In future satellite communication systems, demands for faster access and more information are expected to increase because of the continuous growth of the Internet and direct-to-user satellite requirements. Meeting these requirements will require multibeam satellite systems having an onboard active phased-array antenna system. Phased-array-antenna-based communications links are anticipated to deliver high data rates without the risk of the single-point failure of the gimbaled motors and transmitters used in reflector-based systems. Phased-array antennas contain a multitude of radiating elements, typically arranged in a rectangular or triangular tessellation. Beams are formed by electrically adjusting the relative phase of the radiating elements using ferrite or semiconductor devices. Phased-array antennas have been developed mainly for radar applications but are being used more now for space-based communications applications because of their advantages in scanning, reconfigurability, weight, and power.
Theoretical model of phased-array-antenna intersymbol interference. Left: quadrature phase-shift keying/binary phase-shift keying (QPSK/BPSK) worst-case bit error rate performance. Right: System loss-QPSK and BPSK.
Operation of high-rate, high-frequency phased-array systems has been shown theoretically to degrade performance on the order of 3 to 4 dB at large scan angles (see the graphs). Determining the extent of these degrading system effects requires system-level characterization techniques, which are a battery of specialized tests that include modulation and antenna correlations under dynamic operating conditions. Once these degradation effects are fully characterized, mitigation techniques will be proposed and tested. The specialized tests are being performed experimentally in NASA Glenn Research Center’s Far-Field Anechoic Chamber. In addition, Glenn engineers are using Matlab/Simulink to perform theoretical computations.
In the past year, static characterization tests have been initiated using a 91-element receive array. The tests performed so far include the effects of beam-steering transients and wide-angle scanning. Wide-angle scanning degradation is caused by the introduction of intersymbol interference into the link. The intersymbol interference is due to the timing error between signals radiating from the array elements caused by a physical 360° limitation in the phase shifters. The results of the wide-angle scanning characterization performed to date indicate <1-dB performance savings for medium data rates of 220 Mbps, but they validate the model and expectation of 2- to 3-dB savings for higher data rates as envisioned by NASA’s major enterprises.
Transient effects in the radiofrequency (RF) communications channel are observed when the beam of a phased-array antenna is switched from one pointing direction to another. Such effects can be attributed to electrical ringing of the antennas’ phase shifters or to system implementation effects such as the nonsimultaneous switching of the phase shifters. In particular, testing has revealed implementation-specific transient effects in the RF channel that are associated with digital clocking pulses that occur with transfers of data from the beam-steering controller to the digital electronics of the phased-array antenna being tested. Effects of the digital clocking pulses are observed prior to the beam switch, and they have a significant effect on the RF path of the system. Observed changes to the RF channel clearly increase the average bit error rate in the individual symbols during beam-switch transient events, however, a potentially more serious effect is the loss of receiver synchronization with the communications signal brought by the transients.
Phased-array antennas are highly advantageous in a low-Earth orbiting satellite system where high data rates direct to users on the ground are required. Experiments are being conducted at Glenn to characterize high-data-rate phased-array links in a simulated low-Earth-orbit environment. These experiments characterize phased-array antennas interacting with various modulation techniques at high data rates, and other variables to provide a more complete understanding of the operational characteristics that are experienced in a low-Earth-orbit environment. In these experiments, the low-Earth-orbit environment is simulated by dynamically scanning the array while simultaneously rotating the range pedestal in the complementary direction. The optimal beam switch pattern for when the satellite passes over a user can be verified. The following block diagram shows this dynamic system emulation capability.
Dynamic system emulation.
Long description of figure 2.
A savings of 2 to 3 dB in array-based system performance could reduce the number of antenna elements by 25 to 30 percent, saving cost and prime power. Once system losses have been identified and mitigated, system designers no longer need to design added link margin to cover these losses. Thus, addressing these losses in advance reduces the antenna size, yielding cost, power, and mass savings. Glenn’s facilities provide an enviroment to operate the beam-steering control of array antennas in a manner expected for flightlike, aerospace, or surface mobile scenarios. Dynamic array scanning with synchronized pedestal control allows system engineers to simulate typical array operation and gather firsthand experience in array behavior and expected system performance under realistic conditions.
Find out more about NASA's Space Communications Program.
Glenn contacts: Sandra Johnson, 216-433-8016, Sandra.K.Johnson@nasa.gov; Richard Reinhart, 216-433-6588, Richard.C.Reinhart@nasa.gov ; Dr. Obed S. Sands, 216-433-2607, Obed.S.Sands@nasa.gov; and Dr. Roberto Acosta, 216-433-6640, Roberto.J.Acosta@nasa.gov
Authors: Sandra K. Johnson, Richard C. Reinhart, Dr. Obed S. Sands, and Dr. Roberto J. Acosta
Headquarters program office: OSF, Space Communication and Data Systems
Programs/Projects: Earth and space science missions in near Earth orbit, constellation and ad hoc networks, space communications
Last updated: June 25, 2003
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