Antenna designs for MIMO systems

This chapter describes the development and testing of antenna elements for use in a Multiple-input-multiple-output (MIMO) application. MIMO wireless systems have antenna arrays at both the transmitter and receiver terminals. This project focuses on the terminal MIMO antenna array design. The requirements of the antenna element at the terminal are as yet not well-defined but will include (for commercial reasons) low antenna profile, low volume, light weight and low cost while maintaining good electrical properties such as return loss and isolation (-20dB). The pattern coverage for a terminal should be as omni-directional as possible since the location of the base station is often unknown. However, the patterns need to direct energy away from the circuit board of the device, i.e. laptops, PDAs and handsets, so that electromagnetic interference (EMI) is reduced. The radio environment on electrically small platforms is changing rapidly. Until recently one radio was used in isolation and was usually connected to only one antenna. The situation today is very different: there is usually more than one radio used at once for example a handset may have 4 cellular bands, GPS and Bluetooth TM. Sometimes WLAN radios are also present. This means that more RF filtering of signals is necessary. It is also becoming common for each radio to use more than one antenna in order to create diversity or for MIMO applications. The changing situation is described in Figure 1.1.