Which statement best defines Eb/N0 in digital communications?

• A. Energy per bit to noise power spectral density; determines achievable BER for a given modulation
• B. Carrier frequency stability
• C. Bandwidth efficiency per symbol
• D. Channel impulse response length only

Answer: (A)

Eb/N0 is the ratio of the energy per information bit to the noise power spectral density. This metric matters because it directly relates how much signal energy is available to overcome the noise, which largely determines the achievable bit error rate for a given modulation. Since each symbol carries log2(M) bits in an M-ary scheme, Eb = Es / log2(M) and Eb/N0 = (Es/N0) / log2(M); this lets you compare different modulations on a common energy-per-bit basis. Higher Eb/N0 generally means lower BER for a given modulation. The other statements don’t fit because carrier frequency stability relates to oscillator accuracy, bandwidth efficiency per symbol concerns how tightly you pack bits in frequency, and channel impulse response length deals with multipath duration—not the energy-to-noise ratio per bit.