4. Conclusion

bridge represents the ON state, while its absence represent OFF state. The operating modes are given in Table 1. When the CR-UWB antenna works in mode 1, it is a UWB antenna which can be used in underlay mode in a very low power. In this case, the IR-UWB technology can be used to transmit and receive desired signals. Furthermore, it can also be used as a sensing antenna in CR communication systems. As the CR-UWB antenna operates at mode 2 to mode 8, it is a UWB antenna with different notches. In these cases, the CR-UWB system can be implemented by using OFDM-UWB technology to switch ON/OFF the different carries to make the antenna to prevent the unwanted narrowband signals. Thus, the designed antenna can be used for various CR-UWB communication systems to sense and to prevent interferences. It can also switch between the overlay and underlay modes by change the antenna operating modes. In addition, the CR-UWB antenna can be used for UWB, band-notched

The radiation patterns of CR-UWB antenna in mode 4 and mode 5 are shown in Figure 21. It is found that the CR-UWB antenna has omnidirectional radiation patterns in the H-plane and it

Figure 21. Radiation patterns of the CR-UWB antenna. (a) E-plane of mode 4. (b) H-plane of mode 4. (c) E-plane of mode

UWB and multiband communication systems.

88 Cognitive Radio

5. (d) H-plane of mode 5.

In this chapter, UWB antennas for CR communication have been reviewed. The DMS-based stop-band filter, UWB antenna, band-notched UWB antenna and reconfigurable UWB antenna have been discussed to construct a CR-UWB antenna. The CR-UWB antenna has been realized by integrating desired DMS-based stop-band filters and radio frequency switches into a UWB antenna. The DMS-based filters were used to create the notches to filter out the unwanted narrowband interference signals, while the switches control the reconfigurable modes of the antenna. The antenna is designed step by step and it is analyzed in detail. The results showed that the CR-UWB antenna can be used to switch between different modes and it can be used as UWB antenna, band-notched UWB antenna and multiband communication systems.
