Research On Antennas And Its Decoupling Techniques For Wireless Communication Systems

Increasing the data transmission rate is the main direction of the development of wireless communication technology.As the major technique to enhance the channel capacity and spectral efficiency of wireless systems,MIMO technique greatly push the limits of data transmission rate,and is therefore a research hotspot in the field of wireless communication.Antennas are indispensable terminal components of the wireless systems,and its performance determines the overall quality of the system.With the continual miniaturization of wireless systems,the distances between antenna elements in MIMO systems are increasingly close,and the mutual coupling between antenna elements become more and more severe,which dramatically reduces the performance of multiple antennas and weaken the advantages of MIMO systems.In order to improve the isolation between antenna elements while maintaining the size of the antenna system small,this paper mainly conducts the study on antennas and its decoupling techniques for wireless communication systems.Firstly,a two-element single band MIMO antenna with overlapped structure of antenna elements is optimized in this paper for antenna miniaturization.The area of clearance region of the antenna is reduced by more than a half compared to its coplanar counterpart.Due to the compactness of the antenna structure,the mutual coupling between the antenna elements becomes extremely strong.The isolation is only about 2.5 dB in the operating frequency band of 2.4 GHz to 2.484 GHz.In order to increase the isolation between the overlapped elements,a microstrip decoupling network is designed based on the coupled resonance theory of microstrip line.The overlapped antenna obtains an isolation improvement of about 15 dB through this microstrip decoupling network.Although the isolation improvement provided by the microstrip decoupling network meets the requirement of application,it is not practical enough to be used in a size-limited mobile communication terminal.For the miniaturization of decoupling circuit,a lumped decoupling component whose size is only 2mm×1.2mm×1mm is designed for the overlapped antennas using LTCC technology.This LTCC device is highly suitable for various compact mobile terminals.The basic principle for designing the LTCC device is to convert the LC components of a planar decoupling circuit into a single LTCC device which contains multilayer metal and dielectric.Finally,the measured results show that the LTCC decoupling device provides an isolation improvement of more than 20 dB for the overlapped antennas.At the same time,the area of the decoupling device is only about 4% of the microstrip decoupling network.In order to verify the versatility of LTCC devices in various antenna decoupling applications,this paper then design a dual-band two-element coplanar antenna operating at WLAN 2.4 GHz/5.2 GHz and a two-element coplanar chip antenna system based on commercial chip antenna working at WLAN 2.4 GHz.First,an LTCC decoupling device that solely improves the low band isolation without affecting the high band performance is designed for the dual-band two-element coplanar antenna.The measurement results show that the LTCC device improves the isolation of 5 dB for the low band performance of the antenna.Subsequently,an LTCC decoupling device for the two-element coplanar chip antenna system is designed using the same theory and design method,which increases the isolation between the chip elements by 4 dB to 16.5 dB.Both of the two LTCC decoupling devices have the same size of 2mm× 1.2mm×1mm.These three aforementioned antenna decoupling cases cover many sorts of application scenarios such as single band,dual band,overlapped,coplanar,printed antennas and chip antennas,which convincingly demonstrates the versatility of LTCC devices in the applications of antenna decoupling.