IEEE Aerospace and Electronic Systems Magazine - November 2020 - 34

Evolution of Quantum Radar Concept to Noise Radar Concept
Rðt 0 Þ ¼

1
2p

À1

rn ðvn ; t 0 Þ eivn t0 dvn

(6)

where rn ðvn ; t 0 Þ is experimentally estimated (via experimental values of its I and Q components) cross-correlation
for the nth central frequency. In this case, complex representation of the range profile also has the form of analytical signal with I and Q components being its real and
imaginary parts tied via Hilbert transform. In case of
SF-NR, its range resolution may be also estimated with
the help of (3), where PSD width B is defined by a single
frequency step width Df and total number of steps, N:
B ¼ NDf:

(7)

In realistic applications, usage of N noise signals gives
such benefits as absence of range ambiguity and improvement of radar immunity against external EM interferences
and jamming [9]-[12]. The latter property is common for
any coherent radar, but NR has a unique antijamming
property: robustness with respect to a coherent singlefrequency interference.
In this way, we may see a similarity between NR and
QR concepts: in the latter case, a comparison of the
reflected signal and the idle (or reference) signal is also
required if one has to retrieve the information about the target, as it is described in more detail in [1], [2], [13]-[15].
It worth noting that in earlier research on QR, the conclusions on the advantages of QR are derived from comparison of their performance with that of old classical
radar with noncoherent signal reception and processing.
In the latter case, for efficient detection of a distant target,
the integration of the received signal envelope (noncoherent integration) is used (e.g., Albersheim's equation). Theoretically, it was clear that coherent integration should
drastically enhance the radar sensitivity; however, it was
rather difficult to implement such reception of radar
returns in practice. When the related technology has been
developed, the pulse-coherent processing became a commonly accepted standard in radar design. It is well known
from any radar textbook that in classical pulse radar with
noncoherent reception of radar returns (i.e., time integration of the signal envelope rather than its amplitude and
phase) the SNR at the receiver output is enhanced as a
square root of Tx pulses number N, while for the pulsecoherent radar, the SNR is increasing proportionally to the
first power of N, i.e., similar to the case of QR.
Recently, comparisons of QR and NR efficiency have
been done for the case of coherent processing in both quantum and classical modes of the QR designed, which have
shown some advantage of QR over the classical radar [7],
[8]. The drawback of the results published in [7] has been
briefly discussed in the section " Quantum Radar. " Unfortunately, microwave measurements done in [8] were not performed accurately enough: both Tx and Rx antennas in the
experiment layout does not guarantee " NO reception " by
34

Rx antenna of multiple reflections of the photons transmitted by Tx antenna. Moreover, the X-band waveguide itself
(without special filter with sharp enough frequency
response) cannot properly filter out different photons. So,
both measurements in [7] and [8] might be done with more
care toward removing factors, which could bring a mistake
in the measurement results obtained.
I would be glad to see experimental validation of the
QR theory since, in my opinion, QR, or better quantum
sensor, may have another applications, such as in a supershort working range, quantum sensors in " microrobotics, "
super-short range meters, etc.

STEPPED-FREQUENCY QUANTUM RADAR WITH RANGE
RESULUTION
Recently, several methods for generation of entangled (or
quantum correlated) signals directly in the microwave frequency band were proposed [7], [8]. This gave more chances in design of QR for long-range applications. However,
in such and similar applications, range resolution capability is an extremely important property for any radar system. In the classical radar, range resolution may be
provided by using either short pulses or signal modulation
aiming expansion of the PSD bandwidth of the transmitted
signal, and application of the pulse compression technique
for coherent reception of radar returns. In this case, range
resolution is defined by (3). Another quite popular and
practical method is the stepped frequency technique,
described in the " Noise Radar " section, which also provides range resolution (3) defined by overall frequency
bandwidth (7).
In QR, range resolution may be implemented by transmitting short pulses of quantum entangled photons. However, one can easily fall into the inconsistent situation
mentioned earlier in the " Quantum Radar " section: for
achieving a required range resolution, one has to use
rather short pulses, which may lead (according to the
Fourier transform properties) to transmitting sounding signals actually having a bunch of frequencies rather than a
single frequency. To go around this difficulty, we suggest
using a stepped frequency concept for design of SF-QR,
which will enable its range resolution ability.
A block diagram of the suggested SF-QR is shown in
Figure 3, which may be considered as SF QENR, as well.
As it is seen, the suggested SF-QR block-diagram is quite
similar to that of SF-NR shown in Figure 2. The main difference between those radars is in the transmitter units. In
SF-QR, instead of usage of VCO with special modulation
lows in SF-NR, we suggest to use generator of quantumcorrelated signals, e.g., generators suggested in [6]-[8],
having a possibility of stepped switching of their central
frequency. In SF-QR, we suggest to use a receiver similar
to that in SF-NR, which perform analog (in hardware)

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