HD Radio Boosters and Single-Frequency Networks
This is important because of the mitigation of multipath on the receiver end. If the receiver, in the field, encounters two IBOC signals that are on the same frequency, with synchronized data, there are two characteristics that will determine whether or not there is a destructive multipath effect:
■ One signal is considerably stronger than the other (say 15dB or more) in which case the weaker signal doesn't have any effect on the receivers detection of the subcarrier state changes on the stronger signal, or
■ Both signals are of nearly the same strength, and they are delayed by some amount of time because of different distances between the receiver and the various sources.
If that difference in time (also called the delay spread) is within the guard interval, the receiver can correlate the symbol changes in both received signals, and essentially they add. If the delay is outside (or longer) than the guard interval, then the picture gets blurry; the symbol changes are too far apart, and intersymbol interference (ISI) is caused. From the Harris/NPR article, Anders Mattsson states that the symbol length for HD Radio is 2.9ms, the guard interval is 156µs, and that "it seems that multipath delays of up to the guard interval of 156µs, should be acceptable. Ideally it should be less than 78µs." This leads to a practical limit of about 15 miles in difference between the two IBOC sources; considering that the receiver could be located directly between the two, that leads to a practical limit of about 30 miles between transmitter sites (based on 78µs).
Let's look at some real world examples now of HD Radio boosters that have actually been built and performance-tested. The first will be the KCSN booster noted by John Kean in the Harris/NPR article. It's interesting for several reasons, not the least of which is that the booster has a higher ERP than the main signal. It's also more of an old fashioned booster design in that it takes advantage of terrain shielding. Kean notes that the two transmitters made use of modulation synchronization and frequency synchronization.
Part of NPR's evaluation process of the KCSN booster design involved the development of software that was used to predict interference between the two digital sources. This software was used for these purposes:
■ The propagation time from the main, and from the booster, on a grid across the area of interest;
■ Compared the calculated field strength of main and booster on the same grid:
■ Determined the field ratios and delay spread that could result in ISI, at each point on the grid;
■ Mapped the points on the grid in a color-coded fashion.
The predicted results are shown below. Delay spreads were noted if greater than 75µs.
Clearly the predictions showed that, in a roughly oval shape centered about the KCSN-FM1 transmitter site, that the digital performance was expected to be good. NPR took measurements of the on-air system inside of the box shown in the figure. The northern part of the box is an area that encompasses the Hollywood hills so (based on my prior experience) I would expect to see trouble there just based on the terrain. However, of special interest is the southern part of the box, along I-10, just north of Los Angeles proper.
- continued on page 3
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