Study on components for RF MIMO

Abstract

Recent developments in wireless communications have shown that by using multiple antenna elements at both the transmitter and receiver, it is possible to substantially increase the capacity of a wireless communication system without increasing the transmission power and bandwidth. This structure of using multiple antenna elements is termed a Multiple-Input Multiple-Output (MIMO) system. This thesis covers different basic microwave components within this Multiple Input Multiple Output (MIMO) front end, and proposes a novel concept for its implementation. In the beginning of this thesis, different structures for implementing 3-dB hybrid couplers are presented. The branch line coupler which is the most common form of planar quadrature hybrid couplers is reduced in size using a T-shaped slow wave structure. Another form of hybrid coupler is the patch hybrid coupler which has a simple structure with the narrow branch lines eliminated. Additionally, the novel dual band design using stub loading and the broadband design using impedance steps are proposed to make it versatile for more applications. In the second part of the thesis a new broadband phase shifter is proposed. The basic structure comprising of simple open circuit and short circuit stubs provides a bandwidth of 50% with small variation in phase, while the use of multi section stubs was found to improve the bandwidth. The wide band phase shifter was designed and verified to have a bandwidth of about 100% for a maximum phase deviation of ±3.2º, and maximum insertion loss of 2.1 dB. It was also found that when the structure was implemented using CBCPW (Conductor Backed Coplanar Waveguide), a broadband response of 36% was achieved. Based on the hybrid couplers and phase shifters described previously, several Butler Matrices were then investigated and presented in the third part of the thesis. A compact Butler Matrix using size reduced elements together with a novel topology was proposed, which only occupied 29% of a conventional one without any loss in performance. The novel rectangular patch hybrid couplers and phase shifters were used to realize a wideband Butler Matrix with flat coupling covering a bandwidth of 24%. The rectangular patch hybrid coupler optimized using Jumping Gene Evolutionary Algorithm together with the broadband parallel stubs phase shifters formed a Butler Matrix that gave even wider bandwidth. The fourth part of the thesis begins with the novel concept of a broadband switch-less front end. A new topology which replaces a dummy load of a distributed power amplifier with a feedback low noise amplifier is presented, and can be used as an RF front end from 0.8 GHz to 2.6 GHz, negating the need for a switch. RF MIMO front ends are normally implemented with the beam-forming network such as a Butler Matrix placed between the amplifiers and antennas. Hence the amplifiers are placed between the beam-forming network and antennas according to this concept, thus improving the performance in terms of power loss and noise figure as well as allowing operation for beam steering or spatial diversity. Finally, a conclusion is drawn from the research work and a vision for future work on MIMO front end for use in modern wireless communication systems is given.