• Services identification. Satellite networks can provide a wide range of services (videoconferencing, Web browsing, e-mail, telemedicine, and so on) that fixed networks currently provide. For example, in this issue Franchi et al. discuss Inmarsat's services. In addition, others have conducted demonstrations with Ka band systems, 2,3 including Holzbock and Senninger in this issue.
• Payload key technologies. To improve payload performance, the use of on-board processing (OBP) capabilities based on digital signal processing techniques 4 are well accepted nowadays, having already been tested. 5,6 The use of multibeam antennas and steerable beam technology enables more efficient coverage and alleviates user terminal requirements. The intersatellite link (ISL) allows interconnection among different units, realizing a real network in the space. Marinelli and Giubilei deal with these aspects in this issue.
• Spectral efficiency and interference. One of the most limiting factors for satellite systems is achieving spectral efficiency—providing coverage as similar as possible to terrestrial cellular systems. Although using the frequency reuse technique in cellular-like systems reduces the needed bandwidth, it introduces co-channel interference as the frequency reuse factor is higher. 7
For satellite systems, adopting multispot coverage proves even more critical than for landmobile systems. The beam-to-beam interference level depends only on the angular separation between two beams using the same frequency, and the orography doesn't mitigate this impairment.
• Radio access. One open issue concerns selecting the radio access scheme. In the frame of the UMTS/IMT2000 standardization process several proposals have been submitted, mostly based on code-division multiple access, or CDMA (see http://www.itu.int/imt/2_rad_devt/index.html). In the case of Ka band systems, time-division multiple access (TDMA) is preferred in some cases. 8,9
• Channel impairments. A critical aspect involves communication channel impairments. When mobility is introduced, the time variability proves more critical, and phenomena such as shadowing arise. If a frequency beyond 10 GHz is used, tropospheric attenuation must also be considered. 10
• User terminals. To get maximum market penetrability and real integration between terrestrial and satellite segments, the terminal must be as inexpensive, small, and power efficient as possible. The goal is to have the same terminal connect transparently to one of the two system components.
• Spectrum use. The history of wireless communications has so far demonstrated the need to explore and use even higher frequencies to satisfy traffic requirements. As shown in this issue, the Ka band is well accepted. Exploring the EHF band in both the lower portion (40 to 50 GHz) 3—which Motorola plans to use in its MStar system—and the higher portion (90 GHz) 11 will open new possibilities in the next few years.
• Evolutionary constellations. Starting from the classical constellations as described in the previous section, new and hybrid concepts 11 could synthesize the advantages of different configurations targeting even more capillary coverage with high data-rate capabilities and performances.
• Integration. Integration between terrestrial and satellite components and between fixed and mobile networks represents a key issue. Of course, the integration can be pursued at different levels (terminal, services, network, and so on), each implying different technological requirements and constraints. Searching the maximum number of common elements between the components can facilitate this process. The intersegment handover procedure represents a critical issue (see http://www.euroskyway.alespazio.it/accord.htm).
• Network architecture and protocols. Network aspects are perhaps the most relevant, aiming to adapt protocols and procedures peculiar (and well assessed) to the fixed networks and to the interconnection among different networks. Three articles in this issue cover this topic, one on UMTS (Delli Priscoli) and the other two on a Ka-band system (Blefari-Melazzi and Reali and Iera et al.).
Michele Luglio is a research and teaching assistant at the University of Rome "Tor Vergata" where he works on designing satellite systems for multimedia services both mobile and fixed, in the frame of projects funded by the European Union and European Space Agency. He teaches signal theory and collaborates in teaching digital signal processing and elements of telecommunications. He received an MS in electronic engineering in 1990 and a PhD in telecommunications in 1994 at the University of Rome "Tor Vergata." He received the Young Scientist Award from the 1995 International Symposium on Signals, Systems, and Electronics (ISSSE 95) and is a member of the IEEE.