Gain of multipath fading components For the Rayleigh fading model, I want use four multipath components,  each component has its delay and gain. 


Can the gain in $\textrm{dB}$ of any of the individual four components be positive value?  


I can understand that it can be zero or maybe negative indicating reduction in its amplitude due to delay and effects of channel, but I can not understand how it can be positive. 


The overall signal resulting from the four components at the receiver may be with positive gain. This is reasonable, due to constructive addition of maybe inphase components, but how can a single component be with positive gain?
It can have a positive gain, it all depends on your definition of 0dB. Sometimes, the first path is assumed to be 0dB, then it can happen that later paths have a higher gain than the first one, leading to positive path gain. 

However, in reality, there is always the path loss between the transmitter and the receiver, which is normally added to all paths similarly. 

So, for modeling and simulation, it is possible to have positive path gain. However, in reality the signal at the receiver will always have less power than the transmitted signal.