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HD Radio: Proving Performance with Modulation Error Ratio
MER measures how off the actual readings are from the reference points, as Q1 through Q4 show in Figure 3 after demodulation.
The farther off the readings, the more difficult it is for the receiver to correctly detect the signal. If they are too far off then they can't be detected well, resulting in bit errors. Too many bit errors leads to a total failure of the demodulation and decoding process.
It is interesting to note that peak-to-average power reduction actually generates noise in the constellation, but that particular noise ultimately has little effect on the ability of the receiver to properly demodulate the data. David Hershberger, senior scientist for Continental Electronics, wrote in his paper "IBOC Signal Quality Measurements" that "Peak-to-average (PAR) reduction algorithms may introduce enough deliberate distortion to digital signals that the effects of a transmitter on MER are obscured. Special measurement methods can separate the effects of a transmitter from that of PAPR reduction." And further: "The PA(P)R reduction noise is large compared to other OFDM systems. A textbook evaluation of MER ... will result in the PA(P)R reduction noise dominating the measurement. For this reason NRSC has proposed several modified MER measurements which result in metrics which are closer to the true system performance." Both the Continental and Nautel implementations of MER effectively subtract noise generated by PAR from the measurement results.
The Nautel implementation of MER uses an RF sample from the transmitter output to derive the MER. (It's the same sample used for the adaptive pre-distortion.) The measurement is fairly granular in that it allows you to see the MER of each partition (or group of subcarriers) and therefore how each partition is being affected by some external influence. Some examples will demonstrate this.
In Figure 4, we see a partition fairly far removed from the center frequency, with a high MER. In Figure 5, we see a partition closer in, with somewhat of a lower MER.
- continued on page 3
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