The concept of Noise Power Ratio (NPR) has been around since the early days of frequency division multiplexed (FDM) telephone systems. The NPR is simply a measure of the "quietness" of an unused channel in a multi-channel system when there is random activity on the others. Noise and intermodulation distortion products fall into the unused channel causing less than ideal performance. Originally used to check 4-kHz wide voice channels in FDM links, the same concept is useful today in characterizing multichannel wideband communication systems – but there are some important differences in the modern measurement techniques.
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Editorial
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