How is the spectral density of a digital modulation channel expressed?

The spectral density of a digital modulation channel is expressed in decibels per millivolt per hertz (dBmV/Hz) because this unit effectively conveys the power spectral density of a signal relative to its bandwidth. This measure allows for the representation of very large or small signal values on a more manageable logarithmic scale, which is particularly useful in telecommunications where power levels and signal strengths can vary widely.

Using dBmV indicates the voltage level measured in millivolts, normalized to 1 millivolt, and the 'Hz' part signifies that this measurement is related to a frequency bandwidth. This combination is essential for analyzing how much signal power is available across various frequency components for a digital modulation channel, which directly impacts the performance and integrity of the communication system.

In contrast, the other options do not adequately represent the normalized power spectral density relevant to digital modulation channels. The use of volts per meter, watts per decibel, or volts squared per hertz does not correlate well with the practical considerations needed for assessing performance in digital telecommunications, making the chosen unit particularly suitable for these types of analyses.