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HD Radio Boosters and Single-Frequency Networks
The multipath effect shown in the spectrum above is caused by the difference in time between reception of the main, and that of the booster.
To minimize ISI at this location, the engineers from iBiquity added 20µs of delay in the transmission path of the booster (WD2XAB-FM1). Making note of the solid green, contiguous lines in both figures 7.2, and 7.3, one can see that there is an improvement in the after measurements.
Stepping back now for a moment to the Harris/NPR paper; the following conclusions are drawn at its end: "Digital radio lends itself naturally to SFN implementations. The main advantage is the potential of very flexible coverage and easy expansion - simply add more transmitters. Depending on the length of the guard interval, some care will be needed to avoid excessive multipath. The hybrid IBOC system warrants some further studies with respect to the analog part of the signal, before it is clear how well SFN will work in this case." You may have thought of this question as well: If one station were to use a digital-only booster, what will be the effect on analog-only receivers near the HD Radio booster, where the ratio of analog to digital will be far, far less than 20dB? The WKLB HD Radio booster experiment (NAB Fastroad) endeavored to answer that, along with looking at the booster results for HD Radio.
Like the WD2XAB booster experiment mentioned previously, a time alignment was performed in the case of the WKLB booster. Figure 8.7 shows measurement data taken before, and (slightly to the right) after the time alignment was performed on the booster transmitter. Digital signal augmentation and improvement was noted between the main and booster sites, but actually made worse as one went north beyond the booster location. This is because of delay spread that exceeded 75µs.
- continued on page 5
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