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HD Radio Boosters and Single-Frequency Networks
One common desire of all broadcasters is to extend the station's coverage. Within the framework of our licensing system, that isn't easy to do; however, having your coverage optimized within your allotted coverage area is a realistic goal. The idea of the Single Frequency Network (SFN) is nothing new, and has been implemented in both the AM and FM bands. Interestingly, iBiquity's HD Radio system, and the open source DRM system both lend themselves readily to SFN implementations.
Research has been done in the implementation of SFNs with HD Radio. I'm going to talk about three different examples. I have used two papers in research for this article, and I encourage you to read them for even greater details. The first was prepared by Anders Mattsson of Harris and John Kean of NPR Labs. The second was published by NAB Fastroad.
My own experience with SFNs goes back to an earlier part of my career working in the San Francisco Bay Area. The station group I worked for had three boosters operating on the combined system on Mt. Diablo, which was designed to serve the east bay portion of the greater Bay Area. (This booster system was developed in 1989 by several well-known chief engineers plus the consulting group Hammett and Edison. More details can be seen by studying FCC facility IDs 86911, 28878, or 59990.) This system provided frequency and modulation synchronization for the boosters, and thus minimized the effects of multipath in locations where the desired to undesired ratio was fairly low (between 10 and 20dB). Later, as the chief engineer of KIOI, I developed a booster for the east bay that relied on off-air reception, amplification and re-broadcast (see, for example, FCC facility ID 4085).
What I learned through the experience of building boosters (we didn't refer to them as SFNs) and listening to them for countless hours driving around is that, despite your best efforts in synchronizing the frequencies, and modulation, that terrain shielding was actually the key to their success. I would even go as far as saying (and this is purely anecdotal) that 90 percent of a booster's technical success is due to the terrain shielding factors. The geography of the Bay Area lends itself to the success of VHF booster systems.
It turns out that HD Radio (and DRM) can both be used successfully in the construction of SFNs, also by taking advantage of terrain shielding, frequency and modulation synchronization. According to Anders Mattsson (in the first article I mentioned) digital radio in general has a distinct advantage, not in the transmission mode, but in the design of the receivers: "This poor performance (of analog SFNs) is not due to any inherent limitation in SFNs. In fact, it is because of the lack of equalizers in traditional FM receivers. At the moment the receiver can handle the multipath, SFNs offer many potential advantages. Since all digital systems such as HD Radio and DRM already have equalization, the old limitations are gone."
Even so (as one would expect) there are practical limits to what can be done with HD Radio SFNs, and they mainly relate to what is known as the guard interval in OFDM. Recall that the IBOC transmission scheme uses Orthogonal Frequency Division Multiplexing (OFDM), where parallel data streams are transmitted by way of multiple low-level carriers, themselves modulated in a standard fashion (such as QAM or QPSK). In the specific application of IBOC, hundreds of subcarriers are used; the modulation scheme is QPSK. One of the big advantages to OFDM is its robustness, and that is in part attributable to a long symbol length. Symbol length is the amount time that the individual carriers are in a state that is detected by the receiver. Part of the symbol length, though, is called the guard interval. This is the amount of time that needs to elapse prior to the next symbol. In other words, there is not a continual flow of symbols; between changes, a certain amount of time expires -- the guard interval.
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