Aerospace and Electronic Systems Magazine May 2017 - 49

Wang

Figure 4.

Comparisons between proposed LPI FDA and basic FDA.

method, the advantage of this scheme yields a spoiled beam pattern
in both range and angle dimensions, not just in an angle dimension. However, note that the formed high-gain beam pattern will
be coupled in the range and angle dimensions (similar to Figure 3)
due to the coupling range and angle response in the FDA. This coupling response limits its capability in some specific applications;
for instance, suppressing or locating range-dependent interferences
or targets. Therefore, there should be future work to decouple the
range and angle response in beamforming the high-gain beam pattern. A potential solution is to further adopt a multiple-input multiple-output (MIMO) technique [24]. In doing so, we can formulate
FDA-MIMO radar schemes.

LPI RADAR USING DM BEAMFORMING

Figure 3.

Transmit beam pattern comparisons of FDA and phased array with
12 elements, where Δf = 30 kHz, f0 = 10 GHz, and d is equal to a half
wavelength.

When uniform excitations are adopted, the far-field radiation pattern can be expressed as [23]
a0 ( r ,θ , t ) =

M −1

e

m=0

j 2πΔft − jmβ f 0 d sin θ

e j βΔf m r e − jα m e − jmβΔf m d sin θ ,

(9)

where β = 2π/c0. The sets of spoiled frequency increments {Δfm}
and phases {αm} should be judiciously designed to create a lowgain, "spoiled" beam pattern in both range and angle dimensions.
Following Ref. [9], we can design the values in a multidimensional gradient search routine using randomly selected quadratic
phase as a starting value to minimize the array gain. Similarly,
additional low-gain basis patterns can be formed by applying a
linear-phase progression to the fundamental basis pattern in an
incremental way. When compared with the phased array-based
MAY 2017

Along with technological development, integrating radar and
communication are becoming necessary because radar and wireless communication systems have increasingly similar radio frequency (RF) front-end architectures [25], [26] and greater demand
on bandwidth against a strictly finite RF electromagnetic spectrum
resource [27]. By using a joint waveform for both applications, the
occupied spectrum would be very efficient, and both applications
could be operated simultaneously. There are many areas of applications that could benefit from the availability of integrated radar
communication systems; thus, it is necessary to develop physical
layer secure techniques for this integrated system.
The DM technique using phased arrays was proposed in [15].
By changing the phase of each element, a symbol with the desired
phase and amplitude, enabling distinct modulation constellation
points that could be decoded by an intended receiver, can be created along a particular direction, while purposely distorting the
constellation to be difficult to demodulate in other undesired directions. This technique has been further extended in [28] to the dualbeam DM technique for physical layer secure communications. In
[29], the authors introduced a far-field DM beam pattern synthesis
approach, which could be separated into information patterns describing the information energy projected along each spatial direction and interference pattern, respectively. This implies that highgain beams can be coherently formed in the desired direction, but

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Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine May 2017