noise power ratios (i.e., C/N below \(-20\) dB) thanks to the processing gain provided by the large spreading values (up to 256), and an evolved GEMMA waveform on the forward link, in order to deal with values of C/N below \(-15\) dB. These extended C/N ranges enable thepossibility to equip the terminals with small low-gain antennas, e.g., an 8 \(\times\) 8 patch array, and still be able to operate in typical GEO Ku and Ka band scenarios. This, combined with the offered data rates ranging from some kilobits per second to some tens of kilobits per second with larger flat antennas and the possibility to manage a very high number of devices which sporadically transmit few data bursts, makes it suitable to develop a simple and low-cost terminal for IoT and medium-data rate scenarios. We report the following laboratory results for Use Case 2 (UC#2) corresponding to a very small antenna and very low power consumption terminals, with antenna sizes in the orders of few centimeters (e.g., 6 \(\times\) 6 cm), transmitting over the GEO Ku band. The low EIRP and power consumption calls for a waveform with a high SF and a low power spread. A scenario with terminals transmitting an IURA waveform with an SF = 256 and a chip rate of \(R_{c}=220\) kchip/s, with a minimum received C/N = \(-23\) dB and limited power randomization (3 dB), is representative of this use case. For this scenario, the following remarks are in order:  
(i) Thanks to the E-SSA multiple access, a certain number of simultaneous transmitting terminals (STT), each with the same EIRP, access the same spectrum resource (a band \(B\)) at the same time. The EIRP(\(\theta\)) towards a direction which is \(\theta\) degrees off-axis w.r.t. to the maximum radiation direction depends on the antenna radiation pattern. Hence, the overall off-axis EIRP density at \(\theta\) degrees is given by EIRP(\(\theta\)) \(-10\)log\({}_{10}\)(\(B\)) + \(10\)log\({}_{10}\)(STT). (ii) A number of international regulations such as ITU, FCC and ETSI define the maximum level of the off-axis emission density (OAED) that can be emitted using Ku FSS bands by all terrestrial terminals operated within a given channel. (iii) Due to the adoption of small antennas such as a patch array of 64 elements, the antenna radiation pattern is denoted by a very wide main lobe and high side lobes, increasing the EIRP density in the off-axis direction compared to the case where a bigger antenna is used. To this end, Figure 19 compares the OAED mask (dashed line) provided by the ITU-R recommendation [26] and the off-axis EIRP density (red line) that would be obtained by an IURA terminal in UC#2, that is, considering an EIRP = \(-13\) dBW and a 64-element patch array antenna with a maximum gain of about 21 dB. The minimum distance between the latter and the OAED mask denotes, on the logarithmic scale, the maximum STT (STTmax) that can be operated before violating the constraint. Such value turns out to be STTmax = 15, and the aggregated EIRP density is represented by the yellow curve. Table 2 summarizes the relevant performance of the IURA waveform. It is worth to point out how the E-SSA protocol makes it possible the use of such very small antennas. Indeed, the latter call for a high spreading factor in order to compensate the low EIRP with the processing gain at the receiver, and thus to successfully close the link budget. This would not be possible with non-spread ALOHA-based protocols, where, in addition, the collisions would further degrade the performance without being able to provide 15 STTs. Furthermore, orthogonal access schemes, such as time-division multiple access, would yield a waste of band and imply a higher signaling. Hence, the E-SSA protocol stands as an effective and suitable solution also for this use case.