# On the Energy Performance of Iridium Satellite IoT Technology
## 3 Iridium Overview  
The Iridium satellite system was originally conceived to comprise 77 satellites, which inspired the name for the system, since the atomic number of the iridium chemical element is 77. The Iridium system finally consisted of 66 satellites. While most previously existing satellite communication systems used GEO satellites, Iridium was designed based on LEO satellites. This feature was a consequence of the need to offer worldwide voice and data communications coverage for lightweight handheld devices. For such use cases, LEO satellites require lower transmission power and offer lower propagation delay. The altitude of Iridium satellites was around 780 kms. The system became commercially available in 1998. In 2019, the launch of a new generation of Iridium satellites, called Iridium Next, was completed. There are currently 75 Iridium Next satellites in orbit, comprising 66 operational satellites, and nine on-orbit spare satellites. Note that Iridium Next satellites are fully compatible with the initial Iridium system. The altitude of 70 such satellites is 625 kms, whereas the altitude of the remaining five satellites is 720 kms [16]. Figure 1 illustrates the architecture of the Iridium satellite system. An Iridium subscriber unit (i.e., the Iridium modem of the user device) communicates via a radio link with the Iridium satellite constellation. Data from the user device may be routed through several satellites until they are transmitted in the downlink to an Iridium terrestrial gateway or to another user. An Iridium gateway is connected to the Internet, where data can reach an application server. Each satellite supports three antenna arrays supporting 16 beams each, providing a total of 48 spot beams on the Earth's surface, effectively defining 48 covered cells. Note that Iridium satellites are not geostationary (i.e., they do not remain in the same location). Iridium satellites travel at 26,804 km/h, resulting in a complete orbital period of 100.13 min [9]. As a consequence, from a given ground location, a satellite is visible for seven minutes. When a satellite becomes unreachable for a user, an attempt is performed to hand communication to another satellite. In order to enable intersatellite links, satellites are equipped with additional antennas. Iridium uses a portion of L-band spectrum, ranging from 1616 MHz to 1626.5 MHz, for communication. The frequency band between 1616 MHz and 1626 MHz is used to support duplex user communication. The band between 1626 MHz and 1626.5 MHz is  
Figure 1: Iridium satellite system architecture. used for simplex downlink signaling. The duplex band is divided by means of frequency division multiple access (FDMA) into 240 frequency channels of 41.67 kHz bandwidth each, with an operation bandwidth of 31.5 kHz and guard bandwidth of 10.17 kHz. A frequency channel may in turn be shared among different users by means of a time division multiple access (TDMA) scheme. A TDMA frame is defined with a total duration of 90 ms, which comprises an initial signaling simplex header, four uplink timeslots, and four downlink timeslots. The simplex header has a duration of 20.32 ms, whereas each uplink or downlink timeslot has a duration of 8.28 ms. The remaining time up to the total duration of 90 ms corresponds to guard times before and after the simplex header and each timeslot [9]. A frame supports duplex communication for four simultaneous users by using time division duplex (TDD). Each timeslot contains a burst composed of three parts: an unmodulated tone, a unique word, and the data field. The unique word is transmitted by using binary phase shift keying (BPSK) modulation, whereas the data field is transmitted by means of quadrature phase shift keying (QPSK) modulation [16]. One frame carries a total of 4500 bits, leading to a bit rate of 50 kbit/s. A user duplex timeslot carries a total of 414 bits.