What is meant by channel knowledge? Difference between partial and full?

Question

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?

Answer 1

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:

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.