Circular Polarization Diversity Implementation for Correlation Reduction in Wideband Low-Cost Multiple-Input-Multiple-Output Antenna

In this paper, a multiple-input-multiple-output (MIMO) antenna featuring circular polarization diversity, and designed on a common coplanar ground is presented. The proposed antenna design utilizes a coplanar waveguide (CPW) feeding technique with three parallel coplanar ground planes, and two feedlines in-between. For circular polarization (CP), quasi-loops are created by etching slots on the outermost ground planes. With this configuration, circular polarization diversity is induced in the MIMO antenna with the left-hand CP (LHCP) from one antenna and the right-hand CP (RHCP) from the other. The total footprint of the antenna radiator is only <inline-formula> <tex-math notation="LaTeX">$0.78\lambda _{0} \times 0.55\lambda _{0} = 0.42\lambda _{o}^{2}$ </tex-math></inline-formula>. Experimental results show perfectly overlapping impedance and axial ratio bandwidths of 39.3% (4.5 GHz to 6.7 GHz). In addition, average in-band isolation <inline-formula> <tex-math notation="LaTeX">$\vert S_{21}\vert \le -15$ </tex-math></inline-formula> dB without any added complexity or active circuit elements. The peak realized gain of the antenna is 5.8 dBic with broadside radiation pattern in the +z- direction and envelope correlation coefficient (ECC) of 0.005. The antenna is suitable for multiple applications in the C-band that includes WLAN (5 GHz, 5.2 GHz and 5.8 GHz) and WiMAX (5.5 GHz).


I. INTRODUCTION
Multiple-input-multiple-output (MIMO) technology significantly improve the capacity of wireless communication systems [1]- [3]. This technology has the ability to cater the need of high data rates without additional power requirements and therefore, has emerged as a propitious choice for modern wireless communications. MIMO systems generally employ more than one antenna at the transmitting and the receiving ends for sending and receiving multiple signals over the same channel. This approach effectively enhances the channel capacity, spectral efficiency and radio communication links, especially in a rich multipath environment [4], [5]. Giving to the relation K = min(M ,N ) for a M × N MIMO antenna, The associate editor coordinating the review of this manuscript and approving it for publication was Kwok L. Chung . the signal throughput of a communication system can be improved up to K times only with the low level of correlation between the closely spaced antennas. [6]. In particular, the isolation between the antennas in a compact system is a serious concern due to its adverse effects on the system performance. To fulfill a continuous increase of demands for high capacity, compact size, and to mitigate the issues pertinent to antenna correlation, extensive research efforts have been focused towards the improvement and implementation of techniques aiming at further performance enhancement of the MIMO systems.
Various isolation techniques and decoupling circuits can be applied to reduce the antenna correlation [7]. Among them, one option is the spatial diversity technique, in which antennas with the same radiation characteristics are placed at some distance from each other. It is a common and effective VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ way of isolating the antennas, which is however hindered in size-constrained scenarios. Similarly, using other isolation techniques, such as parasitic elements, neutralization lines, orthogonal feeding of the antennas, electromagnetic bandgap (EBG) structures and decoupling circuits adds to the detrimental circuit complexity and the overall size of the antenna [8], [9]. In addition to multiple antennas used in the MIMO systems, polarization of the antenna is yet another crucial consideration in improving the signal integrity of the communication system [10]. In relation to this, antenna polarization diversity techniques were considered to attain lower level of far-field correlation between the antennas and rendering a chance to establish parallel sub-channels.
In [11]- [13], MIMO antennas with polarization diversity were presented, in which linearly (horizontally and vertically) polarized antennas were deployed. Unfortunately, with the increasing pervasion of wireless technology and modern infrastructure, linearly polarized (LP) antennas are prone to high path losses, absorption losses, Faradays rotation effects and increased sensitivity to alignment between transmitting and receiving antennas. On the other hand, circular polarization (CP) exhibits certain advantages in terms of signal propagation. Some notable features of CP include reduced multipath effects and low sensitivity to spatial alignment between the antennas at the transmitting and receiving end of the system [14]- [18].
In [19], it has been shown that MIMO antenna with dual CP or CP diversity can dramatically improve the system capacity by exploiting low far-field correlations, especially in a dense multipath environment. Several MIMO antennas adopting the CP polarization diversity technique have been proposed [20]- [22]. The dual-polarized features are achieved using added circuit elements such as hybrid couplers and diodes. Apart from the complexity of the circuit, these components need additional DC-biasing which adds to the cost, physical size and implementation of the antenna in consumer electronics. Moreover, these antennas do not comply with the bandwidth requirement of modern communication systems and their complex excitation mechanisms lead to reduced polarization purity. Polarization-reconfigurable structures are another way of reducing the correlation between the antennas. However, this approach is not straightforward to implement and involves high power circuit elements and complex feeding mechanisms. The benefits of this approach are contingent upon certain conditions which makes it less attractive in the context of size, cost-effectiveness and power requirements [23].
In contrast to the aforementioned MIMO structures, in this paper, a simple technique for exploiting circular polarization diversity is presented, which enables reduced cost and implementation complexity. The proposed antenna is comprised of three parallel placed coplanar ground plane with two microstrip feedline in-between the ground planes. Two quasi-loops current paths for the travelling wave are generated in the outermost ground planes which generates CP with opposite sense of polarization. The symmetry of the inner ground plane is broken along the longitudinal axis of the feeding line. With this configuration of the antenna, an inverse correlation between the radiated field is engineered which leads to low level of envelop correlation coefficient (ECC) of 0.005. Furthermore, the antenna maintains wide impedance and axial-ratio bandwidths of 39.3% (4.5 GHz to 6.7 GHz). Moreover, the proposed antenna can be operated with bidirectional (indoor applications, such as tunnels, subways etc.) as well as unidirectional radiation pattern characteristics in the C-band. The major contribution of the work include: (i) development of a new circularly polarized MIMO antenna in a parallel configuration, excited by a coplanar waveguide feed with a common coplanar ground plane; (ii) adaptation of circular polarization diversity for reducing the far field correlation between the two closely spaced antennas; (iii) demonstration (both numerical and experimental) of successful circular polarization diversity in the MIMO antenna without any added circuit element for altering the sense of polarization; (iv) achieving high performance with topologically simple, small size and low-cost structure without involving any additional isolation technique.

II. ANTENNA DESIGN AND MIMO CONFIGURATION A. ANTENNA DESIGN
The geometrical configuration and parameterized top view of the proposed antenna is shown in Fig. 1. The CP MIMO antenna is comprised of two closely spaced mirrored wide-slot type antennas fed by a coplanar waveguide with a common middle ground plane. The antenna is printed on the FR4 substrate (ε r = 4.4, h = 1.5 mm, tan δ = 0.02) with the external dimensions of W s × L s = 0.78λ 0 × 0.55λ 0 . The size of the antenna is calculated in free-space wavelength at the lowest operating frequency of the antenna. The MIMO antenna arrangement is initiated by designing a wideband right-hand circularly polarized (RHCP) antenna fed by a single-port 50-Ohm coplanar waveguide (CPW) with a coplanar gap g. CP is induced by etching a slot of modified rectangular-shape in the coplanar ground plane (CG1) which forms a quasi-loop path for the current flow. The middle ground plane (CG2), is made asymmetric along the longitudinal axis of the feeding line. To realize MIMO operation, the geometry of the slotted ground plane (CG1) is mirrored and placed coplanar to the asymmetrical ground plane (CG2). By mirroring the slot topology (CG3), the surface current rotation on the quasi-loop is phase-shifted by 180-degrees. This leads to exciting left-hand circular polarization (LHCP). Further details on the design evolution and operating mechanism of the antenna are explained in the next section.

B. ANTENNA EVOLUTION
The design process of the proposed CP MIMO antenna involves several stages. In the first stage, a rectangular slot is etched in the coplanar ground plane to the left of the microstrip line. This creates a current path along x and y-direction. The impedance matching at this stage shows a   resonance in the 5 GHz band but with poor matching. Also, the AR response in Stage 1 (Fig. 4) shows some circular polarization, which is due to the field components forming a partial loop along W s2 and L s2 (See Fig.1). In Stage 2, the slot length is extended in the -y-direction and a stub is protruded along the length of the CPW feedline. This result in lengthening the current path by increasing the perimeter of the loop. Following the preliminary design, the antenna is optimized in two steps. First, the impedance bandwidth is optimized with the objective being maximization of the operating impedance bandwidth. In the second step, the antenna is re-optimized to minimize its axial ratio (AR). A penalty factor is added to the cost function to enforce the condition |S 11 | ≤ −10 dB over the operating frequency range. The single element antenna operates in the frequency range of 3.1 GHz to 7.2 GHz with RHCP. In the third design stage, the MIMO configuration is implemented as depicted in Fig. 2. A second microstrip line extension is added to the right of the ground plane (CG2) with the coplanar gap g, and the plane of the CG1 is altered and CG3 is formed. Note that at this stage, the dimensions of all the geometrical parameters are the same as that of the single element.
The small spatial distance between the two resonating elements leads to high mutual coupling, which degrades the electrical characteristics of the antenna. To operate the antenna  effectively, three further optimization steps are performed, aiming at improvement of antenna impedance bandwidth, AR and isolation (|S 12 |), respectively. The S-parameter and axial ratio performance through the development stages are shown in Figs. 3 and 4, respectively. The scattering parameters show that the antenna impedance bandwidth has been reduced in the MIMO configuration. The average isolation (S 12 ) between the two radiators is roughly −16 dB, and it is attained without involving any special isolation improvement techniques. The final geometrical parameters are listed in Table 1.

C. ANTENNA OPERATING MECHNISAM AND POLARIZATION DIVERSITY
The circular polarization operation mechanism is explained using the magnetic surface current distribution illustrated in Fig. 5. The current distribution at the center frequency of the MIMO antenna in Fig. 5(a) corresponds to the RHCP, whereas Fig. 5(b) shows the LHCP. Note that the impedance bandwidth and axial ratio of the single-element design shown in Fig. 3 and Fig. 4 are wider than for the MIMO design. For the single element, the horizontal current on the asymmetrical ground plane (CG2, cf. Fig. 1) and the vertical current on the microstrip line monopole extension also contribute to the antenna CP bandwidth. In the case of MIMO antenna, the horizontal currents flowing from Port 1 to Port 2 are phase-shifted by 180-degrees. Due to the opposite direction of the current on the edge of the common coplanar ground plane between the two antennas, the out-of-phase radiation under the far-field conditions leads to current cancellation as depicted using X in Fig. 5. As a result, the antenna operating bandwidth is reduced but the isolation between the two antennas is improved at the same time. The size of the middle ground plane along the x-axis and the inherent characteristics of the polarization diversity also adds to the VOLUME 8, 2020 level of isolation. The traveling waves quasi-loop current (indicated with arrows) formed at the edges of the modified rectangular slot generates the circular polarization. The counter-clockwise rotation of the fields in the antenna at the left-hand-side ( Fig. 5 (a)), radiates RHCP, whereas the antenna on the right-hand-side ( Fig. 5 (b)) radiates LHCP.

D. PARAMETRIC ANALYSIS
Parametric analysis of the proposed antenna is performed based on the variable sensitivity. The parameters controlling the size of the slot etched in the coplanar ground plane play a vital role in attaining the wideband CP characteristics.
When the value of the Ls 1 is decreased by 1 mm from the optimized value, the slot size and hence the current path on the edges of the slot is shortened. This results in the upward shift in terms of both S-parameter and AR. Similar behavior is observed when varying L s2 which also controls vertical components of the current flowing on the edges of the slot. The horizontal current components are controlled by the width of the slot. Although the impedance bandwidth is not very sensitive to W s1 , the AR is affected when it is increased from the optimized value. W s2 is one of the critical parameters in tuning the antenna as it controls the perimeter length of the quasi-loop. This can be observed from the S 11 and AR responses shown in Fig. 6, when the values are altered. By reducing the size of the perimeter, the frequency is shifted upward and vice versa. Another parameter of interest is the common ground plane labeled as L g1 . As it can be seen from the impedance matching response, reducing it results in enhancing the antenna impedance matching but the impedance bandwidth is narrowed and the AR bandwidth is almost the same but the value degrades slightly from the 3 dB criterion.

E. PATTERN DIRECTIONALITY
The proposed MIMO antenna can be operated with both bidirectional and unidirectional radiation characteristics. If the antenna is fed using CPW without a ground plane underneath the substrate, the antenna is bidirectional. The radiation pattern of the antenna excited from Port 1 (left-hand-side) is illustrated at 5 GHz and 6 GHz in Fig. 7, which shows RHCP and LHCP in the ±z-direction. Similarly, the antenna excited from Port 2 (right-hand-side), yields LHCP and RHCP in the ±z-direction as depicted in Fig. 8. A minor tilt in the main beam direction is due to the middle asymmetrical ground plane.
Due the asymmetrical configuration of the CPW along the length of the feedline, the current distribution of the  vertical and horizontal ground plane combines to form tilted current that slightly deviates from the main beam direction away from the broadside. It is clearly seen that the antenna has a stable radiation pattern throughout the operating bandwidth with a peak realized of 3.15 dBic. The antenna with bidirectional radiation pattern can be effectively used for various applications such as in tunnels, subways and other indoor environments. As unidirectional radiation characteristics are generally desirable for some applications, therefore, the antenna is also validated for directional pattern. The directionality of the antenna is changed by employing a planar reflector placed at a distance of a quarter wavelength. It is important to mention here that all other geometrical parameters were kept the same as that of the bidirectional design.

A. S-PARAMETERS AND AXIAL RATIO
The proposed MIMO antenna has been prototyped and characterized in the anechoic chamber of Reykjavik University, Iceland. To achieve unidirectional radiation pattern and to ensure constructive interference between the transmitted and reflected wave, a flat reflector is added at a distance H = 14.5 mm, which is approximately a quarter wavelength at 5.2 GHz. For mechanical support, a rigid plastic of permittivity close to 1 is used. The top view and perspective view of the antenna prototype are shown in Fig. 9.
The measured reflection coefficient of the antenna shown in Fig. 10 (a) indicates that the minimum impedance   bandwidth of the antenna is 39.3% (4.5 GHz to 6.7 GHz). The AR bandwidth of the antenna covers the same operating range with 100-percent overlap between impedance and AR bandwidth as observed in Fig. 10 (b). Figure 11 shows the simulated and measured |S 12 |. The minimum in-band isolation recorded is approximately −18 dB with average value of −15.5 dB throughout the operating band of the antenna. It should be reminded that this isolation is attained without any additional decoupling elements or employment of isolation improvement technique. VOLUME 8, 2020

B. ENVELOPE CORRELATION COEFFICIENT AND DIVERSITY GAIN
ECC is an important parameter that characterizes the performance of the MIMO antenna in terms of correlation between the two antennas. As the channel capacity of a wireless system depends on this performance metrics, the lower the correlation the better the channel capacity and the signal integrity. Here, ECC is evaluated using the far-field radiation patterns as: The ECC is shown in Fig. 12 (a). The fields radiated by the proposed antenna with polarization diversity are almost entirely uncorrelated due to the opposite sense of rotation. A low level of ECC < 0.005 is observed. The slight shift in the main beam direction (due to the asymmetrical geometry of the CPW) contributes to the low level of correlation between the two antennas. The diversity gain of the antenna is calculated using equation (2) and plotted in Fig. 12 (b). It can be oberved that the proposed antenna has almost maximum diversity gain of 9.997. the low level of ECC and high diversity gain are attributed to the polarization diversity of the proposed antenna.

C. REALIZED GAIN AND RADIATION PATTERN
The simulated and measured realized gain of the antenna is shown in Fig. 12 (c). The measured peak realized gain is 5.8 dBic with average in-band variation of 1.55 dB. The gain drop at the lower edge of the operating frequency is due to the destructive interference with the reflector. The radiation pattern of the antenna has been measured for both ports. Figure 13 and Fig. 14 illustrates the pattern in terms of co-polarized and cross-polarized fields for antenna excited from Port 1, in the xz-plane and yz-plane at four different frequencies. The antenna exhibits a steady radiation pattern in the entire operating band with a difference between co-pol and cross-pol of more than 20 dB in the main beam direction. The slight beam tilt in the main beam direction is due to the asymmetrical configuration of the coplanar waveguide ground planes. This adds to the lower value off ECC, because along with polarization diversity, pattern diversity is induced. For port 1, the main beam direction shift is approximately +10-degrees. The patterns for Port 2 in the xz-plane and yz-plane are shown in Fig. 15 and Fig. 16 respectively. The shape of the pattern is the same as that of the port 1 in both planes. The tilt in the main beam direction is −10-degrees.

D. ANTENNA EFFICIENCY AND TARC
The antenna total and radiation efficiencies are shown in Fig. 17. The maximum radiation efficiency of the antenna is approximately 92% while the total efficiency more than 86% within the overall operating band. The efficiency graph illustrates that the antenna retains a relatively stable in-band  efficiency. Moreover, to analyze the behavior of the individual port reflected signal, the total active reflection coefficient (TARC) is studied. The calculation of the TARC is carried out according to the technique described in [31] as    The phase angle θ of port 2 excitation accounts for port-toport coupling and also random signal combining. A set of curves is shown in Fig. 18 for different phased signal excitation of Port 2. The curve characteristics changes slightly with the phase change due to the effects of mutual coupling and random phase signals but the impedance bandwidth remains almost intact. The average TARC values indicate that the antenna retains the impedance bandwidth, whereas, in the worst case, the TARC value of less than −8 dB is observed when the signal is 180 0 out of phase.

IV. BENCHMARKING
For benchmarking, the proposed MIMO antenna has been compared with the recent state-of-the-art MIMO antenna designs with polarization diversity, cf. Table 2. It should be emphasized that, to date, there is a limited amount of work published on the antenna with CP diversity. The comparison is carried out in terms of the type of polarization, isolation, antenna footprint, gain, impedance bandwidth, and AR bandwidth. The antenna operating frequencies are also included in the table for fair comparison. The size of the antennas are shown in millimeter. As it can be observed from the data in Table 2, the proposed design is competitive with respect to almost all considered performance figures. In particular, in comparison to the CP designs reported in [22], [24], [27], [28], the proposed antenna is better in certain aspects including the impedance bandwidth and the AR bandwidth. It should be emphasized that apart from the gain of the antenna, the impedance bandwidth and AR bandwidth of the proposed MIMO antenna outperform all the benchmark designs reported in the recent literature.

V. CONCLUSION
In this communication, a new design of circularly polarized wideband MIMO antenna with polarization diversity has been presented. The MIMO antenna is developed from a single-point coplanar waveguide fed wideband CP antenna. For the single element design, one-side of the coplanar ground plane is systematically modified to induce circular polarization, whereas the other side of the coplanar ground plane is made asymmetric along the length of the feedline. For MIMO implementation, another feedline is added next to the asymmetrical ground plane in a parallel configuration, and the position of the modified ground plane is switched to alter the sense of polarization. With this formation of the radiators, the multi-element antenna features two senses of circular polarization which renders polarization diversity without any additional circuit element. The design has been prototyped and validated experimentally. The inverse correlation between the radiated fields result in a low level ECC < 0.005. Moreover, a stable radiation (bi-directional/unidirectional) characteristic in the broadside direction, wide impedance and AR bandwidth, relatively high gain and compact size is realized.