Communication Technology

Coherence/Coherent Time/Coherent Frequency

Sometimes, we see some word that sound like simple, but hard to get clear understanding. To me, the word 'Coherence/Coherent' is one of those words. Without clear understanding of this words, I got confused with many other terms that came from this word, like 'Coherent time', 'Coherent Channel', 'Coherent frequency' etc.

What dose 'Coherent' mean ?

It mean 'Something stays same (does not change)'.

What does that 'Something' mean ?

It would mean different things depending on situation (context).

Based on this, you can derive the meaning of some of common terminology you would hear or see in various materials on communication theory.

Coherent time : It means a certain span of time during which 'something' stays same. As I said, the word 'something' would have different meaning depending on situation. For example, if you heard of this when you are talking about some traveling wave (propagating wave), the "something" may mean 'phase delay'. So in this case, the Coherent time would mean 'a certain time span in which the phase delay of traveling wave does not change. If you hear of this when you talking about fading stuff, 'coherent time' would mean 'a certain time span during which the fading property remain unchanged'.

Coherent frequency : Same logic, It means a certain span of frequency during which 'something' stays same. As I said, the word 'something' would have different meaning depending on situation. For example, if you hear of this when you talking about fading stuff, 'coherent frequency' would mean 'a certain frequency span within which the fading property remain unchanged'.