A Compact Ultra-Wideband MIMO/Diversity Antenna with Band-Rejection Capability

Abstract

In recent years, ultra-wideband (UWB) technology has attracted the devotion of researchers due to its unlimited advantages. These advantages include higher channel capacity, better wall-through imaging, localization, medical imaging and reliable transmissions. Along with all these advantages, UWB technology suffers from some limitations. Major limitation of this technology is very low spectral mask i.e. -41.3dBm/MHz. Due to its very low power level, it undergoes severe multipath fading in highly scattering environment. The solution to manage this fading effect is MIMO (multiple-input-multiple-output) technology. UWB-MIMO systems mitigate multipath fading very effectively. Furthermore, such systems offer more channel capacities as compared to narrowband systems. However, on the other side, increasing trend of miniaturization of portable devices leads to increased mutual coupling among MIMO antenna elements. In order to enhance isolation level, distance among MIMO elements has to be increased by compromising design compactness. So, designing an UWB-MIMO antenna with compact size and minimum isolation achieved is a very exciting challenge. Moreover, UWB suffers from interference with already existing communication standards i.e. wireless local area network (WLAN) (5.15–5.825 GHz). So, we have to introduce the band-rejection capability in MIMO systems to castoff the interfering bands. ii iii This thesis focuses on the study of UWB-MIMO antenna design for wireless communications systems. Two different designs have been proposed in this research work. In first design, semi-circular annular slot antenna is used for entire UWB coverage (3.1-10.6 GHz). These slots are symmetric- ally fed by orthogonally placed 50Ω micro-strip transmission lines. Uniform radiation patterns and better polarization characteristics are achieved by symmetric feeding. Two small stubs are used on both sides of microstrip lines for better impedance matching at higher frequencies. Furthermore, for most of the band, isolation better than 20dB is achieved by inserting two slots in between two antenna elements. Vertical slot helps in reduction in coupling at lower frequencies i.e. 3-5.5 GHz while horizontal slot enhances isolation at higher frequencies i.e. 5.5-10.6GHz. For band-rejection at the WLAN (5.15–5.825 GHz), an inverted U-shaped slot is etched in the feeding transmission line. The overall antenna size is 58 × 27 . Second design is a compact ultra-wide band antenna with enhanced isolation and band-rejection capability that is presented for portable devices. Rectangular stepped slots are etched on the back side of antenna to cover entire ultra-wide band. These slots are off-centered fed by 50Ω micro-strip transmission lines so there is no need of stub matching. Use of inverted T-shaped slot near upper edge of ground plane maintains isolation level better than 24.5dB in the lower band i.e. 3-5GHz. A z-shaped slot is inserted on the back side of antenna to reject WLAN. Proposed antenna size is 26 × 23mm2 with planar geometry. Both designs have achieved better impedance matching, minimum isolation among the ports and stable radiation patterns. Good agreement among measured and simulated results makes both proposed designs promising for UWB applications.