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"Everyday Engineering"

"Everyday Engineering" was a technical column posted regularly on the VACETS forum. The Chair of of this column is Dr. Hoang Viet-Dung. For more publications produced by other VACETS  members, please visit the VACETS Member Publications page or Technical Columns page.

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Satellite Communications Systems

This will be the first of several articles on satellite communications systems. Before I lead you into this exciting journey, I like to point out some limitations and constraints I have put on these papers. First, the emphrasize and subject matter of the articles will be on systems that serve mobile users. In this regard, I will not cover the Very Small Aperture Terminal (VSAT) systems, or those huge C-Band TDMA Earth station (e.g.., INTELSAT) systems. As a consequence to this, direct broadcast services for special events such as the World Cup will not be mentioned. However, I will find time for an article on one VSAT network later. Being one of the earlier integrators and testers for the K-Mart VSAT Data Network, these systems are quite "dear" to me. Second, I am taking the user's point of view: that is, I am more interested in looking at the services these systems will be providing. The technical details are given only as an afterthought. Third, I am a system engineer. Link budget, transponder intermodulation noise, and equivalent isotropically radiated power (e.i.r.p.) are Greek to me as Greek can be.

Most systems discussed are still in the planning stages. Available literature provides very little detail on the architecture and other technical data of these systems. More often, top-level rather than quantitative information are given. Furthermore, due to closely held proprietary data, it is very difficult to obtain technical information from the vendors. The detail information obtained are very "soft": there are many occasions when specific numbers vary from one documentation to another. For examples, the altitudes of the satellite orbits are tens or hundreds of miles different from one document to another, or the bit- error-rates for data transmissions are several orders of magnitude different from one study to another. In general, technical details of new designs or recent changes in the designs are seldom published, leading to the variations in the technical specification in subsequent documents as mentioned above.

The systems are classified loosely into two groups: the geostationary Earth orbit (GEO) and the mobile satellite systems (MSS). The MSS is further subdivided into the low Earth orbit (LEO), and the medium Earth orbit (MEO). The main difference between the GEO and the MSS is that the MSS satellites move relatively to a point on the Earth, while GEO satellites stay stationary with respect to the coverage area. Added to the difficulty in the definition, American Mobile Satellite Corporation (AMSC) calls its Mobile Satellite (MSAT) system the geostationary mobile satellite service (GEO MSS ?).

GEOs move in an easterly direction in an orbit at an altitude of approximately 35,800 km directly above the equator, at the same speed as the Earth and appear to remain over the same stationary point on the Earth's surface. Due to the spacing requirement (GEO satellites are spaced 20 apart), only a small finite number of satellites can be "parked" in this equatorial orbit. A significant disadvantage of GEOs for real time applications is the relatively long range of communications. A quarter of a second round-trip propagation delay is expected, making voice communications a problem. GEOs provide a broad region of coverage. However, coverage of the Earth's polar regions is minimal or nonexistent. INMARSAT for example can cover to about +800 latitudes. At such a high altitude, only three or four GEO satellites are required to provide global coverage. GEOs have the advantages of a relatively constant coverage under the satellite, and a relatively constant pointing or elevation angle to the satellite from an Earth terminal. This slow satellite movements lead to minimal Doppler effects (change of frequency in the transmitted waves).

LEOs and MEOs operate at a much lower altitudes, generally between a few hundred (LEOs) to around 10,000 km (MEOs). The orbits are essentially circular or mildly elliptical in shape. Because of the low satellite altitude, the coverage area under a given LEO or MEO will be relatively small compared to a GEO. LEOs and MEOs are moving relative to a point on the Earth, and will circle the Earth in a few hours. All LEOs must operate in a store-and-forward mode since it will require many LEOs to provide uninterrupted coverage to a fixed region of the Earth. However, due to their shorter propagation delay, LEOs are much more amiable to real-time communications. MEO may be effective as a compromise between LEO and GEO. MEO system will require fewer satellites than LEO, being at a much higher altitude. With delay somewhere between LEO and GEO, MEO is also good for most cellular applications.

A MEO system will need 12 satellites (Odyssey) to provide North America coverage, while a LEO system will need 24 satellites (ORBCOMM - US coverage only), or 48 (GLOBALSTAR - global coverage), or 66 (IRIDIUM - global), or 840 (Teledesic - global). Needless to say, network management (routing, switching, etc.) is a challenge for these systems.

Crudely speaking, the network architecture/topology of these systems is very much alike. It consists of numerous hand- held or portable terminals (user terminals), a small number of feeder or control/telemetry Earth stations (control center/gateway stations), and the satellite constellation. The control/gateway stations interface with the user Hosts via private or public networks.

The carrier frequencies allocated to these systems are rather limited. They are licensed in the US by the FCC (Federal Communications Commission). Competition will be fierce. It is my belief that few of these systems will survive beyond the concept phase. The mobile links (between the user terminals and the satellites) will operate at the L-band (about 1600 MHz for uplink, and about 1500 MHz for downlink), the feeder links (between the control center/gateway Earth stations and the satellites) will operate at the S-band (about 2.5 GHz), the C-band (4-6 GHz), the Ku-band (12-14 GHz), or the Ka-band (20-40 GHz). These higher bands (Ku and Ka) are used for "cross links" as well (for store and forward transmissions between satellites). The higher bands offer greater bandwidths but the technology needed to implement them is much more expensive.

Generally, lower frequencies imply lower propagation "path loss" and atmospheric attenuation. They also permit lower cost Earth terminal technology. But the size (area of aperture) of the required satellite antenna, which is proportional to the square of the wavelength, can be quite large. The size of the antenna depends greatly on the allowable transmit power: the lower the transmit power, the smaller the antenna. The transmit power in turn dictates the maximum possible antenna gain, which affects the data rates. The desire for mobility requires that the antenna on the user's hand-held terminal to be quite small. Consequently, most MSS systems will provide low data rates (1.2-4.8 kbps).

Another problem facing these MSS providers is the multiple- access technology to be used. While most systems will implement the code-division-multiple-access (CDMA) technology, IRIDIUM opts for the time-division-multiple- access (TDMA). For those who are not familiar with these terms, it is sufficient to say that multiple-access allows many users to occupy the transmission medium (the spectrum or frequency band) at the same time. TDMA accomplishes this by giving each user a time slot in the transmitted frame. While the available spectrum can be subdivided into several TDMA "bands", CDMA signal is spread across the entire available bandwidth. CDMA techniques, used for years in radio local area networks, are less susceptible to interference, and are more robust. However, their implementation will be more costly. I may add quickly that most MSS do not use a pure CDMA or TDMA scheme, but rather a combination of CDMA and FDMA (frequency-division-multiple- access) or TDMA and FDMA. Until there is some agreement in the multiple-access technology used, spectrum sharing and licensing rules can not move forward.

Of the satellite communications systems mentioned only International Maritime Satellite Organization (INMARSAT) system is operational, with AMSC MSAT system running close behind (perhaps as early as mid 1995). All MEO and LEO systems are still in the planning stages with the anticipated operational date sometime in the calendar year 1998 at the earliest. A big problem facing the LEO systems is the large number of satellites required, hence higher investment. Investor reluctance will remain a significant factor.

Despite the many predicted problems, these MSS will become part of our lives: business executives will find a need to conduct business using "cellular phone" while on a cruise in the middle of the ocean.

Viet-Dung Hoang, Ph.D.

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