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A UWB Antenna Combined with a Reconfigurable
Bandpass Filter for Cognitive Radio Applications
M. Al-Husseini A. Ramadan M.E. Zamudio C.G. Christodoulou
A. El-Hajj K.Y. Kabalan
Abstract A single-port small-size antenna for cog-nitive radio
(CR) applications is presented. The an-tenna is based on an
ultra-wideband (UWB) designand has a reconfigurable bandpass filter
integratedin its feed line. The UWB operation is requiredfor the
sensing task in a CR system. Upon acti-vating the filter, the UWB
frequency response istransformed into a tunable narrowband one,
whichis needed for the communication task.
1 INTRODUCTION
An increasing demand on radio spectrum has re-sulted from the
emergence of feature-rich and high-data-rate wireless applications.
The spectrum isinitially scarce, and the current radio spectrum
reg-ulations make its use inefficient. This necessitatesthe
development of new dynamic spectrum alloca-tion policies to better
exploit the existing spectrum.
According to the current spectrum allocation reg-ulations,
specific bands are assigned to particularservices, and only
licensed users are granted ac-
cess to licensed bands. Cognitive radio [1] is ex-pected to
revolutionize the way spectrum is allo-cated. In a CR network
following the hierarchicalaccess model [2], the intelligent radio
part allowsunlicensed users (secondary users) to access spec-trum
bands licensed to primary users, while avoid-ing interference with
them. One approach to thisspectrum sharing between primary and
secondaryusers is spectrum overlay. In spectrum overlay
CR,secondary users search for unused frequency bands,called white
spaces, and use them to communicate.
Thus, in a cognitive radio system, there is need
for an antenna to be used for monitoring the spec-trum
(sensing), and communicating over a cho-sen white space
(communication). When used forcommunicating, the antenna must be
frequency-reconfigurable, to be tuned to the band selectedfor
communication.
Recently, there has been some research on thedesign of antennas
for cognitive radio systems. In[3], the authors propose a system
that combinesa wideband and a narrowband antennas into the
ECE Department, American University of Beirut,Beirut 1107-2020,
Lebanon, e-mail: [email protected]
ECE Department, University of New Mexico, Albu-querque, NM
87131, USA, e-mail: [email protected]
same volume. The wideband antenna is a CPW-fed printed
hour-glass-shaped monopole that oper-ates from 3 to 11 GHz. The
narrowband antennais a microstrip patch printed on the reverse
sideof the substrate, and connected to the widebandantenna via a
shorting pin and designed to oper-ate from 5.15 to 5.35 GHz. The
antenna presentedin [4] is also dual-port, where one port
interfaces
an egg-shaped UWB printed monopole and is usedfor sensing, and
the second port, which is usedfor communicating, connects to a
matched 40mm-long 1mm-wide microstrip line. Electronic switchesare
placed along this microstrip line to make
itfrequency-reconfigurable.
In this paper, a single-port antenna for CR ispresented. The
antenna is basically UWB, whichmakes it sensing-capable, and has a
reconfigurablebandpass filter embedded in its feed line.
Whenactivated, the filter can transform the UWB fre-quency response
into a tunable narrowband one,which is suitable for the
communication operationof the CR system.
2 ANTENNA CONFIGURATION
The configuration of the presented antenna isshown in Fig. 1. It
uses a 30 35 1.6 mm3
Taconic TLY substrate with a dielectric constantr = 2.2, and
features a partial rectangular groundplane, a rectangular patch,
and a curved match-ing section between the microstrip feed line and
thepatch. The filter, whose design relies on the work in[5], is
based on a symmetrical defected microstrip
structure (DMS) implemented in the feed line ofthe UWB antenna.
A closer view of the filter part,with detailed dimensions, is shown
in Fig. 1(b). Ithas a T-shaped slot, which by itself, has
bandstopcharacteristics. However, when placed between apair of
gaps, which act as capacitors, a bandpassstructure results.
For the purpose of achieving frequency recon-figurability, three
pairs of gaps are symmetricallyplaced around the T-slot, and seven
electronicswitches, each 0.5 0.25 mm2 in size, are placedacross the
slots as shown in Fig. 1. Six switchingcases are considered, as
indicated in Table 1. Case0 corresponds to all the switches being
ON. In this
978-1-4577-0048-4/11/$26.00 2011 IEEE
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7/25/2019 A UWB Antenna for Cognitive
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(a)
(b)
Figure 1: (a) Antenna configuration, and (b) closerview of the
embedded filter
case, the effect of the filter is canceled, bringingback the UWB
response of the antenna. The fre-quency characteristics of the
filter depend on thedimensions of the slots, and on the switching
state.
Case Switches in OFF state0 None (all ON)1 S0, S1, S62 S0, S1,
S53 S0, S2, S54 S0, S3, S55 S0, S3, S4
Table 1: The six adopted switching cases.
3 RESULTS AND DISCUSSION
The design was simulated using Ansoft HFSS. Aprototype was
fabricated, and the reflection coeffi-
cient was measured for the adopted switching cases.Without loss
of accuracy, copper tapes were usedto represent switches in their
ON state. The com-puted and measured reflection coefficient plots
forthe six switching cases are given in Figs. 2 and 3,respectively.
Good agreement is witnessed betweensimulated and measured results.
The operation ofthe antenna makes it suitable for employment
incognitive radio applications, where Case 0 could beused for
sensing the channel (to determine the whitespaces), and the other
cases for communicating inthe corresponding white space. Further
resonancescan be obtained by including more gaps around theT-slot
and appropriately choosing their locationsand widths.
Figure 2: Simulated reflection coefficient.
Figure 3: Measured reflection coefficient.
Since the antenna is a printed monopole, it is ex-pected to
offer omnidirectional radiation patterns.This is verified in Fig. 4
where the normalized H-and E-plane gain patterns are shown, for
switchingCase 4 (f = 7.6 GHz). For this case, and at thisfrequency,
the computed peak is 5.4 dB. Similarlygood gain values are recorded
in the other operationbands for the other switching cases.
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Figure 4: Radiation patterns at 7.6 GHz (switchingcase 4) in the
H-plane (solid line) and the E-plane(dashed line).
4 CONCLUSION
A single-port reconfigurable antenna for CR appli-cations was
presented. The design features a band-pass filter embedded in the
feed line of a UWBantenna. Electronic switches are incorporated
onthe filter to activate/deactivate it and control itsbandpass
frequency. With this configuration, theantenna can be used to sense
a UWB band, andlater communicate over a selected narrow
band.Improved narrowband frequency reconfigurability
can be realized by including more gaps with op-timized
parameters to extend the filters operationrange.
References
[1] J. Mitola and G. Q. Maguire, Cognitive radio:making software
radios more personal, IEEEPers. Commun., vol. 6, no. 4, pp. 1318,
Aug.1999.
[2] K.-C. Chen and R. Prasad, Cognitive RadioNetworks, John
Wiley & Sons, West Sussex,United Kingdom, 2009.
[3] E. Ebrahimi, and P.S. Hall, A dual port wide-narrowband
antenna for cognitive radio, Thethird European Conference on
Antennas andPropagation, pp. 809812, Mar. 2009.
[4] M. Al-Husseini, Y. Tawk, C.G. Christodoulou,K.Y. Kabalan,
and A. El-Hajj, A reconfig-urable cognitive radio antenna design,
The2010 IEEE AP-S International Symposium onAntennas and
Propagation, pp.14, 1117 July2010.
[5] M. Kazerooni, A. Cheldavi, and M. Kamarei,A novel bandpass
defected microstrip struc-ture (DMS) filter for planar circuits,The
2009Progress in Electromagnetics Research Sympo-sium (PIERS2009),
pp. 12141217, 1821 Au-
gust 2009.
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