What does Eb/N0 physically represent?

• A. Energy per bit to noise density.
• B. Energy per symbol to noise density.
• C. Signal power to noise power.
• D. Bandwidth per noise density.

Answer: (A)

Eb/N0 represents the energy carried by each information bit relative to the noise level per unit bandwidth. Eb is the energy per bit, and N0 is the noise power spectral density (noise per hertz of bandwidth). So this ratio tells you how strong the signal is on a per-bit basis compared to the background noise across the channel. It’s a key performance figure because many digital communication metrics, like bit error rate, depend on how much energy per bit you have amid the noise, not just overall power.

For a modulation with M symbols (each symbol carrying log2(M) bits), Es = log2(M) · Eb, so Eb/N0 = (Es/N0) / log2(M). This shows how increasing the number of bits per symbol (higher-order modulation) can reduce Eb/N0 for the same Es/N0, impacting error performance.

If we compared to the other ideas: energy per symbol to noise density would be Es/N0, which isn’t Eb/N0; a simple signal-to-noise ratio compares total powers without tying them to per-bit energy; and a bandwidth-per-noise density term doesn’t describe energy per bit at all.