**5. Conclusion**

22 Wireless Communications and Networks – Recent Advances

system using the proposed pattern reconfigurable antennas, compared with 1x1 and 2x2 systems using reference dipoles. As listed in Table 2, 2.28 bit/s/Hz and 4.13 bit/s/Hz of the average capacity enhancement are achieved in LOS and NLOS scenarios, and 2.51 bit/s/Hz and 3.75 bit/s/Hz enhancement for 95% outage capacities. In the NLOS scenario, the received signal is mainly contributed from reflection and diffraction of the concrete walls and the desk partitions, arriving at the direction of endfire. The diversity gain in the endfire increases the channel capacity. Considering the insertion loss introduced from the non-ideal PIN diodes, better performance of the proposed antenna can be achieved by using high quality switches, such as micro-electro-mechanical systems (MEMS) type switches with less

insertion loss and parasitic parameters.

Fig. 26. CCDFs of channel capacity in LOS condition.

Fig. 27. CCDFs of channel capacity in NLOS condition.

This chapter has introduced the now trend of antenna design in MIMO systems. From a view of antenna design, it is difficult to achieve good performance by using traditional antennas in space-limited MIMO systems. The mutual coupling between the antenna elements deteriorates the independence among multiple channels in MIMO systems. For both techniques of TD and SM, the benefit of multiple channels is difficult to be obtained due to the space limitation. However, the usage of miniaturized mobile terminals is popular and their size is getting much smaller. Here comes the contradiction between the antenna performance and the ministration of mobile handsets.

In this chapter, we are aimed to solve the space problem of antennas in MIMO systems. Two effective solutions are introduced here. The first one is to use polarization, an important spatial resource, to take the place of antenna element. Two orthogonal polarized antenna elements can be arranged together with acceptable isolation. In this way, the space between antenna elements is saved, making the overall antenna system more compact. As an important practical application, two types of dual-polarized antennas are presented and analyzed. Isolation enhancement methods are proposed, such as the feed design and operation modes design. The proposed antennas show the advantages of compact structure, high ports isolation and easy fabrication, and are suitable to be adopted in the space-limited MIMO systems.

Considering in the opposite way, the antennas with better performance in the original space is another solution in space-limited MIMO systems. The reconfigurable antenna is a prevalent type of antenna nowadays. Switching mechanism is added to achieve selectable polarizations, radiation patterns and other property. Different antenna configurations and corresponding signal processing methods are selected due to the channel information. The switching mechanism is the most important issue. Based on the dual-polarized slot antenna design, the PIN diodes are used to achieve polarization selection. A pattern reconfigurable antenna is design by using a switchable CPW-to-slotline feeding structure. In order to prove the benefit of the antenna selection, we design an experiment of channel capacity in a typical indoor environment. The results show that the channel capacity improves in both LOS and NLOS scenarios, especially in NLOS scenario. The reconfigurable antenna shows the potential application in space-limited MIMO systems.
