Delivering broadcast content via SFN

Over the past several months, ION Media Networks and several partners have been testing technology to enable the use of a single-frequency network (SFN)
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Over the past several months, ION Media Networks and several partners have been testing technology to enable the use of a single-frequency network (SFN) to improve broadcast coverage in New York City. After 9/11 and the destruction of the broadcast location used by most stations atop the World Trade Center, there was a great need for coverage improvement.

Since that time, ION and many other broadcasters have been using a transmitter location from the Empire State Building. However, having so many broadcasters crowded onto one tower has made it virtually impossible to replicate the coverage once provided by facilities at the World Trade Center. There is insufficient space on the tower for appropriate antennas to accommodate all the transmitters. An alternative is desperately needed.

The alternative

ION and its partners performed testing during the summer and fall of 2007. The testing involved sites provided by Richland Towers and used technology that has been proposed to the Advanced Television Systems Committee (ATSC) by Samsung and Rohde & Schwarz. This system was shown at NAB2007 as part of the Samsung and Rohde & Schwarz A-VSB demonstration. However, ION wanted to see how the technology would stand up in a difficult reception environment like New York City. It also wanted to investigate whether an SFN system in New York could be a long-term alternative to other solutions such as the Freedom Tower project.

First, the basics

Click image to enlarge.

So what is a single-frequency network? In a nutshell, an SFN is a broadcast system that allows for the operation of multiple transmitters on the same frequency. (See Figure 1.) These transmitters are controlled by not interfering with each other, even though they occupy the same spectrum. To understand how the technology works, you have to know a little bit about the way the DTV standard was originally designed.

The ATSC standard was not originally meant to be used in SFN mode. In fact, one of the properties of an ATSC exciter is that its output appears to be somewhat like random noise regardless of the input data stream. This is done purposefully. If there is a potential for interference, uncorrelated noise can be much less damaging than correlated signals or periodic signals. Given this property of the ATSC signal, if two transmitters are using the same frequency in the same location, they will jam each other, even if they are presented the exact same, perfectly synchronized input. Somehow the exciter outputs must be synchronized together so that they emit the output signal at the same time. The ATSC system adds a framing structure to the data being transmitted. In normal ATSC exciters, this framing structure is not linked to any particular point in time. So an exciter is free running, and it begins the frame and all of its internal randomizing and coding processes at a nondeterministic point in time.

In order to overcome these properties of the ATSC transmission system, exciters in an SFN system must be controlled so that they each treat the input stream in the exact same way. If this is done properly, the transmitters will no longer behave as jammers. Instead, the multiple signals received at a particular location will look like echoes of the same transmitter. In most receive locations, the equalizer in the receiver can cancel the echoes, and the signal can be decoded normally.

Synchronization is key

Synchronization is the first step in setting up an SFN. The system for the New York City testing used a device called a VSB Frame Initialization Packet (VFIP) inserter. The VFIP inserter takes a transport stream input and inserts a special packet into the stream. The packet signals when the downstream exciters should start their framing sequence. All the exciters in each transmitter in the network receive the VFIP and are then synchronized to begin framing the data at the same point in time.

Then there is one more job to do, and the VFIP takes care of this too. The ATSC exciter has a trellis coder as part of its processing. The trellis coder normally starts in an initial condition that may not be deterministic. In an SFN, the trellis states of all the exciters in the network need to be set the same way. So the VFIP contains a special byte pattern. When the pattern is sent through the exciter, it forces the trellis coder to be set to a known state regardless of its initial condition. In summary, the VFIP is able to signal the exciter so that all the relevant processes are synchronized to a known point in time.

Testing the SFN in New York City

Now that we know how to accomplish synchronizing ATSC exciters for SFN operation, we can look at the practical setup of the SFN testing in New York City. (See Figure 2.)

Two transmitters were used for the tests. The main transmitter was located in West Orange, NJ, and a secondary transmitter was located in Manhattan at Four Times Square. The main transmitter already provides coverage to most of the market. The secondary transmitter's job was to improve the coverage in Manhattan and parts of Long Island where the New York City skyline was acting as a shield to the main signal.

Measurements were taken at 32 test locations throughout the New York City market area, including locations in the Bronx, Brooklyn, Long Island, Manhattan, Queens, Staten Island and northeastern New Jersey. At each test location, field strength, signal quality, ability to receive a picture and various other measured parameters for the SFN were taken. There was a noticeable improvement in the areas to the east of Manhattan (including Long Island and Queens) with the secondary transmitter on, compared with measurements taken only from the West Orange main transmitter signal. In areas where the West Orange signal was dominant, there was no negative impact caused by the Four Times Square transmitter at any of the tested locations.

Interference

Even with the SFN synchronization, there are areas where interference is predicted. This is due to the fact that in some cases, the relative strengths and time spacing of multiple signals received might be out of the range of the equalizer in the receiver.

Care is taken in the design of the SFN to minimize those areas by controlling various antenna parameters and adjusting the fixed delay between various transmitters. In our test network, those interference areas were designed to be mostly in areas over water. However, there were some important areas where interference was predicted, notably in northeastern New Jersey and Staten Island. Yet, our field measurements showed better than expected results at the tested sites with the secondary transmitter on. Some sites in the areas to the north of Manhattan, including the Bronx, showed coverage challenges for the SFN. As part of our continued testing, another site is being investigated for this area.

Conclusion

The two-transmitter SFN system worked well. The system provided signal strength and quality improvements at nearly all of the test locations. In many locations, the signal strength measurements were very close to those from the transmitter on the Empire State Building. With another one or two transmit sites, the SFN might offer an attractive alternative to a single transmitter site.

Brett Jenkins is the director of technology strategy and development at ION Media Networks and is currently the chair of one of the ATSC ad hoc groups working on a new standard for mobile TV broadcast.