Signal Distortion in Multibeam Broadband Active
Transmit Arrays with Time Domain Beamsteering
Randy L. Haupt and Manoja Weiss
Ball Aerospace
Westminster, CO, USA
Abstract—An 8 beam planar phased array can transmit 8 signals
in 8 different directions. The array is assumed to have 8
corporate feeds. The signal transmitted by a beam consists of the
desired signal that is coherently combined from all the elements
plus small amounts from the other 7 signals. These other signals
distort the desired signal. This paper shows examples of these
distortions.
I.
outputs from the appropriate TDUs are combined and sent to
the corresponding elements to be transmitted. Each element is
assumed to have a cosθ element pattern. The normalized far
field pattern with the 8 beams labelled is shown by the u-v plot
at 10 GHz in Figure 2.
INTRODUCTION
One of the advantages of a phased array antenna in a
communications system is the ability to generate multiple
beams [1]. Each of the beams can transmit different signals at
different carrier frequencies. If the array is broadband, then
either the signals and/or the difference between the highest
and lowest carrier frequencies are large. Signals between the
beams may interfere with each other much like intersymbol
interference (ISI). The signal in one main beam may be
contaminated through signals in the sidelobes and/or main
beam of the other beams. Making the beams orthogonal is not
a practical solution.
Figure 1. Diagram of a corporate fed multibeam array.
Intermodulation products generated by power amplifiers in
an active phased array radiate unintended beams when phase
shifters are used to scan [2] [3] [4] [5]. Using time delay units
instead of phase shifters avoids any frequency squint in the
main beams.
This paper shows the signal distortion resulting from the
interactions between the multiple beams. The broadband
planar array has 8 beams transmitting 8 different signals at the
same carrier frequencies. The binary phase shift keying
(BPSK) signals in the far field main beam show significant
distortion from the intended transmitted signals.
III. BROADBAND MULTIBEAM ARRAY MODEL
Assume that a 10x10 element multibeam communications
array transmits 8 different signals in 8 directions. The array
scans a 45 degree cone over a frequency range of 8 to 12 GHz.
It has a square grid with an element spacing of 1.46 cm. The
BPSK signals have a data rate of 300 bps.
Figure 1 is a diagram of the feed network for the 100
element planar array. There are 8 signals for the 8 beams.
Each beam is split 100 different ways and sent to a total of 800
time delay units (TDUs) that steer the beams. Next, the
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Figure 2. Array pattern in u-v space at f=10 GHz.
IV. SIGNAL DISTORTION EXAMPLE
An example of the BPSK signal transmitted by this array
on a 10 GHz carrier is shown in Figure 3. The eight signals
receive an appropriate time delay for each of the 100 elements
in the array. The 8 time delayed signals are combined prior to
the element. Examples of the signals at 6 of the 100 elements
are shown in Figure 4. Combining the 8 signals that have
different bit streams results in a composite signal that has a
time varying amplitude envelope with an amplitude as high as
8 if they all add in phase.
GHz, but each signal could have a different carrier frequency
over a wide bandwidth. Also, this example assumed that the
BPSK signals were not orthogonal.
Figure 3. Example of a BPSK signal.
Figure 5. Far field signals in each beam.
Figure 4. Signal at elements 1, 11, 22, 33, 44, and 55.
REFERENCES
The composite signals at each element radiate to form 8
different beams. Figure 5 shows the signal radiated in the far
field by each of the 8 beams. These signals are distorted
representations of the desired transmit signals. The amplitude
distortion results from sidelobe and main beam interference
from the other beams, as well as interference between the
different signals. This distortion does not include
intermodulation products generated by nonlinearities in the
power amplifiers or channel distortion. The dynamic range of
the signals at each element determines the dynamic range of
the power amplifiers.
[1]
[2]
[3]
[4]
[5]
IV.
CONCLUSIONS
The beams of a multibeam array are in general not
orthogonal, so they may interact in such a way as to cause an
amplitude modulation on a constant envelope BPSK signal.
The example presented used the same carrier frequency at 10
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