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  Switchable UWB/Multi-Narrowband Antenna for Cognitive Radio Applications Elham Erfani Department of Electrical Engineering, Urmia University Urmia, Iran Mahmoud Niroo-jazi and Tayeb A. Denidni INRS-EMT Université de Québec Montréal, Canada &  Abstract   — A new integrated switchable UWB/multi-narrowband antenna is proposed for cognitive radio applications. This antenna consists of an elliptical CPW-fed slot as a sensing antenna, three λ  g/2 slots with microstrip feed as communicating radiator and a microstrip-to-CPW transition to integrate two structures. The reconfigurability is achieved by inserting PIN-diodes across the CPW feed line and the entrance of three slots. Depending on the DC-biased states of switches, the proposed antenna can be operated in the wideband or switchable narrow band states. The obtained simulation results are shown in this paper. I.   INTRODUCTION In the traditional fixed spectrum policy, some parts of the available spectrum have been allocated to one or more assigned users. The rapid progress of wireless technology and increasing demand in bandwidth lead to more scare and crowded radio spectrum. As possible alleviation to improve the spectrum utilization, cognitive radio networks (CRNs)  based on  Dynamic Spectrum Access (  DSA ) and spectrum management technique have been proposed [1]. In aspect of hardware, the CRNs require a sensing antenna to scan the spectrum and a reconfigurable frequency antenna to communicate in suitable holes spectrum. In the recent years, different antenna designs for CR applications have been reported in [2-4]. A technique is based on integrating these two antennas into a same substrate with two ports [2, 3]. One  port feeds a very wideband antenna and the other one is for a reconfigurable antenna. In another technique, the sensing and communicating antennas are realized by switching between a narrow-band and UWB resonators so that two structures are fed with one port [4]. In this work, an UWB antenna is incorporated with three switchable narrow band slot radiators into the same substrate. The UWB antenna is fed by a Co-Planar Waveguide (CPW) technology while a microstrip line is used for narrow band resonators. To integrate two structures, a broad band microstrip-to-CPW transition is used. The slots are designed to have three different resonant frequencies at 4.5GHz, 5.5GHz, and 7GHz. The reconfigurability is achieved by inserting PIN- diodes which cross the CPW feed line and the entrance of three slots. This antenna can operate in four frequency bands depending on biasing switches. II.   ANTENNA STRUCTURE  The proposed printed switchable frequency slot antenna is illustrated in Fig. 1. This antenna composed of a microstrip line, the microstrip to CPW transition, three λ  g/2 slots, an elliptical CPW-fed slot and eight switching PIN-diodes. To detect the available operating frequencies in the environment and identify the holes spectrum, a wide band antenna is necessary in CR systems. An elliptical CPW-fed slot as a wide  band antenna is selected that is able to scan from 3.1GHz to 10.6GHz. The omini-directional radiation pattern of this antenna makes it appropriate for real time monitoring frequency spectrum. The elliptical slot dimension, distance  between the bottom of the tuning stub and the lower edge of the elliptical slot are optimized to obtain the best matching impedance. To achieve the communication antenna, three λ  g/2 slot with microstrip feed line are inserted in the proper position on the ground plane .  The width of slots is set to 2mm. A symmetric stub is used inside the slot to reduce the effective length of the resonant slot by folding the slot current distribution and it also suppresses the 2 th  and 3 th  harmonics of the first resonance. The slots are designed to provide three different resonant frequencies at 4.5GHz, 5.5GHz, and 7GHz . For incorporating two structures, a broadband microstrip-to-CPW transition is used. This transition is designed based on electromagnetic coupling between the electric field of open- circuit microstrip in the back side of the substrate and the magnetic field of the CPW that has been loaded with an elliptical slot on the other side of the substrate. The transition length and width are set at 5.8mm and 2.2mm to achieve best impedance matching over entire UWB frequency spectrum. In Fig. 1. Configuration of the proposed switchable UWB/multi-narrow  band antenna. (a)   Top view   (b) Bottom view 978-1-4673-0462-7/12/$31.00 ©2012 IEEE  this design, the frequency switching is achieved by placing 2 PIN-diodes at the entrance of each slot and 2 PIN-diodes across the CPW feed line structure. D1 and D2 switches are  placed across the CPW feed line where they have minimum effect on the reflection coefficient of UWB antenna, while other diodes are inserted underneath the microstrip across the slots to maximally control the matching of narrow band antennas. Depending on dc biased states of switches, the  proposed antenna can operate in UWB or switchable narrow  band states. This antenna has 4 modes of frequency operation. When the switches at the entrance of all narrow band slots are on state, UWB operation is obtained by deactivating the two switches across the CPW line. In other states, by connecting the PIN diodes cross CPW line and managing the other switches, narrow band operating modes are obtained. The  possible operating modes are summarized in TABLE I. To bias PIN-diodes as shown as an inset in Fig. 1, a narrow metal pad is prepared inside each slot with a distance of 0.25mm from the side edges. In this design, the used switches were GMP4201. According to the datasheet of the diode, a 0.09Pf capacitor and 0.6ohm resistor are used in the off and on states, respectively . A piece of RO4350B substrate ( ε r=3.48, tang γ =0.0037) with thickness of 0.662mm and dimension of 70mm × 100mm was used to etch the antenna on it. The  proposed structure is simulated by   Computer Simulation Technology software CST. The simulated reflection coefficients of this antenna in four states are depicted in Fig. 2, validating the expected operating multiband. The radiation  pattern in sensing and communicating operation are shown in Fig. 3 and Fig. 4. The curves demonstrate good performances for the antenna at the different operating modes so that making this antenna suitable for CR systems. . III.   CONCLUSION  In this work, a novel integrated CPW-fed slot with three λ  g/2 slots antenna has been introduced which can operate in UWB and switchable narrowband states for CR application. In the UWB operation mode, the antenna covers the frequency range of 3.1 GHz to 10.6GHz with good radiation performance. For three other narrow band modes centered at 4.5GHz, 5.5GHz and 7GHz, the antenna provides 200MHz, 600MHz and 430MHz bandwidths, respectively. In these modes, the antenna has almost the same performance as the UWB radiator. IV.   R  EFERENCES   [1]   J. Mitola, III, Cognitive Radio for Flexible Mobile Multimedia Communications, in  proc .  IEEE Int. Workshop on Mobile Multimedia Communications (MoMuC),  pp.3-10, 1999. [2]   E. Ebrahimi, J. R. Kelly, P. Hall, Integrated Wide-Narrowband Antenna for Multi-Standard Radio,  IEEE Trans.   on Antennas Propag.,  vol. 59, no. 7, pp. 2628-2635, 2011. [3]   Y. Tawk, J. Costantine, K. Avery, C. G. Christodoulou, “Implementation of a Cognitive Radio Front-End Using Rotatable Controlled Reconfigurable Antennas,  IEEE Trans. on Antennas and Propa., vol. 59, no. 5, pp. 1773-1778, 2011. [4]   M. R. Hamid, P. Gardner, P. S. Hall, F. Ghanem, Vivaldi Antenna with Integrated Switchable Band Pass Resonator,  IEEE Trans. on Antennas and Propa., vol. 59, no. 11, pp. 4008-4015, 2011.   Fig. 3.  Simulated UWB antenna radiation pattern at 4GHz, 6GHz and 9GHz (a). xz- plane (b). yz- plane Fig. 4.  Simulated multi-narrowband slot antenna radiation pattern at 4.5GHz, 5.5GHz and 7GHz, (a). xz- plane (b). yz- plane.   TABLE I. DIFFERENT STATE OF THE PROPOSED ANTENNA. States: State I State II State III State IV D 1 &D 2  off on on on D 3 &D 4  on off on on D 5 &D 6  on on off on D 7 &D 8  on on on off Res. Freq. (GHz ) 3.1-10.6 4.5 5.5 7 Operating mode Sensing Communicate Communicate Communicate Fig. 2. Simulated reflection coefficient of UWB and multi-narrowband slot antenna .
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