Digital Signal Processing Reference
In-Depth Information
Narrowband continuous-time modulated radio
phase/amplitude modulation
optimize spectral efficiency
Symbol rate and transition
smoothness determines
spectral footprint.
High information density,
multiple bits/s/Hz.
noise floor
@ receiver
time
frequency
Pulse-based low-PSD wideband radio
phase/PPM modulation
vulnerable to interference
Symbol rate decoupled from
multipath resolvability.
Wide spectral footprint is
determined by pulse shape
instead of symbol period.
T
symbol
noise floor @ receiver
< 1 bit/s/Hz
time
T
pulse
frequency
Figure 5.2.
A pulse-based radio system decouples symbol rate from multipath re-
solvability. However, the wideband character of the transmission makes
it very vulnerable to in-band interference.
approach in single- and multicarrier systems, where the minimum transmission
bandwidth is determined by the symbol rate itself. Moreover, in the latter case,
no stone
7
is left unturned in the pursuit of maximum bandwidth efficiency.
8
By making the pulse period (T
p
) smaller than the interval between two sym-
bols (T
s
), a pulse-based radio decouples the symbol rate from the duration of
the transmitted pulses.
This has significant consequences for the multipath resolvability of the sys-
tem: even if the symbol rate is lower than the coherence bandwidth (B
c
)of
the channel, it is theoretically possible for a receiver to acquire multiple sam-
ples of the channel and resolve different delayed versions from the same pulse
with a time accuracy which is higher than the symbol rate T
s
. Separately re-
solved pulses can afterwards be joined together in order to reduce the variance
on the combined signal-to-noise ratio as seen by the receiver. In a standard
continuous-time modulated system, merely increasing the sample rate of the
7
Stone: conceptual metaphor for sidelobe. Also known as a small rock outside this context.
8
In a continuous time transmission system, the spectral mask of the symbols is commonly restricted without
introducing isi by using a raised cosine rolloff filter.
