A UWB Antenna for Cognitive

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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

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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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A UWB Antenna for Cognitive