What is the RF requirement for a Satellite IoT direct-to-device communication system?

The RF requirements for a Satellite IoT direct-to-device (D2D) communication system enable a small, low-power IoT sensor on the ground to communicate directly with a satellite in Low Earth Orbit (LEO) without any terrestrial infrastructure, closing an extremely challenging link budget across a 500-2000 km path. The key RF requirements are: transmit power and antenna (the IoT device typically transmits at +20 to +30 dBm (100 mW to 1 W) using a small omnidirectional or hemispherical antenna with 0 to +3 dBi gain; the satellite uses a larger antenna (phased array or reflector) with 20-35 dBi gain to compensate for the low device EIRP), operating frequency (L-band (1.5-1.6 GHz) and S-band (2.0-2.2 GHz) are the primary bands for satellite IoT because: the path loss is lower than at Ku/Ka-band (approximately 20 dB less than at 12 GHz), omnidirectional device antennas are practical at L/S-band (the wavelength is approximately 20 cm, allowing a simple wire or patch antenna), and regulatory allocations for mobile-satellite service (MSS) exist in these bands), link budget (the uplink (device to satellite) is the challenging direction because of the low device EIRP; example for a LEO satellite at 600 km altitude: EIRP_device = 30 dBm + 3 dBi = 33 dBm, path loss at 1.6 GHz and 600 km = 20 x log10(4pi x 600000 x 1.6e9 / 3e8) = 159 dB, satellite G/T = -5 dB/K (small satellite antenna with T_system = 500 K), available C/N0 = 33 + 228.6 - 159 - (-5) = 107.6 dB-Hz; this C/N0 supports very low data rates using spread spectrum techniques), and modulation/coding (to close the link with the limited C/N0: use narrow bandwidth (1-10 kHz channels) to concentrate the signal energy, use strong FEC (turbo codes or LDPC with rate 1/3 to 1/6), and use BPSK or GMSK modulation that tolerates low Eb/N0 (approximately 2-4 dB with coding gain); achievable data rates: 100 bps to 10 kbps depending on the link margin).