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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