Digital Signal Processing Reference
In-Depth Information
Multipath components 3/4 suffer
from destructive interference.
tx symbol interval
transmitter
receiver
time
symbol interval
receive module 1
multipath signal 1
symbol interval
receive module 2
multipath signal 2
ver
i
fyin
g
different template
puls
e
offset positions...
receive module 3
searching signal...
= ignore receive slot (already in use)
Figure 5.9.
During synchronization, a pulse-oriented radio can perform an actively
controlled search for the best performing correlation slots over the in-
terval between two pulses. This requires a lot of synchronization time
overhead, while the result is valid for only a short period of time in a
fast varying channel.
the input of the receiver can be considered as a genuine filter. Signal com-
ponents (e.g. interference) that arrive during the off-state of the window are
effectively blocked. However, a small alignment error of the receive window
also eliminates the wanted signal components.
Even after a successful lock has been achieved, the receiver of a pulse-based
radio system should be able to track the clock offset of the transmitter with
respect to its own time reference. Failing to do this will result in the pulses
shifting outside the receive window which again results in signal loss. Several
techniques have been proposed to solve the tracking problem. For example, in
the early-late delay-locked loop (dll) approach [Foc01], the receiver attempts
to detect clock skew between the transmitter and the internal clock reference by
performing three correlations on each received pulse. One correlator is ahead in
time with respect to the reference correlator, while the other is shifted slightly
later. If the receiver is not perfectly aligned to the stream of arriving pulses, the
power emerging from the early and late detectors becomes unbalanced which
allows the receiver to take the appropriate countermeasures. Remark that this
technique is in fact a very primitive form of the i/q phase tracking loop used
by coherent receiver architectures. The early-late synchronization technique is
also a prime example of how to completely make a mess of things. Of course,
