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Developments in semiconductor and communications technologies over the past decades has enabled proliferation of mobile devices which can communicate wirelessly at data at rates that enable video transmission. These data rates enable the much awaited capability by mobile devices to access TV broadcast of news, entertainment, weather and traffic. Such mobile devices include mobile phones, smart phones, laptop computers, portable DVD players, and, automotive TV receivers. Viewing the content in the well known traditional TV format and schedule is appealing to the public as it integrates the entertainment and information access both at home and when mobile. Extending TV broadcast services to mobile devices brings exciting and interesting business and technology opportunities to many industry segments. These include content providers, advertisers, broadcasters, broadcast transmitter and test equipment manufacturers, consumer electronics receiver and display manufacturers. Reception of TV signals by mobile devices is already popular and well established in several parts of the world including Europe, Korea and Japan. Market size estimates by market research companies point to sizeable market estimates.
Mobile DTV Standard - Background The ATSC Mobile DTV standard has been released in October 2009 [1]. It was developed by the ATSC (Advanced Television Systems Committee). Although the newest and most widely used name for this standard is Mobile DTV, this new standard has the good fortune of having several names associated with it. Technical moniker for the standard is A/153 as this is the document label used by ATSC. Another name used to for this standard is ATSC M/H (Mobile/Handheld) standard. The original digital TV standard was released in 1995 by the ATSC [2]. With FCC mandated transition to digital TV services in 2009, it is now being used in widely throughout the US. This standard is often shortly referred to as A/53 due to its ATSC document label number. In the industry jargon the standard and the technology conforming to this standard are generally referred to as the "Legacy ATSC". The digital TV broadcast signals transmitted according to this standard are referred to as the "Main Service". Over the years it became evident that the Legacy ATSC technology is not suitable to transmit broadcast TV to mobile devices. The Legacy ATSC waveform cannot address the challenges in reception signal quality that a typical Mobile device sees. Mobile devices can travel through areas where received signal quality is reduced by multipath and low signal levels, they do not have and elevated antennas as home TV receivers, and, they can move at high speeds resulting in Doppler shifts in the received signals. The Mobile DTV standard provides modifications to the digital TV broadcast signals for being able to facilitate access of mobile devices to TV broadcast.
Allocating payload bits to Mobile DTV The Main Service ATSC waveform is designed to carry approximately 19.4 Mbps over a 6 MHz wide broadcast channel. A typical HDTV (High Definition TV) program uses about 10 Mbps of payload capacity, and a typical SDTV (Standard Definition TV) program uses about 5 Mbps ( please note that these estimates vary based several system parameters - you will see slightly different numbers in different sources). Broadcast stations can choose the number and combinations of HDTV or SDTV signals they transmit. There are many combinations HDTV and SDTV streams are available to broadcasters. For example a single HDTV signal may be transmitted. Another alternative is to transmit one HDTV and one SDTV signal. Yet another alternative is to transmit only one SDTV signal. In each of these cases there is left over payload capacity that can be allocated for broadcasting to mobile devices. In Figure 1 we illustrate these scenarios.
Figure 1. A few alternative allocation scenarios for ATSC Digital TV payload.
Mobile DTV Standard - A few top level features: The Mobile DTV standard uses a backward compatible technique for inserting the Mobile DTV content into the ATSC Digital TV payload stream. This is accomplished by using the internal structure of the ATSC A/53 data flow and inserting packets that a legacy ATSC receiver that is built in conformance with the A/53 standard will normally ignore. New TV's build after the standardization of Mobile DTV will know the key identifiers for the new stream and can extract and decode the new Mobile DTV stream as well as the Legacy ATSC stream. The Mobile DTV signal transmit waveform is designed to facilitate reception of Mobile DTV signals in challenging signaling environments where Legacy ATSC signals cannot be reliably received. The key features include use of strong forward error correction techniques and increased amounts of training data. The use of PCCC ( Parallel Concatenated Convolutional Coding) and SCCC ( Serial Concatenated Convolutional Coding) enables reception at lower signal strength levels and in environments where the received broadcast signals are exposed to strong interference. The use of increased amount of training data facilitates reception in the presence of received signal Doppler shifts due to mobile device mobility, and, enhanced equalizer performance in multipath environments. PCCC and SCCC codes can be decoded by Turbo decoding techniques. Coding gains of up to 11-12.5 dB over Legacy ATSC have been reported due to use of 1/4 rate PCCC, and, 1/4 and 1/2 rate SCCC codes. Mobile DTV uses MPEG-4 video coding technology unlike the legacy A/53 standard that is based on MPEG-2. MPEG 4 is a higher efficiency coding system that reduces the required transmission bit rate relative to MPEG-2 for similar metrics of video quality. Although computational requirements for MPEG-4 is higher, the increased complexity is easily absorbed by receiver platforms used in mobile devices. A key benefit that MPEG-4 avails is the capability to pack more channels into the bands that are available for Mobile DTV in Figure 1. Use of MPEG-4 typically allows 2 Mbps of capacity drawn from the Digital TV payload stream to support the standard 416x240 resolution of M/H service for one channel. Through choice of key system parameters the amount of payload data taken from the Main Service actually has a fine granularity of as low as 100-150 Kbps. Specific numbers depend on several key system parameters. Furthermore there are ample amounts of "knobs" built in to the standard that allows trade-off of transmission rate, modulation/demodulation latency, and, protection against channel impairments. A key concern for mobile receivers is battery life. The mobile DTV standard provides the capability for receivers to go to sleep mode periodically there by saving power. Mobile DTV content is transmitted in a time sliced manner. The tuner and demodulator of receiving mobile devices can be powered off during the time intervals that content that is not of interest to the specific mobile device is being transmitted. The time interval that the receiver must be kept on scales in proportion to the M/H data being carried thus enabling efficient power saving opportunities.
Receiver design challenges At the PHY layer, Mobile DTV shares the same fundamental 8-VSB waveform as the Legacy ATSC standard. Significant effort has been spent on PHY layer design by the ATSC receiver design industry for more than a decade since the Legacy ATSC standard became available [2]. Several variations for key algorithms such as phase an symbol synchronization, AGC, AFC, and, equalization have been proposed throughout the years. Their benefits and limitations are well understood. The questions related to choosing best algorithms for how processing Mobile DTV waveform are mostly unanswered at this time. We anticipate that algorithms and receiver design tradeoff points will become available throughout the next few ears. Design choices typically tradeoff performance with receiver gate count, power consumption, and capabilities of platform of choice. Areas of unanswered questions for Mobile DTV include such design issues associated with synchronization, channel estimation, Turbo decoding, and power savings modes. Although first wave of products can leverage already established radios designed for supporting Legacy ATSC. However specific upgrades are needed to support efficient Mobile DTV receiver functionality and performance. At TosiaComm we understand that it will take time and wide based industry collaboration to understand the transmission and reception design issues and optimize solutions to these issues. Well designed transmitter networks and receivers will enable efficient and successful deployment of Mobile DTV technology. We are glad to be one of the trail blazers is addressing design challenges of Mobile DTV. Conclusions The exciting new Mobile DTV standard brings in many opportunities to technical and business communities. Content providers, broadcasters, advertisers are offered a new channel to access mobile device users. Equipment manufacturers are provided the prospect to diversify and differentiate their product portfolios. Engineering community will have the opportunity to address the challenges of answering many interesting questions about how to best design and implement Mobile DTV transmission and reception systems.
[1] ATSC-Mobile DTV Standard, Document A/153, Parts 1-8. 15 October 2009. Advanced Television Systems Committee, Washington, D.C., [2] ATSC Digital Television Standard, Doc. A/53 Parts 1-6, Advanced Television Systems Committee, Washington, D.C., Also please check TosiaComm DTV Products page to see our current products in the DTV area.
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