**1. Introduction**

The primary concerns of wireless communication systems are higher data rates, improved quality of service, more channel capacity, low power consumption and less interference with other systems. Ultra Wideband technology is the most promising technology to meet the requirements of wireless communication system because of its advantages, such as higher data rate transmission, low cost, high security, and less power requirement [1]. However, multipath fading and frequency interference with other communication systems are the important problems should be well solved for UWB systems.

In an indoor communication application, like other wireless communication systems, the UWB system performance is also restricted by multipath fading due to rich scattering environments which cause inter-symbol interference. In present times, digital communication using multiple input multiple output (MIMO) technology has emerged as a breakthrough for a wireless system. The MIMO system employs multiple antennas at the transmitter and receiver. It makes use of the rich multipath environment to mitigate the multipath fading effect. And it improves the range of communication and system capacity (data rate) without the need for additional bandwidth or transmitted signal power [2, 3]. Hence, the UWB system with MIMO technology is a viable solution to reduce the multipath fading effect and to improve the quality of service, the range of communication and system capacity [4].

The electromagnetic interaction between the radiating (antenna) elements in multiple antenna or MIMO system is known as mutual coupling. The closely spaced antennas, especially in portable devices, inevitably cause strong mutual coupling between antennas. The mutual coupling is undesirable which causes fluctuations in the input impedance of individual antenna element i.e., impedance mismatch which degrades the radiation efficiency, deviations in antenna radiation pattern due to the high correlation between antenna signals and decreases the channel capacity of MIMO system. Since the mutual coupling has a considerable impact on the MIMO system performance, the reduction of mutual coupling between antennas and enhancement of isolation between ports is imperative. However, placing multiple antennas in a space-limited portable wireless device is a big challenge for antenna designers [5]. Hence, designing compact UWB-MIMO antenna exhibiting bandnotch function and less mutual coupling is very much needed. Various designs were proposed in the recent years to suppress the effects of mutual coupling in ultrawideband MIMO antennas [6–15]. Methods include, placing radiating elements perpendicular to each other and adding two long protruding stubs to ground [6], use of tree-like structure on the ground plane [7], etching a T-shaped slot and a line slot on the ground [8], adding a Y-shaped slot on the T-shaped protruded ground plane [9], placing two shorts at 45 degrees between the microstrip lines and in the opposite direction [10], protruding ground structure [11], T-shaped metallic stub [12, 13], adopting wideband neutralization line [14] and using modified ground structure along with T-shaped slot on the ground [15].

Ultra-wideband is an emerging technology for short distance low power communications. It makes use of short duration pulses which have very low power spectral density for transmission of data. Since the UWB system is operating from 3.1 to 10.6 GHz, it could easily interfere with existing narrowband communication system like Wireless Local Area Network (WLAN-5.15–5.825 GHz). So, ultrawideband antenna with integrated frequency notching function at the interfering frequency band is a feasible solution to mitigate the frequency interference [16]. The power spectral density of UWB and other narrowband systems is shown in **Figure 1**. The frequency interference produced by a UWB transmitter to a narrowband system is very negligible because of the transmitted signal emission power (power spectral density) is very less compared with narrowband systems. But, when a UWB receiver is located very near to the narrowband interferer, the interference caused is very high. So, a notch at the interfering frequency is needed to suppress its effect. The traditional RF filter circuits using lumped elements can be used to implement this frequency notching feature but, it increases the system complexity, cost and occupies more space when integrated with other microwave circuits in the portable device. Another viable solution is to design a UWB antenna with an integrated band-notched feature to mitigate the frequency interference which decreases the complexity and cost of the UWB system. The idea of designing the UWB antenna with band-notched characteristic is shown in **Figure 2** [17, 18]. The basic UWB antenna is having impedance bandwidth from [*fL*-*fH*] as shown in **Figure 2(a)**, where *fL* and *fH* are the lowest and highest 10 dB cut-off frequencies of *S11*, respectively. A band-stop resonant structure also has the impedance bandwidth from [*fL*-*fH*], but with notch (resonant) frequency at *fN* to stop the unwanted frequencies as shown in **Figure 2(b)**. The UWB antenna combining with a bandstop resonant circuit forms band-notched UWB antenna is shown in **Figure 2(c)**. The band-notched UWB antenna does not interfere with existing communication systems which are operating at *fN*. Hence, the design of UWB antennas with

*The basic design of UWB antenna with band-notched characteristic: (a) UWB antenna, (b) Band-notch*

*UWB-MIMO Antenna with Band-Notched Characteristics for Portable Wireless Systems*

The band-notch characteristics for UWB systems can be obtained by integrating band-notch resonator (like slots, split ring resonators, strips and stubs) to the UWB antenna. The possible locations for integration of band-notch resonator are: on or adjacent to radiator or feed line or on the ground as given in **Figure 3**. The length of the band-notch resonator controls the notch center frequency. However, the desired notch band is obtained by proper tuning of length and width of the resonator. The total length of the band-notch resonator should be λ/2 or λ/4 corresponding to the notched-band center frequency as given in equations (1) and (2) [19], where

band-notched function is needed.

*resonant structure and (c) Band-notched UWB-antenna.*

**Figure 1.**

**Figure 2.**

*The power spectral density of UWB and other narrowband systems.*

*DOI: http://dx.doi.org/10.5772/intechopen.93809*

λ is the guided wavelength.

**51**

*UWB-MIMO Antenna with Band-Notched Characteristics for Portable Wireless Systems DOI: http://dx.doi.org/10.5772/intechopen.93809*

#### **Figure 1.**

In an indoor communication application, like other wireless communication systems, the UWB system performance is also restricted by multipath fading due to rich scattering environments which cause inter-symbol interference. In present times, digital communication using multiple input multiple output (MIMO) technology has emerged as a breakthrough for a wireless system. The MIMO system employs multiple antennas at the transmitter and receiver. It makes use of the rich multipath environment to mitigate the multipath fading effect. And it improves the range of communication and system capacity (data rate) without the need for additional bandwidth or transmitted signal power [2, 3]. Hence, the UWB system with MIMO technology is a viable solution to reduce the multipath fading effect and to improve the quality of service, the range of communication and system

The electromagnetic interaction between the radiating (antenna) elements in multiple antenna or MIMO system is known as mutual coupling. The closely spaced antennas, especially in portable devices, inevitably cause strong mutual coupling between antennas. The mutual coupling is undesirable which causes fluctuations in the input impedance of individual antenna element i.e., impedance mismatch which degrades the radiation efficiency, deviations in antenna radiation pattern due to the high correlation between antenna signals and decreases the channel capacity of MIMO system. Since the mutual coupling has a considerable impact on the MIMO system performance, the reduction of mutual coupling between antennas and enhancement of isolation between ports is imperative. However, placing multiple antennas in a space-limited portable wireless device is a big challenge for antenna designers [5]. Hence, designing compact UWB-MIMO antenna exhibiting bandnotch function and less mutual coupling is very much needed. Various designs were proposed in the recent years to suppress the effects of mutual coupling in ultrawideband MIMO antennas [6–15]. Methods include, placing radiating elements perpendicular to each other and adding two long protruding stubs to ground [6], use of tree-like structure on the ground plane [7], etching a T-shaped slot and a line slot on the ground [8], adding a Y-shaped slot on the T-shaped protruded ground plane [9], placing two shorts at 45 degrees between the microstrip lines and in the opposite direction [10], protruding ground structure [11], T-shaped metallic stub [12, 13], adopting wideband neutralization line [14] and using modified ground

Ultra-wideband is an emerging technology for short distance low power communications. It makes use of short duration pulses which have very low power spectral density for transmission of data. Since the UWB system is operating from 3.1 to 10.6 GHz, it could easily interfere with existing narrowband communication system like Wireless Local Area Network (WLAN-5.15–5.825 GHz). So, ultrawideband antenna with integrated frequency notching function at the interfering frequency band is a feasible solution to mitigate the frequency interference [16]. The power spectral density of UWB and other narrowband systems is shown in **Figure 1**. The frequency interference produced by a UWB transmitter to a narrowband system is very negligible because of the transmitted signal emission power (power spectral density) is very less compared with narrowband systems. But, when a UWB receiver is located very near to the narrowband interferer, the interference caused is very high. So, a notch at the interfering frequency is needed to suppress its effect. The traditional RF filter circuits using lumped elements can be used to implement this frequency notching feature but, it increases the system complexity, cost and occupies more space when integrated with other microwave circuits in the portable device. Another viable solution is to design a UWB antenna with an integrated band-notched feature to mitigate the frequency interference which decreases the complexity and cost of the UWB system. The idea of designing

structure along with T-shaped slot on the ground [15].

capacity [4].

*Innovations in Ultra-WideBand Technologies*

**50**

*The power spectral density of UWB and other narrowband systems.*

#### **Figure 2.**

*The basic design of UWB antenna with band-notched characteristic: (a) UWB antenna, (b) Band-notch resonant structure and (c) Band-notched UWB-antenna.*

the UWB antenna with band-notched characteristic is shown in **Figure 2** [17, 18]. The basic UWB antenna is having impedance bandwidth from [*fL*-*fH*] as shown in **Figure 2(a)**, where *fL* and *fH* are the lowest and highest 10 dB cut-off frequencies of *S11*, respectively. A band-stop resonant structure also has the impedance bandwidth from [*fL*-*fH*], but with notch (resonant) frequency at *fN* to stop the unwanted frequencies as shown in **Figure 2(b)**. The UWB antenna combining with a bandstop resonant circuit forms band-notched UWB antenna is shown in **Figure 2(c)**. The band-notched UWB antenna does not interfere with existing communication systems which are operating at *fN*. Hence, the design of UWB antennas with band-notched function is needed.

The band-notch characteristics for UWB systems can be obtained by integrating band-notch resonator (like slots, split ring resonators, strips and stubs) to the UWB antenna. The possible locations for integration of band-notch resonator are: on or adjacent to radiator or feed line or on the ground as given in **Figure 3**. The length of the band-notch resonator controls the notch center frequency. However, the desired notch band is obtained by proper tuning of length and width of the resonator. The total length of the band-notch resonator should be λ/2 or λ/4 corresponding to the notched-band center frequency as given in equations (1) and (2) [19], where λ is the guided wavelength.

**Figure 3.** *Possible locations of resonant structure on UWB antenna.*

$$L\_{Notch} = \frac{c}{2f\_{Notch}\sqrt{\varepsilon\_{\text{eff}}}},\tag{1}$$

ultra-wideband MIMO antenna with low mutual coupling is needed. In this chapter, we have presented compact isolation-enhanced planar UWB-MIMO antenna with single band-notched characteristics at WLAN band [31]. Ansoft HFSS v.13 is used to carry out the proposed antenna design, optimization, and simulations. For validating the simulation results, all the proposed antenna has been fabricated and tested using the Agilent N5224A PNA, Anritsu MS2037C vector network analyzer and an anechoic chamber. In Section 2, UWB-MIMO antenna with single bandnotched characteristic is discussed. Finally, conclusions of the work are given in

*UWB-MIMO Antenna with Band-Notched Characteristics for Portable Wireless Systems*

**2. UWB-MIMO antenna with single band-notched characteristic**

Compact ultra-wideband MIMO antenna exhibiting band-notch characteristics at WLAN band (5 to 5.9 GHz) for portable wireless devices is presented in this section [31]. The following sub sections discusses the detailed description of the

The geometry of the proposed single band-notch UWB-MIMO antenna and photograph of the fabricated antenna are shown in **Figure 4(a)** and **(b)**. The design is printed on an FR4 substrate having dielectric constant (*εr*) of 4.4, a thickness of 0.8 mm, and a loss tangent of 0.02. The overall size of the proposed antenna is L

planar monopole radiating elements, denoted as PM1 and PM2 having sizes LR WR as shown in **Figure 4(a)**. Both PM1 and PM2 are fed by the 50-ohm coplanar waveguide having dimensions FL1 WF. And, the common ground is formed by joining LG WG and LG L reduced ground planes and is also printed on the same side of the substrate. The planar monopoles PM1 and PM2 are positioned perpendicularly to each other to reduce the mutual coupling between the elements and to improve the isolation between the antenna ports. A long rectangular strip of size SL SW is extended from the common ground plane between the monopoles to further enhance isolation and improve the impedance bandwidth of the antenna. The ground strip extends the current path which shifts the first resonance frequency to lower band and blocks the surface currents to minimizes the mutual coupling. An inverted U-slot resonator is placed on the feed line to create a bandnotch function at 5–5.9 GHz. The antenna optimized dimensions are given as follows: (unit: mm): D1 = 5.1, D2 = 6.1, D3 = 11.2, FL1 = 9.5, FL2 = 1.5, FL3 = 0.4, L = 26, LG = 8, LR = 10, SL = 18, SW = 1, W = 40, WF = 1.8, WG = 3.2, WR = 11, U1 = 7.8, U2 = 0.4, and UW = 0.3. **Figure 5(a)** and **(b)** shows the simulated *S*-parameters such as *S*<sup>11</sup> and *S*<sup>21</sup> of the Antenna 1 (UWB-MIMO antenna without ground strip), Antenna 2 (UWB-MIMO with a ground strip), and the proposed antenna. It can be observed that the proposed ultra-wideband MIMO antenna is operating from 2.2 to 11.4 GHz with good impedance bandwidth except at notch band from 5 to 5.9 GHz. Also, the

mutual coupling of less than 20 dB is obtained over the entire UWB band.

of surface currents on the ground plane and near-field radiation leads to poor impedance matching and high mutual coupling, which restricts the performance of MIMO antenna. The ultra-wideband MIMO antenna without and with ground strip

Since the ground and radiating elements are having smaller dimensions, the flow

. The antenna comprises two identical rectangular

Section 3.

proposed design.

**2.1 Antenna design**

<sup>W</sup> h mm<sup>3</sup> = 26 <sup>40</sup> 0.8 mm<sup>3</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.93809*

**2.2 Study of MIMO antenna**

**53**

or

$$L\_{Notch} = \frac{c}{4f\_{Notch}\sqrt{\varepsilon\_{\text{eff}}}},\tag{2}$$

$$
\lambda = \frac{c}{f\_{\text{Notch}} \sqrt{\varepsilon\_{\text{ff}}}},
\tag{3}
$$

$$
\varepsilon\_{\sharp \overline{\mathcal{Y}}} = \frac{(\mathbf{1} + \varepsilon\_r)}{2},
\tag{4}
$$

where *c* denotes light speed, *fNotch* represents notch center frequency, *LNotch* is the total length of slot or strip, *εeff* indicates effective dielectric constant and *ε<sup>r</sup>* denotes dielectric constant.

Several investigations were reported earlier to create band notch function at WLAN band for ultra-wideband systems in [19–30]. Methods include inserting λ/4 and λ/2 slot resonators on the ground plane [19], using a pair of ground stubs locating along the edge of the ground plane [20], inserting open stub in the printed folded monopole [21], etching folded U-shaped slots in the feed line of the antenna [22, 23], incorporating SRR slots on radiating element [24], quarter-wave stub connected to the ground [25], adding protruding two rectangular stubs on the ground plane [26], with a slot of length 1.0 λ in the radiator [27], open-ended quarter-wavelength L-shaped slots were etched on the rectangular radiating patches [28], using C-shaped and Z-shaped slot resonators on the ground [29], employing elliptical SRR on the radiating element [30].

The antenna designs presented in the above literature exhibiting acceptable isolation and notching characteristics, but some designs were not compact enough and few are a bit complex. So, the design of simple and compact band-notched

*UWB-MIMO Antenna with Band-Notched Characteristics for Portable Wireless Systems DOI: http://dx.doi.org/10.5772/intechopen.93809*

ultra-wideband MIMO antenna with low mutual coupling is needed. In this chapter, we have presented compact isolation-enhanced planar UWB-MIMO antenna with single band-notched characteristics at WLAN band [31]. Ansoft HFSS v.13 is used to carry out the proposed antenna design, optimization, and simulations. For validating the simulation results, all the proposed antenna has been fabricated and tested using the Agilent N5224A PNA, Anritsu MS2037C vector network analyzer and an anechoic chamber. In Section 2, UWB-MIMO antenna with single bandnotched characteristic is discussed. Finally, conclusions of the work are given in Section 3.
