Designing OFDM Radar Waveform for Target Detection Using Multi-objective Optimization - Advances in Heuristic Signal Processing and Applications - page 37

Fig. 3.1 Principle of adaptive waveform design in a radar operating in the multipath scenarios

the target-detection capability can be significantly improved by exploiting multi-
ple Doppler shifts corresponding to the projections of the target velocity on each
of the multipath components. Furthermore, the multipath propagations increase
the spatial diversity of the radar system by providing extra “looks” at the tar-
get and thus enabling target detection and tracking even beyond the line-of-sight
(LOS) [ 28 ]. To resolve and exploit the multipath components it is common to use
short pulse, multi-carrier wideband radar signals. We consider OFDM signaling
scheme [ 34 , 36 ], which is one way to use several subcarriers simultaneously. The
use of an OFDM signal mitigates possible fading, resolves multipath reflections,
and provides additional frequency diversity as different scattering centers of a target
resonate at different frequencies [ 27 , 54 ].
First, we develop a parametric OFDM measurement model for a particular range
cell, to detect a far-field target moving in a multipath-rich environment. We assume
that the radar has the complete knowledge of the first-order (or single bounce) spec-
ularly reflected multipath signals. Using such knowledge of the geometry, we can
determine all the possible paths, be they LOS or reflected, from the range cell un-
der test. However, in practice the target responses reach the radar only via a limited
number of paths depending on the position of the target within the range cell. There-
fore, considering all the possible signal paths and target velocities, which represent
themselves as varying Doppler shifts at the radar receiver, we convert the OFDM
measurement model to an equivalent sparse model. The nonzero components of the
sparse vector in this model correspond to the scattering coefficients of the target at
the true signal paths and target velocity.
The formulation of a sparse-measurement model transforms a target-detection
problem into a task of estimating the nonzero coefficients of a sparse signal. To