The most harmful effect of multipath distortion is the Inter-Symbol Interference (ISI). Due to this effect, the effective bandwidth is reduced. Furthermore, if the channel is not static, the multipath effect turns probabilistic.

In order to cancel multipath, adaptive filtering (when the impulse response is unknown) or a matched filter (deterministic case) may be used.

HI:

as u said , in deferent times so if the earlier signal which came first has a phase deferent from the lag signal which came late ( the lag signal is the same earlier signal but it has a phase shift because it take a longest path) then the sum of them (amplitude ) will be suffered and the signal receive will be attenuated .

The multipath should be analyzed in different perspectives , such as:

. 1 - Assuming multipath beams are coming from the antenna face , with traces of multiple attenuation by reflection or refraction , but logically is the same frequency with different phases . In this case you should not skip the radii of Fresnel . such as residual power of neighboring signals.

. 2 - Assuming the antenna is directional and is installed on the same tower with other antennas in the same frequency band, in this case it is necessary to consider the pattern of radiation and analyze the effects against the back of the side lobes - see data sheets. This is because the multipath interference can manifest itself in two ways:

- To a greater degree, when the polarization is the same in all the antennas and the separation between frequencies is minimal.

- Low grade, when the polarization is different also the separation between frequencies is greater, which implies a good distribution of frequencies from the design of the links. As in the microwave radios cellular telephone base station.

3 -. Multipath links in line of sight, it becomes saved with high performance antennas, because the main lobe of minimum opening

In short, you are adding sinusoids together, when multipath occurs. The very simplest example of this will be when a given signal is reflected by a surface along the propagation path, exactly 180 degrees out of phase with the direct signal. If the direct and the reflected signal reach the receive antenna at equal strength, perhaps because the surface was highly reflective, that frequency component of the RF channel will be completely obliterated.

Of course, there are different levels of severity of multipath distortion. Equalizers in receivers are used to try to counter the effect of time delays caused by multipath. Also redundancy in transmitting data, with forward error correction convolutional schemes, attempt to provide copies of data bits, so the receiver can replace those that might have been corrupted.

Precisely stated, multipath means that a single signal from an RF transmitter can travel multiple paths to arrive at a receiving antenna. It is important to think about the propagation path (also called communication channel) as convolving the transmitter signal with an impulse response function representing the comm channel. The impulse function for a comm channel typically as delay, frequency-dependent loss, and frequency dispersion. Thus, in a very formal sense, you must take this convolution process into account. However, what simplifies our lives as comm system designers  is the fact that only 1-4 paths usually have any practical effect on the quality of the received signal, sometimes called multipath distortion.

It answer your question directly, there is no concept of "The Signal" in a multipath environment. We must be more definitive by refering to the "signal of first arrival" or the "signal of highest magnitude." Such is the complication of multipath propagation.

Multipath can create situations were the receiver demodulator can become confused or inoperable. Think of it as one comm channel (multipath signal) jamming another.

The major effect of most multipath is time delay and signal amplitude differences among the multipath signals. If the signal structure has an RF carrier and the delay time is such that two nearly equal signals reach the receiver with carrier phases 180 degrees out of phase, we get what is called "Frequency Selective Fading." An additional complication is that, due to a variety of reasons, the direct path signal may actually be weaker than a signal from a longer path so a robust system would process all received signals of significant amplitude.

To mitigate multipath effects, think about filtering signals in the time domain (rather than the frequency  domain). You can imaging building multiple receivers that receive each signal according to its time delay relative to a local receiver clock. The RAKE receiver, for spread spectrum, is a clever implementation of a reciever that can "tune in time."

We can use methods like designing the signal waveform so we can apply matched filtering to optimize reception of the desired signal. We can also use spread spectrum correlation methods to track and demodulate each multipath signal. My favorite is spread spectrum

If you have no choice over the design of the transmitted waveform and must work with an existing system, You choices for multipath mitigation become very limited.

Hope this helps. Good luck.

So far, all the answers point to the effect of multipath seen in a general way; however asker has not specified which of the technologies required response. This because the multipath on the line of sight links promote interference; but if the context of cellular telephony, then enrich communication.

Multipath weakens the signal strength and causes phase shifting