In my column last month (“Get Ready for ATSC 3.0, But Don’t Ignore 1.0”), I compared ATSC 1.0 and ATSC 3.0 reception. While it is being rolled out, ATSC 3.0 is likely to require similar antennas to those currently used for ATSC 1.0 since signal-to-noise requirements are likely to be the same. I received some email from readers who complained that even though they had sufficient signal strength for ATSC 1.0 reception, multipath was a problem, and they were looking to ATSC 3.0 to fix that. In this column, I’ll show how ATSC 3.0 handles multipath differently than ATSC 1.0 and also look at some other features in ATSC 3.0 that broadcasters can use to improve reception.

MULTIPATH AND SPEEDING TRAINS

Multipath occurs when multiple copies of the same signal reach the receiver. These copies could be from other transmitters in a distributed transmission system (DTS) or, more likely today, from reflections from other objects. These objects include stationary objects like buildings and bridges and moving objects like trucks and airplanes. The impact of multipath on reception depends on the difference in time and the difference in signal level between the signals. 

ATSC 1.0 is a single carrier transmission system. One 8VSB carrier has to carry the ATSC data rate of 19.392 Mbps. Multipath is visible on a spectrum analyzer as ripple (Fig. 1) in the 8VSB spectrum. By measuring the ripple it is possible to determine the time difference between the signals creating the multipath. (See my Jan. 1, 2000 article “Comparing 8VSB and COFDM for DTV Broadcasting” for details on how to do this. Fig. 1 is missing, but is the same as Fig. 1 here.)

ATSC 1.0 receivers deal with multipath using adaptive equalizers. In the simplest sense, an adaptive equalizer digitally stores copies of the incoming signal and shifts them in time to cancel out the multipath. As a result, the range of multipath delays and signal levels an ATSC 1.0 receiver can handle is dependent on how much the receiver manufacturer is willing to spend on the equalizer. The “coupon-eligible” DTV converter boxes sold during the analog-to-digital transition had to meet certain standards, but those do not apply to TV sets sold today. This variability in receiver performance makes ATSC 1.0 DTS design difficult. 

With ATSC 3.0’s OFDM, multipath performance is controlled by the broadcaster. In OFDM, the data is divided among thousands of carriers so each carrier has to carry a much smaller amount of data. Multipath appears as interference between OFDM symbols; ATSC 3.0 handles multipath by inserting a guard interval between symbols. The guard interval (GI) is created in the time domain by taking samples belonging to the last part of the OFDM symbol and prepending them as a cyclic prefix to the original symbol. 

This extra information allows the receiver to fill in the information lost due to multipath. ATSC 3.0 offers 12 selections of GI sample size. The relationship between the number of samples and the allowable time difference between multipath signals depends on the number of carriers as explained in my Jan. 1, 2000, article. 

For a detailed explanation the cyclic prefix and how it works, look up “The Cyclic Prefix for OFDM” at dspillustrations.com. This website also offers a digital communications tutorial using a computer sound card for hands-on experience. Fig. 2, which shows how the cyclic prefix is generated, is from this website. 

The use of OFDM in ATSC 3.0 doesn’t eliminate the need for an equalizer to remove distortion in the signal caused by multipath or frequency response variations in the receive system itself. The ATSC 3.0 standard has pilot carriers with known characteristics that can be inserted at regular and scattered intervals in frequency and time to make equalization easier. More pilot carriers will make it easier to lock on the channel with varying multipath. 

Adding guard interval reduces usable symbol time and adding pilots reduces the number of carriers available for data. The optimum guard interval depends on the maximum multipath time difference. Testing of ATSC-MH (mobile-handheld) in the San Francisco area found a very long echo from the San Francisco-Oakland Bay Bridge caused reception problems in some areas—a long guard interval would fix that. When multiple transmitters are used in a DTS, the spacing of the transmitters and the signal overlap will determine the guard interval required. The ONEMedia App (see details later) will provide the guard “distance” for a given system configuration. 

There is another element to multipath, which I did not mention earlier—Doppler. Doppler is the change in frequency as an object moves towards or away an observer, (as with a train whistle increasing in pitch as it approaches and decreasing as it moves away). 8VSB performance with Doppler was poor and varied by receiver, resulting in some viewers losing reception due to reflections from planes passing overhead.

Reflections with Doppler can cause loss of an OFDM signal as well, but the standard offers options to deal with it. Earlier, I mentioned OFDM divides the data among many carriers. The FFT size options are 8K, 16K and 32K, roughly corresponding to 6,900, 13,800 or 27,600 carriers. A larger FFT size is more efficient, but because the carriers are spaced closer together it will be more susceptible to Doppler. An 8K FFT is most robust, allowing mobile reception at any terrestrial speed likely to be found in private or public transportation, at least in the United States.

The impact of reflections, either as static multipath or as Doppler, once outside the range of the FFT or guard interval to handle it, is the same as noise. As a result, a robust signal with a lower SNR (signal-to-noise ratio) requirement will be less affected by multipath or Doppler. One way to take advantage of this is to use an FFT of 16K with LDM. While the robust LDM layer will experience more Doppler interference with the 16K FFT, if the required SNR is much lower, it can still work in many mobile environments while preserving the benefit of a 16K FFT for the enhanced layer.

IMPULSE NOISE

One problem that has plagued ATSC 1.0 reception, especially at VHF, is impulse noise. ATSC 1.0 has a data interleaver to spread data over time to reduce the impact of brief signal loss, but that time is limited to a small fraction of a second. ATSC 3.0 has three time interleaver modes: None, Convolutional Time Interleaver (CTI) and Hybrid Time Interleaver (HTI). Extended Time Interleaving (ETI) is available for physical layer pipes (PLPs) using QPSK. 

The length of the interleaving depends on how the ATSC 3.0 PLP is configured, and can be much larger than that available with ATSC 1.0. With so many options, it will likely take extensive field-testing at VHF (perhaps including low VHF) to determine what configurations work best. Using CTI and HTI should not impact data capacity, but the longer the time interleaving, the greater the latency. 

ATSC 3.0 also includes a frequency interleaver, which would aid in reception in the event a single carrier or group of carriers was interfered with or otherwise lost. 

To learn more about the options available in ATSC 3.0, see ATSC Recommended Practice A/327, “Guidelines for the Physical Layer Protocol,” available from the ATSC web page under “Recommended Practices.” To play around with the options, ONE Media (the Sinclair company focused on ATSC 3.0), has created the app “NextGen TV Calculator” to determine channel capacity and performance of an ATSC 3.0 signal with different configurations, including guard interval. Visit onemediallc.com for IoS and Android download links. Fig. 3 shows a sample of the input and output from the ONE Media app on Android. The input and output data extends above and below what’s shown in the screenshot. The output, for example, includes final required SNR under Gauss, Rice and Rayleigh channels for the configured input.

I welcome your comments and questions on ATSC 1.0, ATSC 3.0 or broadcast RF technology in general. Email me at dlung@transmitter.com. 

This month I decided to take a break from talking about TV transmission to focus on TV reception. I receive emails from non-engineers with questions about TV transmission and reception who happen to stumble upon my articles.

People are starting to realize the incentive auction and repack will impact viewers’ TV reception. This month I’ll explain the impact of the incentive auction, channel sharing and the repack on over-the-air TV reception.

Stations may sell their spectrum and go away, meaning the programming they carried may no longer be available. Stations may sell their spectrum and “channel share” with another station. These stations’ primary channels will be available on another channel, perhaps at reduced quality, but some multicast channels on either of the stations sharing the channel may disappear.

Viewers who watch TV via translators or over LPTV stations may find they are no longer available. Finally, during repacking, stations’ coverage may change, temporarily or permanently, making reception more difficult in some areas.

Will your favorite DTV program channel disappear after the incentive auction? In part, that will depend on that channel’s popularity and viewership. When talking about “channels,” it is easy to get confused. A station has only one RF channel (their spectrum), but may run several virtual program channels on it—28.1, 28.2, 28.3, 28.4. These are transmitted in the same spectrum, but each takes some of the data bandwidth.

While stations can’t discuss their auction plans during the FCC’s quiet period, the value of the spectrum versus the value of the programming will likely determine whether or not the program channel survives the auction.

CHANNEL SHARING MISUNDERSTOOD

Stations can sell their spectrum, but retain their license, through channel sharing. This is probably the most misunderstood aspect of the incentive auction. First, stations are not, strictly speaking, sharing spectrum. The “sharee” station has no spectrum; the sharer has 6 MHz, one full channel. An ATSC DTV channel cannot be less than 6 MHz. It cannot be divided. What can be divided is the data rate transmitted on that channel—19.392 Mbps.

Viewers watching two or more stations sharing a channel are likely to notice some reduction in picture quality. How noticeable it is will depend on the total number of program streams sharing one RF channel; how many of the program streams are in HD; the way the bits are allocated between program streams; and the content on the channel.

In the early days of legacy ATSC, expert OTA viewers used TSReader software to see what bit rate stations were using to transmit their HDTV programs. Stations that sacrificed HDTV bit rates to add additional program streams were criticized. Now that some stations are transmitting two HDTV program streams and one or two SD program streams in the same 19.392 Mbps, most stations have at least one SD multicast, but there don’t seem to be as many complaints. Why?

Fig. 1: Statistical multiplexing bandwidth allotment The answer is statistical multiplexing (“statmux”). Each program is given the bit rate it needs for the video on the air right now. If the scene isn’t changing—a talking head, for example, or slide without motion—the bandwidth can be used by other programs.

Statistical multiplexing allows multiple video streams to efficiently share limited bandwidth. Each HDTV stream can hit peak bit rates up to the amount left after all the minimum bit rate streams are assigned. Data is also required for audio and program information, but this is a fixed data rate. I’ll cover this in more detail in a future column. Fig. 1 shows the basic concept.

One key point—“statmux” and MPEG- 2 encoding technology has improved significantly since DTV broadcasting began. Stations that are channel sharing will have to spend some of their auction money to upgrade their encoding systems to the latest technology to minimize the impact channel sharing will have on video quality.

As I outlined in my earlier article on the incentive auction and repacking timeline, channel sharing will take place early in the process, within the first year of the 39-month deadline for completing the repack. Your favorite station may give up its spectrum and channel share, but if the station it shares with has good coverage, you won’t lose your programs, at least until the repacking begins.

There are multiple ways viewers could lose access to OTA programs during the repack. The first and worst impacts viewers that receive stations by translators or whose favorite programming happens to be available only on a low-power TV station. These stations are not protected during the repacking, and if they have a channel in the wireless band, they may not be able to find a replacement channel in the more densely packed TV band after the repack.

WHERE’S THE MONEY?

While replacement channels may be available in rural areas, Congress and the FCC have allocated zero dollars to pay for the channel change and any new equipment that might be required. Local governments, civic organizations and public broadcasting stations may have trouble finding the money to make the channel change. I’m worried because all of my TV viewing when I’m home is from a PBS TV translator on Channel 50!

Viewers in areas served by full-power stations may find they encounter reception problems during the repack. The reason is many, if not most, stations will have to use transmission facilities with less coverage than they have now while changing channels.

Stations will construct interim facilities, which may later be used for backup, with lower antennas and less power so they can remove and replace the main antenna with one that will work on the new channel.

In some markets, stations may be able to share a broadband antenna, which might result in less loss than a low-height, low-power antenna, but still not be as good as their original antenna.

As I explained in my last column, FCC flexibility in allowing coverage increases in some areas could help stations with directional antenna patterns avoid major coverage loss if they need to share an antenna.

If a large number of stations have to be repacked, given the limits of tower crew availability and weather, some stations may have to live with reduced coverage until late or even after the 39-month repack period ends. Stay tuned!

I welcome your comments and questions. Email me at 
dlung@transmitter.com.

NEW YORK—More and more folks are embracing over-the-air television. Recent findings from market researcher GfK indicate that 17 percent U.S. TV households rely on “broadcast-only” television reception, up from 15 percent in 2015. Concurrently 25 percent now have no cable and satellite reception.

“The fact that a statistically significant increase in broadcast-only reception occurred over just one year may be further proof that the cord-cutting/cord-never phenomenon is accelerating,” said David Tice, senior vice president in GfK’s Media & Entertainment practice. “If you include homes that have no TVs at all—about 3 percent of all households—then less than three quarters, or 73 percent, of U.S. homes continue to have pay TV service, with the attendant implications for all stakeholders—not just the pay TV services themselves, but also networks, content providers, and advertisers.”

The research, from GfK’s 2016 “Ownership and Trend Report” from The Home Technology Monitor, shows that 17 percent of U.S. TV households now rely on broadcast-only or over-the-air reception, up from 15 percent in 2015. Another 6 percent say they only use Internet services such as Netflix, Amazon Prime, Hulu, or YouTube and do not have traditional broadcast or pay TV reception at all; this compares with 4 percent a year ago.

Further, TV households with a resident between 18 and 34 years old are much more likely to be opting for alternatives to cable and satellite; 22 percent of these homes are using broadcast-only reception, versus 17 percent of all U.S. households, and 13 percent are only watching an Internet service on their TV sets, versus 6 percent of all TV homes. Overall, 38 percent of 18-to-34 households rely on some kind of alternative TV reception or video source, versus 25 percent of all homes.

On the other hand, households with at least one resident age 50 or above have higher rates of subscribing to cable or satellite services. More than eight in 10, or 82 percent, have some sort of pay TV subscription, versus 75 percent of all U.S. TV households. The difference comes almost exclusively in levels of cable subscription, with 46 percent of 50+ homes paying for cable reception, compared with a U.S. average of 41 percent.

Broadcast-only reception is more common in TV households earning under $30,000 per year—26 percent, versus 17 percent among all TV homes—and those with Hispanic residents—24 percent. Households with incomes of $50,000 a year or more post higher levels of satellite subscription—27 percent, compared to an average of 21 percent.

The study was conducted among 3,009 U.S. households, including representative levels of non-TV, non-internet, cell-phone-only, and Spanish dominant homes. 

Charles W. Rhodes The FCC is proposing 11 scenarios for a 600 MHz Band Plan following the spectrum auctions. Which of these will be adopted will be determined by the outcome of the auction in 2016.

Each scenario for the 600 MHz band starts with Channel 21 (512–518 MHz). The highest channel number ranges from 26 to 44, depending on how much spectrum is offered for sale by broadcasters and then resold to broadband operators. The re-allocated spectrum is divided into blocks of 5 MHz each. There could be from two to 12 pairs of blocks. Pairs consist of one block for uplink transmissions from cellphones to base stations; and a second for downlink transmission by base stations to cellphones.

INTER-SERVICE INTERFERENCE
I have restructured the data in Fig. 1 into Figs. 2 and 3. Fig. 2 shows the number of UHF TV channels after repacking. It varies from 6 to 23 depending on how many pairs of 5 MHz blocks are re-allocated after the auction.

Fig. 1: The graph is Fig. 23 of the FCC's June 2 R&O, page 453. Light Blue: 5 MHz blocks of spectrum. Orange: Channel 37 reserved for radio astronomy and medical telemetry (hospitals). Diagonally shaded gray: guard bands. Blocks with numbers from 21–36 and 38–44 should be tinted light green. These are the remaining DTV channels. The numbers in a column on the left side are the amounts of spectrum broadcasters might offer to sell. The amount of spectrum that can be re-sold to broadband is less due to the need for guard bands in the 600 MHz band.

In Fig. 1, the striped cells with numbers of MHz; 11, 9, 7 or 3 represent the guard bands. There is an 11 MHz-wide guard band between the cellphone uplink blocks and the base station downlink blocks. There are also some smaller guard bands, notably, a 3 MHz-wide one adjacent to Channel 37; and a 7 MHz-wide guard band between some base station transmit blocks and DTV channels.

These guard bands are vital to protecting against what the FCC calls “Inter-Service Interference” or ISIX. Broadcasters are familiar with the fact that a DTV transmitter radiates power in both channels adjacent to the channel it is licensed to use. This is sometimes called sideband splatter, but it is actually third-order inter-modulation products generated in the high-power amplifier of a DTV transmitter; which gets past the transmitter RF mask filter and is radiated.

The 3 MHz guard bands on either side of Channel 37 (608-614 MHz), which is reserved for radio astronomy and medical telemetry, protect that channel from ISIX. The total bandwidth of all guard bands varies among the 11 FCC scenarios from 14 MHz to 28 MHz. I call such spectrum “lost” because, by definition, it cannot be used either by broadcasters or sold to broadband. Such spectrum is also lost as a source of revenue to the U.S. Treasury Department.

Fig. 2: Number of DTV channels after repacking as a function of the number of pairs of 5 MHz blocks available for auction.BEST LAID PLANS
I expect there will be pressure by white space advocates to let these guard bands also be used for white space services. That scares me because I recall the “good ol’ days” of Citizen’s Band mobile radios. Chaos soon reigned in the CB Band.

If the FCC chooses either eight or 11 pairs of 5 MHz blocks, it will have to purchase 28 MHz more spectrum from broadcasters than it will be able to resell to broadband operators. Since the commission is mandated not to lose money in these auctions, it will have to resell this spectrum at a price well above what it paid for the spectrum it purchased from broadcasters.

The 2012 law that authorized this auction requires the FCC to recover all costs of conducting it and requires the commission to turn over to the U.S. Treasury the net profits. I believe some of the billions of dollars expected from this spectrum auction will never be realized. I suspect that the estimated $44 billion of auction proceeds do not take into account the fact that some spectrum the FCC will buy cannot be resold because it must be used as guard intervals in the 600 MHz band plan.

The best case scenario from this perspective would have the FCC buy 84 MHz of spectrum and to resell 70 MHz of this spectrum. The minimum markup would be 84/70 or 20 percent. Administrative costs, the $1.75 billion to reimburse displaced broadcasters, and the profit for the U.S. Treasury will erode those billions of dollars promised to Congress.

Fig. 3: “Lost UHF Band Spectrum” in MHz. as a function of the number of pairs of 5 MHz blocks available for auction. Note the steep increase in “Lost Spectrum” between 7 pairs and 8 pairs of 5 MHz auctionable blocks. Fig. 3 plots the number of DTV channels in the 600 MHz band after repacking as a function of the number of pairs of 5 MHz blocks auctioned by the FCC. The outstanding feature of this plot is the steep decline in the number of DTV channels between the scenario with seven pairs of 5 MHz blocks (16 DTV channels) and the scenario with eight pairs of 5 MHz blocks. It would cost broadcasters four channels to allow one additional pair of 5 MHz blocks instead of seven pairs.

When all is said, it appears the best scenario identified by the FCC would provide seven pairs of 5 MHz blocks. This view has also been expressed by a number of cellphone operators according to the FCC.

CELLPHONE DESIGN
The FCC report provides a very complete analysis of each of the 11 scenarios depicted in Fig. 1 (FCC 14–50, p. 453). As Fig. 1 shows, there are guard bands of 3, 7, 9 and 11 MHz between different kinds or signals. The 11 MHz guard bands between uplink and downlink signals are obviously needed to keep the transmitter output from getting into the receiver input. The others are also required to avoid third-order distortion products from causing interference. For example, the 3 MHz band stop filters keep received signals in Channels 36 or 38 out of Channel 37.

Every kind of filter attenuates every signal within its pass band (insertion loss). With each 1 dB of insertion loss, the receiver’s noise figure increases by 1 dB, and sensitivity decreases by 1 dB. Worse yet, where there is more than one filter in the signal path, the insertion loss of each filter is additive. In Fig. 1, you will see that each scenario requires at least one 11 MHz filter and some require two or three filters. Filters not only cost in receiver performance, they cannot be manufactured as an integrated circuit so they take up space and add slightly to the weight of handheld cellphones. The fewer the number of filters in a handheld cellphone, the less it will cost and weigh, all things cellphone users can appreciate. As Fig. 1 shows, the number of filters varies significantly for the various scenarios.

There is yet another variable of importance not shown in Fig. 1, but was covered in the FCC report. The bandwidth over which the antenna of a handheld cellphone is efficient varies between the scenarios in the FCC Plan for the 600 MHz Band. Antenna efficiency directly affects battery life (time between recharges), as well as the sensitivity of the receiver. Scenarios that provide more than eight pairs of 5 MHz blocks will involve these antenna bandwidth problems. Where the efficiency of a simple passive antenna is poor (lots of signal bandwidth), the antenna can be automatically tuned to improve its efficiency. However this automatic antenna tuner feature requires added circuitry and therefore adds to the manufacturing cost. The insertion loss of this reduces the power to the antenna, which translates to increased power drain on the battery when transmitting. When receiving, it adds desensitization of the received signal.

Stay tuned.

Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via email atcwr@bootit.com.