# Is Satellite Ahead of Terrestrial in Deploying NOMA for Massive Machine-Type Communications?
## 3 From S-MIM to F-SIM
### F-SIM Physical Layer  
The E-SSA waveform specification adopted in S-MIM and F-SIM is based on the 3 GPP W-CDMA uplink waveform [13; 14; 15]. The payload is carried by the physical layer data channel (PDCH), while pilot symbols for unknown parameter estimation (e.g., time, frequency and phase) at the receiver and an optional signaling field providing information on the actual carrier format are carried by the physical layer control channel (PCCH). The same forward error correction (FEC) scheme (Turbo code with rate 1/3) and BPSK modulation are adopted on both channels. Similar to 3 GPP W-CDMA, the two channels are spread and mapped to the I and Q components of a complex signal which is, in turn, scrambled by a complex long spreading code. As described in [5], differently from S-MIM, F-SIM defines four possible channel sizes: 2.5, 5, 10 and 40 MHz (the bandwidth actually occupied is, respectively, 2.34, 4.68, 9.36 and 37.44 MHz, with a 22% square root-raised-cosine chip shaping filter roll-off factor). Different spreading factors are defined, ranging from 16 to 256, in order to adapt to the link budget and the required data rate. Finally, different packet sizes are defined, from 38 to 1513 bytes, in order to minimize the amount of padding bytes in each packet sent, and to optimize the use of the bandwidth. As an example, for channelization of 10 MHz and a spreading factor of SF = 16, the burst duration varies from 2 (38 byte payload) to 75 ms (1513 byte payload), and the minimum C/N for reception is \(-15.2\) dB (at PER = \(10^{-4}\)). For the same channelization and a spreading factor of SF = 256, the burst duration varies from 32 to 240 ms (300 byte payload), and the minimum C/N for reception is \(-27.3\) dB (at PER = \(10^{-4}\)). Similar to S-MIM, F-SIM uses dual-BPSK modulation with the FEC coding rate 1/3, as shown in Figure 2. The Turbo code from 3 GPP Release 99 specifications has been adopted [13]. The uplink burst composition is depicted in Figure 3 and further described hereafter. The PDCH carries the random access channel (RACH) data burst followed by a CRC. The PCCH carries physical layer signaling and reference symbols to allow coherent demodulation of the PDCH channel. The physical layer signaling conveys the transport format indication (TFI), particularly information associated with the spreading factor and data burst length used in the modulation process. Each channel is scrambled with an orthogonal variable spreading factor (OVSF) code, and a final scrambling is performed using Gold codes. Different scrambling codes can be used in the system, but, in general, a single scrambling code for each satellite beam is used. The preamble is composed of a sequence of 96 symbols, spread by a pseudorandom noise (PN) code of the period and spreading factor equal to the spreading factor used for the current packet. With the F-SIM physical layer being very close to the terrestrial 3 GPP W-CDMA return link standard, there are also benefits from its suitability to be embedded in low-cost handheld user terminals. One of the W-CDMA uplink features is to have a moderate peak-to-average envelope fluctuation (see Figure 3 in [16]), as it is based on a power unbalanced dual-BPSK modulation, resulting in an asymmetric 8 PSK constellation (see Figure 2 in [16]). This, combined with the use of low-code rate FEC, makes F-SIM suitable for driving the user terminal solid-state power amplifier (SSPA) in a moderately compressed mode (i.e., 2-3 dB compression mode) with no appreciable performance degradation. It should be remarked that when link margins are available, it is preferable to randomize the transmit power to achieve a higher throughput [17]. This power randomization approach has been implemented in F-SIM-based networks.