Research On Miniaturized MIMO Antennas With High Isolation

Multiple input multiple output(MIMO)technology has played an important role in wireless communication systems,because it can increase the channel capacity and link quality significantly without additional bandwidth or increasing transmit power.As crucial devices in MIMO systems,MIMO antennas have received increased attention.Researches on the analysis of miniaturized MIMO antennas with high isolation have been done in this dissertation.The main contents of this dissertation are listed as follows:1.A compact single-band MIMO antenna with high isolation characteristic is studied and designed.The MIMO antenna consists of two same monopoles which are printed symmetrically.A complementary split ring resonator(CSRR)is etched on the ground between the two monopoles.The CSRR,which is fed by the monopole through a microstrip line,works as an isolatiton structure to reduce the mutual coupling and improve the impedance matching characteristic.A prototype operating around 5.8 GHz was fabricated and measured.The results show that the fabricated antenna is with return loss of less than 20 and mutual coupling of less than-24 dB.The radiation patterns,gains,efficiencies and envelope correlation coefficient(ECC)were also measured and good performances are achieved.2.A novel dual-band MIMO(multiple input,multiple output)antenna for WLAN(wireless local area network)applications is studied and designed.The MIMO antenna contains two dual-band antenna elements,each of which comprises a T-shaped monopole and a special ?-shaped stub resonator.Two operating bands with center frequencies of 5.5 GHz and 2.5 GHz,are crested by the monopole of T shape and the stub resonator of ? shape,accordingly.The ?-shaped stub also works as an isolation structure at the higher band,which can simplify the dual-band isolation design into a single-band problem.Moreover,the isolation is enhanced at the lower band by inserting a metal strip which can cancel out original coupling.The inserted metal strip is the only additional decoupling structure in this design and has a simple texture with a compact size.The measured and simulated results reveal that the designed MIMO antenna can cover all the 2.4/5.2/5.8 GHz WLAN operating bands and within the recommended bands the isolations exceed by 20 dB.3.A tri-band MIMO antenna resonating at 2.4,3.5 and 5.8 GHz is studied and designed.The proposed antenna consists of two identical tri-band antenna elements and a ?-shaped metal line.Each antenna element comprises a T-shaped monopole and two inverted-L ground stubs;the T-shaped monopole is fed by a microstrip line to produce the middle-frequency band,while the two Inverted-L ground stubs arranged in close proximity are driven by the T-shaped radiator to generate the lowest and highest frequency bands,respectively.The longer inverted-L ground stub,which is the resonant element for 2.4 GHz,can reduce the mutual coupling for higher bands though suppressing near-field radiation.It indicates that a tri-band isolation design is simplified into a single-band problem by self-decoupling analogy.Moreover,isolation enhancement at lower band is achieved by a ?-shaped metal line to neutralize original coupling.The neutralization line is the only additional isolation structure in this design.In this way,the proposed MIMO antenna covers the 2.4,5.8 GHz WLAN and 3.5 GHz WiMAX bands,with the measured mutual coupling lower than-18 d B.4.A compact printed U-shape ultra-wideband(UWB)antenna with triple band-notched characteristics is studied and designed.The proposed antenna,with compact size of 24×33 mm2,yields an impedance bandwidth of 2.8-12 GHz for VSWR<2,except the notched bands.The notched bands are realized by introducing two different types of slots.Two C-shape half-wavelength slots are etched on the radiating patch to obtain two notched bands in 3.3-3.7 GHz for WiMAX and 7.25-7.75 GHz for downlink of X-band satellite communication systems.In order to minimize the mutual coupling between the band-notched structures,the middle notched band in 5-6 GHz for WLAN is achieved by using a U-slot defected ground structure.The parametric study is carried out to understand the mutual coupling.Surface current distributions and equivalent circuit are used to illustrate the notched mechanism.The performance of this antenna both by simulation and by experiment indicates that the proposed antenna is suitable and a good candidate for UWB applications.5.A very compact ultra-wideband(UWB)multiple-input multiple-output(MIMO)antenna with high isolation is studied and designed.The proposed antenna,consisting of two UWB slot antennas,has a very compact size of 22×26 mm2,which is smaller than most of UWB antennas only with single antenna element.A T-shaped slot is etched on the ground to improve the impedance matching characteristic in the low-frequency and reduce the mutual coupling for the frequencies ?4 GHz.By etching a line slot to cancel out original coupling,isolation enhancement at the 3–4 GHz band is achieved.The antenna possesses a low mutual coupling of less than –18 dB over the operating band from 3.1–10.6 GHz.The performance of this antenna both by simulation and by experiment indicates that the proposed antenna is a good candidate for UWB applications.