Displays for Mobile Communications Enabling New Markets and Applications

Wireless Personal Communications 22: 199–212, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

Displays for Mobile Communications Enabling New Marketsand Applications

RAINER KUHNThree Five Systems Inc., 5490 Conestoga Court, Boulder, CO 80301, U.S.A.E-mail: [email protected]

Abstract. Displays directly affect the functionality of wireless terminals and resulting applications available tousers of next generation wireless networks. Limitations in the human visual system and in direct view displaytechnologies, enable a host of new types of display technologies to be developed that offer original equipmentmanufacturers (OEMs) new product design opportunities. Without these advances in display development, thecomplete multimedia experience of next generation networks will go unfulfilled. This paper describes the currentstate of display technology for the mobile communications market, illustrates some emerging technologies thatshow promise for incorporation in future mobile terminals, and lastly elaborates on microdisplay technologies andhow they can enable the true 3G experience in mobile terminals.

Keywords: microdisplays, miniature displays, LCD displays, multimedia terminals, wireless multimedia, wear-able internet devices, wearable computers, 3G, 4G, GPRS, UMTS.

1. Introduction

The market trend of smaller and smaller handsets is indisputable. “Thin is in” as they say.The other indisputable fact is that richer and richer content is driving the need for largerdisplays and with higher resolutions. These two opposing forces are clashing to the pointwhere the handset size is almost totally dominated by the active area of the display. TheKyocera QCP6035 Palm OS-based phone is a good example of this. These clashing forceswill not see a reprieve any time soon. In fact, as all of the pieces of the puzzle come together;network bandwidth, infrastructure, semiconductor technology, terminal design, content, andinteroperatibility standards, one piece of the puzzle that should not be forgotten is the displaytechnology element.

In fact, the display can be a key differentiator to end users when selecting a product. Theirmonitor at work or their TV at home establishes user’s display quality expectations. Sure,consumers want access anywhere, small devices, long talk times and standby times, contentthat is affordable and easy to access, but users are very vocal about the quality of the displayin their wireless terminal.

Where we were once limited by network bandwidth and display quality, this is now chang-ing so that network bandwidth and display technology now enables many new applications(Figure 1). This includes applications that range from narrowcasted content, inquiry based sys-tems (WAP), to fully interactive systems that are available in today’s initial wireless networks[1].

But more importantly, new display technologies (e.g. microdisplays) help support full con-tent delivery available on 3G and 4G networks such as desktop equivalent e-mail functionality,personal information services like location-based content, and also engaging entertainment

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Figure 1. Content vs. bandwidth landscape.

Figure 2. Pre-Post I-mode launch content demand.

multimedia content (e.g. video, gaming, 3D, etc.). In fact, initial research concluded by NTTDocomo (Figure 2) shows that consumers are truly compelled by entertainment content suchthat it far surpassed news, financial services, shopping, city guides, travel and other content interms of appeal [2].

For the first time in the history of mobile terminal development, network bandwidth in-creases and display technologies have concurrent availability paths that enable carriers tooffer a new level of service to their customers. Quarter video graphics array (QVGA) dis-plays are already the minimum standard for market entry, and now Super Video Graphics

Displays for Mobile Communications Enabling New Markets and Applications 201

Figure 3. Mobile device technology landscape.

Array (SVGA) displays are entering the market. This can be seen in innovative terminals likeCompaq’s iPaq handheld PC [3], Nokia’s 9210-class personal communicator [4], Kyocera’sQCP6035TM Palm-based phone [5], and the Hitachi Wearable Internet Appliance [6] (POMA).Of note is Hitachi’s WIA platform, which is the first mobile communications device to offertrue SVGA (800 × 600) resolution image quality in a portable wireless communication device.This is accomplished via a beltworn WinCE platform with optical mouse and an innovativeheadmounted display using a microdisplay. And this is just the beginning.

This paper will now offer a glimpse into display technologies that are on the near term andlong horizon and identify some trends and issues regarding their application in next generationterminals.

2. Display Technology Landscape

There are a number of display technologies that support the mobile communication market(Figure 3): passive matrix liquid crystal displays (LCDs), active matrix LCDs, organic lightemitting diode (OLED) displays, and microdisplays. Each of these technologies offers benefitsthat make them attractive to wireless terminal developers, but each also has its limitations. Thispaper will now discuss some of the most notable features and benefits of each offering.

2.1. PASSIVE MATRIX LCDS

Even today, current handset offerings and the installed base of mobile phones are dominatedby passive matrix displays. These displays typically have sub-QVGA resolutions, some offerintegrated driver functions, and generally all consume very low power at a cost-effective pricecompared to other technologies. In many cases each passive matrix LCD is custom configuredfor each phone type. There are two main types of passive matrix display architectures in usein mobile phones, transmissive and transflective (Figure 4). Transmissive displays utilize abacklight and transmit light through the array. Transflective displays utilize ambient light anda front liht to reflect off the surface of the display. Power consumption of 25 mW is quitecommon and most displays are 1.5–2.0′′ diagonal in size. Resolution is typically limited tosub-QVGA resolution.

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Figure 4. AMLCD cell construction example.

In addition to low resolution, passive matrix displays have typically suffered in terms ofcolor performance, support for full motion video, and aperture ratios. The poor aperture ratiosare due to the gate and drain lines used to create the array structure. In color transmissive dis-plays, aperture ratios can be as low as 20–30%. Advances have occurred in recent years withthe introduction of transflective passive matrix displays that offer low power consumption,plus high aperture ratios ranging from 50–90+%. This provides a much more seamless im-age with less pixelation. Additionally, active addressing now enables support for full-motionvideo.

2.2. ACTIVE MATRIX LCDS

Whereas Passive Matrix displays typically offer resolution of ∼110 lines per inch, the sweetspot for Active Matrix LCDs (AMLCD) is up to 150 lines per inch. The full color, video capa-ble performance of AMLCDs make them a natural selection for wireless terminal developersas the features available on mobile phones proliferate. Amorphous Silicon, Low TemperaturePolysilicon (LTPS), thin film diode, and Active Matrix OLED (AMOLED) technologies makeup the majority of AMLCD displays used in mobile products.

The development of AMLCDs has yielded continuous improvements over the past decade.While initially the apporach of transmissive and reflective display providers was to simplyscale down the size and resolution of AMLCDs of those used in PCs and LCD monitors, theseplatforms now command significant R&D resources offering differentiating wireless marketspecific features to product developers. Special emphasis has focused on ease of integration,optimizing for power consumption, minimizing the display footprint.

Of special note is the Thin Film Diode (TFD) AMLCD technology that is being pio-neered by Seiko Epson. The LCD array structure utilizes a metal-insulator-metal structurethat eliminates gate and drain lines, improving the aperture ratio of its predecessors. An all-digital interface eliminates digital-to-analog (DACs) converters, enabling lower-power powersupplies, driver circuitry, and reducing memory capacity. Additionally, the TFD platform uti-


Figure 5. AMOLED cell structure example.

lizes a new color modulation approach that enhances color depth through dithering. With thisapproach, Seiko epson is able to deliver 262,144 colors from 16 grayscales. The net effectis an 800 mW solution which includes both the display (5 mW) and illumination system(∼800 mW) when running full motion video.

This is certainly an exciting trend, but at 800 mW it will be a challenge to meet tomorrow’smobile handheld product multimedia + talktime requirements.

2.2.1. AMOLEDThe other emerging display technology showing promise in the mobile communications mar-ket is Active Matrix Organic Light Emitting Diode technology (Figure 5). Whereas passivematrix approaches have been used in the past for mobile phones, active matrix approaches arerequired for full color displays. AMOLED technology utilizes a low temperature polysiliconsubstrate to create the active matrix array, and then an organic electroluminescent layer isused to create the red, green and blue sub-pixels. Lastly, an electron transport layer, emittinglayer and hole transport layer is constructed to encapsulate the array. The result of whichis a very bright, environmentally robust display technology. AMOLEDs are not a panaceahowever. AMOLEDs have significant issues with lifetime due to the deterioration of theorganic compounds over time. Lifetimes of 10,000 hours are common [7]. Additionally, thecost structure of AMOLEDs remains higher than standard AMLCDs due to the immaturefabrication infrastructure and potentially lower yields. What’s more, sunlight readability willalways be an issue due to the emissive nature of AMOLED technology.

2.3. EMERGING TECHNOLOGY

As each of these technologies has been described, one or more shortcomings appear thatcreates an opportunity for new alternative display technologies. Two of those technologies arePlastic Substrate-based displays and E-ink.

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Figure 6. Plastic substrate display example.

2.3.1. Plastic Substrate-Based DisplaysPlastic Substrate-based displays (Figure 6) offer advantages over glass-based displays in termsof improved shock resistance, reduced thickness and weight. Additionally, the display size isvirtually unlimited due to the more tolerant fabrication process. What adds to their attractive-ness is that organic material is used to create the transistors of the display, which enables thedensity of the display to be double that of a standard AMLCD. All in all, it could be a veryattractive technology for the future, but not until 2004–2005 [8].

2.4. E-INK

Another emerging technology of note is E-ink (Figure 7). E-ink utilizes a process wherebymicrocapsules are coated onto a plastic surface. Each microcapsule has positively chargedwhite particles and negatively charged black particles. A negative charge applied to the topsurface attracts the white particles and creates a white image. Once the charge is removed,the particles maintain their orientation reducing the power consumption down to zero. Thistechnology will undoubtedly attract the E-book development community, but as this technol-ogy matures, it could appear in a variety commercial and consumer mobile communicationproducts. Only time will tell [9].

This bring us to the last category of displays to discuss in this paper: Microdisplays.

2.5. MICRODISPLAYS

What is a microdisplay? In its simplest definition a microdisplay is any display less than 1′′diagonal that is brought to the eye for viewing or is projected onto a wall (Figure 8). Wewill only discuss the near eye type of microdisplay in this paper. But before we cover thetechnologies, lets discuss the motivating factors behind why a microdisplay would be used ina mobile communication device.

The fundamental reason to use a microdisplay is not a technical one, rather it is due tolimitations of the human visual system [10]. The human visual system is able to discernapproximately 60 pixels per subtended degree of visual arc. Based on this constant, if weuse the following formulas [11], we can determine the maximum number of pixels per inchthe human eye can resolve at a given distance away from the eye.


Figure 7. E-ink display example.

Figure 8. Example of three five SVGA microdisplay.

2.5.1. Image Distance Versus Resolution CalculationE = discernment of human eyeEm = minutes of minimal angle acuityD = reading distanceH = height of the displayAh = horizontal angle of visual arcPpi = pixels per inch

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Figure 9. Display size versus resolution requirement at 10′′ from device.

em = ahtan(ah/2) = (h/2)/dppi = 1/(2 ∗ d ∗ tan((em/2) ∗ (1/60)).

If:d = 10 inch distance from the display, then the ppi = 343.77,d = 15 inch distance from the display, then the ppit = 229.18,d = 20 inch distance from the display, then the ppi = 171.89.

More simply stated, the following chart (Figure 9) indicates the required screen size perresolution in order for a person to effectively discern the content.

The direct result of this limitation in the human visual system is that product size is directlyaffected. In the past, if you wanted a large image, you needed a large display. At least that’s theway it was in the past. Microdisplays change this quite dramatically by creating virtual images(Figure 10) that are presented to the user. The images are created through the use of advanceddiffractive and prism-based optical designs. The image size is quite literally decoupled fromthe device size enabling product developers to maintain small products, yet produce largeimages. This is especially compelling given the rapid growth of internet gaming, mobile digitalimaging, location-based content and other personal entertainment applications [13, 14].

Microdisplays will find application in mobile products through a variety of user paradigmsincluding monocular and binocular headmounted displays, embedded viewers, accessoryviewers and other approaches (Figure 11). Just as consumers have become accustomed tousing ear-buds for mobile listening, so too Three Five believes that consumers will adoptvarious new product designs that enhance mobile video content delivery.

From a technology standpoint, there are a variety of technologies vying for leader-ship in the neareye microdisplay market. The two technologies leading in design wins aretransmissive displays and reflective liquid crystal on silicon (LCOS) displays.


Figure 10. Creating a virtual image.

Figure 11. Three five microdisplay-based wireless communication product design concepts.

2.5.2. Transmissive MicrodisplaysAs stated previously, in a transmissive modulated microdisplay configuration, the light sourceis placed in back of the microdisplay. Light passes through the LCD microdisplay whereit is modulated at each pixel to produce an image. Two major transmissive approaches areavailable: Poly-silicon (p-si) and Silicon-on-Insulator (SOI) modulators. P-si technology is,in many ways, an extension of the traditional AMLCD manufacturing techniques used fordirect-view LCDs. To reach the pixel densities required for microdisplays, developers use p-si and single-crystal silicon (x-si) which have electrical characteristics superior to those ofamorphous silicon (a-si) TFTs used in traditional LCDs.

Poly-silicon microdisplays can be obtained from four to five major Japanese LCD manu-facturers. For virtual display applications, some offer products in the 0.50′′ to 0.75′′ size range,but p-si processing limits how small the on-screen transistors can become. Consequently, highdensity displays have poor aperture ratios (percentage of light passing area to light blockingarea), that produce pixelated images and require higher power consumption. This is referredto as the “screen door” effect. This filtering approach reduces the spatial resolution of a full-color display and is evident when presenting a single-color full screen, such as a red image.Since only one third of the pixel is active for a red image, the display is only using one thirdof the total active pixels. A p-si display with an advertised resolution of QVGA may in factonly use 25,600 of its active 76,800 pixels.

One way to eliminate pixelation is to drive a display in field sequential mode temporallyversus spatially (Figure 12). This poses a problem for p-si microdisplays because they can-not be operated at fast enough refresh rates to enable field sequential color operation. Fieldsequential operation is an approach to producing a full-color display that relies upon the se-quential illumination of the microdisplay with red, green and blue light from a side-mounted

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Figure 12. Spatial versus temporal (time sequential) color generation methods.

or rear-mounted light source. If done fast enough, the human eye cannot discern betweenthe separate color fields that compose the full-color image. There is one transmissive-basedmicrodisplay technology that utilizes a field sequential illumination process and that is SOItechnology.

Using single crystal silicon to enable better electrical characteristics than poly-silicon,screen transistor size can be reduced to allow higher density, smaller sized microdisplays.In spite of this modification, SOI wafers still suffer from pixelation however. Additionally,their cost basis is higher due to the higher SOI wafer costs and proprietary manufacturingprocess. In manufacturing SOI displays, a “lift-off process” is used to create an interim product(a thin, transparent and fragile backplane), when the silicon circuitry is separated from thesubstrate. This SOI backplane (Figure 13) is bonded to a glass substrate to create the display.It is this complicated “lift-off” process that has had a major impact on device yields. Flicker[11] and high power consumption are other drawbacks of the SOI approach. Inthe mobilecommunications market where power consumption is of key concern, SOI-based displaystypically consume up to double that of reflective LCOS-based solutions.

Reflective technologies utilize an external light source to reflect and modulate light off thefront of the microdisplay (Figure 14). The advantages of this aproach over transmissive dis-plays include a higher fill-factor per pixel, more efficient utilization of light, and less complexmanufacturing requirements.

2.6. REFLECTIVE MICRODISPLAYS

There are two primary types of reflective microdisplays; Dynamic Nematic-based LiquidCrystal (DNLCOS) and ferroelectric-based (FLC) liquid crystal.

These two technologies utilize two distinct modulation approaches, FLC of which is digitalall the way into the display (binary), and the other DNLCOS is digital up to the display,but then incorporates an analog drive scheme to deliver full-color, full-motion, high-refreshrate video images. With FLC-based microdisplays, gray level is obtained via time divisionmultiplexing (TDM) techniques. Although no digital to analog signal conversion is necessary,


Figure 13. Silicon on insulator microdisplay cell structure example.

Figure 14. Reflective LCOS microdisplay optical block diagram.

digital binary microdisplays require a more complex and faster running interface than analog,which drives up cost and power consumption. This requires fast liquid crystal material, a thincell gap (distance between the silicon layer and coverglass), and has the drawback of making ifdifficult to achieve a wide color gamut. The other drawback is narrow environmental storagerange. FLC-based displays exhibit hard failures when exposed to environmental conditionsabove the specified range. This makes them highly susceptible to failure when stored in hotcars in the middle of the summer (83 ◦C) [15].

Perhaps a word should be mentioned about refresh rates at this time. Refresh rates are bestdescribed through a few examples. For instance, the refresh rate of a typical film is 24 framesper second. Most people can tolerate this slow frame rate because the images presented onlarge movie screens are dark and the periphery of the image is often out of focus. Thereforeficker is not noticeable for a large majority of viewers. Comparatively, most televisions utilize

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Figure 15. Color breakup example.

a 60 Hz frame rate. This slow rate can also be tolerated because the picture tube displaysalternate odd and even fields (rows) of information. The human eye averages out the alternatefields such that flicker is acceptable for the majority of users. The third example is computermonitors. Due to the fact that users are in close proximity to the screen, information is high incontrast, brightness of the LCD or CRT display is high, this makes flicker very perceptible. Toeliminate flicker, RGB color filters are rapidly refreshed spatially to present images to users(typically 60–110 Hz). The Video Electronics Standards Association and defacto industrystandards have worked to overcome these objections by recommending that CRT displays notonly meet the 60 Hz specification (Variant 59.94), but also the 85 Hz refresh rate specificationin VESA Monitor Timing Standard 1.0 (Rev. 0.7). Additionally, the 85 Hz refresh rate criteriahas been adopted by Microsoft in their current Windows operating system specifications andtoday the majority of CRT monitors support refresh rates in excess of 85 Hz.

With field sequential displays, the flicker problem is even more accentuated. As describedearlier, in field sequential displays, users are presented the red, green and blue fields serially.If these fields are refreshed too slowly, users can perceive not only flicker, but also motionartifacts and color breakup. Color breakup is when the eye can detect the color components,or trailing effects, rather an a stabile image. Research has shown that frame rates of less than93 Hz can result in the full combination of motion artifacts, color breakup and flicker for alarge number of users (Figure 15). Flicker causes discomfort to users in the form of eye fatiqueand headaches, which may result in user rejection of products. Therefore, the typical 60–95 Hzused in CRTs and LCDs doesn’t work for field sequential displays. Research indicates that theminimum performance standard requirements are frame rates 100–120 Hz (300–360 RGBcolor fields per second).

Most field sequential microdisplays, including FLC-based displays, fall short of supportingthe 100–120 Hz refresh rate that Three Five recommends for near eye microdisplay applica-tions. This is due to either the slow response time of their liquid crystal or the high powerrequirements of driving FLC displays at this refresh rate. Three Five has solved the flickerand color breakup problem by offering microdisplays that support frame rates of 120 Hz (360


color fields per second) while maintaining low power consumption. What’s more, nematicliquid crystal microdisplays offer lifetimes in excess of nearly 1M hours (MTTF), robustenvironmental performance of –10–+70C, and a variety of optical design options, and OEMshave a solid solution for use of microdisplays in consumer electronic products [16].

3. Conclusions

After looking at all of the available display technologies, it is often difficult to discern thehype from the facts. Truth-be-told, selecting displays for tomorrow’s mobile communicationdevices involves a careful evaluation of user needs, content delivery requirements, technologyroadmaps, supply assurance and bill of material tradeoffs. It is our belief that future mobilecommunication devices will be comprised of a variety of display technologies and types, andin some cases multiple display types may end up in the same product.

Microdisplays can play a role in the future mobile communication applications landscape.Especially in applications that require small product size, yet demand a rich multimedia expe-rience. Mobile gaming, digital imaging devices, location-based services are all enabled withmicrodisplays.

Product designers should be encouraged to start by identifying the image size requirementfrom a human factors standpoint along with the desired product size that consumers are willingto accept. When these two factors have been reconciled, the right display solution will becomeclear.

References

1. C. McLaughlin, MCG Consulting, “Generation 3 Mobile Displays”, Society for Information DisplayConference 2001, San Jose, CA, June 2001.

2. NTT Docomo, Demo Mobile 2001 Conference Presentation, Scottsdale, AZ.3. Compaq Ipaq Website, http://www.compaq.com/showroom/handheld.com4. Nokia Website, http://www.nokia.com/phones/9210i/index.html5. Kyocera Website, http://www.kyocera-wireless.com/kysmart/kysmart_series.htm6. Xybernaut Website, http://www.xybernautonline.com/eCommerce/Poma/Plac_Poma.htm7. D. Fellowes, R. Draper, C. Bradford, C. Reese and O. Prache, Night Vision and Electronic Sensors Direc-

torate, “Performance Test Results for Miniature SVGA Color Organic Light Emitting Diode Displays”, Ft.Belvoir, Virginia, eMagin Corporation, Hopewell Junction, New York.

8. Business Week, August 2001 issue.9. E-ink Website, http://www.eink.com

10. James Melzer and Kirk Moffitt, Headmounted Display Designs for Users, Magraw Hill, 1997.11. P.G.J. Barten, “The SQRI Method: A New Method for the Evaluation of Visible Resolution on a Display”,

SID Proc., Vol. 28, No. 3, p. 253, 1987.12. Toru GanNichida, “Short- and Long-Term Effects of HMD Use”, Environmental Analysis and Technology

Dept., Technical Support Center, Sony, Tokyo, Japan, ISSN/0001-0966X/01/3201/� SID 01 DIGEST.13. E. Peli, “The Visual Effects of Head-Mounted Display (HMD) Are Not Distinguishable from Those of

Desk-Top Computerdisplay”, Vision Research, Vol. 38, pp. 2053–2066, 1998.14. J.E. Farrell et al., “Predicting Flicker Thresholds for Video Display Terminals”, Hewlett Packard Co., PRO

SID, Vol. 28, No. 4, pp. 449–453, 1987.15. T. Chang, N. Gough, D.D. Parghi, R. Vohra and S. Yang, “Improved FLC Mixtures for Consumer Electronic

Products”, Displaytech Inc., Microdisplay, PRO 2.7, pp. 39–42, 2001.16. P. Gleckman and Miller Shuck, “Optical Characteristics of a High Performance LCOS Virtual Dislay”, Zight

Corporation, Microdisplay, PRO 1.4, pp. 10–13, 2001.

212 Rainer Kuhn

Rainer Kuhn is director of sales and marketing for Three Five Systems Personal DisplaySystems. As director, Rainer is responsible for business development, product marketing,strategic planning, and sales. He has been in the microdisplay industry for four years andhas more than 20 years of marketing and sales experience in the technology fields of wirelesscommunication, computing peripherals and network communications. Prior to Three Five,he was with Zight Corporation where he served as the director of marketing and businessdevelopment, Lexmark International as manager of worldwide marketing, and Siemens in theWireless Terminals and Information and Communication Networks Groups. His experiencein the high-tech industry spans direct-to-consumer, business-to-busienss, channel-based, andOEM direct/rep/distributor business models serving North America, Europe, Asia-Pacific andLatin America. Rainer received his B.A. degree in business management from the Universityof South Florida.

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Displays for Mobile Communications Enabling New Markets and Applications