Global communication often relies upon satellites to relay information. A typical satellite communications system is illustrated in FIG. 1. To send information on a forward channel from a hub 30 to a client 32, the hub converts the information into a radio frequency uplink signal 34 which it transmits to a satellite 36. Satellite 36 transmits a downlink radio frequency signal 38 to client 32. Satellite 36 produces the downlink radio frequency signal 38 by amplifying the uplink signal and shifting it to a different frequency. The client 32 processes the downlink radio frequency signal 38 to reproduce the information. To send information on a return channel from client 32 to hub 30, the process is reversed. Client 32 transmits an uplink signal 40 to satellite 36 and then satellite 36 transmits a downlink signal 42 to hub 30. In this context, the term client may include a fixed client or a mobile client such as a client onboard an aircraft in flight. The hub is simply another client that may include management functions.
Typically, the client antennas are highly directional. In other words, the gain is much stronger in the direction the antenna is aimed, called the boresite, than the gain in other directions. When receiving, a directional antenna is much more sensitive to signals arriving from the boresite than it is to signals arriving from other directions. Several factors limit the gain of antennas as a function of angular offset from the boresite. Beamwidth is commonly defined as the angular span between points three dB down from the boresite peak gain (half power). Antennas provide discrimination through higher gain along the boresite than in other directions. Beamwidth varies inversely with aperture width, or span in physical terms. Wider aperture results in smaller beamwidth. In some applications, such as mobile users, large antennas are impractical. Smaller antennas have larger beamwidths and thus less discrimination towards adjacent satellites when compared to larger antennas. Beam steering error occurs between the intended and the actual absolute pointing of the antenna boresite, both at the hub and the client, and for both the transmit boresite and receive boresite, which are not necessarily coincident.
Communication must contend with interference from users of adjacent satellites. FIG. 2 illustrates interference associated with an uplink. Due to the limits of directional antennas, although unrelated clients 44 and 46 are aiming their signals 48 and 50 at satellites 52 and 54 respectively, attenuated signals 56 and 58 will also arrive at target satellite 36. These interfering signals are then included in the downlink signal transmitted from satellite 36. As shown in FIG. 3, a similar interference issue applies to downlink communications. Downlink signals 60 and 62 from adjacent satellites 52 and 54, intended for unrelated clients, arrive at client 32 along with the intended downlink signal 38 from satellite 36. The lower gains based on the client 32 receive antenna radiation pattern towards satellites 52 and 54 attenuate but do not entirely remove the interference signals.
The received radio frequency signal is the combination of the encoded radio frequency signal with useful information, noise, and various attenuated interference signals. The presence of noise and interference signals may occasionally result in errors when the receiver decodes the received signal to recover the information. The error rate is influenced by the ratio of the intended signal strength to the noise and interference signal strength, the information rate, the modulation, carrier frequency, guard bands, symbol rate, and error-correction or coding parameters.
A common practice for communication via geosynchronous satellites is to establish the boresite gain, modulation parameters, and coding parameters based on managing interference to adjacent satellites. A model of the antenna radiation pattern is developed for each antenna based on range measurements to predict the off-axis gain in relation to the boresite gain at various relative azimuth and elevation off-axis angles from boresite. A combination of satellite position, skew angle, beam steering error, etc. resulting in an estimate of the attenuation to adjacent satellites is determined. A boresite gain that will satisfy the regulatory maximum interference to adjacent geostationary satellites in this worst case condition is selected. Modulation and coding parameters are then selected based on maximizing information rate with acceptable error rate while compliant under the worst case. Lower discrimination towards adjacent satellites results in increased bandwidth (spectrum) requirements for smaller antennas than larger antennas, for any given situation.