1. Introduction

In 2002, FCC allocated the unlicensed frequency spectrum from 3.1 to 10.6 GHz for ultra-wideband (UWB) technology [1]. After this allocation, ultra-wideband has received attention from wireless communication experts owing to its advantageous features like wider bandwidth, low cost, low susceptibility to multipath fading, reduced probability of detection and intercept and potentially high data rates. In a highly dense and dynamic environment, the UWB systems suffer from multipath fading due to reflection and diffraction. This multipath fading results into the degradation of signal-to-noise ratio (SNR) and channel capacity.

An effective method to resolve these multipath fading issues is the incorporation of antenna diversity techniques in wireless communication systems. Several types of diversity, such as space/spatial, pattern and polarization diversity, have been already proposed and implemented to receive multiple signals [2–4].

In a diversity scheme, the power or signal-to-noise ratio of the received signal is optimized by the selection or combining of output signals in several ways like selection combining, equal gain combining or maximal ratio combining. The detailed description of diversity combining techniques is available in [5, 6].

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For good diversity performance, the received signals should have very low correlation between them [7]. An increase in correlation reduces the combining efficiency. In a spatial diversity scheme, a large separation (compared to wavelength) between the antennas is used to achieve decoupling between signals. This large space requirement limits the use of this diversity method. To overcome this drawback, other techniques such as pattern or polarization diversity [8, 9] are investigated. These alternate techniques involve the use of two or more antenna elements with different radiation patterns [10]. An UWB system with polarization diversity technique has potential applications in advanced instruments used for microwave imaging, radar and high-speed data transfer. Some UWB polarization diversity antennas are already reported in the literature [11–28]. However, the application of those available structures is limited due to their large dimensions, multilayer structure, complex feedline, complex geometries, etc.

Among the various bandwidth enhancement techniques, the use of fractal geometries is proven to be a good method. Fractal antenna structures have a compact size and wideband performance due to properties of self-similarity, space filling and effective energy coupling properties [29].

In this chapter, a compact CPW-fed UWB fractal antenna with polarization diversity performance is presented. The bandwidth of the antenna structure [29] is enhanced by loading the coplanar ground planes with a quarter wavelength long rectangular notches. Two identical copies of this antenna structure are arranged orthogonally to achieve good interport isolation and orthogonal polarization diversity performance without affecting the UWB performance. In the following sections, antenna design description is followed by discussion of frequency domain analysis results, time domain analysis results and diversity performance parameter calculation. Finally, it is concluded with major findings of this chapter.
