Integrated Multi-Operational Antenna System Design for CR Applications

A four-port multi-operational antenna system is designed for Cognitive Radio application. It consists of a sensing ultra-wideband antenna and three communicating narrowband antenna. The CPW fed monopole is a sensing antenna. It covers 2-11 GHz of the UWB band. The three narrow band antennas are operating at 5.834 GHz, 6.42 GHz, 7.355 GHz, and 8.786 GHz respectively. The mutual coupling is below-15 dB. This integrated UWB/NB antenna system can also be a good candidate for C-band, Maritime radio navigation, and X-band applications.


Introduction
In the last few years, most of the allocated radio spectrum is underutilized for the exponential growth of wireless communication users. For efficient spectrum utilization, Cognitive Radio (CR) emerged as an efficient solution [1,2]. The antenna is a vital part for CR system [3][4][5][6][7][8][9][10][11][12]. This system needs a UWB antenna for continuous spectrum sensing [13]. However, if any channels are available or vacant, the communication is established through a narrowband (NB) antenna. Several communication bands are classified within UWB. To use these bands, more number of NB antennas is needed. However, UWB and the number of NB antennas are responsible for large physical dimensions and coupling issues in a CR antenna system [14][15][16][17][18][19]. Ebrahim etal. proposed a two-port antenna system of dimension 68 x 54 x 0.79 mm 3 which can operate in quad-band mode and covers 3.1-11 GHz band for sensing operation [20]; In another work [21], the system of dimension 80 x 65 x 1.58 mm 3 operates in single-band mode and used 2-5.5 GHz band for sensing purpose. A various integrated antenna system for cognitive radio is reported in [22,23]. However, such an antenna system suffers from lower sensing bands, poor gain, and isolation, also larger antenna size. Now a day, researchers are using extra circuitry to improve isolation [24]. Without adding any extra circuitry and components to achieve high isolation is an open challenge. Further, reconfigurable antennas are gaining more popular [25][26]. The reconfiguration can be done by using switching components (i.e p-i-n diodes, varactor diodes, etc.). In [27], the authors highlight the drawbacks of such kind of switching techniques. Again, the number of NB states is limited to the underutilization of available system areas [28].
A compact four-port antenna system is proposed in this article. This CR system is maximally meets the above challenges.

Antenna design
The Integrated UWB/NB antenna system is shown in Fig. 1. It consists of one sensing and three communicating antennas integrated in a proper manner.

A. Ultra-wideband Sensing Antenna
We choose a CPW fed monopole antenna for spectrum sensing (Fig. 2). It helps to easy integration of narrowband communicating antennas within the available area. The geometrical parameters of the CPWfed monopole antenna are well documented in [13]. The lower cut-edge frequency (UWB antenna) is determined by using Equation -1.
where the equivalent cylindrical height, radius, and the gap of the monopole antenna are represented with l, r, and g. Empirically, we fixed the value of k to obtained ݂ .

B. Narrow-band Communicating Antenna
Three NB antennas are chosen for communication purposes are shown in Fig. 3. These are well-known slot antennas reported in [3,19]. The side edge of the hexagonal patch antenna is 6 mm (Fig. 3 (c)).

Results and discussion
CST 2019 is used for simulating and analyzing the system performances. The system is simulated by using a low-cost FR-4 substrate (ɛr =4.4 and loss tangent=0.02) of size 45 x 40 x 1.6 mm 3 . GHz, and (d) 10.12 GHz Figure 4 shows the surface current distribution of the UWB monopole antenna at different frequencies. It helps in identifying the areas where the current amplitude is minimal. So we, incorporate the NB communicating antennas in that location (Both center of ground plane and the radiator) where the magnitude of the current is less. Also, they indicate the formation of higher order modes as frequency increases. It helps to increase the NB states by efficiently utilizing the system area. Figure 5 shows the reflection coefficient vs. frequency plot of the proposed antenna system. It observed that the UWB antenna operates from 2 GHz to 11 GHz, and the NB antennas operating at 5.  Figure 5. Reflection coefficient of the integrated UWB/NB antennas system Figure 6 illustrates the isolation plot of the proposed antenna system. The isolation is less than -15 dB throughout the Ultra-wideband range. Figure 7, 8, and 9 shows the simulated radiation patterns for the individual antennas. Table I indicates the performance and applications of NB antennas.

Conclusion
A small four-port antenna system of dimension 45 x 40 x 1.6 mm 3 is proposed. The major contribution is that the number of NB states increases by proper utilization of the available system area. The coupling between the antennas below -15 dB is obtained.