In the late 1980s, satellite systems appeared to be a promising approach for constructing telephony systems with worldwide coverage. At that time, conventional cellular telephony was not very widespread and its cost was relatively high. These facts made room for satellite-based systems. However, by the time satellite-based systems were ready for deployment, the market penetration of cellular telephony was so big that little space was left for satellite phones. However, satellite telephony is not completely without future or benefit: It is still an efficient way to link mobile users existing in regions of the world without communications infrastructure. Furthermore, it may be the only available mobile telephony system in many regions of the world, as there are countries in which conven- tional cellular systems have a limited coverage.
In this section, we study two examples of satellite-based mobile telephony systems:
Iridium and Globalstar. Iridium was an ambitious project aiming for worldwide coverage using a dense constellation of LEO satellites. However, the project was finally abandoned in 2000. Globalstar, which on the other hand had a better fate than Iridium, is a simpler system and its coverage also depends on the existence of ES.
7.4.1 Iridium
The Iridium project [3–5] was initiated by Motorola in the early 1990s. The project aimed to offer coverage to every place on the planet through a dense constellation of LEO satellites. The Iridium satellites employ significantly richer functionality than simple
‘bent-pipe’ satellites by enabling intra-satellite communication for relaying of control
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Figure 7.10 A VSAT communication system via an intelligent satellite
signaling and phone calls. The project initially called for use of 77 LEO satellites. This was the fact that gave it the name Iridium, since Iridium is the chemical element with an atomic number of 77. Despite the fact that the number of satellites was later reduced to 66, the name Iridium stayed probably due to the fact that the marketing people preferred it to Dysprosium, which is the chemical element with an atomic number of 66. Nevertheless, this decision did not seem to favor the project’s fate, as Iridium was finally abandoned for economic reasons in 2000.
The Iridium system comprised four main components: the satellite constellation, the system control facilities, the gateways and the subscriber units. These are described below, along with a number of issues relating to the operation of Iridium.
7.4.1.1 The Iridium Satellite Constellation
Iridium employs 66 LEO satellites, about 700 kg each, that orbit the Earth at an altitude of 780 km above its surface. This altitude was chosen in an effort to minimize delay (which is discussed later), enable portable ground units and provide for an acceptable satellite lifetime (8 years for an Iridium satellite). This is the time it takes for a satellite to consume the fuel, which powers the engines that combat friction with the atmosphere and keep the satellite in proper orbit. Iridium satellites have an orbital period of 100 min.
Iridium satellites are divided into six polar orbital planes with each plane having 11 satellites. Each orbit has an inclination of 86.58with respect to the equator. In each plane satellites rotate in the same direction with the exception of planes 1 and 6 which are counter-rotating. Co-rotating and counter-rotating planes are spaced 31.68 and 228 apart.
Each satellite is able to maintain up to four Inter-Satellite Links (ISLs), two of which are permanent and involve the two adjacent satellites in its plane. The other two ISLs are dynamically established with the two satellites in the adjacent orbital planes. Exceptions to this fact are the satellites in planes 1 and 6. These maintain only three ISLs, due to the fact that the rapid relative angular speed of a pair of counter-rotating satellites from these planes does not allow them to establish ISLs between each other. Finally, ISLs operate at frequencies between 22.5 and 23.5 GHz at a link speed of 25 Mbps.
Each Iridium satellite is equipped with an antenna comprising three panels: the first is perpendicular to the direction of the satellite’s travel, and the next two are 1208 and 2408 displaced relative to the first one. As the satellite moves in its orbit, the footprint of each panel obviously moves on the Earth’s surface. Each panel transmits 16 beams resulting in a total of 48 beams per satellite. Combining this with the total of 66 satellites used by the system, one can see that Iridium provides 3168 beams overall. However, only 2150 beams are used to provide global coverage, due to the fact that that there is a significant overlap among the beams of satellites from adjacent orbital planes when these satellites are above areas near the poles. Since global coverage can be achieved without such an overlap, a satellite’s beams are reduced near those areas in order to conserve power.
7.4.1.2 Frequency Reuse
Iridium employs frequency reuse like conventional mobile telephony systems. It divides bands into groups, called clusters. Each cluster contains beams that can use the same
frequency. The principle of operation is the same as that of frequency reuse schemes of cellular systems. Adjacent beams are not allowed to use the same frequency. Iridium uses a frequency reuse factor of 12. Beams that use the same frequency channels can be found as follows: (a) starting from the center of a beam, move two beam centers; (b) make a turn of 608; and (c) move two cells.
7.4.1.3 MAC
Iridium employs a combination of TDMA and FDMA as its multiple access technique both for uplink and downlink. These use QPSK for modulation. The FDMA component is attributed to the above-mentioned frequency reuse scheme. The system uses the spectrum from 1616 MHz to 1625.5 MHz. Of this bandwidth, 10 MHz are used to constitute a total of 240 41.67 kHz channels. The bandwidth of these channels totals 10 MHz as the additional 500 kHz are used for establishing guard bands between adjacent channels.
Each guard band has a width of 2 kHz.
The TDMA scheme comprises 90 ms frames each of which contains four pairs of slots supporting four full-duplex channels at a rate of 4800 bps. Additionally, half-duplex data channels of 2400 bps are supported. The specific details of the TDMA frame structure were not published in open literature [5]. The same holds for the nature of the voice codec used.
7.4.1.4 System Control Facilities
The operation of Iridium is assisted by two System Control Facilities (SCFs) which are responsible for maintaining control of the constellation of the 66 satellites. Each satellite is monitored via the SCFs which manages their operation in order to ensure correct performance within orbit. Furthermore, the network formed by these satellites is also monitored by the SCFs, which informs the constellation in the event of a malfunctioning node.
7.4.1.5 Gateways
Gateways are ESs that interface Iridium to external communication networks, such as the PSTN for voice calls. Such an interface extends the coverage of Iridium since it enables Iridium subscribers to place/receive calls from PSTN users. Gateways perform a number of operations, such as subscriber location, billing and call setup.
7.4.1.6 Numbering
As in cellular systems, Iridium subscribers are assigned a home gateway which contains a permanent record regarding the subscriber’s identity. The numbers that can identify an Iridium subscriber are the following [4]:
† Mobile Subscriber Integrated Services Digital Network Number (MSISDN).This number is the permanent number assigned to the Iridium subscriber. In order to dial a number to establish a voice call with the subscriber, the MSISDN is preceded by two more fields: (a)
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The Iridium country code (ICC), which is a four-digit number that identifies the Iridium network; and (b) a three-digit geographical code that is used to identify a user’s home country in cases of gateways that serve more than one country.
† Temporary Mobile Subscriber Identification (TMSI). This number is used to achieve confidentiality of the user’s MSISDN. In order not to send the MSISDN over the airwaves, this is mapped to the TMSI which is sent instead. The TMSI changes periodically to increase security.
† Iridium Mobile Subscriber Unit (MSI).This number is permanently stored in the user’s Subscriber Identity Module (SIM) that resides within his phone and uniquely identifies the subscriber.
7.4.1.7 Call Management: Subscriber Location
In order for Iridium to be able to serve roaming users, a method for determining the location of subscribers is needed. This is made possible through the concept of home and visitor gateways. The process of subscriber location is depicted schematically in Figure 7.11. For the purpose of illustration, assume a user registered in Europe travels to North America. The user’s home gateway will be the European one, while his visiting gateway will be the North American one. When the user arrives in North America and switches on his phone, communications will be established with the closest Iridium satellite (point A in Figure 7.11). The satellite will connect the user to the local North American gateway (point B) which will of course recognize the user as a visitor and create a relative entry in its database. Furthermore, the visiting gateway will determine the subscriber’s home gate- way via his TMSI. Next, the visiting gateway will instruct the satellite above it to contact the home gateway (point C) and (a) inform it of the new location of the subscriber, (b) ask for permission to allow call access to the subscriber. The latter is necessary in cases of subscribers with pending bills or stolen phones. If the home gateway grants call access to the subscriber (point D), the latter is ready to make/accept calls via the North American gateway.
Figure 7.11 Subscriber location in Iridium
7.4.1.8 Call Management: Call Setup
When a call arrives for a user outside the area of its home gateway, then a joint operation of the home and visitor gateways ensures call setup. Returning to the case of the previous example, assume that a European PSTN user makes a call to the Iridium subscriber while the latter is still in North America. The PSTN user will obviously dial the Iridium ICC, which will lead the call to the European Iridium gateway. This is the home gateway of the Iridium subscriber. Thus, it will check its database and determine that the subscriber’s current location is in North America. Consequently, it will use the satellite above it to contact the North American gateway. This operation will probably pass through one or more ISLs. The North American gateway is the subscriber’s visitor gateway. It will check in its database, determine that the user is in its area and use the satellite above it to relay the call to the subscriber’s terminal. When the latter goes off hook, a corresponding message is relayed to the visitor gateway via the satellite above. The visitor gateway then sends this message via the satellite constellation to the home gateway. Upon recep- tion of this message, the home gateway starts sending voice packets to the satellite above the subscriber’s location, which in turn relays them directly to the subscriber. Thus, the call is established. It can be seen that the call setup process in Iridium is very similar to that of the AMPS system.
7.4.1.9 Handoffs
There are three types of handoffs in Iridium: intra-beam, inter-beam and inter-satellite.
Intra-beam handoff occurs when the satellite beam that serves a subscriber has to change its operating frequency as it approaches another geographical region. This could be due either to (a) regulatory issues that do not allow use of this frequency in the specific geographical region or (b) interference reasons. The latter is true in situations when the beam is too close to that of another satellite that uses a beam of the same frequency. In any of the above cases, the satellite will inform the user to change to the new beam frequency. In this handoff scenario, the intelligent unit is obviously the Iridium satellite.
Inter-beam handoff involves two different beams of the same satellite. Recall that in conventional cellular systems, terminals operating within a specific cell constantly monitor link quality to adjacent cells. Upon finding a link with better quality, a handoff to this cell is made. The same principle describes inter-beam handoff in Iridium: an Iridium terminal in operation constantly monitors the link quality of two adjacent beams. When the term- inal detects an alternative beam with a better signal quality than that of the current beam, the terminal initiates a handoff to the new beam. In this handoff scenario, the intelligent unit is obviously the Iridium terminal.
Inter-satellite handoff involves two satellites. Due to the rapid movement of Iridium satellites relative to ground units (typical LOS times are 10 min) handoffs between satellites are very common. When an Iridium terminal goes out of coverage of the satellite above it (e.g. satellite A), it is approached by a new satellite (e.g. B) to which it will be handed. The handoff procedure is a responsibility of the local gateway since it knows both the satellite and terminal movements. The gateway will thus send a message to satellites A and B asking for release and acceptance of the terminal, respectively. After the handoff is
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made, satellite B contacts the Iridium terminal in order to notify it of the frequency to use.
In this handoff scenario the intelligent unit is obviously the gateway.
7.4.2 Globalstar
Globalstar [6,7] is a satellite-based telephony system that aims to enable users to talk from virtually anyplace in the world. However, the word ‘virtually’ has a definite meaning. This is due to the fact that contrary to Iridium, which enables true worldwide coverage through the use of ISLs for call routing, the operation of Globalstar depends on the presence of a Globalstar gateway in range of the satellite that serves the user. This is because gateways are necessary in order to connect users, since no ISLs are used, as in Iridium. This fact, which is shown in Figure 7.12, limits the coverage of Globalstar. On the other hand, it constitutes an advantage in terms of cost and system simplicity. Furthermore, since typical Globalstar gateways can have a range of many kilometers, few such stations are needed to support the system. When every gateway is operational, Globalstar can cover most of the Earth’s surface, except for the regions in the middle of the oceans where ESs deployment is not possible or costs a lot and those near the poles, for reasons that are described later.
The frequency bands used by Globalstar are shown in Figure 7.13.
Figure 7.12 Operation of Globalstar
Figure 7.13 The frequencies used by Globalstar
The Globalstar system comprises three main components: the satellite constellation, the gateways and the subscriber units. These are described below, along with a number of issues relating to the operation of Globalstar.
7.4.2.1 The Globalstar Satellite Constellation
The Globalstar system comprises LEO satellites that operate in eight planes. These satel- lites use LEO orbits with an altitude of 1400 km. Each plane contains six satellites and has an inclination of 528 with respect to the equator. This fact explains the system’s inability to cover regions with latitudes beyond 708 in both hemispheres [7]. However, this is not as bad as it sounds, since most of the population (thus subscribers) are located outside those areas.
Each Globalstar satellite employs 16 beams and the same frequencies are reused within each beam. Satellite orbit is monitored and maintained by using a GPS system, which also supplies accurate time to the satellite [6]. The satellite is powered by rechargeable solar batteries and the system controlling the satellite’s altitude is driven by small thrusters. Of course, when a satellite runs out of fuel it will eventually fall back to Earth. The life expectancy of a Globalstar satellite is around 8 years. Finally, both satellites and ground units in Globalstar implement antenna diversity in an effort to increase performance.
7.4.2.2 MAC
Globalstar uses CDMA as a MAC technique. The forward link (uplink from gateway to satellite and downlink from the latter to the user terminal) uses a chip rate of 1.2288 Mcps and a spreading factor of 256, which results in a peak information transmission rate of 4800 bps [6]. The forward link also employs a pilot channel that is defined by the same spreading code for all gateways. The pilot channel is used by Globalstar terminals to synchronize with gateways. The reverse link (uplink from the user terminal to the satellite and downlink from the latter to the gateway) uses a spreading code of length 215. Finally, the CDMA nature of Globalstar demands that the signal of all users reaches a gateway with the same power. This has led to use of closed-loop power control in order to combat the near-far problem.
7.4.2.3 Gateways
A gateway is a special fixed ES. Apart from its main functionality which is to link satellites, it contains HLRs and VLRs for user location management and performs opera- tions relating to security, billing and interfacing to PSTN and GSM.
7.4.2.4 Handoffs
As was mentioned above, Globalstar satellites use the same frequency in adjacent beams or overlapping beams of different satellites. This fact enables soft handoff, which is similar to that of conventional CDMA-based cellular systems. When a Globalstar terminal is covered by another beam or satellite, it reports this to the gateway. Due to satellite movement, the Globalstar terminal is soon likely to experience a better link quality at
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the new beam. The gateway is thus informed that a soft handoff may take place and starts transmitting the same information at both beams. When the terminal is within coverage of the new beam, the system is informed of this fact and it drops the connection to the beam of the departing satellite.
7.4.2.5 Subscriber Units
Globalstar terminals can be either single or dual mode units that also support conventional cellular systems, such as GSM.