A two port frequency reconfigurable antenna for cognitive radios is presented. It is composed of a disc monopole with two ports that excite at opposite sides; one port is very wideband and consists of a coplanar feed line and the other port is tunable narrowband and consists of a microstrip feed line with defect slots in its ground plane. The slots act as a filter that suppresses frequencies outside the desired band, its operating frequency band can be tuned by varying the length of the slots. The two ports are decoupled by at least 10 dB through the considered frequency range. It is believed that the proposed antenna can be a good candidate for cognitive radios in generic small base stations where the narrowband port is used for operation and the wideband one is used for sensing the spectrum. For accuracy, it is important that the sensing (measurement of the interference noise) is made with the same polarisation as the operation and the proposed antenna achieves this by having the two collinear ports. To examine the presented approach, simulated and measured results are presented and good agreement is reported.
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F. Ghanem, P.S. Hall and J.R. Kelly
the corresponding blocking resonances become far enough from each other to create a passband between them. Based on these properties, a filter has been designed and integrated into the microstrip feed line, as shown in Fig. 1, to pass frequencies between 5 and 6 GHz and suppress frequencies outside. The other port has two arms that enhance the excitation of the disc at the bottom frequencies. Fig. 3 gives simulated and measured return loss at the two antenna ports.
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Proposed antenna: The proposed antenna is shown in Fig. 1. It is composed of a disc monopole antenna excited by two collinear ports to ensure that the polarisation for both the sensing and operation modes is the same. The two ports excite very wideband modes; one with coplanar and the other with microstrip feed lines. To transform a wideband response to a narrowband one, defect slots have been used in the ground plane of the microstrip port to pass frequencies in the WLAN bands and filter out frequencies outside. By printing a slot under a microstrip line, a notch is created at a frequency proportional to the length of the slot length. The behaviour of a defect slot is similar to that of a conventional microstrip resonator except that the resonances are in opposition, the first introduces blocking resonances while the second gives passing resonances. As can be seen in Fig. 2, by disposing two slot resonators next to each other and varying the distance between them, the corresponding blocking resonances shift in frequency. If the slots are close enough to each other,
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Fig. 1 Schematic of proposed two-port antenna a Front view b Back view
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Introduction: With the advances realised in the wireless communications domain, the number of users is increasing rapidly, which will lead to the saturation of important parts of the spectrum. In addition, it has been shown that a large portion of the spectrum is not adequately used [1]. One of the solutions to the spectrum saturation problem consists of the cognitive radio (CR) concept, where the radios have the capability of learning and adapting to the environment where they operate. One of these adaptations involves changing the operating frequency [2– 4]. In the future, it is suggested that the spectrum will be deregulated and cognitive radios could operate in any frequency band within the available spectrum. To this end, CRs must sense the spectrum to find a suitable frequency band to operate in, and then use it to transmit and receive data. While there is no agreed scheme for the operation and the sensing modes in cognitive radios yet, one possibility involves having an antenna with its front-end used for continuous sensing and another antenna with its front-end for the operation. To make sensing of the whole spectrum quick, it is suggested that the sensing antenna could be very wideband and the front-end would perform the frequency subdivision, scanning and filtering to achieve a convenient noise level for measuring the interference. It is also suggested that the operation antenna should be narrowband to help filter out-of-band signals [5]. In this Letter, we present a new two port antenna that integrates the two antennas needed for CRs in the same space. It consists of a disc monopole with two ports disposed at opposite sides; one port consists of a coplanar feed line that gives very wideband operation and the other port is a microstrip feed line with defect slots in its ground plane. The slots act as a filter and are designed to suppress frequencies outside the desired band; the operating frequency band can be tuned by varying the length of the slots. To ensure that the sensing is measuring interference at the same polarisation as the operation, the two antenna ports must be collinear. The other advantage of having the two ports at opposite sides is the low mutual coupling it gives since the two ports are decoupled by at least 10 dB through the whole of the considered range.
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Fig. 3 Simulated and measured S11 of antenna at the two ports a Narrowband port b Very wideband port
As can be seen from Fig. 3, the proposed antenna exhibits narrowband and very wideband responses at the two ports. The narrowband port passes frequencies between 5 and 6 GHz and filters out frequencies outside while the return loss of the other port is below 26 dB from 3.6 to 10 GHz. The suppression provided by the filter is quite important since S11 is 21 dB at 3 GHz and the gain suppression has been found to be 8 dB at the same frequency. It should be noted that the resonances in the narrowband port at 2.5 and 9.5 GHz are due to the fact that the filter is designed to block frequencies that extend to 3.5 GHz from the operating band edge. To block more frequencies and suppress these resonances, more poles may be added. While the proposed antenna has been presented to demonstrate its functionality, the wideband port could easily be designed to cover the entire UWB band (3– 10 GHz) by making the disc larger. In this case, the proposed antenna could be used for cognitive radios in the UWB spectrum where the wideband port is used for sensing and the narrowband port is used for operation in one of the UWB sub-bands. The two antenna ports have reduced mutual coupling since the S21 parameter is below 210 dB throughout the UWB range, as shown in Fig. 4. Good agreement between simulated and measured results is obtained. Finally, the response of the narrowband port can be varied
ELECTRONICS LETTERS 21st May 2009 Vol. 45 No. 11
by modifying the slot length. Fig. 5 gives the antenna response of the redesigned narrowband port in the band 4 – 5 GHz. 0
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Conclusion: A new two port antenna has been presented for cognitive radios. It is composed of a disc monopole with two ports that excite a tunable narrowband mode and a very wideband mode, respectively. Simulated and measured results have shown good agreement, which makes the proposed antenna a good candidate for CRs where the two ports are used for the sensing of the spectrum and operation.
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# The Institution of Engineering and Technology 2009 3 April 2009 doi: 10.1049/el.2009.0935
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F. Ghanem, P.S. Hall and J.R. Kelly (Department of Electronic, Electrical and Computer Engineering, School of Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom)
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[email protected] References 1 Cabric, D., Mishra, S.M., and Brodersen, R.W.: ‘Implementation issues in spectrum sensing for cognitive radios’. Proc. Asilomar Conf. on Signals, Systems, and Computers, Pacific Grove, CA, USA, November 2004, Vol. 1, pp. 772– 776 2 Gardner, P., Hamid, M.R., Hall, P.S., Kelly, J., Ghanem, F., and Ebrahimi, E.: ‘Reconfigurable antennas for cognitive radio: requirements and potential design’. IET seminar on wideband, multiband, antennas and arrays for civil or defence applications, March 2008, London, UK 3 Malik, Q.M., and Edwards, D.J.: ‘Cognitive techniques for ultra wideband communications’. IET seminar on ultra wideband systems, technologies and applications, London, UK, April 2006 4 Cabric, D., Mishra, S.M., Willkomm, D., Brodersen, R., and Wolisz, A.: ‘A Cognitive radio approach for usage of virtual unlicensed spectrum’. 14th IST Mobile Wireless Communications Summit, Dresden, Germany, June 2005 5 Aberle, J., et al.: ‘Automatically tuning antenna for software-defined and cognitive radio,’ Proc. Software Defined Radio Technical Conf., Orange County, CA, USA, November 2005
It should be noted that to prevent the transmitted power from the operating port being fed to the sensing front-end, RF switches must be added after the wideband port in order to avoid saturation and burning of the sensing front-end.
ELECTRONICS LETTERS 21st May 2009 Vol. 45 No. 11
