A reconfigurable dual port antenna system for underlay/interweave cognitive radio

An antenna system that is reconfigurable in frequency is presented in this paper as a novel dual port design that serves both undelay and interweave cognitive radio. This 25×40×0.8 mm3 system is composed of two wide slot antennas: the first is designed as an ultra-wideband (UWB) antenna with controllable band rejection capabilities, while the second antenna is reconfigurable for communication purposes. Three slots are etched into the patch of the UWB antenna to obtain band notching in wireless local area network/Xband/International Telecommunication Union bands (WLAN/Xband/ITU) bands which can be controlled by a positive-intrinsic-negative (PIN) diode across each slot. The configuration states of these three diodes are all useable that produces seven band rejection modes plus the UWB operation mode. The second antenna is configured by five PIN diodes to operate either in Cband, WLAN or Xband regions which results in three interweave modes when setting the first antenna for UWB sensing. The design is simulated by computer simulation technology (CST) v.10. S21 results shows good isolation while input reflection coefficient and realized gain results prove system’s scanning, filtering and communication capabilities. This system is new that it gathers the undelay/interweave operation in a single design and when considering its large number of operation modes it looks adequate for many cognitive radio applications.


INTRODUCTION
Since 2002, the year when the (3.1-10.3 GHZ) range is declared for ultra-wideband (UWB) by Federal Communications Commission (FCC) [1], this range has gained a growing interest in the world of wireless communications. Features of this range including depressed consumption of power, broad operation band and high throughput [2] made it so favorable for a verity of applications in wireless communications including internet of things (IoT), radar, and medical imaging [3], [4]. Despite of its merits, the main challenge for those want to utilize this wide range is the congestion with many coexist wireless technologies that has been already licensed within the borders of the UWB rang like WiMax (3.3-3.6 GHz), C-band (3.7-4.2 GHz), wireless local area network (WLAN) (5.15-5.825 GHz), Xband (7.25-7.75 GHz) and International Telecommunication Union (ITU) (8.02-8.4 GHz) [5].
Fortunately, cognitive radio, offers a good solution to this problem. This technology which is capable to reduce or eliminate interference effect of wireless technologies that share or use the same frequencies has two directions to address the problem; underlay and interweave [6]. The interweave approach

SYSTEM DESIGN
The design in this work is constructed over a two-sided 0.8 mm Rogers RT/Duroid 5880 substrate. It occupies a 40×25 mm 2 of this substrate that has a relative permittivity of 2.3 and tangential loss of 0.0009. Final geometry of this dual port system, that is illustrated in Figure 1(a) for the front plane and Figure 1(b) for the back plane, is mainly carried out through a four-step procedure. The first step is to design the UWB antenna named Antenna#1 for UWB scanning. The next is to etch the slots responsible of rejecting undesired bands into the patch of this antenna. The third step is to design the second antenna named Antenna#2 accountable for communication in the coveted frequency bands and finally controlling the operation of the system by proper configuration of its switching elements.

UWB antenna
For both underlay and interweave systems, the UWB antenna is the corner stone. This design depends a wide slot antenna with a rectangular radiating stub that pass through many progressive modifications to achieve the goal of scanning the 7.5 GHz allocated for UWB range. For this antenna named Antenna#1, the main modifications with the corresponding input reflection coefficient are shown in Figure 2.

Notch creation
For underlay CR, interference bands have to be excluded from the UWB radiation. To accomplish this objective, three slots are etched symmetrically along the longitudinal axis Antenna#1 radiating stub. These openings are implemented as λg/2 slots [21], where the length of slot (Lslt) basically determines the notched frequency (fn).
Where : guided wavelength, c: velocity of light, : effective permittivity, and : relative permittivity. The first slot takes the shape of an inverted (3) and used to exclude X-band. Two face-to-face 3-shaped slots are linked to get a total slot length that causes prohibiting the WLAN frequencies. The third slot is formed by joining an inverted Y and H shape slots and results in blocking the ITU spectrum. The effect these slots produce on S11 response of Antenna#1 when changing a specific parameter in each slot is shown in Figure 3. This effect is illustrated for parameters Rs1, Rs2 and Ls1 in Figure 3

Communication antenna
As an interweave system, another antenna has to be built for communication purposes. This antenna -named Antenna#2-is designed as reconfigurable slot antenna. This antenna composed of several sections that when joined/disjoined result in changing radiator length and thus affecting the resonance frequency. This antenna can be configured to radiate in C-band, WLAN or Xband regions. Altering the parameters regarding the sections of this antenna affects its operation as shown in Figure 4. The parameters that are selected here are r2p2, r4p2 and L5p2 and their effect on S11 is illustrated in Figures 4(a), 4(b) and 4(c) respectively.

System configuration
All modes underlay or interweave operation modes of the proposed system have to be controllable. The system is controlled by switches that can be configured to decide its operation mode. PIN diodes are among the most commonly used elements for switching purposes in reconfigurable antennas [22]. HPND-4005 PIN diodes [23], [24] are utilized for the eight switches of this design. These diodes can be configured to act as short/open circuit due to the values of Rs/Cp of their forward/reverse biasing equivalent circuits illustrated in ON/OFF states as illustrated in Figures 5(a) and 5(b). All operation modes of this antenna with the related switch configuration and targeted bands are tabulated in Tables 1 and 2.

RESULTS AND DISCUSSION
The design is capable to handle both undelay and interweave functionality that is evidenced by input reflection coefficient results of this antenna system. Working in the undelay modes, only Antenna#1 is considered, here good S11 response that covers the whole UWB is seen in Figure 6(a) where the all-PIN diodes are enabled. A large operation bandwidth is shown in each of the curves of Figure 6(b) with a complete exclusion of the targeted interfered band (WLAN, Xband or Cband) from the UWB by disabling the intended switch (S1, S2, or S3). Dual-band notch can be obtained by reverse biasing any two diodes in Antenna#1 that results in one of the curves of Figure 6(c) where each show two notches that almost blocks the frequencies of two of the three targeted bands. Finally, when all switches act as open circuits, Figure 6(d) shows that S11 at the WLAN, Xband and ITU regions is above the -10 dB line that leads to the triple band rejection mode. On the other hand, to enable interweave operation, both antennas of this dual port antenna system have to be considered. All interweave modes requires enabling the three diodes of Antenna#1 in order to achieve the scanning functionality. Antenna#2 input reflection coefficient curves in Figure 7 show an S22 response below the -10 dB line for C-band when the switch pair S4 and S5 is enabled. While having only S6 in the ON state causes the antenna to act as WLAN antenna. Moreover, having S6 in ON state while enabling the pair S7 and S8 shift the operation region to the Xband. Figure 8 presents the outcomes related the realized gain of the antenna system in its underlay operation. It exhibits nearly a flat gain in the UWB mode with a peak value exceeds the 4 dB value as illustrated in Figure 8  Surface current distribution can interpret the operation of this dual port system both in underlay or interweave modes of operation. Figures 9(a), 9(b) and 9(c) show how current is concentrated around the λg/2 slot at modes U#2, U#3 and U#4 respectively. Switching OFF the PIN diode across the slot in each of these modes causes the cancellation of radiation at the intended band. On the other hand, current distribution is very high in specific sections of Antenna#2 in each of the interweave modes I#1, I#2 and I#3 as clarified in Figures 10(a), 10(b) and 10(c) respectively. These sections, which affect the effective length of the patch, are responsible to configure this antenna to radiate in the desired communication band. In multi-port antenna systems, the interaction between the antennas or how to affect each other is important to judge its proper operation. An indication to that is the mutual coupling usually expressed by the value of the forward or reverse reflection coefficients. The antenna in this design exhibits low levels of coupling which is indicated by S21 curves of Figure 11. S21 value in those curves is generally below -15 dB along the UWB range and for all interweave modes except mode I#1 at which S21 rises to -10 dB at 3.9 GHz. The design is compared to recent works, but it should be mentioned that none of them combines underlay and interweave operation in a single system. So, two distinct comparisons are considered for the designs in the two directions of cognitive radio. Table 3 compares the proposed design to recent works as an undelay system considering the size of the antenna, the number of PIN diodes in the design, its modes of operation, the notched bands, and the rejection states. Then, as an interweave system, Table 4 lists the main comparison topics which again take the size and the number of configuration elements (PIN diodes for switching and varactor diodes for tuning) but focuses on communication instead of rejected bands. Thus, beside the capability of this design to work as an underlay/interweave system which is a novel aspect when compared to previous cognitive radio systems, it provides a large number of operation modes with high flexibility to choose the rejection or communication bands where it all comes in a simple and compact construction.

CONCLUSION
This paper presents a dual port antenna system that is applicable for cognitive radio. The two antennas of this compact design are reconfigurable in terms of band rejection for the first and selecting band of operation in the second. Antenna#1 can work an UWB antenna or can be used for band rejection by three PIN diodes controlling the notching slots etched into its patch. The ON/OFF state of these diodes leads to cancel/achieve notching effects that results in a no-rejection mode covers the whole UWB plus seven single/dual/triple band rejection modes to block WLAN, Xband and/or ITU regions. On the other hand, having Antenna#1 with no band rejection to scan the UWB, Antenna#2 can be reconfigured by five PIN diodes to radiate in either Cband, WLAN or Xband. The results regarded S11, S22 and realized gain ensures good performance of the antenna system to work as UWB antenna, notch the desired bands or to communicate in the intended bands. Moreover, S21 results ensures the feasibility as dual port system by low coupling values. This system that can choose between eleven operation modes looks promising to fill the requirements of cognitive radio as compared to recent designs especially when considering its novelty of gathering underlay and interweave functionality in a single design.