Radio Channel Impulse Response Measurement Systems
Multipath
Multipath, as the name implies, is a phenomenon whereby radio
signals travel more than one route between the transmitter and
receiver. This is illustrated in Figure 1 for which there are
two paths, aline-of-site route (path 1) and a reflected route (path
2). Because path 2 has a longerdistance of travel, the signal
information traveling along that path is delayed in relation to
path 1. This can result in what is called frequency selective
fades, meaning that, at certain frequencies, the signal may
attenuate or disappear. This is illustrated in Figure 2 in which there are
two sinusoidal signals, one traveling along path 1 (signal 1 - blue
plot) and the other traveling along path 2 (signal 2 - green plot).
The two signals look the same and have the same amplitude but
signal 2 is delayed such that it has a different phase in
relationship to signal 1. At the receiver, these two signals are
summed together to produce the signal shown in red. The resulting
summation, in this case, gives a signal that is smaller in
amplitude than either of the two individual signals, thus resulting
in an attenuation of the received signal. If the two signal are
in-phase (lined up with each other in time), the resulting received
signal will be larger than either of the two individual signals
(coming from the two different paths). As the receiver moves, the
path lengths will vary in relation to each other and, therefore,
the phase will change in relation to each signal coming from the
different paths. This results in a fluctuation in amplitude of the
received signal. For narrow band signals (using only a very narrow
section of the frequency spectrum) these power fades can result in
a total loss of signal (depending on the degree of fade and the
sensitivity of the receiver). A real world example of this is
sometimes seen when listening to a radio station in an automobile.
It is not uncommon to loose a weak signal completely when pulling
up to an intersection, only to have it return again by pulling
forward a few feet.
The impulse response for the above example would appear
something similar to the plot shown in Figure 3. There are two
impulses representing the two different paths. The first impulse,
in time, represents the direct path. The second impulse represents
the reflected path and is smaller in amplitude due to additional
attenuation along the route. The time difference between the two
impulses represents the additional time it takes to travel along
the reflected path.
High speed digital radio signals have a wide bandwidth which
when subjected to multipath can result in periodic frequency fades
and intersymbol interference. Because the information in a high
speed digital signal is spread over a wide bandwidth, multipath
generally results in distortion of the signal but not the total
loss of signal power. The biggest problem is that there are
overlaying copies of the signal each having a different delay. This
means that the symbols used to represent the digital information
show some overlapping. When severe enough, this overlapping results
in intersymbol interference, making it difficult to demodulate the
signal without bit errors.