Propagation from one radio to another will always be a net loss.  You'll never receive more power than you transmit.  However, in talking about a channel model, the gains of clusters and taps are generally treated as relative values, absent the net path loss term.  The typical approach is to have the first cluster be the 0 dB reference and other clusters are usually less than that, indicating a decaying power delay profile. However, there's nothing preventing the shortest path having a lower signal than a longer path (e.g. through a wall vs. a strong reflection around the wall).  Thus, if the first cluster is 0 dB, then the second cluster would be positive.  That doesn't really mean anything physically.  It's all just an arbitrary reference normalized to the first cluster.

Note that for a faded multipath, you don't get gain due to "in phase" conditions.  You have to treat those as power sums, not voltage sums.  That's different than gain at the source due to beamforming, etc. where phase of the unfaded signal is used to provide gain (over an isotropic radiator) in the antenna array.  

Thanks alot Prof. Feogelle for your answer

I agree with Prof. Michael Foegelle and Vijay. As Prof Michael said, that propagation from a transmitter to a receiver over a single path will be always a net loss. Moreover, the taps (path gains) in a multipath scenario are treated relative to each other with the strongest one having a maximum value of 0 dB (or still even negative) and all other path gains are considered relatively less than that (more negative), Just reconsider the reflection from a wall example.

Even if we consider multipath with a Line of Sight component (Rician Fading), still the gain of the strongest path will be negative in reality due to still some loss (free space path loss).  As Vijay mentioned the simple equation, that y=hx+n, then assume a n=0, we get y=hx. If we put h>0, this implies that y>x i.e. received signal is stronger than the transmitted signal, which violates the basic energy principle also (can only be assumed if the channel is adding a gain which actually doesn't happen in reality).  

Dear Student,

If You want to improve the gain means, you should  introduce repeaters (Amplifiers) in between transmitter and revceiver.In general Transmitting power is always higher than received power.

best wishes

You really have to look at the mechanism that creates the multipath channel model. The multipath channel model is used to simulate the propagation media where signal is transmitted by the transmitter and multiple rays arrive at the receiver as a result of reflections (from the flat obstacles), diffraction (from the edges of obstacles) and scattering (from changing medium with different characteristics. The strongest rays arrive at the receiver due to the reflections. Since the reflection coefficients range between 0 and 1, the gain of individual components can never be positive (10 log g can be between -inf dB when g is ~0 and ~0 dB when g is ~1). Hence, gain of individual rays can never be positive.

For radio frequency propagation, it is hardly ever a direct LOS path consisting of a single ray. Multiple rays are propagated from the transmitting antenna- some of these are direct, some are scattered, some are diffracted and some suffer multiple reflections.

Now, let us consider the desired receiver radio antenna and system. All carriers within a given bandwidth (BW) are accepted by the antenna. The front end tunes within  the IF 3-dB BW all carriers. 

Different carriers received through different paths will have different degrees of attenuation and phases.

If the phases of two signals are same (constructive), the amplitude of the resultant signal will be more than the original (0dB). The general practice while designing a robust RF network is to consider a fading variation in signal to the tune of +10 dB to -40 dB.