| In the modern world,communication systems are the infrastructure of a nation’s economy and society.Our country’s information and communication industry continues to achieve new leapfrogs,providing a strong impetus for the sustained and healthy development of the economy and society.To meet the growing demand for wireless communications,it is vital that the system possesses a high transmission rate and high reliability.However,the availability of radio spectrum resources is limited;as a result,a multiple input multiple output(MIMO)wireless access technology has been proposed as a means for increasing efficiency without occupying additional spectrum or increasing transmission power.MIMO technology uses multiple antennas for communication,which increases the dimensions of the signal and significantly enhances the channel capacity and transmission reliability.MIMO technology has demonstrated success in 4G,5G and other wireless communication systems,and it will also be integral to various wireless communication systems in the future.As the economic status continues to grow,personal communication terminals become more popular and the internet of things is introduced.The design of MIMO antennas in terminal equipment is receiving increasing attention.The requirements for miniaturization and portability of mobile terminals impose severe space constraints on the design of MIMO antennas.The closely arranged multiple antennas can,however,produce a significant amount of mutual coupling,which will seriously degrade the performance of the system.Therefore,reducing the mutual coupling in the multi-antenna system has become a research hotspot in academia.Based on actual application requirements,in this thesis the decoupling techniques for compactly arranged small antenna arrays are studied,and the decoupling and matching networks(DMNs)and the loaded parasitic element(LPE)are focused on.Main contributions of this dissertation are summarized as follows.1.The integrated design of eigenmode decoupling and matching network for the two-element array.With the eigenmode decoupling network cascaded,the array scattering matrix is diagonalized to achieve decoupling.In this regard,the scattering matrix of the four-port eigenmode decoupling network must conform to certain mathematical properties.The introduction of virtual ports complements the symmetry of eigenmode decoupling network circuit in the general implementation form.We then apply the even-odd method to obtain scattering parameter characteristics of the five-port network.The scattering matrix of original four-port network can be restored by the cascading formula.This general realization form is demonstrated to be effective in decoupling its eigenmodes.Similarly,an analysis of a two-port network generated by cascading the array with the eigenmode decoupling network can be carried out,resulting in the derived impedances of ports.Using the degrees of freedom of the decoupling network parameters,the integrated design of eigenmode decoupling and matching network can be yielded.This method avoids the cascading of matching networks and simplifies the design.A decoupling and matching network has been developed for a two-element array with 0.1 λ spacing,and the measured results show that the network performs well at two ports.2.Compact and dual-band eigenmode decoupling and matching network of the two-element array.It is possible that the parameters of eigenmode decoupling matching network determined by its network parameter matrix cannot be physically realized when applied to a two-element array.The eigenvector of a symmetrical scattering matrix does not change when cascading a symmetrical network.The antenna is first cascaded with the impedance transformation network,followed by the decoupling and matching network,which is also able to decompose and match the eigenmodes.A new miniaturized eigenmode decoupling and matching network is thereby formed by combining the symmetrical impedance transformation network with the decoupling matching network.For a two-element array with 0.1 λ element spacing,in the traditional method a ring circuit with a circumference of 1.5 λ is used and a cascaded matching network outside is still needed.However,in the proposed method,only a ring circuit with a circumference of 0.87λ is used,and no external circuit is required.The decoupling performance is better,and the circuit area is greatly reduced.For the dual-band two-element array with the element separations of 0.1 λ in the lower band and 0.14 λ in the higher band,the DMN can be obtained by replacing the transmission line of original circuit with the stub tapped transmission line.For both applications,the measured results have good port performance.3.Hybrid decoupling method for the isosceles triangle array.Due to the symmetry of isosceles triangle array,the couplings between the array elements can be categorized into two types,i.e.coupling between the vertex element and base elements as well as coupling between the two base elements.By introducing the loaded parasitic element on the symmetry axis,there is no coupling between the vertex element and its base elements,and the array can be considered to be divided into two almost irrelevant parts.Thus,the impedance matching of vertex element as well as the decoupling and matching of base elements can be implemented independently.A novel conjugate decoupling matching network is developed,in which the neutralization section deploys a transmission line with two stubs.In the admittance matrix of coupled network,the imaginary parts of principal and auxiliary diagonal elements can be independently cancelled.These two procedures of eliminating couplings are complementary.By adjusting the position of parasitic unit,we can find the most convenient conjugate matching network for the physical realization.The decoupling and matching technique for the isosceles triangular array with the maximum spacing of 0.2 λ is carried out.The measured results show that the method is effective.4.The DMN for the equilateral triangle array consists of an impedance transformation section and a star-shaped neutralization section which eliminates couplings between the antennas while occupying a smaller area.The impedance transformation section converts odd-mode and even-mode conductances of the antenna to the impedance of feeding line,and the neutralization section eliminates the odd-mode and even-mode susceptances to complete the decoupling and matching of the antenna.The star-shaped circuit utilizes the area surrounded by the antennas in a more efficient manner than traditional triangle circuits.This facilitates folding of the TL and miniaturization of the circuit.A design formula is given for each module of the circuit after it has been analyzed analytically.A DMN for the array with a spacing of 0.15 λ is simulated and manufactured.Decoupling circuits occupy a small area with less than 0.25 λ diameter,and measured results confirm good port performance. |
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