In this paper, we analyze a technique for accurately detecting transmitted data symbols contained in a modulated signal that has been degraded by a linear dispersive channel and additive Gaussian noise. The approach uses an adaptive equalizer which provides preliminary decisions to an adaptive canceller. The canceller output is used to remove the interference from an adaptive matching filter, resulting in the desired signal. Channel equalization attempts to invert the channel transfer function, while avoiding excessive noise enhancement. However, cancellation (as used in echo cancellers), attempts to generate a replica of the interfering signal and subtract it from the actual received signal containing the sum of the desired signal and interference. The cancellation approach, unlike equalization, offers the possibility of removing interference without enhancing the level of noise already present in the received waveform. Simulation results for transmission over practical channels show significant improvement of linear cancellation over both linear forward and decision-feedback equalization.

Adaptive cancellation of intersymbol interference for data transmission

The general usage of multibeam antennas in satellite communication system is reviewed, and design constraints for a six-beam reconfigurable satellite antenna system are considered. These show that losses in the variable beam-forming network (BFN) Iimit performance achievable with a conventional common-power-amplifier/receiver system. An alternative design for an active BFN is presented, and relative performance predicted at 4/6, 11/14, and 20/30 GHz.

Technology Considerations for the Use of Multiple Beam Antenna Systems in Communication Satellites

The problem of synthesis of an optimum shape of a planar aperture and optimum amplitude-phase distribution over this aperture corresponding to a given power pattern is considered. The functional, which ensures minimization of the mean-square deflection of the magnitudes of the given pattern and of the synthesized one in the area of the main lobe and minimization of the sidelobes, is used as the optimization criterion. The problem of determination of the optimum functions of the shape or a planar aperture and of the excitation over this aperture is reduced to the determination of solution of the system of two nonlinear integral equations. The numerical algorithms of solution of these equations are based on utilization of the method of successive approximations and the Newton-Kantorovich method. The numerical results of synthesis of partial power patterns with rectangular and triangular outlines are presented.

Synthesis of amplitude-phase distribution and shape of a planar antenna aperture for a given power pattern

Hybrid antennas, the combination of a radiating or receiving aperture with an array feed, have been the subject of considerable recent interest because they offer some of the versatility of phased array antennas at a much lower cost. Adaptive nulling in hybrid antennas has not received as much attention as other important aspects, such as compensation for reflector surface distortion, scanning, pattern calculation, and the design of multibeam antennas. In this report we investigate adaptive nulling in hybrid reflector antennas using the Generalized Sidelobe Canceller algorithm. A general framework is presented for the computer simulation of narrow band adaptive nulling in hybrid reflector antennas that takes account of the details of reflector scattering and polarization. The general framework is then illustrated by the results of calculations performed for an offset paraboloid with a planar feed located in the focal plane of the paraboloid.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a275797.pdf

Adaptive Nulling in Hybrid Reflector Antennas

Two problems of multifixed-beam satellite antennas have been 1) gaps in coverage in a given field of view, and 2) use of polarizations which are sensitive to cross coupling caused by precipitation in the earth's atmosphere. These problems can be solved, and in addition, large frequency reuse can be achieved, by application of cluster feeds to an offset Cassegrainian antenna. New features described here include a) interleaving of cluster feeds, b) use of linear orthogonal polarizations distributed to lie in local vertical and local horizontal planes on the earth's surface, c) clusters of keystone-shaped feed elements, d) a system for interconnecting uplink and downlink beams, and e) a method for evaluating aberration-sensitive antenna parameters. It is found that aberrations are negligible for beams aimed within the contiguous United States.

Multifixed-beam satellite antenna with full area coverage and a rain-tolerant polarization distribution

A TEM lens antenna is described that provides the capability of generating 61 low-sidelobe dual-polarized beams over a 17° field of view, at both 4 and 6 GHz. Isolation between these beams is examined, and it is concluded that 6 of the available 61 beams may be used simultaneously in many different positions, while maintaining at least 27 dB isolation, for a total of 12 times frequency reuse.

E. Matthews

Papers

Design tradeoffs for muitibeam antennas in communication satellites