| In recent years,mobile devices featuring big screen,compact size and low cost have become a part of everyone's daily life.As video applications consume more and more network capacities,the wireless network standards are moving close to the bottleneck of the theoretical limit of wireless channel decided by Shannon formula.Newer wireless network access protocols including 802.11 and Wi MAX rely heavily on the diversity combining of independently fading signal paths to reach high diversity gain and capacity gain,supporting large number of antenna units at both transmitting and receiving ends,which brings challenges on the designing and researches about multiple antenna systems.In this dissertation,we propose several multiband,multi-beam,multi-polarized MIMO(Multiple Input Multiple Output)antennas for wireless access points.The results are discussed and the performances are calculated and analyzed,which are summarized as follows:1.A planar directional triple-band antenna is proposed.The antenna consists of a pair of bigger rings for the lower band,a pair of smaller rings for the middle band,and an arrowshaped dipole for the higher band.The triple-band antenna element is printed on the back side of a substrate and excited by an S-shaped strip line which is etched on the front side of the substrate.Simulated and measured results show that the bandwidth for return loss > 10 d B covers WLAN 2.4-GHz band,Wi MAX 3.5-GHz band and WLAN 5-GHz band.High gain and stable directional radiation patterns are obtained.2.A triple-band four-beam MIMO antenna is investigated for access points.The antenna covers 2.4–2.5,3.4–3.7,and 5.1–5.9 GHz frequency bands with high gains and directional radiation patterns for WLAN/Wi MAX applications.The MIMO antenna consists of four triple-band antenna elements rotated around a circle for space and angular diversities.The antenna is composed of four MIMO antenna units for the coverage of a conical area.The triple-band multi-beam MIMO antenna is designed and the MIMO performance is evaluated.It is shown that low capacity loss and high diversity gain can be obtained with the low correlation and low branch power ratio between the MIMO antenna units.3.A dual-polarized triple-band MIMO antenna is proposed for WLAN/Wi MAX access points.The dual-polarized antenna consists of two identical crossed triple-band dipoles,each of the triple-band dipole is composed of a pair of small isosceles trapezoid patches operating for higher band,a pair of T-shaped branches for middle band,and a pair of arc branches for lower band.The two dipoles are printed on the two side of a square substrate,forming a compact planar configuration.The triple-band dual-polarized antenna is placed above a metal reflector to achieve high gain and directional radiation patterns.Measured results show that the dual-polarized triple-band antenna covers the WLAN 2.4-GHz band,Wi MAX 3.5-GHz band,and WLAN 5-GHz bands with return loss > 10 d B and isolation > 20 d B.4.Three dual-polarized multiple antenna systems for wireless access points are investigated,which include 1)An eight-port triple-band MIMO antenna,which consists of 4 identical triple-band dualpolarized antennas,each printed on one corner of a square substrate.The isolation between each pair of ports are all better than 15 d B and low mutual coupling is achieved.The overall dimensions are still very compact considering all the eight ports are implemented on the same substrate.2)A four-beam dual-polarized MIMO antenna,which includes four 5-degree tilted sectors to have coverage of 360 degrees.The coverage can be increased by the high gain antenna for each sector while the coupling between different sectors is suppressed.Each of the four dual-polarized antennas can be assigned a different channel to operating as a two-port MIMO antenna and communicate with different clients,thus implements space-division multiple access(SDMA).3)A thirty-two-port antenna,which consists of four directional 8×8 MIMO antennas facing the four different directions to provide sufficient multiple antenna capability for the 360-degree area.The tilted angle is 5° and the worst isolation is still greater than 15 d B.Calculated results show that the low correlation coefficient and high channel capacities are achieved.Simulated and measured results show that for all the three multiple antenna systems above,the frequency band for WLAN 2.4-GHz,Wi MAX 3.5-GHz and WLAN 5-GHz are all covered with return loss > 10 d B.High gain and stable directional radiation patterns are also achieved.Methodology of evaluating the performance of multiple antenna systems have been discussed and improved for MIMO antennas in this dissertation.Different combinations of multiple antennas with space diversity,polarization diversity and angular diversity are investigated,which should be useful in the area of wireless network access for large public places with high density clients where enhanced frequency reusability and reduced maintenance efforts are preferred. |
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