Constant differential phase components for multi-way system
Student thesis: Doctoral Thesis
Related Research Unit(s)
Mobile communications has witnessed rapid development in the variety and speed in services provided. The transition from voice services in the first generation (1G) to data transmission in third generation (3G) networks is a clear indication that the demands expected by customers will be ever increasing. The number of mobile users in the world had already exceeded 6 billion in 2012, and the rate of growth of mobile subscribers is beyond the maximum capacity of current mobile networks. An enormous amount of information is transmitted via these wireless channels which causes heavy congestion. Therefore sophisticated multiplexing and modulation techniques have been deployed to increase its capacity with recent developments edging towards multi-way systems, like 802.11n and 4G LTE. This is primarily because multi-way networks have shown potential improvement in capacity and stability. Attention of multi-way system has focused on MIMO (multi-input and multi-output) systems, the capacity of MIMO systems can increase by several factors. However the increase in capacity relies on increased feeding network/front-end components, with errors in each component that propagate through the system. The error in each component can be minimized through its precise and careful manufacture which also increases its cost. However the error due to cascade connection which propagates through each section have never been considered. This thesis presents three approaches that alleviate the above technical challenges. First, the integration of multi-functionality in a component is proposed to compress the physical size and reduce performance degradation. This concept is demonstrated using versatile port-feeding network for single section branch line coupler. Its primarily function is to overcome the narrow bandwidth of the coupler; band-pass filtering is the additional function. The versatile port-feeding networks results in a 5-6 fold increase in bandwidth compared to the original (~10%), while the coupler with additional band-pass response also exhibits good magnitude and phase response. Secondly, apart from the function integration, arbitrary phase difference is another possible solution for practical applications. Previous hybrid couplers have output phase differences that have either been 0°, 90° or 180°. Other phase differences could only be realized using additional phase shift elements. This increases circuit size and cost, and also degrades overall performance due to interconnecting mismatch losses. The proposed uni-planar arbitrary phase difference coupler eliminates these additional components thereby maintaining small size and good performance. Theoretical analysis is presented with measured results for two microstrip couplers for verification. Thirdly the concept of multi-way and poly-phase is proposed to eliminate the cascaded networks found in MIMO antennas. Traditional 2-way passive differential phase shifters are comprised of a reference line and a delay line. To realize passive, multi-way, poly-phase, differential phase shifters, a single reference line is not possible and a cascaded network becomes necessary. This results in both a cumbersome and complex structure, with large phase error that propagates through the network. Novel phase shifters are presented here that allows both multi-way and poly-phase differential phase shift using just one common reference line. These phase shifters are constructed using a newly proposed phase shift component in all element designs, where each element maintains a constant phase shift with adjacent elements. This characteristic for differential phase shifters offers a simple solution to multi-way, poly-phase systems, such as those found in phased arrays. Finally, this thesis concludes with further work that was subsequently discovered as a result of this work that could further improve MIMO and other multi-way systems.
- MIMO systems, Mobile communication systems