Adaptive antenna technology has enjoyed considerable development since its introduction in the 1950s. The Aerospace Corporation has followed this research and has received several U.S. patents for innovative concepts.
One development devised a means to incorporate a priori antenna design information into the adaptive cancellation process. For example, an array of antenna elements can be arranged in a straight line to produce a low-sidelobe pattern. The positions and levels of these sidelobes are known from the array design; therefore, if interference is detected, the system can determine which sidelobe contains the interference by sequentially sampling them all with a separate antenna beam obtained from the same array of elements. This beam can then be subtracted from the quiescent array pattern. The technique, known as "sidelobe annihilation," avoids the usual time-consuming recursive determination of the adaptive weight values and has potential application in future wireless systems.
Other research has focused on protecting a very-high-data-rate (greater than 500 megabits per second) uplink antenna. In this case, the spacecraft has a low-sidelobe, high-gain antenna with a very narrow beamwidth. The combination protects against interference not generated in the immediate vicinity of the ground terminal. Interference that is generated near the terminal is reduced by repositioning the uplink antenna so that interference arrives at a low-gain portion of the main beam. This repositioning is guided by a low-level coded signal attached to the desired signal. In interference-free conditions, this signal can be used to maintain beam pointing to offset variations in satellite attitude. This approach is advantageous because it obviates the need for auxiliary antennas and adaptive circuitry and avoids the problem of achieving effective cancellation over the wide bandwidth used by the high-data-rate communications.
More recently, a patent was granted for a technique that reduces interference to signals transmitted by an antenna. Aerospace researchers believe it is the first time that adaptive antenna concepts have been applied to transmitting antennas. This development was prompted by the advent of wireless services that use code-division multiple-access modulation, a technique in which multiple channels are independently coded for transmission over a wideband channel. The capacity of such systems can be impaired by multipath components—signals that follow different paths to the receiver. The time delays of multipath signals interfere with the direct signal path, which in turn decreases the isolation between system users and increases channel interference. The standard approach—use of a so-called RAKE receiver—provides the necessary time-delay alignment of the direct and multipath signals. The new Aerospace approach uses the transmitted signals as "radar" signals to detect signal components bouncing off nearby obstacles, such as buildings. These same obstacles that reflect the transmitted signal also generate multipath components. By adaptively processing these signals and reducing their magnitude, the multipath generation will be reduced along with the degradation to other transmitted signals.