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

220 Ultra Wideband – Current Status and Future Trends

isolation in printed antennas.

without the EBG reflector.

**3.3. Using neutralizing line** 

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

**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

**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

**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,

involve an intricate fabrication process with cells shorted to the ground through vias.

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 increase the isolation between antennas.

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 increased by 30 dB.

**UWB-MIMO Systems** - In [46], a diversity antenna operating at a frequency range of 3.1-5.8 GHz is designed consisting of two orthogonal half circles with the radiators placed symmetrically with respect to a protruded T-shaped ground plane, which has a slot at the upper center portion of the ground plane. This slot helps in enhancing the isolation and matching the impedance. In [47], this technique is used in real sense where circular slot antenna with a stepped ground plane is proposed. A stepped ground plane generates nonplanar connections and discontinuous interfaces between the elements and the system ground planes. This strategy has effectively decreased the mutual coupling and provided 10 dB enhancements in isolation characteristics. The antenna consists of four radiating elements and operates over the range of 2-6 GHz.

Multiple-Input Multiple-Output Antennas for Ultra Wideband Communications 223

**Narrowband MIMO Systems** - The technique of using stubs to get better isolation also deals with the ground plane instead of the radiating elements. One or more stubs are inserted to enhance the isolation. To the best of our knowledge, there is no scientific

**UWB-MIMO Systems** - The method of inserting stubs is mainly found in the literature for UWB-MIMO antennas. For instance, in [54], two elements diversity planar antenna, with three stubs on the ground plane to improve the isolation, has been proposed particularly for PDA phone. The 10 dB return loss bandwidth is achieved from 2.27 GHz to 10.2 GHz and isolation is always more than 15 dB. Similarly, another printed UWB diversity antenna consisting of two square radiators and a cross stub placed between them on the ground is presented in [55]. The 10 dB return loss bandwidth of the antenna ranges from 3.1 GHz to 10.6 GHz and the isolation

**Narrowband MIMO Systems** - The method of using heterogeneous elements is sometimes used for multi-band antennas in narrowband systems. The objective is then relatively

**UWB-MIMO Systems** - In [56], a vector antenna system has been presented. This vector antenna system comprises a center-fed loop antenna and two orthogonal bow-tie antennas in the plane of the loop. This antenna system has large form factor and operates in the frequency range of 3.6-8.5 GHz. The isolation between the antennas is more than 15 dB and reduced mutual coupling is obtained exploiting the advantage of orthogonal components of electric field. The capability of antenna system for UWB operations is authenticated by time domain measurements. It is shown that the vector antenna can provide nearly the same

A lot of UWB antennas and MIMO antennas have already been presented in the literature. But few publications have been presented on the design and characterization of MIMO antennas for UWB applications as presented in the previous section. This section deals solely with our contributions towards UWB-MIMO antennas. It presents the defined objectives and consequently the followed approaches to achieve these goals. The designs and structures of the proposed different types of MIMO antennas for UWB applications exploiting spatial, polarization and pattern diversities are described. The analysis and evaluation of performance of these proposed designs are provides taking the special parameters into account which are necessary to characterize UWB-MIMO antennas. Finally,

publication presenting the use of this technique for narrowband MIMO systems.

between the two ports is higher than 18 dB within 3.3 GHz to 10.5 GHz.

**4. Some contributions towards UWB-MIMO antennas** 

a solution to enhance isolation with reduced size antenna is presented.

**3.7. Using heterogeneous elements** 

different than to realize a MIMO channel.

capacity as a traditional spatial array.

**4.1. Introduction** 

**3.6. Inserting stubs** 

#### **3.5. Using spatial and angular variations**

**Narrowband MIMO Systems** - The technique of using spatial and angular variations relative to the antenna elements of array is very commonly used to reduce mutual coupling. It is well demonstrated that by increasing the space between the radiating elements decorrelates them and even the spacing greater than or equal to ߣȀʹ gives mutual coupling less than -20 dB, where ߣ is free space wavelength at the center frequency [48]. However, the spacing becomes less than ߣȀʹ in the case of compact MIMO antennas for portable devices, thus it requires considering the mutual coupling effects to be compensated [49]. Therefore, in addition to separating the radiating elements by some distance, positioning of the radiating elements at different angles with respect to each other helps to reduce mutual coupling by exploiting the diversity in polarization. Chae et al. [27] has presented the detailed study using this technique. Further, the same technique is described and employed in [50].

**UWB-MIMO Systems** - Being very simple technique, it has not some specific constraints relating to the bandwidth but with size of the antenna. First of all, this technique is used for UWB diversity antenna by Wong et al. in [51] where the antenna consists of two truncated square monopoles orthogonally and symmetrically printed on the two sides of a T-shaped protruded ground plane as shown in Table 2. This antenna operates over 2.3-7.7 GHz giving isolation more than 20 dB. Using T-shaped ground plane also indicates that the modification of ground plane is an additional technique used together with polarization diversity to enhance the isolation. Recently, Chen et al. have used the similar technique [52]. It presents very compact UWB diversity antenna exploiting polarization diversity. The antenna elements are fed orthogonally and are designed for the lower band of UWB, i.e., 3.1-4.8 GHz. The isolation between two antennas is greater than 20 dB across the bandwidth. Also, the same research group has presented a detailed analysis of two suspended UWB plate antennas operating over 3.0-6.0 GHz in [53] for UWB-MIMO systems. They tested two configurations; (i) when shorting walls are vertically positioned (ii) when shorting walls are horizontally positioned. The effects of the variation of distance between antennas on mutual coupling, isolation and impedance matching are presented.
