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Multipath Signal Processing


The majority of propagation channels are multipath. In communication systems the multipath effect of a propagation channel is taken into account by combining the energy of multipath signal components in Rake-receiver. A separate one-path receiver, including time synchronization schemes, receives a signal of each path. This approach is efficient in the presence of several well-resolvable components of a multipath signal, i.e. they are apart from each other in time by several chips. However, frequently, components of a multipath signal are irresolvable. The multipath signal processing with a cluster structure (difference of time positions of neighbor path signals is less than PN chip – signals are irresolvable) results in substantial energetic losses.

There are developed different methods of searching, tracking, and receiving a multiple signal, paths of which can be irresolvable. The methods are based on the original algorithm of estimating a number of components of a multipath signal.



Fig. 1 Time distance between path signals – 1 chip.

Fig. 1 shows the simulation results of one of methods of estimating a number and time position of signal paths. A two-path fading channel is simulated, and average powers of path signals are considered to be equal. At selected time distances between path signals, they are, as a rule, irresolvable. Here there are presented a curve of the ideal synchronization when two fingers are set exactly to time positions of path signals, and a curve of one-path DLL, at which a single one-path receiver is set with accordance to the position of correlation function maximum. It is evident that the use of the developed algorithm substantially improves the interference stability of the reception as compared with the one-path reception.
In the presence of path signals with close time positions data streams, by which the carrier is recovered, (complex envelope estimation) of signals of each path, are distorted due to the influence of path signals on each other.

There is developed an algorithm of estimating a complex envelope by decorrelation that eliminates the influence of signals of neighbor components of a multipath signal on each other.



Fig. 2. Time distance between path signals – 1/2 chip.

Fig. 2 illustrates the efficiency of a quasicoherent reception of a two-path signal. Path signals have an equal average power and perform fading independently at 166 Hz frequency, corresponding to a movement speed of a mobile user equal to 100 km/h. It is obvious that the use of decorrelation substantially improves the interference stability of the reception.
There are also developed a simplified approach to the reception of a multipath signal with a cluster structure. This method provides the interference stability that is close to the potentially attainable at the minimum number of used fingers.

 

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