What is coherent bandwidth, ISI and equalization?

I'm studying digital communication where I have met many new concepts (I used to be a electronic engineer)
- What is coherent bandwidth?
- what is ISI?
- what is equalization

I really need your helps because the final exam is coming
Thank you in advance!

Added after 2 hours 39 minutes:

In addition, I find that for many wireless system, the DL direction always uses frequency band less than UL direction.

Can you give me the reasons why?

Comm. question

first , i dont know what do u mean by coherent B.W .... coherent comm. system is a type of comm. systems in which we must have carrier at the Rx for demodulation ... the B.W for this sytem differs from that of noncoherent system


about ISI it stands for InterSymbol Interference .... when te symbols ("1" , "0" ) are transmitted , the square wave shape becomes distorted and this leads to interference between pulses

-equalization : means to make an effect at the reciever opposite to the channel effect ( for example , if channel is represented by Low pass filter ) , at the reciever the equalizer should give hih gain to high frequency components and low gain to low frequency components to compensate the channel effect ....

for the last question .... i guess u mean that the carrier frequency for UL is less than carrier frequency for DL .... that is because as frequency increases , power required increases .... so in mobile system for example, UL should be at less frequency because the mobile phone has limited power .... but the frequency band is the same in both directions

Comm. question

Hi

Coherent bandwidth is a characteristic of the propagation channel and is considered as the minimum bandwidth within which all signals propagating are almost non-distorted. Well saying it easy, it is the portion of the bandwidth where you can consider the channel has flat response.

Information is transmitted in symbols which are supposed to be limitted in time to lets say T seconds, but because of the channel (multipath fading basically) this symbol is extended to a time T+dT , which means the symbol components during this extra time causes interference to the next symbol, this is know as ISI.

Well channel equalization are the techniques employes to undo the effects of the channel.

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Sal