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HD Radio: Proving Performance with Modulation Error Ratio
In February 2010, iBiquity released a paper on techniques for measuring the quality of IBOC transmissions. A quality-metric known as MER (Modulation Error Ratio) was introduced by the National Radio Systems Committee as a standard. If you don't read the entire document, it's important to note that stakeholders in the United States have agreed to this methodology.
After three years we are now seeing the introduction of MER metering for IBOC transmitters. Nautel recently made this available for its NV and NX series transmitters; Continental also has MER metering in its current HD Radio exciter, the 802Ex FM/HD. Nautel recently presented a webinar on its MER techniques.
According to information presented in the webinar, Nautel has implemented the FM MER technique described in the iBiquity paper in its NV series transmitters. There's no standard for AM, however, so Nautel has implemented what it refers to as a textbook standard methodology for its AM transmitters MER measurement.
So what are the implications of a low MER value, and how is MER measured? Conceptually (at least) it's pretty straightforward. Recall that the IBOC subcarriers are in groups (or partitions) consisting of both QPSK (for the data) and BPSK for the reference carriers. See Figure 1.
In an ideal situation, when QPSK carriers are demodulated the result would be perfect placement inside the constellation, resulting in easy error-free detection, (Figure 2). In the real world though, when the QPSK carriers are demodulated they show up close to the right (reference) spot, but not exactly right on.
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