Radio Channel Impulse Response Measurement Systems
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Radio Channel Impulse Response Measurement Systems |
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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, a line-of-site route (path 1) and a reflected route (path 2). Because path 2 has a longer distance 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. |
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