TOOLS OF XCELLENCE
eamforming is a signalprocessing technique that utilizes an array of sensors to achieve directionality, increase the strength of transmitted signals and improve the quality of received signals. Communications, radar, countermeasures, weapons systems, oil and mineral exploration, medical imaging and direction finding make extensive use of beamforming. In direction finding, we steer the beamformed antenna to locate the arrival angle of a signal source. We can use two or more arrays to triangulate the exact location of the source, which is essential for many signal intelligence and counterterrorism efforts. The accuracy of this technique depends on the exact settings of gain and phase among the beamforming channels. We used a Pentek product built around a Xilinx® Virtex®-6 FPGA with native and custom IP to achieve these fine adjustments that improve system performance and accuracy.
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PRINCIPLES OF BEAMFORMING We typically use beamforming with an array of sensors or antennas to improve receptivity in a specific direction, for example, from a single cell phone as shown in Figure 1. The signal from this source arrives at each antenna based on the distance between the source and the antenna, so the antenna signals have relative phase and amplitude offsets. The beamforming process adjusts the gain and phase of each antenna signal to compensate for the different delays in the signal paths. The adjustments align the signals from each antenna with the signals arriving from one particular direction. When the signals are summed together, the nonaligned signals arriving from other directions cancel each other, while the signals from the beamformed direction add constructively for greatly improved signal-to-noise ratio. In this way, by electronically adjusting the gain and phase in each path, we effec-
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tively steer the antenna toward the direction of the signal source. EIGHT-CHANNEL SYSTEM In this system, we arranged eight antennas in a linear array, as shown in the overall block diagram of Figure 2. The antenna frequency here is 2.5 GHz, so each antenna signal needs to be amplified, filtered and then downconverted to an intermediate frequency (IF) so that an A/D converter can digitize it at a reasonable sampling rate. It is mandatory to use synchronous sampling across all eight channels in order to preserve a fixed phase relationship for beamforming. We then downconvert samples from each A/D to the baseband’s complex I+Q signals in a digital downconverter (DDC), which also includes channel-specific phase and gain adjustments for the beamforming “weights.” Finally, we add together all eight baseband signals in summation blocks to produce the beamformed
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Figure 1 – Typical cell phone beamforming system
Third Quarter 2012
Xcell Journal
59