Adaptive Interference Nulling and Direction of Arrival Estimation in GPS Dual-Polarized Antenna Receiver - Adaptive Signal Processing in Wireless Communications - page 417

Table 14.1 Weight Vector in Single and Multiple MVDR Methods

Multiple Output Beamforming

MVDR Beam 1 Beam 2

W 1H 0.0182 + 0.0395i -0.0013 + 0.0010i 0.0011 + 0.0019i

W 1V 0.7563 - 0.2890i 0.0435 - 0.0011i 0.0831 - 0.0081i

W 2H -0.0518 + 0.0866i -0.0010 - 0.0009i 0.0008 - 0.0006i

W 2V 0.5980 - 0.0485i 0.1228 + 0.0152i 0.1466 - 0.0469i

W 3H -0.0320 + 0.0181i 0.0013 - 0.0022i -0.0016 + 0.0029i

W 3V 0.0921 + 0.2146i 0.1681 + 0.0541i 0.1335 - 0.0646i

W 4H 0.0566 - 0.0037i 0.0028 + 0.0015i -0.0046 - 0.0041i

W 4V -0.0948 + 0.1721i 0.1371 + 0.1086i 0.0906 - 0.0839i

W 5H 0.0964 - 0.0094i -0.0028 + 0.0023i 0.0066 - 0.0027i

W 5V -0.1696 + 0.0930i 0.0606 + 0.1661i 0.0521 - 0.1188i

W 6H 0.0268 + 0.0249i -0.0015 - 0.0032i -0.0000 + 0.0079i

W 6V -0.1854 - 0.1926i -0.0099 + 0.1735i 0.0256 - 0.1356i

W 7H -0.0652 - 0.0562i 0.0030 - 0.0006i -0.0065 - 0.0023i

W 7V -0.0871 - 0.2303i -0.0281 + 0.1067i -0.0131 - 0.1407i

W 8H -0.0167 - 0.1025i -0.0004 + 0.0024i 0.0038 - 0.0030i

W 8V 0.1837 + 0.0734i -0.0131 + 0.0330i -0.0267 - 0.0809i

all the horizontally polarized interferers. In both cases, this is achieved by reducing the
weights of the horizontally polarized antennas to approximately zero values. The weight
vector values are given in Table 14.1. It is clear that while the horizontal weight at each
antenna assumes a small value to suppress the horizontally polarized interferers, the
vertical weights are organized as a beamformer to spatially cancel the vertical compo-
nents of the RHCP interferers and to receive the GPS signals with unit gain.
In the next example, one of the interferers arrives from the same direction as one of
the GPS satellites, e.g., 27 degrees of elevation angle. In this case, the optimum antenna
array has to maintain a unit gain toward this direction, while trying to null a very
close interference. Figure 14.4 depicts the array responses at the eight dual-polarized
antenna arrays when two 20 dB JNR interferers arrive from 50 and 27 degrees. In this
case, the single MVDR method will maintain a unit gain at 27 degrees, permitting the
nonintentional GPS signal along with the interference to be received with equal sensi-
tivity. Further, the null placed by the beamformer at 50 degrees appears shallow, with
only -10 dB depth, and as such, the respective jammer may not be fully suppressed. On
the other hand, with the multiple MVDR beamforming method, only the beamformer
toward 27 degrees acts similar to the single MVDR beamformer and fails to cancel the
interference. The other three beamformers show regular array responses and success-
fully suppress the two interferers with approximately -30 dB null.
In space-time processing, where spatial and temporal processing are applied jointly
or sequentially, jammers that escape the spatial nulling can be dealt with in the tem-
poral domain using excision filters [19, 40]. However, depending on the jammer signal
characteristics, temporal and spatial filtering may not be able to provide proper jammer
mitigation without compromising the desired signal.