dual feed, single element antenna for wimax mimo applicationfigure 3: example of wimax usb antenna...

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2008, Article ID 219838, 5 pagesdoi:10.1155/2008/219838

Research ArticleDual Feed, Single Element Antenna forWiMAX MIMO Application

Frank M. Caimi and Mark Mongomery

SkyCross, Inc. 7341 Office Park Place, Suite 102 Viera, FL 32940, USA

Correspondence should be addressed to Frank M. Caimi, [email protected]

Received 11 December 2007; Accepted 10 April 2008

Recommended by Seong-Youp Suh

A novel u-shaped single element antenna having two feed ports is compared with two equal length monopoles separated by adistance equivalent to the width. A discussion of relative performance metrics is provided for MIMO applications, and measureddata is given for comparison. Good impedance match and isolation of greater than −10 dB are observed over the operatingbandwidth from 2.3 to 2.39 GHz. The antenna patterns are highly uncorrelated, as illustrated by computation of the antennapattern correlation coefficient for the two comparison monopoles.

Copyright © 2008 F. M. Caimi and M. Mongomery. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

1. INTRODUCTION

With the continued proliferation of wireless devices comesthe increasing need for data services for e-mail, world wideweb access, video on demand, and streaming video. Forthose in the industry, this means providing reliable serviceswithout interruption, delay, and data corruption. Wirelesscarriers are forced to maintain an infrastructure to meetincreasing needs for this reliability and higher data rates,creating the need for communications systems with highercapacity. Meeting these needs has been a challenge, butdevelopments in technology continue to provide solutions oralternatives.

Multiple input, multiple output (MIMO) communica-tions protocols are one such development along the path,and have demonstrated significant data communicationscapacity improvements by exploiting multiple independentspatio-temporal channels. These channels are found in signalenvironments where scattering and rotation of the electricvector of the signal are common—buildings, offices, homes,and so forth. Perhaps surprisingly, these techniques wereused for military acoustic communication in the oceansprior to the coining of the term MIMO, since signals travelby discernibly different paths and often many paths areneeded to ensure communications reliability. In order toexploit these channels, the antenna system should meetcertain requirements. This paper reviews metrics used for

characterization of antennas used in MIMO systems andcovers advances in antenna technology that allow a singleantenna to be used having multiple feed ports.

2. ANTENNA METRICS FOR MIMO SYSTEMS

In order to fully exploit independence of signal channels forthe communications system, it is commonly believed thatthe transmitter and receiver must utilize multiple transducers(antennas) having certain characteristics. Since the signalsmay arrive from different directions, the antennas shouldexhibit characteristics that allow for highest independenceof the received signals. One such characteristic is antennapattern independence—measured by the pattern correlationcoefficient [1] (although that alone is not sufficient, as itdoes not include the effect of the communications channel).The pattern correlation coefficient can be calculated from fullsphere antenna pattern measurements and is given in termsof the electrical field components by [1]

ρp =∫ 2π

0

∫ π0A12dΩ√∫ 2π

0

∫ π0A11dΩ

√∫ 2π0

∫ π0A22dΩ

,

where Amn(θ,ϕ)=XEmv(θ,ϕ)E∗nv(θ,ϕ) + Emh(θ,ϕ)E∗nh(θ,ϕ),

and the cross-polarization power ratio X = SvSh.

(1)

mailto:[email protected]

2 International Journal of Antennas and Propagation

The subscript indices (m,n) refer to either antenna port1 or 2, depending on their integer value; Ω(θ,φ) is thespatial angle in steradian as commonly depicted; andE1,v, E1,h, E2,v, and E2,h are the complex envelopes of eitherthe vertical or horizontal field components resulting fromexcitation of either antenna port 1 or 2, respectively. Thecross polarization ratio is defined as the ratio of the meanreceived power in the vertical polarization to the meanreceived power in the horizontal polarization, as shown.

Correlation coefficients less than a prescribed thresholdare desirable, and envelope correlations (ρ2) less than 0.5are preferred for acceptable data capacity according to manystudies.

Antennas should not only exhibit low port-to-port signalcorrelation, but should also exhibit isolation from oneanother so that signals being broadcast or received from onefeed port do not appear at the opposite port or feed. Theisolation is typically measured at the antenna terminals byusing the commonly derived s-parameters. Isolation figuresare typically given in decibels and are designated as S12 or S21depending on whether the measurement is from antenna 1to 2 or vice versa. If the isolation is poor, the source antennawill deliver substantial power into the adjacent antenna’stermination impedance (typically 50-ohms resistive), with acorresponding reduction of the overall radiation efficiency.S21 values of −10 dB are regularly deemed acceptable andresulting in power loss of less than 10%.

Since the antenna near-field coupling is related to thefar-field pattern, it is possible to approximately express thecorrelation coefficient in terms of the s-parameters measuredat each antenna terminal. This is given by [2]

∣∣ρs∣∣ =

∣∣S∗11S12 + S∗21S22

∣∣

(1− (∣∣S11

∣∣2 +

∣∣S21

∣∣2)1/2(1− (∣∣S22

∣∣2 +

∣∣S12

∣∣2)1/2

.

(2)

Comparison of measurements taken by this approachcan approximate those computed using the pattern method,and are much simpler. Most studies and specifications forcorrelation coefficient refer to the envelope correlation whichis approximately equal to the magnitude squared of ρ as givenin (1) and (2). From this equation, however, it is clear thatsmall values for S12 and S21 can result in small values for ρs.There are, however, cases where the correlation coefficientmight be small even if S12 and S21 are larger, depending onthe input match reflected in the S11 and S22 values.

In spite of the ease by which the correlation coefficientcan be computed using the previous equations, predictionof the communications system performance also relies onsome measure of the signal independence resulting fromthe environment. This factor has been investigated and canbe included in the antenna pattern correlation coefficientcalculation to produce a more complete measure of thereceived signal correlation or independence in nonlaboratoryconditions. The method involves the use of a simpleenvironmental model that has been experimentally verified[3]. In this model, the signal power is described as a functionof angle of arrival by probability functions Pv and Ph, which

can be included in (1) as follows:

Amn(θ,ϕ) = XEmv(θ,ϕ)E∗nv(θ,ϕ)Pv(θ,ϕ)+ Emh(θ,ϕ)E∗nh(θ,ϕ)Ph(θ,ϕ).

(3)

Measurement data have been taken to define parametersfor Gaussian forms of Pv and Ph in various environments [4].This data supports the general methodology for computingρ based on a field model (versus that of pattern alone)with good correlation between the two computationalapproaches. Perhaps most interesting is that the correla-tion coefficient depends primarily on the phase differencebetween the two antenna patterns—indicating that goodcorrelation performance requires careful consideration of theantenna placement and design in the case, where multiplesingle antennas are used in close proximity.

Briefly then, the antenna system, its associated pattern,and the environment are all clearly important in establishingindependence between the signals received at each antenna,and therefore the overall system data transfer rate.

3. MULTIPLE VERSUS SINGLE ANTENNADESIGN APPROACHES

A standard approach to achieve MIMO operation is todevelop multiple antennas that are sufficiently separated toachieve a desired level of signal independence and port-to-port isolation. In small handheld terminals, this approachtaxes the industrial design process to accommodate doubleor triple the space allocation commonly reserved for theantenna, as well as requiring additional feedline length.Perhaps the most stressing geometric configurations arefor USB form factors requiring HSPA or WiMAX opera-tional capability, since space allocation is extremely small—nominally 20 mm wide by 10 mm long.

Conversely, if it were possible to develop a single antennahaving multiple feed ports, with equivalent or better perfor-mance characteristics than the separate antenna approach;it would represent a significant advantage by reducing thecost of assembly and manufacture while providing equivalentperformance in an equal or perhaps smaller volume. Severalstudies have been reported in the literature describingpotential antenna designs for MIMO applications in smallhandheld terminals. Generally, these make use of externallumped element decoupling networks between the feed portsto allow matching of even and odd modes to a commonimpedance, thereby producing small cross correlation andmaximum gain over a limited frequency range [5]. In onerecent publication, a ground wall and connecting line wereused in conjunction with shorting pins to develop a highisolation two port MIMO antenna consisting of 2 foldedmonopoles for WiBro service in Korea [6]. Here, we reporton a single antenna with 2 feeds and two unique resonancemodes that occur at the same frequency. Operation of theantenna can be described by an electromagnetic modelthat illustrates the self-induced and conducted currents inrelation to an active reference feedpoint. The opposite,symmetrically located feedpoint of the antenna is isolatedfrom the reference feedpoint by 10 dB or more versus that

F. M. Caimi and M. Mongomery 3

Conventional twoantenna approach

Coupling

Efficiency

Frequency

Isol

atio

n(d

B)

−30

−20

−10

0

Effi

cien

cy(%

)

30

40

50

60

1 2

(a)

Single skycross antennawith iMAT

Coupling

Efficiency

Frequency

Isol

atio

n(d

B)

−30

−20

−10

0

Effi

cien

cy(%

)

30

40

50

60

1 2

(b)

Figure 1: Illustration of a conventional antenna versus a two port iMAT antenna for MIMO application.

Gtotal (dBi) - θ=90 - 2450 MHz

0

30

6090

120

150

180

210

240270

300

330

Port 1

Port 2

Antenna 1Antenna 2

5

0

−5

(a)

Gtotal (dBi) - θ=90 - 2450 MHz

0

30

6090

120

150

180

210

240270

300

330

Port 1

Port 2

Antenna 1Antenna 2

5

0

−5

(b)

Figure 2: Example of antenna azimuthal gain pattern produced by: (a) two closely spaced monopoles and (b) single element iMAT antennaof similar size showing substantially different far-field pattern compared to (a).

Antenna volume: 19 mmwide, 10 mm long,

2.5 mm tall

Board Lengthdedicated to antenna: 10 mm

USB connector

xy

z

Figure 3: Example of WiMAX USB antenna illustrating smallantenna volume of 19 mm wide by 10 mm high and extending awayfrom PCB distal end by 10 mm, [7].

achievable from two separate single antennas occupying thesame or larger physical volume, and polarized equally. This

method will be referred to as an isolated mode antennatechnology or “iMAT” in this paper. A novel feature ofthe method is that the isolation improves as the source isswept through the resonance frequency, as compared with 2adjacent monopole antennas.

The basic advantages of the single element MIMOantenna using iMAT versus the use of multiple antennas areillustrated in Figure 1. The coupling for the conventionalantennas increases in the band region of interest, while forthe isolated mode antenna, it decreases, as shown. In theconventional paired antenna arrangement, the current in theantenna increases toward resonance, which due to couplingto the adjacent antenna results in decreased efficiency.In contrast to the conventional antenna case, the iMATapproach relies on the electromagnetic modal distributionto achieve isolation, and therefore the frequency at which themodes are supported results in improved isolation as well ashigh-radiation efficiency as resonance is approached.

In the following example, the antenna pattern forthe iMAT antenna is also substantially different from the

4 International Journal of Antennas and Propagation

Frequency (GHz)

2.3 2.32 2.34 2.36 2.38 2.4

VSW

R

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Two monopolesiMAT

(a)

Frequency (GHz)

2.3 2.32 2.34 2.36 2.38 2.4

S 21

(dB

)

−20−18−16−14−12−10−8−6−4−2

0

Two monopolesiMAT

(b)

Frequency (GHz)

2.3 2.32 2.34 2.36 2.38 2.4

Effi

cien

cy(%

)

0

10

20

30

40

50

60

70

80

90

100

Two monopolesiMAT

(c)

Frequency (MHz)

2300 2320 2340 2360 2380 2400

Pre

dict

eden

velo

peco

rrel

atio

n|ρ

e|2

0

0.2

0.4

0.6

0.8

1

Two monopolesiMAT

(d)

Figure 4: Comparison of (a) VSWR, (b) S21, (c) radiation efficiency, and (d) envelope correlation coefficient for 2 cases: two monopoles andiMAT antenna over the frequency range from 2.3 to 2.4 GHz.

independent antenna case, as in Figure 2. The antenna pat-tern exhibits independence depending on which feed or portis excited, and therefore the correlation coefficient computedfrom the pattern is generally smaller than with independentseparately located antennas in close proximity. An example ofthe pattern differences as shown in Figure 2. In Figure 2, themeanderline-loaded monopoles are separated by an average10 mm or approximately 1/12 wavelength in vacuum, andconsequently are highly coupled. The antenna pattern istilted away from azimuth due effects from the counterpoise,which accounts for the smaller gain, as compared to a freespace dipole configuration. In addition, the coupling intothe adjacent load resistance accounts for some additionalgain reduction. The far-field pattern differences due toseparate port excitation are generally expected to provide an

improvement in signal independence by sampling differentspatial channels.

The performance of an antenna using iMAT is bestillustrated by example against other designs using the samespace allocation. We have purposely chosen a monopole typeof design, although it is possible to use a balanced antennawith similar results.

4. iMAT ANTENNA DESIGN EXAMPLE

An example of a USB WiMAX antenna design suited for the2.3-2.4 GHz band is illustrated in Figure 3. A USB devicetypically consists of a printed circuit board (PCB) assembly,enclosed in a plastic housing, with USB connector at oneend. In this case, the space available for the antennas is

F. M. Caimi and M. Mongomery 5

assumed to be at the opposite end of the PCB from theUSB connector. The width of the device is 20 mm and thelength and height available for the antennas are 10 mm and2.5 mm, respectively. The majority of the PCB, between theantennas and the USB connector, is taken to be a continuousRF ground.

5. COMPARISON STUDY

A similar antenna designed for the 2.3–2.4 GHz WiMAX/WiBRO band is illustrated in Figure 4. Two different antennasolutions are taken for comparison. The first consists of twomeanderline monopoles in the same form factor as illus-trated in Figure 3. The distance from the ground availablefor the radiator is 10 mm, or λ/12 in free-space; however,the monopoles are made an effective λ/4 length by using ameanderline conductor. The second case considers the iMATsolution that includes a similar meanderline structure forcompactness, but one that maintains a half-wave equivalentelectrical length as a single resonator. The iMAT solution inthis case uses a half-wave mode of a u-shaped element, wherethe location of each feed port is symmetrically positionedand selected for the minimum coupling to the oppositefeed port. A simulation analysis of the current developedby excitation at a single port shows that two fundamentalcompeting modes contribute to a null condition at theopposing port, which is also supported by an analyticapproach using transmission line theory of the u-shapedstructure.

A comparison of measured performance for the twosolutions is shown in Figure 4. Each approach yields goodimpedance match with a VSWR of 2 or better over theband (Figure 4(a)). However, the coupling (S21) for themonopoles is poor—about−4 dB as shown in Figure 4(b). Inas similar application, the addition of the ground extensionbetween the antennas provides marginal improvement of S21to about −6 dB [7]. The iMAT solution has considerablybetter isolation, with S21 values between −10 and −15 dB inthe designated band as shown in Figure 4(b).

The S21 value has a direct impact on efficiency due tosignal lost to the neighboring antenna and its associated load;and so the iMAT solution shows better efficiency than theother solution as shown in Figure 4(c). Similarly, the iMATsolution produces more diverse antenna patterns as indicatedby the graph of correlation coefficients calculated using (1) asshown in Figure 4(d).

6. SUMMARY

Results have been presented for a novel antenna that uses ahigher-order modal excitation from 2 separate feedpoints toachieve isolation of 10 dB or more over a limited bandwidth.The antenna size is similar to that of two identical monopoleor dipole antennas that are closely coupled at less than0.15 wavelength spacing. In addition, the antenna correlationcoefficient for this approach is superior to the case of twoseparate closely spaced monopoles.

REFERENCES

[1] S. D. Parson, The Mobile Radio Propagation Channel, JohnWiley & Sons, New York, NY, USA, 2nd edition, 2000.

[2] S. Blanch, J. Romeu, and I. Corbella, “Exact representation ofantenna system diversity performance from input parameterdescription,” Electronics Letters, vol. 39, no. 9, pp. 705–707,2003.

[3] G. Breit and E. Ozaki, “Phone level radiated test methodologiesfor multi-mode multi-band systems,” in Proceedings of theInternational Wireless Industry Consortium Interactive TechnicalWorkshop, Handset Antenna Technologies for MultiMode-Multi-Band (IWPC ’07), Durham, NC, USA, January-February 2007.

[4] K. Kalliola, K. Sulonen, H. Laitinen, O. Kivekas, J. Krogerus, andP. Vainikainen, “Angular power distribution and mean effectivegain of mobile antenna in different propagation environments,”IEEE Transactions on Vehicular Technology, vol. 51, no. 5, pp.823–838, 2002.

[5] H. J. Chaloupka and X. Wang, “Novel approach for diversityand MIMO antennas at small mobile platforms,” in Proceedingsof the 15th IEEE International Symposium on Personal, Indoorand Mobile Radio Communications (PIMRC ’04), vol. 1, pp.637–642, Barcelona, Spain, September 2004.

[6] K. Chung and J. H. Yoon, “Integrated MIMO antenna with highisolation characteristic,” Electronics Letters, vol. 43, no. 4, pp.199–201, 2007.

[7] Skycross, Inc, iMAT Antenna Whitepaper, January 2008,http://www.skycross.com/.

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