Digital Signal Processing Reference
In-Depth Information
performance, cost and power consumption: the duration of the locking pro-
cess depends on the number of possible window positions in the time frame
between two consecutive pulses, and the number of positions that the receiver
can observe in parallel. In practice, the number of parallel receive paths is kept
very limited, due to the considerable cost in terms of silicon area. So the only
parameter left to play with is the duration of the window: increasing the du-
ration of the on-state of the receive window reduces the number of window
locations and reduces the synchronization time overhead, but it also causes a
proportional increase of the noise and interferer power that is captured by the
receiver. A compromise would be to start the synchronization process with an
increased window length. After the receiver has acquired lock on one of the
multipath streams of the transmitter, the signal-to-noise ratio can be improved
by a stepwise decrease of the window length until the optimal result is achieved
(Figure 5.4). Of course, this implies that the receiver must have disposal over
at least two parallel receiver units, since the continuous operation of the link
has to be guaranteed during the quest for better signal quality. A more detailed
analysis of this topic is covered later in this section.
Finding the location of the pulses is one thing, but after the receiver has ac-
quired a successful lock, it should be able to track the accumulating time error
A wrongly aligned receive window results in complete signal loss.
Q
I
time
Increasing the window length reduces the synchronization time overhead.
Q
I
time
After successful synchronization, the window size is reduced to reject noise and interference.
Q
I
time
Figure 5.4.
The receive window in a pulse-based radio is the equivalent to tun-
ing the band-select filter in a narrowband architecture. Note that
this figure is only a simplified representation of reality, since the
effect of multipath reflections is omitted in this example.
