What is spectral efficiency in communications?

• A. The number of bits transmitted per second per Hz of bandwidth (bps/Hz).
• B. The ratio of throughput to latency
• C. The ratio of SNR to bandwidth
• D. The number of channels per hertz

Answer: (A)

Spectral efficiency is the rate at which information can be sent for each hertz of spectrum, measured in bits per second per hertz. It shows how efficiently the available bandwidth is used to carry data. This is the right way to describe it because it directly quantifies how many bits per second you can achieve for every unit of bandwidth, which is exactly what “spectral efficiency” aims to capture. In practical terms, for a channel with bandwidth B and SNR, the Shannon limit gives capacity C = B log2(1+SNR); dividing by B yields spectral efficiency = log2(1+SNR) bits/s/Hz, highlighting that efficiency depends on the signal environment, not just the raw bandwidth. The other options describe different concepts—throughput-to-latency, a ratio involving SNR and bandwidth, or channel count per hertz—none of which measure how effectively spectrum is used to carry data per unit bandwidth.