Localization

Localization of users in satellite networks is similar to that of terrestrial cellular networks. One additional problem arises from the fact that now the ‘base sta- tions’, i.e., the satellites, move as well. The gateways of a satellite network

maintain several registers. A home location register (HLR) stores all static

information about a user as well as his or her current location. The last known

location of a mobile user is stored in the visitor location register (VLR).

Functions of the VLR and HLR are similar to those of the registers in, e.g., GSM (see chapter 4). A particularly important register in satellite networks is the

satellite user mapping register (SUMR). This stores the current position of satellites and a mapping of each user to the current satellite through which communication with a user is possible.

Registration of a mobile station is achieved as follows. The mobile station initially sends a signal which one or several satellites can receive. Satellites receiving such a signal report this event to a gateway. The gateway can now determine the location of the user via the location of the satellites. User data is requested from the user’s HLR, VLR and SUMR are updated.

Calling a mobile station is again similar to GSM. The call is forwarded to a gateway which localizes the mobile station using HLR and VLR. With the help of the SUMR, the appropriate satellite for communication can be found and the connection can be set up.

5.6 Handover

An important topic in satellite systems using MEOs and in particular LEOs is handover. Imagine a cellular mobile phone network with fast moving base sta- tions. This is exactly what such satellite systems are – each satellite represents a base station for a mobile phone. Compared to terrestrial mobile phone net- works, additional instances of handover can be necessary due to the movement of the satellites.

● Intra-satellite handover:A user might move from one spot beam of a satel- lite to another spot beam of the same satellite. Using special antennas, a satellite can create several spot beams within its footprint. The same effect might be caused by the movement of the satellite.

● Inter-satellite handover:If a user leaves the footprint of a satellite or if the satellite moves away, a handover to the next satellite takes place. This might be a hard handover switching at one moment or a soft handover using both satellites (or even more) at the same time (as this is possible with CDMA systems). Inter-satellite handover can also take place between satel- lites if they support ISLs. The satellite system can trade high transmission

quality for handover frequency. The higher the transmission quality should be, the higher the elevation angles that are needed. High elevation angles imply frequent handovers which in turn, make the system more complex.

● Gateway handover:While the mobile user and satellite might still have good contact, the satellite might move away from the current gateway. The satellite has to connect to another gateway.

● Inter-system handover:While the three types of handover mentioned above take place within the satellite-based communication system, this type of handover concerns different systems. Typically, satellite systems are used in remote areas if no other network is available. As soon as traditional cellu- lar networks are available, users might switch to this type usually because it is cheaper and offers lower latency. Current systems allow for the use of dual-mode (or even more) mobile phones but unfortunately, seamless handover between satellite systems and terrestrial systems or vice versa has not been possible up to now.

5.7 Examples

Table 5.1 shows four examples (two in operation, two planned) of MEO/LEO satellite networks (see also Miller, 1998 and Lutz, 1998). One system, which is in

operation, is the Iridiumsystem. This was originally targeted for 77 satellites

(hence the name Iridium with its 77 electrons) and now runs with 66 satellites plus seven spare satellites (was six, Iridium, 2002). It is the first commercial LEO system to cover the whole world. Satellites orbit at an altitude of 780 km, the weight of a single satellite is about 700 kg. The fact that the satellites are heavier than, e.g., the competitor Globalstar results from their capability to route data between Iridium satellites by using ISLs, so a satellite needs more memory, pro- cessing power etc. Mobile stations (MS in Table 5.1) operate at 1.6138–1.6265 GHz according to an FDMA/TDMA scheme with TDD, feeder links to the satel- lites at 29.1–29.3 GHz for the uplink and 19.4–19.6 GHz for the downlink. ISLs use 23.18–23.38 GHz. The infrastructure of Iridium is GSM-based.

A direct competitor of Iridium is Globalstar(Globalstar, 2002). This system,

which is also operational, uses a lower number of satellites with fewer capabili- ties per satellite. This makes the satellites lighter (about 450 kg weight) and the overall system cheaper. Globalstar does not provide ISLs and global coverage, but higher bandwidth is granted to the customers. Using CDMA and utilizing path diversity, Globalstar can provide soft handovers between different satellites by receiving signals from several satellites simultaneously. Globalstar uses 1.61–1.6265 GHz for uplinks from mobile stations to the satellites and 2.4835–2.5 GHz for the downlink. Feeder links for the satellites are at 5.091–5.250 GHz gateway to satellite and 6.875–7.055 GHz satellite to gateway.

While the other three systems presented in Table 5.1 are LEOs, Intermediate Circular Orbit, (ICO) (ICO, 2002) represents a MEO system as the name indi- cates. ICO needs less satellites, 10 plus two spare are planned, to reach global coverage. Each satellite covers about 30 per cent of earth’s surface, but the system works with an average elevation of 40°. Due to the higher complexity within the satellites (i.e., larger antennas and larger solar paddles to generate enough power for transmission), these satellites weigh about 2,600 kg. While launching ICO satellites is more expensive due to weight and higher orbit, their expected life- time is higher with 12 years compared to Globalstar and Iridium with eight years and less. ICO satellites need fewer replacements making the whole system cheaper in return. The start of ICO has been delayed several times. The ICO con- sortium went through bankruptcy and several joint ventures with other satellite organizations, but still plans to start operation of the system within the next few years. The exact number of satellites is currently unclear, however, the system is shifted towards IP traffic with up to 144 kbit/s.

Irdium Globalstar ICO Teledesic (orbiting) (orbiting) (planned) (planned)

No. of satellites 66 + 7 48 + 4 10(?) + 2 288(?) Altitude [km] 780 1,414 10,390 Approx. coverage global ±70° latitude global 700 global

No. of planes 6 8 2 12 Inclination 86° 52° 45° 40° Minimum 8° 20° 20° 40° elevation Frequencies 1.6 MS 1.6 MS 2 MS 19 [GHz (circa)] 29.2 2.5 MS 2.2 MS 28.8 19.5 5.1 5.2 62 ISL 23.3 ISL 6.9 7

Access method FDMA/TDMA CDMA FDMA/TDMA FDMA/TDMA

ISL Yes No No Yes

Bit rate 2.4 kbit/s 9.6 kbit/s 4.8 kbit/s 64 Mbit/s (144 kbit/s 2/64 Mbit/s planned) No. of channels 4,000 2,700 4,500 2,500 Lifetime [years] 5–8 7.5 12 10 Initial cost $4.4 bn $2.9 bn $4.5 bn $9 bn estimate

Table 5.1 Example MEO and LEO systems

A very ambitious and maybe never realized LEO project is Teledesicwhich plans to provide high bandwidth satellite connections worldwide with high qual- ity of service (Teledesic, 2002). In contrast to the other systems, this satellite network is not primarily planned for access using mobile phones, but to enable worldwide access to the internet via satellite. Primary customers are businesses, schools etc. in remote places. Teledesic wants to offer 64 Mbit/s downlinks and 2 Mbit/s uplinks. With special terminals even 64 Mbit/s uplinks should be possi- ble. Receivers will be, e.g., roof-mounted laptop-sized terminals that connect to local networks in the building. Service start was targeted for 2003, however, cur- rently only the web pages remained from the system and the start was shifted to 2005. The initial plans of 840 satellites plus 84 spares were dropped, then 288 plus spares were planned, divided into 12 planes with 24 satellites each. Considering an expected lifetime of ten years per satellite, this still means a new satellite will have to be launched at least every other week. Due to the high bandwidth, higher frequencies are needed, so Teledesic operates in the Ka-band with 28.6–29.1 GHz for the uplink and 18.8–19.3 GHz for the downlink. At these high frequencies, communication links can easily be blocked by rain or other obstacles. A high elevation of at least 40° is needed. Teledesic uses ISL for routing between the satellites and implements fast packet switching on the satellites.

Only Globalstar uses CDMA as access method, while the other systems rely on different TDMA/FDMA schemes. The cost estimates in Table 5.1 are just rough figures to compare the systems. They directly reflect system complexity. ICO satellites for example are more complicated compared to Iridium, so the ICO system has similar initial costs. Smaller and simpler Globalstar satellites make the system cheaper than Iridium.

5.8 Summary

Satellite systems evolved quickly from the early stages of GEOs in the late 1960s to many systems in different orbits of today. The trend for communication satel- lites is moving away from big GEOs, toward the smaller MEOs and LEOs mainly for the reason of lower delay which is essential for voice communication. Different systems will offer global coverage with services ranging from simple voice and low bit rate data up to high bandwidth communications with quality of service. However, satellite systems are not aimed at replacing terrestrial mobile communication systems but at complementing them. Up to now it has not been clear how high the costs for operation and maintenance of satellite systems are and how much data transmission via satellites really costs for a customer. Special problems for LEOs in this context are the high system complexity and the rela- tively short lifetime of the satellites. Initial system costs only constitute part of the overall costs. Before it is possible to offer any service to customers the whole satellite system has to be set up. An incremental growth as it is done for terres- trial networks is not possible in LEO systems. Operators instal new terrestrial