**3. Techniques to reduce mutual coupling and to enhance isolation**

Theoretical work has proved that mutual coupling has a significant effect on MIMO channel capacity in rich scattering environments. The degree to which coupling induced correlation degrades MIMO channel capacity depends on the multipath's angular power spectral density. Another problem resulting from an increase in mutual coupling is the subsequent decrease of the array's radiation efficiency due to impedance mismatch. Hence, the reduction of mutual coupling becomes very important. Similarly, poor isolation also degrades the array's radiation efficiency due to the leakage of transmitted power from the excited antenna to the port of non-excited antenna. Therefore, the need of good isolation is imperative. The different techniques for this purpose are presented in the literature. Most of these techniques are vital only for narrowband MIMO systems however some of them are presented for UWB-MIMO systems. These techniques are discussed in the sub-sections as follows, and some examples illustrate them. A summarized state of art corresponding to the presented MIMO-UWB antennas is presented in Table 2 detailing the used method, the reference, the antenna layout and the analyzed parameters.

#### **3.1. Using Decoupling and Matching Networks (DMN)**

218 Ultra Wideband – Current Status and Future Trends

reflected back or goes to the other ports.

with a state of the art are now detailed.

analyzed parameters.

Γ�

**2.3. Summary on UWB MIMO antenna characteristics** 

� <sup>=</sup> �������������

�

The TARC of MIMO antenna is calculated by applying different combinations of excitation signals to each port. There is no need to define the TARC as a complex number since the phase reference plane does not have any physical meaning for a multiport antenna. The TARC is a real number between zero and one. When the value of the TARC is equal to zero, all the delivered power is radiated and when it is equal to one, all the power is either

In context of UWB where the whole band approved by FCC is required to be covered in one shot, the design of antenna becomes challenging enough. The characteristics of the antennas are required to be stable for the wide frequency band. Moreover, time domain measurements like dispersion and group delay become significant in addition to conventional frequency domain characteristics. Furthermore, the development of future UWB-MIMO communication systems brings more challenges for the antenna design. MIMO antennas are required to be characterized for mutual coupling, correlation and diversity gain. However, a detailed study on characterization of MIMO antennas for UWB is among the current hot topics of research. Also, the design of UWB-MIMO antenna system is always confronted with the same constraints like cost, size, ease of fabrication and integration with other circuits as in the case of single antenna design. Having the specific parameters used essentially for the analysis of UWB and MIMO antennas, the current research orientations

**3. Techniques to reduce mutual coupling and to enhance isolation** 

Theoretical work has proved that mutual coupling has a significant effect on MIMO channel capacity in rich scattering environments. The degree to which coupling induced correlation degrades MIMO channel capacity depends on the multipath's angular power spectral density. Another problem resulting from an increase in mutual coupling is the subsequent decrease of the array's radiation efficiency due to impedance mismatch. Hence, the reduction of mutual coupling becomes very important. Similarly, poor isolation also degrades the array's radiation efficiency due to the leakage of transmitted power from the excited antenna to the port of non-excited antenna. Therefore, the need of good isolation is imperative. The different techniques for this purpose are presented in the literature. Most of these techniques are vital only for narrowband MIMO systems however some of them are presented for UWB-MIMO systems. These techniques are discussed in the sub-sections as follows, and some examples illustrate them. A summarized state of art corresponding to the presented MIMO-UWB antennas is presented in Table 2 detailing the used method, the reference, the antenna layout and the

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� (24)

**Narrowband MIMO systems** - The achievement of low mutual coupling and good isolation using decoupling and matching networks is well explained by S. Dossche et al. in [28]. As earlier described in previous section, the envelope correlation can be calculated from the farfield radiation patterns as well as from the scattering parameters of the antenna system, assuming uniform propagation channel. The envelope correlation can be written as in (13) for a reciprocal and symmetrical antenna system:

$$\rho\_e = \frac{|\text{ZRe}\{\mathbb{S}\_{11}^\*\mathbb{S}\_{12}\}|^2}{(1 - |\mathbb{S}\_{11}|^2 - |\mathbb{S}\_{12}|^2)^2} \tag{25}$$

From above equation, it is clear that by changing the magnitude and phase of either ��� or ���, the correlation between the two antennas can be decreased. In practice, this can be achieved by using a matching network for connecting the antennas. From the system point of view, it is also important to consider the value of 1 − |���| � − |���| � that takes into account the effective radiated power by the antenna system, and it is maximized by minimizing |���| and |���|. Thus, two matching networks can be used at both sides to minimize ��� and ��� while a decoupling network can be used to make ��� in quadrature with ��� i.e., ��� is pure imaginary and thus the real part of mutual impedance ��� is equal to zero. This can be achieved by using a lossless decoupling network.

Also, Weber et al. have used passive DMNs and studied them in detail [29]. In this context, the method for derivation of the admittance matrices of the DMN is explained. An important feature of this method is the formation of predefined orthogonal system port patterns. The admittance matrix can be converted to an actual circuit layout of the DMN in terms of capacitors and inductors. Recently, in [30], hybrid circuit is used as decorrelation circuit. This circuit provides a straightforward, frequency-independent, and feasible solution if the elements are symmetrically placed. This hybrid circuit introduces a 180° phase shift between the signals from the two antenna branches. A little variant of this technique can be observed in [31] where several parasitic elements have been employed between the radiating elements to reduce the isolation. The reduction is dependent on the length and number of the parasitic elements. At least 10 dB improvement, in isolation is noticed.

**UWB-MIMO Systems** - It can be noticed that lot of work has been presented to get better isolation using DMNs. However, this technique is not tractable for UWB-MIMO systems. The matching networks to design and to realize for multiband, wideband and ultra wideband MIMO systems are enough difficult. Thus, this technique is not employed yet for UWB-MIMO systems in the literature to the best of our knowledge.

#### **3.2. Using Electromagnetic Band Gap (EBG) structures**

As noted by Sievenpiper [32], an electromagnetic band gap (EBG) structure behaves as a high impedance surface. This structure consists of an array of metal protrusions on a flat metal sheet. They can be visualized as mushrooms or thumbtacks protruding from the surface. If the protrusions are small compared to the wavelength, their electromagnetic properties can be described using lumped circuit elements – capacitors and inductors. The proximity of the neighboring metal elements provides the capacitance, and the long conducting path linking them together provides the inductance. They behave as parallel resonant LC circuits, which act as electric filters to block the flow of currents along the sheet. This is the origin of the high electromagnetic surface impedance. Because of its unusual impedance, the surface wave modes on this structure are very different from those on a smooth metal sheet. In this way, EBG structures have the ability of suppressing surface waves propagation in a frequency band which makes them very useful to improve the ports isolation in printed antennas.

Multiple-Input Multiple-Output Antennas for Ultra Wideband Communications 221

this technique worked in both cases, i.e., the line is inserted either between shorting pins of PIFAs or between feeding pins of PIFAs. Later, the same technique has been tested for two square patch antennas in [35]. Recently, the same research group is presented a novel implementation of neutralizing line in [36] based on the same concept. In this new form, space between antennas is not occupied rather folded lines are used between the ground plane and the side of each PIFA without disturbing their initial resonant frequency. This idea is also employed to enhancing isolation by many other researchers of mobile companies due to its simplicity, e.g., Nokia [37], LG Electronics [38], and Samsung

**UWB-MIMO Systems** - Although this technique is very attractive and has provided good results, yet it is not employed to UWB MIMO antennas so far. It could be very difficult to couple the elements operating over the wide range of bandwidth in such a way that they

The researchers have found that the defected ground structure (DGS) is also able to provide a bandstop effect due to the combination of inductance and capacitance [40]. The defects on the ground plane store a fraction of propagating energy and that can be modeled in terms of a simple equivalent reactive circuit as was explained in detail in [41]. DGS has been applied to antenna designs to suppress harmonics, cross polarization of a patch antenna, and to

Narrowband MIMO Systems - In [42], a defected ground structure (DGS) consisting of concentric circular rings in different configurations is presented and its stop band characteristics are examined. Later, this DGS is being employed to reduce mutual coupling between two cylindrical dielectric resonator antennas. About 5 dB suppression has been obtained near the operating frequency around 3.3 GHz. Other variants of this technique could be embedding of slits [43] or meander lines [44] in the ground plane. In [43], the ground plane structure consisting of five pairs of slits etched into the middle of a ground plane of two closely packed planar inverted-F antennas is proposed. These slits are interleaved with metal strips and these strips could be thought of as capacitors. At the same time, some inductance is introduced along the central small connecting strip. Therefore, the structure behaves as a bandstop filter based on a parallel resonator. As a result, such a pattern etched onto the ground plane effectively suppresses mutual coupling. A significant improvement up to 20 dB in isolation is observed in the case of monopole antennas. In [44], it has been demonstrated that meander line embedded ground plane provides better isolation as compared with slitted ground plane. Recently, a combination of two techniques, i.e., DGS and EBG, is presented in [45]. A slitted pattern is etched on the ground plane and three mushroom photonic band gap (PBG) are etched on each wall. Using two techniques together, isolation between the ports of closely-packed antenna elements is

Electronics [39].

cancel out mutual coupling.

increased by 30 dB.

**3.4. Using Defected Ground Structure (DGS)** 

increase the isolation between antennas.

**Narrowband MIMO Systems** - In [33], such structures are used to increase isolation in patch antennas by using very simple EBG structures with the help of a multilayer substrate containing a high and a low permittivity layers. A planar EBG consisting of a double squared ring is printed on high permittivity layer while antenna is printed on low permittivity layer. The isolation is enhanced by approximately 10 dB. Further, a simple line fed microstrip patch array designed on a relatively thick substrate gives very good port isolation by using three periods of mushroom EBG elements in addition to variable offset superstrates in [34]. The isolation was improved by 10 dB. However, bandwidth was increased by 50 MHz by using additional superstrates. Recently, in [35], a mushroom-type EBG structure is designed to minimize the loading effects between the two slot antennas without significantly modifying the radiation pattern and input impedance profile. When the EBG reflector is utilized, the insertion loss between the antennas is increased due to the suppression of the parallel-plate modes in the band-gap. The reduction in antenna coupling at some specific frequency is observed by more than 30 dB in comparison to the prototype without the EBG reflector.

**UWB-MIMO Systems** - Though this technique is widely used for narrowband MIMO systems, yet it has some constraints. The method is not viable for wideband systems because a large number of mushroom-like EBG structures will be required to cover the wide range of frequency. As a result, antennas will require large area to embed these structures for UWB-MIMO systems. Further, an intricate process is required to fabricate such structures. They involve an intricate fabrication process with cells shorted to the ground through vias.

#### **3.3. Using neutralizing line**

**Narrowband MIMO Systems** - The technique of using neutralizing line is based on the concept to neutralize two antennas operating in the same frequency band to enhance the isolation. Originally, this technique is proposed by C. Luxey et al. in [16]. They have used a suspended neutralization strip line physically connected to the antenna elements. This line samples a certain amount of the signal on one antenna element and delivers to the other antenna element in order to cancel out the existing mutual coupling, thus increasing total efficiency. In other words, an additional coupling path is created to compensate for the electrical currents owing on the PCB from one antenna to another. Initially for UMTS PIFAs, this technique worked in both cases, i.e., the line is inserted either between shorting pins of PIFAs or between feeding pins of PIFAs. Later, the same technique has been tested for two square patch antennas in [35]. Recently, the same research group is presented a novel implementation of neutralizing line in [36] based on the same concept. In this new form, space between antennas is not occupied rather folded lines are used between the ground plane and the side of each PIFA without disturbing their initial resonant frequency. This idea is also employed to enhancing isolation by many other researchers of mobile companies due to its simplicity, e.g., Nokia [37], LG Electronics [38], and Samsung Electronics [39].

**UWB-MIMO Systems** - Although this technique is very attractive and has provided good results, yet it is not employed to UWB MIMO antennas so far. It could be very difficult to couple the elements operating over the wide range of bandwidth in such a way that they cancel out mutual coupling.
