Digital Signal Processing Reference
In-Depth Information
IN
ISSR subsystem
equalized QPSK signal
affected bands removed
spectral footprint
c
ross-c
o
rrelatio
n
graph
compar
e
relevant bands
sequence number
locally generated symbol sequences
best correlation result
OUT
Figure 3.2.
The theoretical fundamentals of the issr system are based on the
cross-correlation of the spectral footprints of received data and
locally regenerated symbol sequences.
approach - even for moderate values of
n
- virtually useless for any practi-
cal application. In the next section, a more efficient algorithm to retrieve the
original symbol stream from the corrupted signal will be described. As a final
note, it is worth mentioning that the concept of issr works out quite well
for wideband single-carrier systems, but not for multicarrier systems such as
ofdm. The underlying mechanisms of issr require that information is spread
over a large frequency band, as they rely on the fact that frequency-selective
fading will only wipe out confined parts of the spectrum. Multicarrier systems
such as ofdm concentrate the signal power in smaller subbands. Since each
of these subbands will experience flat fading, the prerequisites for frequency
diversity are no longer fulfilled by the individual subchannels.
