3G Video Cell Phone

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    SECOND-GENERATION (2G)cell phones are quite common in industrialized regions ofthe world, with rapid proliferation continuing as the cost ofservice declines. Now, network providers are seeking thenext generation of technology, typically referred to as 2.5Gor third-generation (3G) networks capable of delivering

    data at much higher rates. This article will help define thechallenge of designing 3G cell-phone products that have a

    video component, and the methods used to design and ver-ify the algorithms for managing errors in transmission andproviding the best-perceived quality of service (QoS).

    Todays data rates are typically 14 to 30 kb/s. The2.5G/3G networks will be able to support 64 kb/s to 2 Mb/sdata rates. The increased data rate is expected to enablemany new applications beyond simple voice-based systems. With increased wireless datarates, the services will expand from voice-only to include: two-way data messag-ing, picture with data message, videoclip with data message, video clipwith audio, and two-wayinteractive video with audio

    (Fig. 1).These services will be

    used to support applica-tions such as simple e-mail, or video content in-cluding news, financialstories, sports highlights,short entertainmentclips, traffic, andweather reports. Do-mestic uses includegames, day care, orsecurity. Two-way in-

    teractive video phone enables a richer communication. Avideo phone may be used for chatting, or for other commonvoice-type communications. However, the extra bandwidthwill cost more, so users are likely to reserve usage wherevideo adds significant value in the communication. For ex-ample, a realtor could perform a walk-through of a house tohelp a prospective out-of-state buyer decide whether to fly

    out and view it personally. Or, a technical-support process

    could be improved because a problem could be viewed live,thus reducing the down time of a manufacturing facility.These high-value applications are examples of how net-work providers hope to increase revenues to survive in anincreasingly competitive voice-only market.

    To support increased data rates, network serviceproviders will upgrade or replace existing networks. In

    addition, consumers will purchase new 2.5G or 3Gcell phones or an integrated de-

    vice, like a camera/cell-phone combination. Thesimple voice-only phonewill become a phone with

    a more flexible key-

    board, a color LCDwith usable resolu-

    tion, and a localwireless link suchas Bluetooth toconnect desktopresources. The cir-cuits of 3G wire-less data networksare estimated byexperts to be sevento 10 times morecomplex than that

    The DesignChallenges Of A 3GVideo Cell Phone

    B Y L E S W I L S O N A N D B O W U

    FEATURE

    1. The increased data rate of 3G wireless networks is expected to enablemany new applications beyond simple voice-based systems, including a two-

    way interactive video cell phone with audio.

    NEXT-GENERATION CELL-PHONE DESIGN

    REQUIRES A STRATEGY TO OPTIM IZE THEQUALITY OF VIDEO AND PICTURES SENTOVER A 2.5G OR 3G NETWORK.

    54 WIRE LESS SYSTEMSDESIG N/M ARCH 20 01

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    of the 2G wireless network(Fig. 2).The increased complexity of video over

    wireless creates new design challenges.For example, the video is compressed at amuch higher ratio than the voice so it be-comes much more sensitive to data errors.The quality of video and pictures sent overa 3G network may be critical to the success

    of some applications. Design teams willneed a strategy to optimize the design and

    deliver adequate QoS for each application.There are technical bottlenecks for wire-

    less multimedia access, such as the unreli-ability caused by the adverse multipathpropagation channel and severe interfer-ence from other transmissions. It is wellknown that the wireless environment is of-ten nonstationary due to fading of thetransmitted signal. Fading is the conse-quence of summation of multiple delayedcopies of the signal with different phase re-lationships due to reflections of the signalfrom various surfaces. Moreover, mobile-phone users may move when talking into

    the phone, so the surrounding environ-mentwhich will affect communica-tionis always changing. This makes mo-bile communication more unpredictablethan wired communication.

    Another challenge in wireless commu-nication-system design is to mitigate theco-channel interference (CCI), which be-comes a severe problem when many usersare using the channel at the same time. Dueto the time-varying nature of the propaga-tion channel and the unpredictability of theinterference, bit-error rates (BERs) will

    vary widely and cause error process to bebursty, which may lead to loss of completevideo frames. Measures are taken in 3Gstandards to combat these effects: turbocoding, transmission diversity, variablerate coders with bit-rate control mecha-nisms, interleaving, and an advancedmulti-user detection algorithm. Propercell-planning and smart-antenna tech-

    niques will also greatly reduce the impact

    of CCI.Bit errors and frame loss in video trans-

    missions tend to cause noticeable picture-quality degradation. Error-correction andconcealment techniques provide methodsfor the decoder to deal with errors in a waythat minimizes the quality loss. Error-cor-

    VIDEO CELLPHONES

    WIRE LESS SYSTEMSDESIG N/M ARCH 20 01

    rection techniques, which remove the er-rors and restore the original information,are more difficult for real-time data thanfor non-real-time data. The real-time na-ture of video transmission means that itcannot tolerate the delay that would be as-sociated with a traditional retransmission-

    based error-correction technique suchas automatic repeat request (ARQ). Delayis introduced in the acknowledgment ofreceipt of frames, as well as in waiting forthe timeout to expire before a frame isretransmitted.

    Unequal-error-protection (UEP) meth-ods can be used to conserve bandwidth andprotect important information for the de-coder.1 For example, the header that gives

    information about image size, quantizationtables, and other parameters essential forreconstructing the compressed imagesneeds better protection. In other situations,

    transmission of compressed images andvideo on a noisy communications channelmight be enhanced by providing higher de-grees of error protection to the more sig-nificant portions of the compressedstream, at the expense of reduced protec-tion of less-significant portions.

    Error concealment is a method of reduc-ing the magnitude of errors and frame lossin the video stream. It does not remove theerrors, but manages them in a way thatmakes them less noticeable to the viewer.

    These methods include temporal conceal-ment, spatial concealment, and motion-

    compensated concealment. With temporalconcealment, the erroneous data in the cur-rent frame is replaced by the data from theprevious frame that is not in error. In videosequences where there is little motion inthe scene, this method will be quite effec-tive. Another method of concealing errors

    2. With increased data rates, the simple voice-only phone will become a phone with amore flexible keyboard, a color LCD with usable resolution, and a local wireless link such

    as Bluetooth to connect desktop resources. The circuits of 3G wireless data networks areestimated by experts to be seven to 10 times more complex than that of the 2G wireless

    network.

    3. A design methodology aided by software simulations in Cadences Signal ProcessingWorksystem (SPW) can be used to simulate 3G wireless video systems. From a high level,

    the solution requires the systems and implementation engineers to use SPW as a common

    design and system-level verification platform.

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    is spatial concealment. Spatial conceal-ment involves interpolating the data thatsurrounds an erroneous block in a frame.This method is most useful if the data doesnot contain a high level of detail. Motion-compensated concealment involves esti-mating the motion vectors from neighbor-ing error-free blocks. This method couldbe used to enhance spatial or temporal con-cealment techniques. The error-correction

    codes in the 2.5G/3G wireless system re-duce the effective BER, which brings the

    number of bit errors remaining in the bit-stream thats sent to the video decoderdown to a level at which the error-re-silience tools can work.

    Video telephony requires medium tohigh bandwidth, low delay (two-way), me-dium-to-high quality, and continuous con-nection. To provide video of acceptablequality to the mobile user, the wireless net-work must provide certain QoS. Frame-delay variation, bit errors, and frame losscan have severe effects on the video qual-ity. A transmission link with a BER of 105

    might be acceptable for non-real-time data

    transmission with some form of error cor-rection. In a video stream, however, this er-ror rate would cause a serious degradationin the quality of the received video. Simi-larly, frame-delay, frame-loss, and rate-control issues also have a significant im-pact on the quality of video received.Simulation is needed to assess the picturequality under different propagation chan-nels and error-correction and/or conceal-ment schemes.

    Algorithm engineers, usually workingclosely with systems engineers, design the

    appropriate signal-processing techniquesto meet the system requirements. Thesemay include channel encoders/decodersand video-compression algorithms. Thealgorithm engineers perform mathemati-cal analysis and simulations to design,evaluate, and refine their algorithms. Thedesigns are at the algorithmic and behav-ioral level and, therefore seldom have anyarchitectural or implementation details.Usually the lowest level of detail at this

    stage is a behavioral fixed-point model.

    A design methodology aided by soft-ware simulations in Cadences Signal Pro-cessing Worksystem (SPW) can be used tosimulate 3G wireless video systems.2

    VIDEO CELLPHONES

    WIRE LESS SYSTEMSDESIG N/M ARCH 20 01

    From a high level, the solution requires thesystems and implementation engineers touse SPW as a common design and system-level verification platform. Hardware orsoftware designers may use the SPW mod-els as executable specifications and sys-tem-level test benches for testing their

    modules. SPW helps specify the algo-rithms clearly and becomes the module-level test bench, thereby increasing thechance of first-time success (Fig. 3).

    The most important step is the clear def-inition of simulation goals. The primaryadvantage of any simulation is the use ofabstraction. Abstraction means reducingthe complexity of a system or a sub-systemto a tolerable level of approximation as

    dictated by the desired result. Using ab-straction, it is possible to design, test, andverify a given subsystem within the con-text of an end-to-end system without all the

    unnecessary implementation details.The greatest challenge with simulations

    is choosing the right level of abstraction. Ifthe abstraction level is too low, the simula-tion run time will be longer than necessary.On the other hand, if the abstraction levelis too high, the simulation results will beless accurate than desired. Given the com-plexity of 3G systems, there is much to begained from thinking ahead and planningthe simulation stages and the required ab-straction level for each stage.

    A complex system such as the 3G com-

    munication device has several compo-

    nents that contribute to the overall qualityof results, and thus the end users percep-tions of the product. In the terminal per-forming image capture, the lens, image

    4.All major elements of the video-cell-phone system will contribute to the overall perfor-mance and should be considered in the simulation studies. This high-level diagram shows

    the components included in the simulation model.

    5.Image-quality algorithms or probes can be inserted into the video-cell-phone system toobserve the cumulative effects of the entire network as it influences the final rendered

    image.

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    VIDEO CELLPHONES

    WIRE LESS SYSTEMSDESIG N/M ARCH 20 01

    sensor, image processing, and image com-pression (possibly MPEG4 or JPEG2000)all contribute to the sourcing of the imageand conservation of the important infor-mation.3 A critical loop may exist betweenthe lens and the compression algorithm. Asthe handheld device moves, the autofocus

    algorithms will attempt to adjust. Whilethe lens moves into focus, the image issomewhat blurred. This may temporallyincrease the efficiency of the video com-pression, and the frame rate may increase.If the autofocus is not yet optimized, the re-sults may indicate false performance, asthe image quality may be lower. Its im-portant to understand interactions in thesystem and how to measure and maximize

    the performance of the algorithms.Additional major elements of the video-

    cell-phone system will contribute to over-all performance and should be considered

    in the simulation studies. The image sensorplays a significant role in source imagequality. After the image is compressed, itis sent via the wideband-code-division-multiple-access (WCDMA) transmitter.The antenna can influence the strength ofthe signal and provide better mobility cov-erage. Beam-forming may be modeled toevaluate the effects on system BER andwhether it can be cost-justified. If trans-mitted video quality is enhanced, beam-forming may provide effective value to theQoS. The wireless channel should be mod-

    eled. The IMT2000 standard provides sev-

    eral channel models for typical mobilityscenarios (Fig. 4).

    Although each scenario and applicationmay have differences, at the receiver sidethe data stream is decoded. Receiver qual-ities will influence the system BER. Nowthe data is decoded, scaled (MPEG4 orJPEG2000), and presentedtypically to asmall, flat LCD. The display should beable to render all the colors and intensitiesrequired for the intended applications.While the wireless channel can be ob-served by measured BER or signal-to-noise ration (SNR), the video channel and

    rendered results are much more subjectiveand may require human blind studies to se-

    lect final designs.To help automate the process of select-

    ing the best video algorithms and their pa-rameters, we use algorithms that attempt tomodel human perception of image quality.Possible algorithms for image perceptionare: modulation transfer-function area(MTFA), integrated contrast sensitivity(ICS), subjective quality factor (SQF),square-root integral (SQRI), folded SQRI,

    folded MTFA, and the ever-popular peaksignal/noise ratio (PSNR). These image-quality algorithms or probes can be in-

    serted into the video-cell-phone system toobserve the cumulative effects of the entirenetwork as it influences the final renderedimage (Fig. 5).

    There are many ways for vendors todifferentiate their products in a given ap-plication. For example, a vendor may in-clude a better-quality image sensor anddisplay, thus the product is capable ofsending and rendering a better image. Thedifferentiation may be better audio quality.Audio quality has a strong perceptive im-pact on...

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