**3.7. Using heterogeneous elements**

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

**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 capacity as a traditional spatial array.

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

#### **4.1. Introduction**

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


Multiple-Input Multiple-Output Antennas for Ultra Wideband Communications 225



**Technique Reference Antenna layout Analyzed parameters** 

The objective is to design a UWB-MIMO antenna that covers the entire range of frequency approved by the FCC for UWB systems (i.e., 3.1-10.6 GHz) with minimum mutual coupling between the constituent antenna elements, thus attaining good diversity performance. The detailed specifications for the desired MIMO antenna for UWB applications are given in Table 3. To achieve the goal, two types of MIMO antennas are envisaged, i.e., homogeneous and heterogeneous MIMO antennas and are being designed. In wireless communications, the antennas are expected to be embeddable or easy to be integrated into wireless devices in system design; therefore the antennas directly printed onto a PCB/substrate are the most promising designs. Such antennas are usually constructed by etching the radiators onto the dielectric substrate of PCB slabs and a ground plane around the radiators. Taking this major argument into account, printed and planar monopoles have been selected as the constituent

Parameter Value

Gain variation Not more than 4 dBi

Group delay Not more than 2 ns Isolation Not less than 14 dB Correlation coefficient Not more than – 15 dB

TARC Not more than – 10 dB Design profile Compact, printed and easy to fabricate

Different types of MIMO antenna systems have been proposed for UWB applications. These antennas are categorized in two groups: homogeneous UWB-MIMO antennas and

Operating BW 3.1-10.6 GHz (± 100 MHz acceptable)

Radiation efficiency High (> 70%) and variation not more than 25%

Inserting stubs

Using heterogeneous elements

Two square patches with cross-shaped stub [55]

Vector antenna with one loop and two bow-ties [56]

**Table 2.** Summarized state of the art in UWB-MIMO antennas.

radiating elements for UWB-MIMO antennas.

**Table 3.** Design specifications for UWB-MIMO antenna.

**4.2. Presentation of proposed designs** 

Multiple-Input Multiple-Output Antennas for Ultra Wideband Communications 225


**Table 2.** Summarized state of the art in UWB-MIMO antennas.

224 Ultra Wideband – Current Status and Future Trends

Two cone-shaped radiating elements [46]

Four elements arranged as 2 × 2 array [47]

Two truncated square monopoles [51]

Two notched triangular radiator [52]

Two suspended UWB plate antennas with horizontal and vertical configurations [53]

Two Y-shaped radiators with three stubs [54]

Using Defected Ground Structure (DGS)

Using Defected Ground Structure (DGS)

Using Spatial and Angular Variations

Using Spatial and Angular Variations

Using Spatial and Angular Variations

Inserting stubs

**Technique Reference Antenna layout Analyzed parameters** 





> -substrate 45×100 mm² -BW: 3-6 GHz -correlation < - 8dB


The objective is to design a UWB-MIMO antenna that covers the entire range of frequency approved by the FCC for UWB systems (i.e., 3.1-10.6 GHz) with minimum mutual coupling between the constituent antenna elements, thus attaining good diversity performance. The detailed specifications for the desired MIMO antenna for UWB applications are given in Table 3. To achieve the goal, two types of MIMO antennas are envisaged, i.e., homogeneous and heterogeneous MIMO antennas and are being designed. In wireless communications, the antennas are expected to be embeddable or easy to be integrated into wireless devices in system design; therefore the antennas directly printed onto a PCB/substrate are the most promising designs. Such antennas are usually constructed by etching the radiators onto the dielectric substrate of PCB slabs and a ground plane around the radiators. Taking this major argument into account, printed and planar monopoles have been selected as the constituent radiating elements for UWB-MIMO antennas.


**Table 3.** Design specifications for UWB-MIMO antenna.

### **4.2. Presentation of proposed designs**

Different types of MIMO antenna systems have been proposed for UWB applications. These antennas are categorized in two groups: homogeneous UWB-MIMO antennas and

heterogeneous UWB-MIMO antennas. The term "homogeneous" refers to the fact that the identical radiating elements constitute MIMO antenna and the term "heterogeneous" is used to indicate that the constituent elements are not identical. A lot of UWB antennas have been presented in the literature. Among these antennas, printed and planar monopoles are very attractive for their efficient UWB attributes. Therefore, we have also selected printed monopoles to develop UWB-MIMO antennas.

Multiple-Input Multiple-Output Antennas for Ultra Wideband Communications 227

� � ���� �� (���� ��), and � � ���� �� (���� ��); where �� is the free space wavelength corresponding to the lower edge frequency. System-1 is designed on the basis of using spatial diversity. The radiating elements are separated by such a value of distance that mutual coupling becomes less than ��� ��. On the other hand, system-2 and system-3 exploit spatial and polarization diversities by placing the radiating elements orthogonally separated by some distance as shown in Fig. 1b and Fig. 1c respectively. The optimized dimensions of system-2 are: � � �� �� , � � �� �� , � � ���� �� and � � ���� �� . Similarly, the optimized dimensions of system-3 are: � � �� ��, � � �� �� and � � �� ��. System-4 is developed to exploit the spatial and pattern diversity therefore it consists of different radiating elements. The dimensions of system-4 after optimization are: � � �� ��, � � �� ��, � � �� ��. Both radiating elements have their own ground planes unlike system-1. It is worth mentioning that CST Microwave Studio is being used for designing and simulating the antennas. Figure 3 illustrates the UWB-MIMO performance of system-1. Figure 3a shows the simulated reflection coefficients of the left (���) and the right (���) monopoles of system-1. As illustrated in figure, the � �� �� bandwidth ranges less than 3.1 GHz to more than 10.6 GHz that confirms the UWB characteristics. Through simulations as well as from geometrical point of view, it is clear that symmetry exists for the antenna elements, i.e. ��� is the same as ���. The radiation patterns (Figure 3c) are nearly omnidirectional at lower frequencies in H-plane. The pattern also follows donut shape at lower frequencies in E-plane, but it becomes more directional at higher frequencies. The transition of the radiation patterns from a simple donut pattern at the first resonance to the complicated patterns at higher resonances indicates that this antenna must have gone through some major changes in its behavior. The maximum absolute gain values and the total efficiency of the left and right monopoles are presented in Figure 3d and Figure 3e respectively. It can be recalled that the total efficiency of an antenna takes into account all the losses in the antenna such as reflections due to mismatch between transmission lines and the antenna, conduction and dielectric losses. From the plots, it can be noticed that gains as well as efficiencies of both elements are the same and it again confirms that the symmetry holds in system-1. The variation of the gain values along the wide range of frequencies is found to be less than 3.5 dBi for both radiating elements. The total efficiency is always more than 75% and the variation is less than 15% throughout the bandwidth of interest. The radiating elements are also characterized for their time domain performance to confirm their capability for UWB operations. The fifth derivative of Gaussian pulse is used to excite the antenna elements as this covers the FCC's defined UWB spectrum efficiently. The width of the pulse to excite the radiating elements is ���� �� where the pulse width is

The time domain impulse response is determined by placing the probes in the far-field zone. Figure 3f shows the impulse response for both elements. It can be seen that the pulse of 0.32 ns wide is received. Figure 3g shows the group delay for both monopoles. A face-to-face arrangement with 500 mm distance between them is made to determine the group delay. The group delay is nearly within 2.2 ns throughout the whole of the required pass band. To attain better diversity performance it is important to keep mutual coupling between radiating elements as minimum as possible. The distance of 43.5 mm for system-1 is being selected on the basis of an optimization. Figure 3b shows the curves of ��� and ���. There exists the reciprocity, so ��� and ��� are equal. Isolation is less than – 10 dB. Figure 3h presents the

measured at 50% of the maximum amplitude.

Among the proposed and studied antennas, three homogeneous 2-elements UWB-MIMO antennas are designed using identical circular disc monopoles (system-1 and system-2) and identical stepped rectangular monopoles (system-3) and one heterogeneous 2-element UWB-MIMO antenna (system-4) is designed using stepped rectangular and circular ring monopole. The geometries of these antennas are shown in Figure 1.

**Figure 1.** Geometries of designed UWB-MIMO antenna.

All the antennas are fabricated on the FR4 substrate of dielectric permittivity of 4.4, dielectric loss tangent of 0.02 and thickness of 0.� ��. The constituent printed UWB antennas i.e. stepped rectangular, circular disc and circular ring monopoles have already been presented in [57], [58] and [59] respectively. The selection of these antennas to design MIMO antennas can be justified by their good performance, size and ease of integration. However, these antennas have been redesigned to adapt the changes in substrate and thereafter optimized to reduce their dimensions as compared with those presented. It is found that partial ground plane and feed gap play vital role in matching the impedance thus increase the BW if optimally sized. The radiating elements are fed by 50 Ω microstrip lines.

First of all, system-1 comprising two identical circular disc monopoles is designed as shown in Fig. 1a. The radiating elements have common ground plane of length of �2 �� and width of �0 ��. The dimensions of the system-1 are: � �0 �� (0.�� ��), � � 4� �� (0.44 ��),

� � ���� �� (���� ��), and � � ���� �� (���� ��); where �� is the free space wavelength corresponding to the lower edge frequency. System-1 is designed on the basis of using spatial diversity. The radiating elements are separated by such a value of distance that mutual coupling becomes less than ��� ��. On the other hand, system-2 and system-3 exploit spatial and polarization diversities by placing the radiating elements orthogonally separated by some distance as shown in Fig. 1b and Fig. 1c respectively. The optimized dimensions of system-2 are: � � �� �� , � � �� �� , � � ���� �� and � � ���� �� . Similarly, the optimized dimensions of system-3 are: � � �� ��, � � �� �� and � � �� ��. System-4 is developed to exploit the spatial and pattern diversity therefore it consists of different radiating elements. The dimensions of system-4 after optimization are: � � �� ��, � � �� ��, � � �� ��. Both radiating elements have their own ground planes unlike system-1. It is worth mentioning that CST Microwave Studio is being used for designing and simulating the antennas. Figure 3 illustrates the UWB-MIMO performance of system-1. Figure 3a shows the simulated reflection coefficients of the left (���) and the right (���) monopoles of system-1. As illustrated in figure, the � �� �� bandwidth ranges less than 3.1 GHz to more than 10.6 GHz that confirms the UWB characteristics. Through simulations as well as from geometrical point of view, it is clear that symmetry exists for the antenna elements, i.e. ��� is the same as ���. The radiation patterns (Figure 3c) are nearly omnidirectional at lower frequencies in H-plane. The pattern also follows donut shape at lower frequencies in E-plane, but it becomes more directional at higher frequencies. The transition of the radiation patterns from a simple donut pattern at the first resonance to the complicated patterns at higher resonances indicates that this antenna must have gone through some major changes in its behavior. The maximum absolute gain values and the total efficiency of the left and right monopoles are presented in Figure 3d and Figure 3e respectively. It can be recalled that the total efficiency of an antenna takes into account all the losses in the antenna such as reflections due to mismatch between transmission lines and the antenna, conduction and dielectric losses. From the plots, it can be noticed that gains as well as efficiencies of both elements are the same and it again confirms that the symmetry holds in system-1. The variation of the gain values along the wide range of frequencies is found to be less than 3.5 dBi for both radiating elements. The total efficiency is always more than 75% and the variation is less than 15% throughout the bandwidth of interest. The radiating elements are also characterized for their time domain performance to confirm their capability for UWB operations. The fifth derivative of Gaussian pulse is used to excite the antenna elements as this covers the FCC's defined UWB spectrum efficiently. The width of the pulse to excite the radiating elements is ���� �� where the pulse width is measured at 50% of the maximum amplitude.

226 Ultra Wideband – Current Status and Future Trends

monopoles to develop UWB-MIMO antennas.

**Figure 1.** Geometries of designed UWB-MIMO antenna.

monopole. The geometries of these antennas are shown in Figure 1.

heterogeneous UWB-MIMO antennas. The term "homogeneous" refers to the fact that the identical radiating elements constitute MIMO antenna and the term "heterogeneous" is used to indicate that the constituent elements are not identical. A lot of UWB antennas have been presented in the literature. Among these antennas, printed and planar monopoles are very attractive for their efficient UWB attributes. Therefore, we have also selected printed

Among the proposed and studied antennas, three homogeneous 2-elements UWB-MIMO antennas are designed using identical circular disc monopoles (system-1 and system-2) and identical stepped rectangular monopoles (system-3) and one heterogeneous 2-element UWB-MIMO antenna (system-4) is designed using stepped rectangular and circular ring

All the antennas are fabricated on the FR4 substrate of dielectric permittivity of 4.4, dielectric loss tangent of 0.02 and thickness of 0.� ��. The constituent printed UWB antennas i.e. stepped rectangular, circular disc and circular ring monopoles have already been presented in [57], [58] and [59] respectively. The selection of these antennas to design MIMO antennas can be justified by their good performance, size and ease of integration. However, these antennas have been redesigned to adapt the changes in substrate and thereafter optimized to reduce their dimensions as compared with those presented. It is found that partial ground plane and feed gap play vital role in matching the impedance thus increase the BW if optimally sized. The radiating elements are fed by 50 Ω microstrip lines. First of all, system-1 comprising two identical circular disc monopoles is designed as shown in Fig. 1a. The radiating elements have common ground plane of length of �2 �� and width of �0 ��. The dimensions of the system-1 are: � �0 �� (0.�� ��), � � 4� �� (0.44 ��),

The time domain impulse response is determined by placing the probes in the far-field zone. Figure 3f shows the impulse response for both elements. It can be seen that the pulse of 0.32 ns wide is received. Figure 3g shows the group delay for both monopoles. A face-to-face arrangement with 500 mm distance between them is made to determine the group delay. The group delay is nearly within 2.2 ns throughout the whole of the required pass band. To attain better diversity performance it is important to keep mutual coupling between radiating elements as minimum as possible. The distance of 43.5 mm for system-1 is being selected on the basis of an optimization. Figure 3b shows the curves of ��� and ���. There exists the reciprocity, so ��� and ��� are equal. Isolation is less than – 10 dB. Figure 3h presents the

surface current distributions when the left monopole of system-1 is excited while the right monopole is terminated with the matched impedance. It can be observed that very low amount of current is coupled to the right monopole at first resonance and second resonance, and it is justified by the value ܵଶଵ of at these frequencies. However, other two resonances have not also good isolations and current is coupled to some extent. Because of the symmetry of system-1, it is not required to show the current distributions when the right monopole is excited. To evaluate diversity performance into detail, the correlation coefficient is calculated from S-parameters as well as from 3D pattern (figure 3i). Finally TARC is calculated and shown in Figure 3j. It meets the requirement giving the values less than – 10 dB.

Multiple-Input Multiple-Output Antennas for Ultra Wideband Communications 229

**Figure 3.** UWB-MIMO antenna performance of system-1.

**Figure 2.** (a) Layout of UWB MIMO antenna (b) Detailed layout of inverted-Y shaped stub (c) photography of the prototype.

It can be noticed that the system-1 is not capable of meeting the specifications defined in Table 3 for group delay as well as isolation. The other solutions, presented previously, have been envisaged both this reason but also in order to improve the compactness. System-2 exploits spatial diversity as well as polarization diversity. The orthogonal configuration results in decorrelating the radiating elements: radiation pattern and isolation are improved while dispersion is mitigated. System-2 using orthogonal topology shows better results as it exploits the polarization diversity, however physical constraint lies regarding the antenna feeding. With comparable characteristics, system-3 constituted by two identical stepped patch monopoles presents a more compact size. System-4 is designed by integrating two non-similar radiating elements, i.e., stepped patch and circular ring monopole. The exploitation of pattern diversity eliminates the need to print the radiating elements orthogonally. Circular ring monopole nearly behaves the same as circular disc because the current distribution in the center of disc is negligible. The performance of these three solutions is presented with more details in [60], [61] and [62]. So far, the combinations of diversities to reduce the mutual coupling or to improve the isolation have been exploited. The antennas can be reduced in size if some special technique is used to overcome the problems of coupling and isolation between the radiating elements. From system-1, a novel UWB-MIMO antenna (called system-5) has been designed with enough compact dimensions taking advantage of a stub which is inserted on the ground plane. The design of stub, a sort of inverted-Y shape, is initiated from the idea of microstrip LC filters. The introduced stub behaves like a stopband LC filter; therefore it suppresses efficiently the currents from the excited port to the inactive port. The best position of the stub is found to be the middle of the ground plane. The performance has been evaluated numerically and experimentally [63]. Figure 4 displays the measured and simulated reflection coeffcients for the antennas with stub and without stub. It is noticed that the measured results

**Figure 3.** UWB-MIMO antenna performance of system-1.

photography of the prototype.

surface current distributions when the left monopole of system-1 is excited while the right monopole is terminated with the matched impedance. It can be observed that very low amount of current is coupled to the right monopole at first resonance and second resonance, and it is justified by the value ܵଶଵ of at these frequencies. However, other two resonances have not also good isolations and current is coupled to some extent. Because of the symmetry of system-1, it is not required to show the current distributions when the right monopole is excited. To evaluate diversity performance into detail, the correlation coefficient is calculated from S-parameters as well as from 3D pattern (figure 3i). Finally TARC is calculated and

shown in Figure 3j. It meets the requirement giving the values less than – 10 dB.

**Figure 2.** (a) Layout of UWB MIMO antenna (b) Detailed layout of inverted-Y shaped stub (c)

It can be noticed that the system-1 is not capable of meeting the specifications defined in Table 3 for group delay as well as isolation. The other solutions, presented previously, have been envisaged both this reason but also in order to improve the compactness. System-2 exploits spatial diversity as well as polarization diversity. The orthogonal configuration results in decorrelating the radiating elements: radiation pattern and isolation are improved while dispersion is mitigated. System-2 using orthogonal topology shows better results as it exploits the polarization diversity, however physical constraint lies regarding the antenna feeding. With comparable characteristics, system-3 constituted by two identical stepped patch monopoles presents a more compact size. System-4 is designed by integrating two non-similar radiating elements, i.e., stepped patch and circular ring monopole. The exploitation of pattern diversity eliminates the need to print the radiating elements orthogonally. Circular ring monopole nearly behaves the same as circular disc because the current distribution in the center of disc is negligible. The performance of these three solutions is presented with more details in [60], [61] and [62]. So far, the combinations of diversities to reduce the mutual coupling or to improve the isolation have been exploited. The antennas can be reduced in size if some special technique is used to overcome the problems of coupling and isolation between the radiating elements. From system-1, a novel UWB-MIMO antenna (called system-5) has been designed with enough compact dimensions taking advantage of a stub which is inserted on the ground plane. The design of stub, a sort of inverted-Y shape, is initiated from the idea of microstrip LC filters. The introduced stub behaves like a stopband LC filter; therefore it suppresses efficiently the currents from the excited port to the inactive port. The best position of the stub is found to be the middle of the ground plane. The performance has been evaluated numerically and experimentally [63]. Figure 4 displays the measured and simulated reflection coeffcients for the antennas with stub and without stub. It is noticed that the measured results

agree with the simulated ones. The important point to be noticed is that the impedance BW remains the same, i.e., 3.2-10.6 GHz. Further, figure 5a gives the measured and the simulated port isolations for the case when there is stub and figure 5b depicts the results when there is no stub. It is clear from the measurements that the insertion of stub has played a vital role in enhancing the port isolation. System-5 is more compact and efficient as compared to system-1, and verifies the constraints given by table 3.

Multiple-Input Multiple-Output Antennas for Ultra Wideband Communications 231

**Efficiency** 

**(%)** 

**Group delay** 

original 3.5 80 1.7 15 - 28 - 10

original 2.5 74 0.8 15 - 20 - 10

original 2.5 73 1.2 14 - 20 - 10

original 2.2 78 1 15 - 20 - 9.5

original -- -- -- 15 -- --

original 3.2 -- -- 18 -- --

original 3.0 -- -- 20 -- --

**(ns)** 

**Isolation (dB)** 

**Correlation** 

**(dB)** 

**TARC (dB)** 

**Topology** 

system-1 [60]

system-2 [60]

system-3 [61]

system-4 [62]

system-5 [63]

[56]

[55]

[52]

**Size (mm²)** 

**Impedance** 

**BW (GHz)** 

<sup>40</sup>×80 3.1-10.6 Nearly

<sup>30</sup>×68 3.2-10.6 Nearly

<sup>35</sup>×85 3.1-10.6 Nearly

<sup>40</sup>×68 3.2-10.6 Nearly

<sup>45</sup>×62 3.3-10.5 Nearly

<sup>45</sup>×37 3.1-5.0 Nearly

**Table 4.** Summary: performance comparisons of presented UWB-MIMO antennas.

3.6-8.5 Nearly

125×1 25

**Radiation** 

**pattern** 

**Variations in** 

**gain (dBi)** 

43×80 3.1-10.6 Distorted 3.5 75 2.2 11 - 17 - 10

**Figure 5.** Measured diversity performance of UWB-MIMO antenna.

#### **5. Conclusion**

Taking a little overview of UWB and MIMO, it makes easier to understand the idea of implementing MIMO technique in UWB communications systems. As per FCC rules, extremely low power is being allowed to be transmitted, i.e. – 41.3 dBm/MHz, and it impedes the development of UWB communication systems with higher data rates or covering longer distances. To overcome this bottleneck, MIMO technique has been considered to be one of the solutions that will improve the reliability and the capacity of UWB systems. However, a number of challenges arise to shape this solution physically. In this chapter, we took the challenges into account related to antennas as their properties play a key role in determining MIMO system performance. Table 4 summarizes the presented UWB-MIMO antennas and compares the performance.


**Figure 4.** Measured impedance characteristics.

**5. Conclusion** 

**Figure 5.** Measured diversity performance of UWB-MIMO antenna.

UWB-MIMO antennas and compares the performance.

Taking a little overview of UWB and MIMO, it makes easier to understand the idea of implementing MIMO technique in UWB communications systems. As per FCC rules, extremely low power is being allowed to be transmitted, i.e. – 41.3 dBm/MHz, and it impedes the development of UWB communication systems with higher data rates or covering longer distances. To overcome this bottleneck, MIMO technique has been considered to be one of the solutions that will improve the reliability and the capacity of UWB systems. However, a number of challenges arise to shape this solution physically. In this chapter, we took the challenges into account related to antennas as their properties play a key role in determining MIMO system performance. Table 4 summarizes the presented

agree with the simulated ones. The important point to be noticed is that the impedance BW remains the same, i.e., 3.2-10.6 GHz. Further, figure 5a gives the measured and the simulated port isolations for the case when there is stub and figure 5b depicts the results when there is no stub. It is clear from the measurements that the insertion of stub has played a vital role in enhancing the port isolation. System-5 is more compact and efficient as

compared to system-1, and verifies the constraints given by table 3.

**Table 4.** Summary: performance comparisons of presented UWB-MIMO antennas.
