I usually focus on RF topics broadcast engineers have to deal with--transmitters, microwaves, antennas, and sometimes satellites. This month, I'll focus on technology that will deliver over-the-air TV to consumers, but that won't use broadcast TV transmitters in the United States.
In most homes, the primary TV set is hooked up to cable TV or a satellite dish. Over-the-air transmissions serve those who don't need or want the extra channels from cable or satellite and provide pictures for portable or second sets. As I mentioned in my weekly RF Report newsletter a while back, when the popular geek forum Slashdot (slashdot.org) talked about the end of analog TV, many of the comments lamented the loss of reception on small handheld TV sets. Don't worry, companies are ready to sell you content you can view on cell phones!
Several technologies have emerged to offer TV to cell phones. The two attracting the most attention are DVB-H, based on the DVB standard covering terrestrial and satellite digital TV transmissions, and the proprietary MediaFLO technology developed by Qualcomm.
In the United States, expect to see MediaFLO transmissions showing up soon on the lower 700 MHz TV channels Qualcomm won in FCC spectrum auctions. Crown Castle is rolling out DTV broadcasting using 1,400 MHz spectrum it won in FCC auctions. I haven't heard what modulation Aloha Partners and other lower 700 MHz licensees plan to use.
Transmitting digital TV and other multimedia content to handheld devices requires different capabilities than transmission to fixed receivers. As the screens are small, data rate can be traded for robust reception, but providers will want to be able to broadcast as many channels as possible in the spectrum available. As viewers move around, reception will drop in and out, so interleaving is a must. However, it can't interfere with the viewers' ability to channel surf.
As every broadcast engineer knows, single tower transmission sites have areas where no reception is possible. If the goal is to reach handheld cell phones anywhere in the service area, multiple transmitters will be needed and the technology has to support this. Watching TV on the cell phone can't drain the battery faster than normal conversations and ideally will consume much less power than a conversation. These special requirements are the reason new technologies such as DVB-H and MediaFLO are needed for handheld DTV.
DVB-H
Since most long-time readers are familiar with the DVB-T terrestrial DTV transmission standard, I'll start with a description of how DVB-H handles these requirements. If you aren't a long-time reader, you can catch up by reading past columns on-line at www.tvtechnology.com and some really old columns at www.transmitter.com
The DVB-H standard builds on the COFDM-based DVB-T transmission system that was touted as a superior alternative to the ATSC 8-VSB system. Through the use of multiple carriers, OFDM-based (Orthogonal Frequency Division Multiplex) systems excel at handling multipath and interference. By choosing the right interval between carriers, OFDM can work well with single frequency networks while meeting the need to handle the rapidly varying multipath encountered with mobile reception.
For the DVB-H standard, the DVB organization chose to use approximately 4,000 (4k) individual carriers per channel. Use of 4K modulation improves mobile reception over 8K modulation while maximum single-frequency network (SFN) coverage is doubled compared to 2K modulation. The standard, however, allows for use of 2K carriers. QPSK or 16-QAM carrier modulation may be used.
In digital transmission systems, coding is important. To overcome the signal dropouts and high impulse noise inherent in mobile operation, the DVB-H standard includes MPE-FEC (multiprotocol encapsulation forward error correction). MPE-FEC provides additional Reed-Solomon coding for the IP datagram and uses the time-slicing buffer (more about this soon) to provide virtual interleaving. An optional in-depth interleaver can be used to increase robustness of 2K and 4K carrier DVB-H signals in noisy and mobile environments. DVB simulations showed MPE-FEC provided a 7 to 8 dB improvement in signal-to-noise performance over unencoded data at packet error rates of one in 100.
Receiver power consumption is an important consideration in handheld devices. DVB-H uses time-slicing to reduce it. The receiver is turned on to grab a burst of data, then shut down until the next burst arrives. This requires high instantaneous data rates. DVB-H literature indicated an 8 MHz channel allows DVB-H data rates of 15 Mbps or higher. Mobile video data services are expected to require data rates of around 384 Kbps. If time-slicing is used to divide the 15 Mbps stream into many slices, each containing a burst of data for one program stream, many program streams can be accommodated, allowing the receiver to remain off while the other program data bursts are sent. With time slicing, it is possible to achieve up to 90 percent power saving for the receiver front-end.
In addition to reducing power, time-slicing allows data to be interleaved between slices, increasing robustness and allowing hand-off from one cell to another without program interruption.
While DVB-H can be transmitted on a dedicated channel, it is designed to share a multiplex with DVB-T and can take advantage of the hierarchical transmission capability of DVB-T.
You can learn much more about DVB-H by visiting www.dvb.org
MEDIAFLO
The Qualcomm MediaFLO technology covers transmission of files and multimedia content from media control to handheld devices, including a system for distributing signals nationwide and inserting regional and local material. Information on the system is available from on the Qualcomm Web site. For this month's discussion, I'll focus solely on the last link between the transmitter and the receiver.
As with DVB-H, MediaFLO uses OFDM transmission with approximately 4,000 (4K) carriers with either QPSK or 16-QAM modulation of the carriers. MediaFLO also uses time division multiplexing, which is similar to what DVB-H refers to as time-slicing, to transmit specific content at specific time intervals. This allows the receiver to be shut down in between these intervals to save power.
One ad-vantage of MediaFLO over DVB-H is that it was designed to use the latest turbo-coding algorithms. They allow more aggressive error correction than is possible using the traditional Viterbi coding present in other systems. Qualcomm claims this allows MediaFLO to double the data rate in the same amount of spectrum as DVB-H. MediaFLO also allows a form of hierarchical transmission, which it calls layering. A content stream can contain a low-bandwidth, robust signal for viewers in poor signal areas and a higher bandwidth, less robust signal for viewers receiving a good signal.
Another MediaFLO feature is that it transmits CRC (cyclic redundancy check) data with each turbo code packet. If the packet is received perfectly, the receiver does not have to calculate the Reed-Solomon outer code, allowing additional receiver power savings.
Time interleaving data is a good way to reduce the impact of impulse noise and changing multipath, but it can also increase channel acquisition time, making surfing difficult. MediaFLO divides the transport stream into super frames, each of which includes Overhead Information Symbols (OIS) and time division multiplex pilots to allow rapid acquisition of the OIS. A Qualcomm tech overview says this allows MediaFLO to switch channels in 1.5 seconds versus 5 seconds for DVB-H.
Qualcomm's MediaFLO was designed specifically for transmitting content to handheld devices. The FLO in MediaFLO stands for Forward Link Only, emphasizing it is different than Qualcomm's 1xRTT and the Verizon EvDO technology for two-way data communication. The transmission part of the system is designed to work on a wide range of frequencies, from 450 MHz to 3 GHz. Qualcomm has obtained spectrum in the lower 700 MHz band (TV channels 52-59) that it plans to use to offer nationwide multimedia services. Transmitters on these channels can transmit up to 50 kW, which Qualcomm says will allow it to use far fewer 700 MHz transmitters to serve an area than what is required to offer cell phone service.
You can learn more about MediaFLO technology on Qualcomm's resources Web page at www.qualcomm.com/mediaflo/news/resources.shtml.
WHAT ABOUT E-VSB?
DVB-H and MediaFLO tests are either underway or planned shortly. Expect to see these services as an added cost option to your cell phone plan within the next few years.
Want your handheld DTV for free? Right now, the high-power requirements for 8-VSB reception, especially for demodulators with enough processing power to handle dynamic multipath, make it unlikely we will see a handheld 8-VSB receiver anytime in the near future. Chip technology continues to improve, so I don't want to say it will never happen.
What about E-VSB? E-VSB works with much lower carrier-to-noise ratios, and if a long enough interleaver is used, it may work okay in a mobile environment, even with a single ATSC transmitter site. It might work for pedestrian reception if multiple synchronized transmitters are used to reduce dead spots. Power consumption, however, is a problem that will likely take some revision of ATSC standards to resolve. Combining time-slicing or time division multiplexing with the need to track a changing multipath environment could be difficult.
I hope the ATSC, broadcasters and manufacturers find a way to solve these problems. Come 2009, I'll miss my old Sony black-and-white Watchman with the little CRT and my newer pocket-sized Casio color LCD receiver. Lugging around a converter box won't work!
Questions and comments are always welcome and appreciated. I read all of them, even if I sometimes have trouble finding time to reply! E-mail me at dlung@transmitter.com.
