IEEE - Aerospace and Electronic Systems - December 2019 - 23

Bockmair et al.
Adaptive nulling: Interferences from other frequency users are sometimes limited to certain spatial directions. The signal-to-interference ratio for
these specific directions is very low and the radar
cannot detect the targets as desired. If this is recognized by a cognitive radar, the radar signal emission
can be restricted toward the interference source
directions in order to concentrate the limited resources time and energy to the other angles of interest.
Adaptive nulling can also be applied as a cooperative method to avoid interferences between the
radars belonging to one military unit or group.
Frequency adaptation: Frequency ranges with the
lowest interference level are identified and selected
for the radar operation. This system capability
requires that the radar continuously observes its
whole operational bandwidth. If necessary, also the
radar signal bandwidth can be adapted to the current
interference situation. Here, a balance between the
bandwidth reduction (leading to degradation of
range resolution) and a larger bandwidth (increasing
reception of emissions from the other users) has to
be performed by the radar resource management.
Dynamic adaptation of the search scheme and emission power level: If a radar identifies interferences with
specific temporal patterns in the received signals, the
own spatial scan pattern can be adapted and synchronized, e.g., to avoid interferences from the main lobe
of the interfering system into the radar's main lobe.
The analysis of interference scenarios has shown that
the interferences originating from the main lobe emissions of the interferer into the radar main lobe are critical, while the mutual interferences from the side lobes
can be mitigated effectively by measures in the signal
processing of the radar receiver. If the radar is coordinated within a network of sensors, the search schemes
and spatial portions can also be adapted cooperatively
among the systems.

MEASURES FOR IMPROVED COEXISTENCE CAPABILITY
ENABLED BY THE SIGNAL PROCESSING OF THE RADAR
RECEIVER
Detection of interfered samples in the receive signal:
For suppression of interferences in the receive signal,
the interference has to be detected first. Therefore, signal characteristics have to be searched that are different
between the unwanted signal and a real target echo,
which is a reflection of the own emitted waveform.
Suppression of interfered samples in the radar signal: Singular interference peaks in the receive signal
can be eliminated if they are identified and the signal can be repaired afterward. For this purpose, the
DECEMBER 2019

Figure 2.
Interference mitigation by ABF: simulation results for a target
without interference (top), a target in presence of broadband
interference (middle), the target visible again when interference is
suppressed by an ABF (bottom).

distorted value can be replaced by the result of an
interpolation of the neighbor values.
Adaptive beamforming (ABF): If the interference is
located in a certain spatial direction, it can be suppressed by ABF on receive [9]. For a radar with multiple separate receive channels, a directional null can be
generated in this direction (see Figure 2 for example).
Time blanking: If a temporal scheme of the interference can be detected, the dimension "time" can be
used to suppress the interference on receive. Here, the
receiver is turned OFF temporarily, when a strong interference arrives, or the associated data is omitted after
the digitalization, respectively. With cognitive capability and a learning process, the expected time of
arrival of the interference signal can be predicted.
Switching OFF the receiver can avoid that nonlinearities arise by saturation in the receiver. However, this
measure produces constraints by periodic events in
the radar data matrix. Generally, data that is assumed
not plausible are discarded by an intelligent radar.
Desensitization of the receiver: If the interference
energy is very high and the receiver uses a sensitivity
versus time control circuit in order to avoid the saturation, the input signal power can be attenuated
(receiver desensitized) and further processed but with
the reduced sensitivity.

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IEEE - Aerospace and Electronic Systems - December 2019

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