The performance of modern satellite communication systems can be significantly improved by the employment of directive antennas with adaptive beam forming and steering on the satellite and at the receiver end. Impairments of the reception caused by multi-path propagation and other influences of noise and interference can strongly be reduced by a limited beam width, appropriate side-lobe reduction and generation of nulls. All these measures finally lead to a better quality of service (QoS). Moreover, the use of multiple independent beams enables spectral domain multiple access (SDMA) and thus an increase of the capacity of the communication system. The limitation of the beam width corresponds to a gain that directly improves the signal to noise ratio. Thus, satellite payload and user terminals can run with lower RF-power.
Communication systems with non-geostationary satellites (LEO, MEO, HEO) offer only a temporally limited visual contact to the individual satellite. For LEO satellites the visibility is less than 15 min. The beam of the terminal antenna must therefore track each satellite and also compensate for any movements of a mobile terminal. The latter must take place in any case, also with geostationary satellites (GEO).
| | Figure 1: Cross section of an active phased array
| | Figure 2: Transmit and receive module of an antenna array with digital beamforming
| | Figure 3: Concept of the high-rate data relay between a GEO satellite and several LEO satellites
| | Figure 4: Sketch of a possible configuration of the satellite multibeam antenna
Beam forming and steering of an adaptive antenna can take place on analog or digital level. The conventional phase and amplitude-controlled antennas (phased arrays) use electronic phase shifters and variable amplifiers (Figure 1). A multibeam architecture is possible, but very complex.
A promising technology for the realization of an adaptive antenna with multibeam capability is the digital beamforming (DBF). In combination with planar radiating elements the possibility of integration with active elements is given to set up complete transmitting and receiving modules (Figure 2). By their flat or also conformal architecture these antennas can be particularly well integrated in the surface of vehicles or airplanes.
In contrast to conventional phased-array configurations, which require complex multibeam architectures, active electronic components are completely avoided on the RF-level. Moreover, by shifting the beamforming procedure into the digital processor an enormous increase of flexibility is obtained. The accuracy can be additionally enhanced by calibration and suitable algorithms for error correction and thus kept independent of external physical influences to a large extent.
In recent years, the amount of data acquired by single low-earth-orbit (LEO) earth observation satellites has grown tremendously. Therefore, the relatively short period of time (typically ~10 min), during which a given earth station can maintain contact to a passing LEO satellite to download all its gathered data, represents an increasingly serious bottleneck. An elegant way to circumvent this bottleneck is the utilization of a satellite in geosynchronous-earth-orbit (GEO) to redirect LEO signals with high data rate to a ground station (Figure 3). Thus, long contact times to LEO satellites based on only one ground station can be obtained, since the majority of LEO satellites will be visible for the GEO satellite for at least 50% of their orbital period. For this purpose, novel receiving antennas are required, that generate several individually trackable beams. Purely mechanically steered antennas are not suitable.
The required high gain for transmissions of signals with high data rate at around 26 GHz and the multibeam capability are realized by combining a reflector with an electronically steerable antenna array as feed (Figure 4). In this way, multiple beams can be generated and the antenna can track LEO satellites independently by switching between different subarrays. A reconfigurable switch matrix connects the outputs of the subarrays involved in the reception to the transponder frontends and the signal processing electronics. Additionally, digital beamforming performed on the level of subarrays can improve the reception characteristics. In cooperation with universities and space industry a novel electronic steerable multibeam antenna with digital beamforming is being designed. A demonstrator showing the basic functionality of the antenna and its qualification for the intended application is in preparation. Environmental tests are performed on some of the most critical components of the antenna.