Communications By Means Of Low Earth Orbiting Satellites

Raymond L. Pickholtz

1 Introduction: Personal communication by satellite

Many organizations have proposed satellite-based personal communications providing global coverage for voice and data to hand-held subscriber units with a direct link to a satellite. Such systems will offer the ultimate promise of personal communications of allowing communications to the person at any time and in any place, literally, rather than to a physical location. The projected system capital costs are estimated variously as being between $1-4 thousand million dollars. While satellite communications for trunked transoceanic telephone traffic has seen successful operation for more than a quarter of a century, the application of satellites to mobile communications applications and for direct broadcasting is a relatively recent development. To some extent, the change in focus was due as much to the "push" of competition from wideband digital, transoceanic optical fiber cable as to the "pull" of satellites being able to service a new market of mobile communications and, it is hoped, multimedia, nomadic personal communications, and computing. The unique opportunities that satellites have are for covering entire globe with minimal terrestrial infrastructure. Thus, initially, at least, satellites can be used for obtaining telephone and data communications in parts of the world where none exists at present.

Traditional communication satellites reside in a geosynchronous equatorial orbit (inclination 0°) at an altitude of 35,784 Km. While such systems offer significant advantages in terms of coverage per satellite and other benefits, this is not the typical approach being taken by current developers of mobile satellite communication systems in general, and for Personal Communications in particular. In the next section, we discuss, briefly, the issues that drive towards Low Earth Orbiting Satellites (LEOS).

2 LEOS, MEOS, and GEOS

During WARC'92, several frequency bands were established for mobile use on a worldwide basis. Since then, there have been numerous proposals for implementing such systems. There are proposals for using Geosynchronous Orbit Satellites (GEOS) at an altitude 35,784 Km, Medium Earth Orbit Satellites (MEOS) at 5,000-10,000 Km, and Low Earth Orbit Satellites (LEOS) at 150-1,500 Km. Of these, LEOS have attracted the most attention because of technical advantages and the novelty of having many satellites, handoffs, and a cellular-like configuration. The advantages include small propagation loss so that handsets could be used for direct communication from a mobile user, and small propagation delay (about 10 ms compared to 250 ms for GEOS) for better performance of voice, data, and other interactive services. In addition, LEOS do not suffer from consistent low elevation angles at high latitudes and the associated propagation anomalies that GEOS do. Disadvantages are that more satellites are required - offset by cheaper launch costs - and increased probability of shadowing and increased Doppler shifts.

LEOS orbits are placed beneath the lower of the two Van Allen radiation belts as shown in Figure 1 so as to minimize the radiation damage to electronic components that would result from a relatively unshielded, lightweight satellite. Extensive ionizing radiation severely reduces useful satellite life. On the other hand, such lower orbits experience slight, but greater atmospheric drag than higher orbits and, this too, reduces satellite life, thereby forcing smaller solar cell arrays and less primary power. LEOS satellites with significantly inclined or polar orbital constellations can easily cover higher latitudes and, to some extent therefore, even avoid blockage from tall structures.

There are additional major issues distinguishing LEOS and MEOS from GEOS. Some of the major issues are due to the fact that the former do not appear stationary to a ground user. Thus, the satellites in a LEO (MEO) orbit will pass overhead from horizon to horizon in a short time. This requires the implementation of a handoff mechanism and tracking. The additional design challenges include required worldwide coverage, maximizing system user capacity, allocating margins of fading, controlling interaction delay, call handoffs, spectrum sharing, maximizing handheld battery life, and restricting user health hazards (transmit power). The frequency bands to be used for LEOS is 1616.0-1626.5 MHz for the uplink and 2483.5-2600 MHz for the downlink if frequency division duplex (FDD) is used and just the former band if time division duplex (TDD) is used. Of five original proposals for "Big" LEOS, four have opted for CDMA in one form or another, and only one has proposed FDMA/TDMA. Because of this, and general interest in CDMA in LEOS, we have focused our effort in this paper on LEOS CDMA and, in particular, how it differs from terrestrial CDMA which has already demonstrated its virtues and has become one of several cellular standards in the United States and elsewhere.

As in cellular communications, maximizing area capacity (users/Km2) requires a sufficient number of spot beams (cells) per satellite necessary and, to minimize interference, consideration must be given to frequency reuse patterns. The ephemeral visibility associated with low orbit means that, for LEOS and MEOS, continuous communication requires a "hand-off' procedure between spot beams and satellites, not unlike cellular terrestrial systems when the user moves from one base station coverage zone to another.

Leos Procedure

Figure 1: Relative flux levels of Van Allen radiation belts [Wu et a I., 1994], To understand the required number of satellites, N, for global coverage, we can consider multiple polar orbits of many satellite per orbital plane. In this case, the orbital planes of each satellite group within an orbit remains fixed (unlike inclined low orbits whose orbits drift due to the oblateness of the earth). We like to minimize N = PS, the total number of satellites to get full global coverage, where P is the number of polar planes and S is the number of satellites per plane. It is possible to solve this geometric optimization problem [Werner et al, 1995] and

Figure 1: Relative flux levels of Van Allen radiation belts [Wu et a I., 1994], To understand the required number of satellites, N, for global coverage, we can consider multiple polar orbits of many satellite per orbital plane. In this case, the orbital planes of each satellite group within an orbit remains fixed (unlike inclined low orbits whose orbits drift due to the oblateness of the earth). We like to minimize N = PS, the total number of satellites to get full global coverage, where P is the number of polar planes and S is the number of satellites per plane. It is possible to solve this geometric optimization problem [Werner et al, 1995] and where y is the earth central angle, i.e., the angle from the earth's center to the edge of a single satellite "footprint" circle on the earth. For example, the original proposed IRIDIUM constellation consisted of 77 satellites: P = 7, S = 11 yielding y = 18.44° and from this we can determine the minimum elevation angle on the ground. This established the satellite orbit altitude for a minimum elevation angle in the ground of 8.2°. Later, due to improvements in design, a slightly higher orbit was chosen so that N = 66 and P = 6, S = 11. For non-polar orbits, it may be possible to use a smaller number of satellites if certain regions of the earth are not covered as well.

LEOS have also been functionally subdivided as "Little LEOS" and "Big LEOS" with the Big LEOS offering wide band voice and data and the Little LEOS offering paging, short message services, utilities meter reading, inventory location, etc.

System (company) s

No. satellites

Orbit inclination (altitude, Km)

Spot beams per satellite

Multiple access/duplexing processing

Launch date

IRIDIUM (Motorola et al.)

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