Investigation Of In-band And Out-of-band Decoupling Techniques For Antennas In Wireless Communication System

In today's mobile communications,more antennas and frequency bands are needed inboth terminal and base station.However,the size of the equipment is expected to be smaller and smaller.As a result,strong mutual coupling exists between closely-placed antennas,which can be divided into two types: in-band and out-of-band coupling.In general,in-band decoupling is mainly used in multi-element MIMO antennas with small spacing.By reducing the pattern correlation of antenna elements,communication capacity can be improved.The out-of-band decoupling is suitable for multi-band base station,where radiation pattern of antenna is distorted by out-of-band coupling.Therefore,it is of great significance and challenge to conduct research on reducing mutual coupling..This thesis has investigated the in-band and out-of-band decoupling techniques in detail and the main work can be divided into two parts as follows:1.Pattern-diversity-based decoupling method and its application to MIMO antennas.A novel decoupling method without adding any decoupling structure is investigated.The decoupling mechanism applied in here is to change the electrical length of the radiators to manipulate the radiation pattern for obtaining pattern diversity and good isolation simultaneously.The design mechanism and procedures for realizing pattern diversity are presented.To validate the proposed method,a 2-element monopole antenna array achieves isolation better than 20 dB at spacing of 0.066?.Then the 2-element MIMO antenna is extended to an closely-placed 8-element one with measured isolations better than 15 dB.Apart from applying to multi-element MIMO antenna,the proposed decoupling method can be used with other decoupling structures to realize dual-band MIMO antenna.A 2-element dual-band MIMO antenna is designed at the spacing of 5 mm.By etching ?/4 open-ended slots on the ground plane,more than 20 dB is achieved over1.7-2.7 GHz and 3.4-3.6 GHz.2.Broadband dual-polarized filtering base station antenna.Filtering antenna is a good way to reduce the interference among different bands.Two broadband dual-polarized filtering dipole antennas are designed for multi-band base station application.For the first filtering antenna,without using complex filtering circuits,the dual-polarized dipole antenna realizes satisfactory filtering performance and enhanced bandwidth by employing only two parasitic loops.Two specific radiation nulls are thus generated and individually controlled by the two parasitic loops.To further improve the upper stopband selectivity and bandwidth,a simple open-ended stub is added to the arms of dipole.As a result,the bandwidth can be tuned from7.4% to 47.6% and the realized gain is decreased dramatically from 8.6 dBi at 2.7 GHz(in-band)to-10 dBi at 2.9 GHz(out-of-band).The suppression level is more than 17 dB.Forthe second filtering antenna,two radiation nulls can be generated by adding simple U-shaped parasitic element close to the arms of dipole.The frequencies of the radiation nulls can be tuned by optimizing the lengths of parasitic element and dipole arms.More than 63%bandwidth and 13 dB suppression level can be realized in this case.