Digital Signal Processing Reference
In-Depth Information
tx symbol interval
transmitter
receiver
time
Slot positions are
...
slot 1
slot 2
slot 3
slot 4
slot 1
slot 2
fixed. Pulse arrival
times are ignored.
Important: each
receiver sees a
virtual frequency-
selective channel!
slot 1
conventional
stationary slot 1
assigned to module 1
QPSK receiver
pulse-based
extension layer
The larger window
signal
combiner
captures more pulses
slot n-1
in the same receive
slot. This also causes
independent fading
in each of the slots.
QPSK receiver
slot n
QPSK receiver
Figure 5.10.
In the proposed pulse-based receiver, the interval between two
transmitted pulses is divided in a number of fixed receive slots.
Each of the receive slots is assigned to a conventional receiver with
pulse-based extension layer. The fact that pulses are being pro-
cessed can be completely ignored in this approach.
receiver, the duration of the receive window is increased. The aim is to cover
the entire period between two successive received pulses using the joint effort
of all receive modules in parallel. Doing this way a fast single-shot synchro-
nization is always guaranteed (see Figure 5.10). For example, suppose that the
interval between two pulses (T
p
=
500 ps) is T
s
=
10 ns, while the system ac-
commodates 10 independent receive branches. It follows that a minimum win-
dow length of 1 ns is sufficient to cover the entire 10 ns time interval at once.
Of course, increasing the receive window length has some inescapable side ef-
fects. Increasing the window length in a heavy multipath (indoor) channel in-
creases the probability that two or more multipath-delayed versions of the same
pulse pass through the same receive window. This is due to a reduced multi-
path resolvability of a receiver with an increased window length. The effect is
comparable to intra-symbol interference in the classic continuous-time modu-
lated systems, and causes a time-varying alternating pattern of constructive and
destructive interference in the affected branch of the receiver. In the proposed
scheme, the receiver does not show any intention to avoid this from happen-
ing. Both the offset and the duration of the receive window remain untouched.
