Signal Processing - September 2016 - 114

subaperture and cross-correlating all the recorded, complex
information. Typically, systems distribute a local oscillator to downconvert the captured field data to a lower intermediate frequency, where it can be digitally recorded and
processed. Although well-suited for imaging at microwave
frequencies, the power, size, and space requirements for distributing the local oscillator, the intermediate frequency processing, and construction of the correlation engines present
significant design challenges at mmW frequencies, which
increase cost.
Distributed-aperture mmW imaging
Martin et al. [4] showed that these challenges can be overRecently, a pupil plane, distributed aperture mmW imager was
come by upconverting to optical frequencies and taking advandeveloped by the University of Delaware and Phase Sensitive
tage of existing optical technology for processing and imaging.
Innovations [4] shown in Figure 9. As opposed to a continuous
Electro-optic modulators were used to modulate received milaperture over which radiation is collected to form an image, dislimeter-wave radiation onto the sidebands on
tributed aperture systems sample the incian optical carrier [33]. Optical upconversion
dent radiation within subapertures. This is
Optical upconversion
allows the use of lightweight, flexible fiber
typically done when a continuous aperture is
allows the use of an
optics to route optical energy before and
prohibitive due to scale, e.g., for radio telesafter mmW encoding, which eliminates the
copy. This approach was particularly taken
optical lens to perform
need for cables to distribute a local oscillator.
for mmWs due to the lack of detection techthe necessary correlation
Even more significant, optical upconnology, such as inexpensive silicon-based
required for image
version allows the use of an optical lens to
detector arrays used for detecting visible
formation.
perform the necessary correlation required
radiation. This approach offers important
for image formation. Digital reconstruction
SWAP-related benefits compared with more
requires discrete spatial Fourier transforms and correlations, the
traditional architectures like compound antenna systems and
number of which increases quadratically with the number of
lens-based imagers, since the overall upconversion system size
subapertures. The phase transformation of a lens combined with
scales in two dimension (2-D) versus three-dimension (3-D).
propagation over a distance physically generates the correlations
Image formation in a distributed array requires recordnecessary for image formation.
ing both magnitude and phase of the incident field at each
content that is otherwise lost in a conventional system. The
wider bandwidth, in addition to the noise suppressing characteristics of the Weiner filter, produces images that appear
sharper than those produced by a conventional imaging system. Therefore, one can extend the region over which the system generates diffraction limited images. In fact, [5] showed
that the DoF of a conventional 94-GHz imaging system can be
extended from 17.4 in to more than 68 in.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 8. Images from a conventional imaging system at (a) 113 in, (b)
146 in, and (c) 180 in. (d)-(f) Images from a system with cubic phase at
the same object distances as for (a)-(c). (g)-(f) Processed images from a
system with cubic phase at the same object distances as for (a)-(c) [9].

114

(a)

(b)

(c)

(d)

Figure 9. A 35-GHz, 30-channel distributed aperture imaging system with
(a) hexagonal and (b) nonredundant distributed apertures and distributed
aperture geometries for (c) hexagonal and (d) nonredundant apertures.

IEEE SIgnal ProcESSIng MagazInE

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September 2016

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Table of Contents for the Digital Edition of Signal Processing - September 2016
