IEEE Signal Processing - July 2018 - 122

ask a specific CPE apparatus to transmit (if silent) to improve
is introduced. The proposed SR is integrated with an IEEE
the detection or tracking performance of the radar. The sig802.22 wireless regional area network (WRAN). The radar
nal transmitted by the CPE devices always follows the IEEE
receiver is based on PR technology and exploits IEEE 802.22
802.22 standard, and the overall system
devices as transmitters of opportunity. But,
should guarantee a preset quality of service
being integrated with the WRAN, it can
There are numerous
for communications. Therefore, the commucontrol the medium access control layer
exciting future research
nication and radar systems work cooperaof the WRAN, selecting some of the cusdirections to be explored
tively in the SR.
tomer-premises equipment (CPE) in a colto make cognitive radars
Supposing that all of the N CPEs are
laborative way with the base station. These
a reality.
selected CPE devices are scheduled to
transmitting to the base station and that
transmit in each frame to improve the radar
the PR would use M ( % N ) of those CPE
performance (active technology), based again upon the knowlsignals for its tracking function, the best set S ideal of the M
edge of the environment.
CPEs could be chosen, for instance, to minimize the MSE
IEEE 802.22 is a new standard based on cognitive radio
of the target state prediction. In [30], it has been proven that
techniques for WRAN that exploits, on a noninterfering and
this is tantamount to minimizing the trace of the prediction of
opportunistic basis, the unused channels in the VHF and UHF
the target state covariance matrix Pk + 1 k at each time instant
bands allocated to television. The architecture of the IEEE
k, so the minimum value of the cost function is given by
802.22 network is composed of a base station that covers a cell
Trace " Pk + 1 k ^S ideal h, . Unfortunately, the radar can rarely use
with a radius up to 30 km, providing high-speed Internet serthe set S ideal, because few CPEs transmit at the same time. So
vice for N CPE devices or groups of devices, up to 512. All of
in the general, purely passive case, the radar should select its
set from among the few that are active at the instant k.
the details on the IEEE 802.22 standard can be found in [59].
Some recent works (e.g., [46] and [64]) analyzed the possibility
In the SR, conversely, the base station can choose n of the
of exploiting IEEE 802.22 equipment as transmitters of opporM sources from among those that are not transmitting, requirtunity for purely PR systems.
ing them to transmit to improve the tracking performance. The
An SR system is not purely passive; it does not simply select
cognitive tracking algorithm selects the set of transmitters for
the best sources of opportunity among those available, based
the subsequent frame by finding the minimum number n that
upon some optimality criterion. As conceived in [65], an SR can
guarantees a performance level such that
m $ Trace " Pk + 1 k ^ S n h, # Trace " Pk + 1 k ^ S ideal h,,

RMSE Position (m)

λ=0
λ = 0.95
Ideal Set

40
30
20
10
0

100

200

300

400
k

500

600

700

Transmitters
Allocated for Tracking

FiguRe 13. The RMSE of the target position.

λ=0
λ = 0.95

3
2
1
0
0

100

200

300

400
k

500

600

700

122

where 0 # m 1 1. The threshold m is used to select how the
desired performance compares with that of the ideal case.
When m = 0, the tracking is purely passive, and the SR does
not select any CPE apparatus. In this case, the SR will not
improve the target-tracking performance. On the other hand,
when the values of m tend to 1, the SR improves the tracking
performance as closely to the ideal case as possible.
Some results of this approach are shown in Figures 13 and
14. The target is moving linearly on a plane from [−400 m,
400 m] to [400 m, −250 m], with a speed of 8.33 m/sec. The
gain of the symbiotic approach compared to the purely passive
one is evident in Figure 13, where the RMSE of an extended
Kalman filter on the target position is plotted as a function of
the time instant k. Comparing the black curve (n = 0) and
the blue curve (n = 0.95), the symbiotic approach obtains a
performance nearly identical to the ideal set. In this example,
N = 256, M = 8, and n goes from zero to a maximum of four.
In Figure 14, the number n of CPE devices selected specifically by the SR is plotted as a function of the time instant k,
calculated on 104 Monte Carlo trials. It is evident that one or
two selected CPE items are enough for the majority of the time
to guarantee performance close to the ideal one.

Open issues and future research directions

FiguRe 14. The mean number of CPE devices scheduled to transmit by
the SR.

(6)

Although fully cognitive radars akin to bat and dolphin biosonars, as conceived in [28], are not yet a reality, some encouraging

IEEE Signal Processing Magazine

July 2018

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