NAB, MSTV: Cellular DTV Plan Termed Non-Viable
February 5, 2010
Last week, MSTV and NAB shot down a proposal from the CTIA-The Wireless Association and the Consumer Electronics Association to move broadcasters to a distributed transmission system.
In late December, CTIA and CEA filed comments with the commission proposing the replacement of single antenna/high-power transmission systems currently used by TV broadcasters with distributed transmission systems (DTS), which use a collection of lower power transmitters operating as a single frequency network (SFN).
The comments were made in response to the FCC's National Broadband Plan Public Notice 26 (NBP 26). CTIA and CEA said this would free up additional spectrum for wireless providers to use to offer broadband services.
In their reply comments [PDF] to the FCC's National Broadband Plan last week, the associations praised CTIA and CEA for recognizing the public interest in protecting consumers' investment in DTV sets, preserving every broadcaster's 19.39 Mbps bit stream, and ensuring that broadcasters would not be required to cover the costs of reallocating spectrum to the wireless industry.
The reply comments did not criticize the intent of the CTIA and CEA concept, only its practicality.
The CTIA and CEA concept would be great for broadcasters, if feasible. A properly engineered DTS would provide consistent signal levels throughout stations' coverage areas that could be received both indoors and out, as well as provide seamless ATSC mobile DTV coverage.
Unfortunately, as MSTV and NAB have pointed out, designing such systems would not be easy, and finding the sites and funding to construct them out would be even more difficult. In addition, the amount of spectrum freed up by the use of DTS systems would be much less than CTIA and CEA anticipate.
CTIA and CEA have estimated the cost of transitioning TV broadcasting to DTS at something less than $2 billion, based on 15 to 20 SFN transmitters per market in 210 markets, and using $435,000 as the price tag for each transmitter required.
This estimate is apparently based on one transmitter, one antenna and related equipment for 15 to 20 sites. However, it is not practical to use one transmitter to carry the signals from the average 8.57 television stations per market. Even if we assume nine channels, which are all adjacent in frequency, this would require a transmitter that could transmit a 54 MHz signal bandwidth at a UHF frequency with sufficient linearity to avoid generating intermod inside and outside the transmitted channel passband. Such a transmitter would be more expensive and be much less energy-efficient than using nine separate transmitters. If we moved to a one-transmitter-per-channel scenario, this would raise the cost of the proposed SFN to an average of $3 million per site, even after allowing for savings from a combined antenna and cooling system. This raises the total cost to almost $16 billion.
Also, MSTV and NAB noted that CTIA/CEA's proposal of 15-20 DTS transmitters would be needed per TV station is incorrect. The two organizations stated that since a typical TV station covers about 25,430 square kilometers, each of the DTS transmitters would need to cover approximately 1,560 square kilometers (service radius of about 25 kilometers) and would require more power. Lowering the service radius to 5 kilometers per transmitter would reduce this problem, but also increases the number of transmitters to 325 or for each station. Each of these 325 sites would need a tower and antenna, as well as a fiber or microwave link from each station's central studio facility. It's clear that in this scenario, even with lower power and lower cost transmitters, the total expenditure would likely exceed $100 billion.
As I've discussed in my RF Technology column, designing a multiple transmitter DTS is not easy. Each transmitter site needs to be carefully timed so that any signal arriving at the receiver outside the range of its adaptive equalizer has to be at least 15 to 20 dB below that of the primary signals inside the equalizer range. Furthermore, the equalizer range and signal rejection capability varies from receiver to receiver.
In studies I've done, problems often arise when the primary site is shadowed, thus allowing weaker signals from transmitters outside the receiver equalizer range to cause interference. In any market with terrain obstructions (and this includes most east coast and west coast markets), transmitters would have to be very precisely placed, and even then there would likely be some loss of service.
In their argument against such a plan, NAB and MSTV have explained the difficulties in obtaining sites for all of the towers required and the environmental impact of these towers.
"Replication of present over-the-air coverage would require near-perfect siting of every tower in a DTS deployment," their comments conclude. "For the reasons enumerated above, that is not possible."
MSTV and NAB also responded to comments filed by the Department of Justice (DOJ) and the National Telecommunications and Information Agency (NTIA). While the associations agreed with many of the DOJ and NTIA comments, they cautioned against interpreting the DOJ/NTIA submissions as endorsing removal of spectrum from television broadcast usage. MSTV/NAB noted that the DOJ comments do not discuss either the competition from mobile DTV to wireless video services or the competition free off-air TV provides as an alternative to video pay services. Finally, MSTV/NAB question the assumption that a lack of spectrum is the key impediment to increased competition in broadband generally, stating "reallocating spectrum for broadband will not necessarily increase broadband penetration or competition."
The MSTV/NAB reply comments note an increasing recognition of the value of off-air broadcasting. They also point out that the "broadcasting" of video to handheld devices using point-to-point broadband wireless Internet connections strains existing wireless broadband infrastructure.
As the number of Smartphones, netbook computers and personal media players and tablets with Internet connectivity increase, the demand for live video will increase as well. ATSC mobile DTV can meet that demand much more efficiently than streaming broadcast video over the Internet.