**2. Antenna theory**

210 Ultra Wideband – Current Status and Future Trends

is 802.11n.

(SNR). Spatial multiplexing in MIMO systems helps in increasing data rate. Beamforming is used either to increase data rate or to strengthen the signal. The applications of MIMO include digital television (DTV), wireless local area networks (WLANs), metropolitan area networks (MANs), and mobile communications. IEEE standard related to MIMO technology

Where one says that UWB is a classical solution to the demand of high data rate communications, there arise the questions while thinking of UWB and MIMO together in the wireless systems. Obviously, it can be answered in terms of the reasons rather motivations behind this combination of two technologies. In effect, it is well known that the main applications of UWB technology are found for WPAN and WBAN (Wireless body area network) in indoor environments where the dense multipath propagation leads to generally detrimental Inter Symbol Interference (ISI). Therefore, to turn this drawback into an advantage, multiple antennas or MIMO techniques can be employed to exploit such rich scattering environments. The more important is that the applications of UWB are limited to the short distance communication due to very low transmission power allowed by the FCC. Hence, using MIMO together with UWB helps in extending the communication range as well as offers higher link reliability. The benefits of UWB-MIMO can be summarized as following [7]-[8]: interference mitigation/suppression, higher data rates, improved link quality, extended coverage, reduced analog hardware requirements, and concurrent localization. Apart from these benefits, there are also challenges for the joint implementation of UWB and MIMO. These challenges include: UWB-MIMO signaling trade-offs, UWB-MIMO channel modeling, the optimization of UWB-MIMO modulation schemes, design of compact and suitable UWB antenna arrays, efficient and cost-effective UWB-MIMO RF circuit design, etc. Among these challenges, the design of compact and suitable UWB diversity antennas has appealed many researchers to work on this topic. The significance of antenna in a wireless communication system cannot be avoided. It also becomes a critical element to be miniaturized along with the other circuit elements. The design of antenna faces a lot of challenges itself in this race. As devices are going to be more compact, therefore the antennas must be positioned within the available space. Moreover, MIMO can be implemented by three ways: beamforming, spatial multiplexing and diversity techniques. Diversity techniques, more specifically antenna diversity techniques (i.e., spatial, polarization and pattern), are adopted for MIMO antenna designs. Hence, the implementation of multi-antenna structures becomes more challenging in the very limited space provided by the small terminals. The limitations to exploit the diversity arise when the antennas are placed in close vicinity. So, it is required to decorrelate their patterns, or in other words, mutual coupling should be minimized. Another challenge is the enhancement of isolation between the access ports of MIMO antennas. In the literature, a relative few MIMO antenna designs have been presented for UWB systems. In this context, this chapter presents an overview of MIMO UWB antennas with the main following objectives: to highlight the additional parameters required to characterize the performance of UWB antennas as well as MIMO antennas, to bring a state of the art in MIMO antennas for UWB

The antennas are an essential part of any wireless system. According to the IEEE Standard Definition of terms for Antennas, an antenna is defined as "a means for radiating and receiving radio waves" [9]. In an advanced wireless system, an antenna is usually required to optimize or accentuate the radiation energy in some directions and suppress it in others at certain frequencies. A good design of the antenna can relax system requirements and improve overall system performance. To describe the performance of an antenna, there are several commonly used antenna parameters, including impedance bandwidth, radiation pattern, directivity, gain, efficiency, and polarization [10].
