**Latest Progress in MIMO Antennas Design**

Yue Li, Jianfeng Zheng and Zhenghe Feng

*Tsinghua University China* 

### **1. Introduction**

Multiple-Input Multiple-Output (MIMO) wireless communication system, which is also called Multiple-Antenna system, is well known as one of the most important technologies and widely studied nowadays (Winters, 1987; Foschini & Gans, 1998; Marzetta & Hochwald, 1999; Raleigh & Cioffi, 1998). The main idea of MIMO wireless communication is to utilize the spatial degree of freedom of the wireless multi-path channel by adopting multiple antennas at both transmit and receive ends to improve spectrum efficiency and transmission quality of the wireless communication systems. MIMO technology is able to extremely improve the transmission data rates and alleviate the conflict between the increasing demand of wireless services and the scarce of electromagnetic spectrum. Two famous techniques of the MIMO systems are spatial multiplexing (SM) and transmit diversity (TD) (Nabar et al, 2002). In the scheme of SM, multiple data pipes between transmit and receive ends provide multiplexing gain to dramatically increase the channel capacity linearly with the number of antennas (Telatar, 1999; Bolcskei et al, 2002). The TD technologies, such as space-time coding, are adopted to improve the link reliability of wireless communication, especially in the multi-path fading channels (Marzetta & Hochwald, 1999; Tarokh et al, 1998; Bolcskei et al, 2001). The channel knowledge is not required in the transmit end for TD technologies. MIMO is the key technology for future wireless communication systems, such as 3GPP LTE, WiMAX 802.16, IEEE 802.20, IMT-Advanced and so on.

Although the spatial degree of freedom is important and has the potential to extremely increase the capacity of the MIMO systems, how to utilize the space resources is still needed to be studied. Physical layer design is the most important issue of wireless communication systems. Among all the components, the antenna is the interface of the MIMO wireless communication systems to the channel, which is the most sensitive part for the spatial degree of freedom. The system performance is directly dictated by the number of antennas adopted in transmit and receive end. The key issue to achieve high channel capacity of the MIMO system is the mutual coupling between antenna elements. In traditional MIMO systems, space-separated antenna array is adopted at the base station or mobile terminal. Nearly half of the wavelength is required to achieve acceptable isolation, about -15 dB for most of the situations. However, for the space is limited in both the base station and the mobile terminal, the mutual coupling between the adjacent antenna elements becomes more and more serious, restricts the performance of MIMO systems (Wallace & Jensen, 2004; Morris & Jensen, 2005). The design of antenna in space-limited MIMO system is still need further discussed. This chapter will focus on this topic.

In this chapter, we provide a comprehensive discussion on the latest technologies of antenna design for space-limited MIMO applications, such as minimized base stations, portable access points and mobile terminals. To solve the contradiction of system volume and antenna performance, two basic methods are proposed to maintain the channel capacity in a reduced system volume, as illustrated in Fig. 1. The first one is to reduce to volume each antenna occupied without decreasing the number of antenna elements. The polarization resource is one of the important space resources. Different from the space-separated antennas, the polarization antenna array can utilize the multiple field components to improve the spatial degree of freedom of MIMO systems within a limited space. And the antennas with different polarizations can locate in the same place to save the space occupied. The ports isolation is the challenge for antenna design. Another one is to enhance the antenna performance in the space-limited MIMO system, without increasing the antenna volume. Using switching mechanism, one more polarization or radiation pattern can be selected due to the channel conditions. Based on the adaptive antenna selection, suitable signal processing methods can be adopted alternatively to achieve better performance. The design of switching mechanism is the key issue for carefully consideration.

Fig. 1. Technical diagram for antenna design in space-limited MIMO system.

This chapter is organized as following. In Section 2, dual-polarized antenna solution is proposed as an example of 2-element polarization antenna array. Two practical designs are present to show the isolation enhancement between ports. Section 3 describes polarization reconfigurable antenna element based on the Section 2. Channel capacity benefit has been validated by experiment. In Section 4, another type of reconfigurable antenna, pattern reconfigurable antenna element is proposed. Section 5 will give a summery of this chapter.

### **2. Dual-polarized antenna**

In this section, we talk about the polarization resource of antenna. The polarization antenna array has been studied in mobile communications for decades. In 1970s, the polarization characteristics of mobile wireless channel had been widely measured and discussed. The results illustrated that the correlation between feeding ports of different polarization antenna elements must be low to satisfy the requirements of diversity, and the volume occupied is much smaller than the space-separated antennas. Thus, more uncorrelated subchannels can be obtained by using polarization antenna array. Further, the orientations of the mobile terminals are commonly not perpendicular to the ground. Polarization antenna

In this chapter, we provide a comprehensive discussion on the latest technologies of antenna design for space-limited MIMO applications, such as minimized base stations, portable access points and mobile terminals. To solve the contradiction of system volume and antenna performance, two basic methods are proposed to maintain the channel capacity in a reduced system volume, as illustrated in Fig. 1. The first one is to reduce to volume each antenna occupied without decreasing the number of antenna elements. The polarization resource is one of the important space resources. Different from the space-separated antennas, the polarization antenna array can utilize the multiple field components to improve the spatial degree of freedom of MIMO systems within a limited space. And the antennas with different polarizations can locate in the same place to save the space occupied. The ports isolation is the challenge for antenna design. Another one is to enhance the antenna performance in the space-limited MIMO system, without increasing the antenna volume. Using switching mechanism, one more polarization or radiation pattern can be selected due to the channel conditions. Based on the adaptive antenna selection, suitable signal processing methods can be adopted alternatively to achieve better performance. The

design of switching mechanism is the key issue for carefully consideration.

Fig. 1. Technical diagram for antenna design in space-limited MIMO system.

**2. Dual-polarized antenna** 

This chapter is organized as following. In Section 2, dual-polarized antenna solution is proposed as an example of 2-element polarization antenna array. Two practical designs are present to show the isolation enhancement between ports. Section 3 describes polarization reconfigurable antenna element based on the Section 2. Channel capacity benefit has been validated by experiment. In Section 4, another type of reconfigurable antenna, pattern reconfigurable antenna element is proposed. Section 5 will give a summery of this chapter.

In this section, we talk about the polarization resource of antenna. The polarization antenna array has been studied in mobile communications for decades. In 1970s, the polarization characteristics of mobile wireless channel had been widely measured and discussed. The results illustrated that the correlation between feeding ports of different polarization antenna elements must be low to satisfy the requirements of diversity, and the volume occupied is much smaller than the space-separated antennas. Thus, more uncorrelated subchannels can be obtained by using polarization antenna array. Further, the orientations of the mobile terminals are commonly not perpendicular to the ground. Polarization antenna array is an effective solution to reduce the polarization mismatch. In traditional cellular mobile communication systems, the system with polarization diversity antennas has a 7 dB gain than the one with space diversity antennas in Line-of-Sight scenarios, and a 1 dB gain in Non-Line-of-Sight scenarios (Nakano et al, 2002).

In MIMO systems, the channel capacity of MIMO system with polarization antenna array is approximately 10%~20% higher than that with space-separated co-polarized antenna array, though the system SNR of polarization antenna array is lower (Kyritsi et al, 2002; Wallace et al, 2003). Another measurement results in micro- and pico-cell show the channel capacity of MIMO systems with dual-polarized antenna elements are 14% higher than that with twicenumbered single-polarized antennas (Sulonen et al, 2003). Similar results are also obtained (Erceg et al, 2006). Of course, the dual-polarized antenna element can be treated as a 2 element single-polarized antenna array. For this application, two important issues must be considered: one is the ports isolation, the other one is the antenna dimension. High-isolated compact-volume dual-polarized antenna is our goal of design.

In resent research, different methods of isolation enhancement are introduced. An air bridge, which is utilized in the cross part of two feedings for high isolation, was proposed in (Barba, 2008; Mak et al, 2007). Different feed mechanisms, feed by probe and coupling through aperture, were used in (Guo et al, 2002). Another isosceles triangular slot antenna is proposed for wideband dual-polarization applications in (Lee et al, 2009). TE10 and TE01 modes are excited by two orthogonally arranged microstrips. The above mentioned methods are difficult to be realized in a compact structure and unable to be adopted in space-limited multiple antenna systems. In this section, we introduce two compact antenna designs with good ports isolation.
