Information Technology Reference
In-Depth Information
first carried out to quickly approach the peak, and then a fine search accurately locates
the encoded frame. Since the pilot is a narrow band signal composed of a series of
sinusoids, the correlation output d ( m ) oscillates rapidly with m , as shown in Fig.2.
Therefore the search is likely to fail since the possibility of falling into a pit between
two peaks is high. To resolve the problem, magnitude of the correlation “envelope”
may be used instead, which is obtained by taking difference between the maximum
and minimum among several consecutive samples in d ( m ). As soon as it exceeds a
predefined threshold, a fine search is invoked.
1
0.5
0
-0.5
-1
-400
-300
-200
-100
0
100
200
300
400
Fig. 2. Correlation between local pilot and the band-pass filtered audio with embedded data
Lag
The key to an efficient search is an appropriate choice of search step size, deter-
mined by the correlation radius of the pilot obtainable from IDFT of the power spec-
trum density, E 2 ( n ), n = 0, 1,
1. Since the pilot in the frequency domain is
composed of L spectral lines corresponding to an m -sequence, E 2 ( n ) is rectangular
shaped whose width is given by
, N 2
f
S
w
=
L
f
=
L
.
(11)
P
2
N
2
So e ( m ) is a sinc function. Define the half-width of the main lobe as correlation ra-
dius:
2
N
1
2
K
=
=
.
(12)
w
t
L
P
Thus, letting the search step be K , and choosing a threshold greater than, say, twice
the highest sidelobe will ensure a reliable search. To further speed up the search proc-
ess, the pilot is weighted with a Hamming window so that the main lobe is signifi-
cantly broadened.
Having identified the encoded frame, the embedded symbols are recovered:
[
]
M
(
n
)
=
S
(
n
)
W
(
n
)
Q
S
(
n
)
W
(
n
)
.
(13)
M
M
The information needed at the receiver includes the frame length N 2 , the modulation
technique used (here QAM), the mark band allocation, the quantization step, and the
pseudo-random sequence for generating the pilot. These may form part of the key.
Search WWH ::




Custom Search