An Integrated Frequency Reconfigurable Antenna for Cognitive Radio Application

A novel integrated antenna that consists of spectrum sensing and frequency reconfigurable antenna, printed on a single substrate is presented here. The integration of both antennas is based on the fact that reasonably large metallization area of UWB antenna can be used as a space for printing another narrowband antenna. Prominently, the second antenna should be placed on a region of large UWB antenna where there is less distribution of E field. The proposed antenna senses the spectrum over a wide frequency range from 2 GHz to 12 GHz via a U shape monopole having partial ground structure. After sensing the spectrum condition, the slot antenna with reconfigurable feeding structure can switch its operating bands among six narrowbands: 5.65 GHz, 3.6 GHz, 5 GHz, 2.94 GHz, 4.5 GHz, 2.15 GHz and two dual bands at 2 GHz, 5.48 GHz and 1.7 GHz, 5 GHz. In addition to this, the proposed antenna can also reconfigure its frequency in wide-bandwidth using two ground stubs. Since wide band response for spectrum sensing and each of eight frequency state works independently, therefore, there is no interference among various signals.


Introduction
The next generation smart systems should have capability to automatically detect the spectrum condition and accordingly change the radio operating parameters to modify system performance such as maximize throughput, mitigate interference, facilitate interoperability etc.In addition, the physical size of these devices must follow the downsizing trend of the system unit.A smart Cognitive Radio (CR) compatible antenna with smaller physical size meets the essential requirement for these systems.Such antenna has a spectrum-sensing antenna for detecting the spectrum condition and reconfigurable antenna for communicating purpose [1], [2].Many antenna designs have been already reported in the literatures, which are compatible with CR [3][4][5][6].Further to decrease the system size, these two antennas must be optimally design on a single substrate.Only a handful of techniques based on integration concept of these antennas, on a single substrate are reported in literature [7][8][9][10][11][12][13][14][15].In these techniques, small narrowband (NB) antenna are integrated in UWB antenna by using various topology viz.using large metallization area of UWB antenna as ground plane for a smaller antenna [7], [8], by utilizing the empty space in between the radiating element, and in between two tapered slots [9], by adding reconfigurable part in the feeding area of Vivaldi antenna [10], by adding reconfigurable band pass filter in feed line [11], by adding JK inverter in the switchable feed line structure [12], by placing the NB antenna at a point where electric field does not disturb the UWB antenna [13], by adding UWB antenna above the reconfigurable antenna [14], by reusing the printed area in different operating modes [14], etc.In all the above designs the numbers of frequency reconfiguration states are limited because of size constraint so there is underutilization of available area.Further up gradation of these antennas to increase the reconfiguration states is quite challenging because of limited space constraints on board.This may compromise the goal of combining the wideband and reconfigurable antenna together for CR systems.
Therefore, in the present work, UWB antenna and reconfigurable antenna are integrated in such a way that they do not disturb the characteristics of each other.The challenging goal in this work is to use the available space optimally for multiple purposes.For space management, the  large metal area of UWB antenna can be used for printing slot antenna and the partially etched ground area can be used for printing reconfigurable feeding structure for the slot antenna.Proper isolation of high field and low field area ensure the least interaction between two antenna and efficient utilization of space for multipurpose.

Design Strategy/Design Evolution
For CR systems, an antenna consists of two parts: a UWB antenna as a spectrum sensor and a reconfigurable NB antenna for communicating purpose.So, a U shape monopole, which is excited by port 1 , is chosen as a spectrum sensor, over a wide spectrum range from 2 to 12 GHz.A reconfigurable slot antenna, which is excited by port 2 , is used as communicating antenna in an operating range between 1 GHz to 6 GHz.The target is to integrate these two individual antennas on a single substrate to use the available space optimally such that space of one antenna can be used for printing two antennas.Importantly, the NB antenna should be positioned in a region of the UWB antenna where there is less concentration of electric field intensity.The high and low field areas on the UWB antenna are isolated by etching a thin slit in order to have space for the NB antenna.This ensures less interaction between the two antennas.The corresponding antenna topology is shown in Fig. 1 and the fabricated prototype is shown in Fig. 2. It consists of two layers.The top layer is the large U shape antenna and the bottom layer consists of partial ground plane and reconfigurable feeding structure.The design steps adopted for complete integration topology are explained below.

Integration Methodology
Here, the methodology adopted to integrate a NB antenna into the large UWB antenna is explained thoroughly.Firstly a U shape monopole antenna namely configuration I is designed on 6  6 cm 2 substrate as shown in Fig. 3(a).The proper selection of monopole shape and partial ground plane size optimizes the impedance bandwidth of UWB antenna.Figure 5 shows the effect of partial ground size on the return loss characteristics in UWB range.The result clearly show that the UWB behavior of antenna improved as we increased the ground plane size from 5 mm to 7 mm.Further increasing the ground size, the UWB response starts degrading, so the optimum size of partial ground plane is chosen as 7 mm for UWB response from 2 to 12 GHz.Figure 4(a) shows the E field distribution in UWB antenna for configuration I.It is observed that the electric field intensity is very high on the outer periphery and very low intensity is found on the central area of the antenna.If one can separate the high E field area from low E field area in UWB monopole without much affecting the S 11 response, than this space can be reused to integrate another NB antenna.Therefore, in configuration-II a very thin slit is etched in the UWB antenna, which separates the region R into two parts R 1 as high E field area, R 2 very low E field area as shown in Fig. 3(b).The E field distribution for configuration-II is supporting the statement that region R 2 can act as nearly passive area for integrating another antenna.Figure 6 shows that S 11 parameter from configuration I to II does not disturb so much and is acceptable of UWB operation from 2 GHz to 12 GHz.Now, in configuration III, two slot antennas (U shape, inverted E shape) are successfully integrated in region R 2 without disturbing the UWB behavior.This configuration is used for spectrum sensing mode and its functioning is explained in detail later.All simulations are done using HFSS ver.16. Figure 4 shows the E field distribution for all the three configurations and Figure 6 shows the comparisons of S 11 parameter.

Mutual Coupling between Ports
In the proposed design, the UWB and the reconfigurable antenna are integrated together but work independently because both antennas are excited by different ports, one at a time.Both operating modes work separately without interfering each other's characteristics.Further, design optimization of integrated antenna is done so that passive   antenna does not interrupt the operating state of active antenna.To prove that both antennas do not interfere with each other's characteristics, coupling coefficient for both the ports are studied.
Figure 7 shows the simulated S parameter.The response clearly shows that S 21 is better than 25 dB from 2 to 12 GHz, which proves that there is no transmission of RF power from port 1 to port 2 .So there is no need to design extra port isolation circuit because there is good isolation in both operating modes and this ensures no interaction of both the antennas.

Antenna Architecture and Working Operation
Now antenna architecture, geometry description and working for each mode is explained in detail below.

UWB Antenna Design
Figure 8 shows the active portion of integrated antenna when operated in spectrum sensing mode (configuration III).In this mode, antenna is fed by port 1 to excite the U shape monopole.The top layer structure consists of U shape monopole and bottom layer consists of partial ground and two symmetrical stubs (S 6 , S 7 ).The two ground stubs namely S 6 , S 7 are used to reconfigure the frequency in wideband (WB) mode.Diode D 6 and D 7 are used to control the state of stubs S 6 and S 7 independently.Three modes

Frequency Reconfigurable Antenna Design
Figure 9 shows the active portion of the integrated antenna when it operates in frequency reconfigurable mode.The two slot antennas are imprinted such that they use switchable feed line structure for their active operation.
Tab. 1. Geometrical parameter of the feeding structure (mm).A number of cases for different combination of these diodes can be envisaged.Figure 9(c) shows the diode state and pictorial view of active portion of feeding structure under different cases.When no diode is forward biased than feed line portion F 1 excites the lower U slot antenna (case 1 ).Now by activating different stubs combination with feed line F 1 , lower U slot antenna can reconfigure its resonant frequency over 4 different bands (case 2 -case 5 ).When diode D 5 is forward biased than feed line portion F 1 F 2 also excites the upper E shaped slot antenna (case 6 ).For this combination two cases are also discussed, i.e. when stub S 1 and S 2 are attached with feed line F 1 F 2 (case 7 ) and stub S 3 and S 4 are attached with the feed line portion F 1 F 2 (case 8 ).The geometrical parameters for the reconfigurable feeding structure are given in Tab. 1.

Results and Discussion
To validate the proposed design, the prototype has been fabricated and results are verified experimentally here.Figure 10 shows the experimental setup for integrated antenna.In the prototype, Infineon BAR-6402 diodes are used here.In Ansoft HFSS 16, PIN diodes are model using lumped RLC boundary to add the effect of actual diodes.

Spectrum Sensing Mode/ WB Frequency Reconfigurable Mode
Antenna works in normal mode or spectrum sensing mode, when both stubs (S 6 , S 7 ) are not attached with the partial ground structure (D 6 D 7 -00).Figure 11 shows the simulated and measured S 11 vs. frequency for normal mode.It is clear that simulated and measured S 11 is better than -10 dB in UWB bandwidth from 2 GHz to 10 GHz.The gain of the antenna is 2.6 dB at 5 GHz as shown in Fig. 12(a).The E and H plane pattern is nearly omnidirectional except a discontinuity at θ = 180°.Figure 12(b) shows the simulated and measured E plane pattern at 5 GHz.When diode D 6 is forward biased, then stub S 6 , reconfigures the operating bandwidth and the antenna works for WB-I.Now the antenna has two WBs from 1 to 3 GHz and from 5 to 7 GHz as shown in Fig. 13.Simulated and measured S 11 is better than -10 dB for 1-3 GHz and 5-7 GHz. Figure 13 shows that E field redistributes under UWB antenna and stub S 6 .The observed value of gain is 2.4 dB at 5 GHz as shown in Fig. 14   biased then stub S 6 and S 7 are attached with the partial ground to tune the operating bandwidth.
In this case, the antenna operates in WB-II.Simulated and measured S 11 is better than -10 dB from 5 to 7 GHz. Figure 15 also shows that E field redistributes and the maximum field is found under UWB antenna and stubs S 6 and S 7 .In this case, the maximum value of gain is 3.6 dB as shown in 3D polar plot of gain in Fig. 16(a).The shape of H plane is nearly omni-directional and E plane is nearly dumbbell-shaped as shown in Fig. 16(b).

NB Frequency Reconfigurable Mode
In this mode, different combinations of stubs are activated at different times to achieve frequency reconfiguration.All the cases are discussed by comparing simulated S 22 with the measured value and analyzing electric field distribution.The 3D polar plot for gain and simulated and measured radiation pattern for each mode is also given for each frequency state.In case 1 , when all the diodes are in OFF condition then feedline section F 1 excites the U slot structure.Figure 17(a) shows the simulated and measured S 22 vs. frequency for this case.The simulated value of return loss (RL) is 30 dB at resonating frequency 5.65 GHz whereas the measured value of RL = 28 dB at 5.68 GHz.Electric field distribution for case 1 is also given in Fig. 17(a) which clearly shows that maximum E field is distributed under F 1 portion.The observed value of gain is 3.43 dB and comparison of simulated and measured radiation pattern is shown in Fig. 17(b-c) respectively.In case 2 , either of diode D 1 or D 2 is forward biased by applying +5 V on V 1 or V 2 pad then antenna resonates at 3.6 GHz as shown in Fig. 18.In this case, the U slot antenna is fed by feedline section F 1 in conjunction with stub 1 or stub 2 .It should be noted here that resonating frequency for feedline structure F 1 S 1 or F 1 S 2 is found the same because geometry of stub 1 and stub 2 are identical so they are symmetrically placed across feedline section F 1 .In this case, the simulated frequency is 3.6 GHz with RL = 28.3dB whereas the measured frequency is 3.69 GHz with RL = 40 dB as shown in Fig. 18.The maximum E field intensity is found under F 1 S 1 structure.The observed value of gain is 2.8 dB and simulated and measured radiation patterns are shown in Fig. 18(c) respectively.
In case 3 , when diode D 1 and D 2 is forward biased by applying +5 V on DC pad V 1 and V 2 then active feed portion is F 1 along with stub 1 and stub 2 .In this case, the simulated frequency is 5 GHz with RL = 30 dB whereas the measured frequency is 5.15 GHz with RL = 30 dB as shown in Fig. 19.In this case, electric field is maximum under the F 1 S 1 S 2 structure and at the corner of the U slot antenna.The observed value of gain is 2.7 dB.The simulated and measured radiation patterns are shown in Fig. 19(c).In case 4 , when either of diode D 3 or D 4 is forward biased then either stub 3 or stub 4 is selected to tune the resonating frequency for dual band.Again in this case, due to the geometrical symmetry of F 1 S 3 or F 1 S 4 structure because of the same geometrical parameters of stub 3 and stub 4 , their resonating frequency is found the same for both cases.So only the results for F 1 S 3 are given here.In this case, simulated dual band is found at 2 GHz and 5.48 GHz with RL = 36 dB, and 35 dB respectively and the measured dual band is obtained at 1.89 and 5.38 GHz with RL =27 and 38 dB, respectively, as shown in Fig. 20(a).The observed value of gain is 2.2 dB and the simulated and measured radiation patterns are shown in Fig. 20(c), respectively.The discrepancies between the simulation and measurement radiation patterns can be attributed to the presence of the diodes, in close proximity to the antenna during the pattern measurements.
In case 5 , when diode D 3 and D 4 are forward biased by applying +5 V on DC pad V 3 and V 4 , then the U slot is excited by T shape feedline structure (F 1 S 3 S 4 ).In this case, the simulated resonating frequency is 2.94 GHz with RL =   In case 6 , upper inverted E shaped slot is excited by feed line F 1 F 2 by forward biasing the diode D 5 .The antenna this time resonates at 4.48 GHz with RL = 36 dB whereas the measured resonating frequency is 4.5 GHz with RL = 40 dB as shown in Fig. 22(a).The observed value of gain is 1.8 dB and radiation patterns are shown in Fig. 22(b-c).In case 7 , when diode D 1 , D 2 , D 5 is forward biased then the slot antenna is excited by feed line section F 1 F 2 in conjunction with stub 1 and stub 2 .The electric field distribution for this case is given in Fig. 23(a), which clearly indicates that maximum electric intensity is found under F 1 F 2 S 1 S 2 structure.The simulated dual band is found at 1.7 GHz and 5 GHz with the measured RL = 30 dB and 31 dB respectively whereas the measured dual band is at 1.66 GHz and 4.88 GHz with RL = 18 dB and 31 dB respectively as shown in Fig. 23(a).The observed value of gain is 2.7 dB and the simulated and measured radiation patterns are shown in Fig. 23(c).
In case 8 , feed line section F 1 F 2 S 3 S 4 excites the slot antenna by forward biasing the diodes D 3 , D 4 , D 5 .In this case, the antenna resonates at 2.15 GHz with RL = 38 dB whereas the measured RL = 27 dB is at 2 GHz as shown in Fig. 24(a).E field distribution also shows that maximum field is distributing under F 1 F 2 S 3 S 4 structure.The observed value of gain is 1.7 dB and the simulated and measured radiation patterns are shown in Fig. 24(c).Simulated and measured parameters are also tabulated in Tab. 2.  Freq

Conclusion
In this work, we integrate the NB antenna into a large UWB antenna without affecting each performance.The UWB antenna, slot antenna was printed onto the top side of the substrate and the reconfigurable feeding structure for the U slot antenna was printed onto the bottom side of the substrate.This has the advantage of reducing space requirements and making the two antennas lying in the same plane required for cognitive radio communication.The arrangement has potential for various CR applications.

Fig. 5 .
Fig. 5. Effect of ground plane size on UWB characteristics.

Fig. 8 .
Fig. 8. Active area of integrated antenna in UWB mode.

Figure 9 (
Figure9shows the active portion of the integrated antenna when it operates in frequency reconfigurable mode.The two slot antennas are imprinted such that they use switchable feed line structure for their active operation.Figure9(b) shows feeding structure which is composed of two feed lines (F 1 , F 2 ), four stubs (S 1 , S 2 , S 3 , S 4 ), 5 PIN Diodes (D 1 , D 2 , D 3 , D 4 , D 5 ) and 6 DC pads (V 1 , V 2 , V 3 , V 4 , V 5 , V gd ).The heart of the proposed design is electronically switchable feeding structure, which is specially designed for frequency reconfiguration using micro strip open stubs.These stubs are placed at proper position, such that these stubs match the antenna's input impedance at different desired frequencies.Five PIN diodes are used to control the activation of different stubs independently.The diode positioning along with DC biasing network are given in Fig.9(b).The orientations of all PIN diodes are such that they share single ground pad.Applying 5 voltages on their respective DC pad and ground signal on V gd pad activate the respective PIN diodes.

Fig. 9 .
Fig. 9. Active area of integrated antenna in frequency reconfigurable mode, (b) layout of reconfigurable feeding network, (c) different cases using different diode combinations.
Figure 11 also shows the very low electric field distribution is observed in the central part of the antenna and maximum on the outer edges of the U shape monopole.
(a).Simulated and measured radiation pattern at 5 GHz has distorted shape as shown in Fig.14(b).If both diodes D 6 and D 7 are forward
the measured frequency is 3 GHz with RL = 35 dB as shown in Fig.21(a).The electric field is found high under F 1 S 3 S 4 structure as shown in Fig.21(a).The observed value of gain is 3.7 dB and radiation patterns are shown in Fig.21(c).