Spectral efficiency

Modulation techniques may be more or less conservative. For example, we illustrated amplitude modulation using two levels, where a low amplitude signified a 0 bit and a high amplitude a 1 bit.

What if we had more sensitive, expensive radios that could differentiate between four amplitude levels: lowest, low, medium and high?

Amplitude level Bits

Lowest 0 0

Low 0 1

Medium 1 1

High 1 1

In this case, we would be transmitting two bits of information in the time it previously took to transmit one. That would represent a 100% gain in spectral efficiency. Spectral efficiency is measured in bits/second/hertz.

Tradeoffs

An aggressive modulation technique will be more error prone than a conservative one, but when it works, it will transmit data faster.
As signal strength diminishes, there will be more errors.

When error rates exceed a certain threshold, radios often switch to a more conservative modulation technique. For example, IEEE 802.11b radios can switch among five modulation schemes, giving five transmission speeds.

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