Digital Signal Processing Reference
In-Depth Information
slot 1
slot 2
slot 1
slot 2
1
1
slot 1
slot 2
slot 1
slot 2
2
2
slot 1
slot 2
slot 1
slot 2
3
3
time
Rectangular receive slots.
Smoothened, overlapping receive slots.
1
Q
2
Q
3
Q
1
Q
2
Q
3
Q
I
I
I
I
I
I
Figure 6.2.
To cope with the transient effect of a multipath pulse stream shifting
out of a receive slot as a result of clock offset, a smoothened receive
window is used. This gives the phase tracking algorithm more time to
compensate for the rotating constellation or to reallocate its resources
to another slot.
This effect is in fact desirable, since it smooths out the transient effects of a
pulse stream that slowly shifts out of the scope of a receive slot that is assigned
to some particular receive unit. This can be understood bearing in mind that the
baseband signal is formed by the combination of several pulses that fall within
the scope of a single receive slot. The compound signal is formed by the vector
sum of all these individual components, each with their own magnitude and
phase delay. If for one reason or another (either due to changes in the length
of the propagation path or due to clock offset), a strong multipath component
shifts out of the receive slot, this will result in a change of both magnitude and
rotation of the baseband i/q constellation plane in that slot. In a receiver with
a rectangular window shape the transition will be very abrupt, without giving
the digital back-end sufficient time to react on this. For a window with smooth
edges, the strong signal component will shift gradually out-of-scope, so the
system has the opportunity to anticipate on this and correct for the varying
rotation in the constellation of the symbol stream. To prevent the creation of
blind spots
in the search space of the slotted receiver, it is strongly advised to
provide for some overlap between the positions of two neighbouring receive
slots. This should also be taken into account by the back-end controller of the
system, though, since the overlap invalidates the assumption of independent
fading between receive slots. The system controller may avoid that overlapping
slots become activated at the same time by different receive units.
