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Keep coming across the term channel knowledge (specifically in the transmitter) in a reading for class but can't really grasp the idea and don't see a clear cut definition. From what I understand it is the idea of knowing certain characteristics about the channel a signal is being transmitted on to provide a more focused beam of energy to transmit signal and this knowledge is often obtained via feedback loop with the receiver. Am I on the right track? Also, what determines partial vs. full channel knowledge?

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The channel is the frequency selective path from transmitter to receiver which can include components such as analog amplifiers, filters, etc and multipath reflections between the transmitter and receiver. This modifies both the phase and amplitude of the various signal components at different frequencies, and can make demodulating the signal challenging.

An easy way to grasp this quickly is to consider the following graphic:

channel distortion

In the upper right hand corner is an ideal "Eye Diagram" which shows the signal as transmitted superimposed on itself at even symbol boundaries. Here we can see how easy it would be to determine if the transmitter sent a "1" or a "0" based on the opening of the eye. That signal as it appears at the transmitter arrives at the receiver through multiple paths each having their own gain and delay which results in the eye diagram at the receiver appearing as shown in the lower left hand corner.

Having full channel information is knowing the gains and delays for each of those paths, in which case we can "equalize" the received signal for the channel and restore the eye diagram to what we saw at the transmitter. Partial channel information is knowing some but not all of the information about the channel. This can be due to temporal changes in the channel characteristics and delays in our ability to monitor it. We can use known sounding patterns to determine the channel based on what we actually receive compared to what we know we should have received, and this is equalization with techniques further detailed here.

For MIMO implementations in particular, the transmitter uses the channel information to maximize data throughput, and this is where the term "partial channel information" may more likely appear since the transmitters knowledge of the channel is from a feedback from the receiver where we would have a delay that is competing with the rate of change of the channel characteristics.

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  • $\begingroup$ I understand the concept of a channel. Is the channel knowledge just the delay and gain associated to each channel path? What is meant by partial channel knowledge? $\endgroup$
    – lceans
    Apr 18, 2022 at 13:10
  • $\begingroup$ @Iceans see my last paragraph $\endgroup$ Apr 18, 2022 at 13:11
  • $\begingroup$ Sorry didn't see the edit. Makes sense now. As I mentioned in my question, I have read that channel knowledge is obtained via feedback from the receiver in most cases? Are there other ways to obtain this info? $\endgroup$
    – lceans
    Apr 18, 2022 at 13:14
  • $\begingroup$ @Iceans yes you are referring to the use of MIMO which is when we would be concerned about the channel in the transmitter (rather than simply equalizing in the receiver). The channel is inevitably changing with time, and the "channel" is VERY dependent on the receiver so we have to send back the channel information specific to the receiver within a range of time. There will be inevitable delay between the receiver and the transmitter which limits us and hence "partial channel information". $\endgroup$ Apr 18, 2022 at 13:17
  • $\begingroup$ @Iceans I am not sure if this will confuse you and if so it's a side comment you can ignore- but a time delay in the feedback will have a low pass response, meaning given a particular delay, the low frequency variations in our channel will still be applicable (usable) while the high frequency components will no longer apply (they will be decorrelated). Thus the "partial info" is typically the lower frequency portion of the frequency domain view of our channel. $\endgroup$ Apr 18, 2022 at 13:24

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