Graphics Programs Reference
In-Depth Information
L
n
=
0
,
±
L
,
±
2
L
…
,
φ ()
=
(4.50)
1
elsewhere
Fig. 4.19 shows a typical sketch for an MLS autocorrelation function. Clearly
these codes have the advantage that the compression ratio becomes very large
as the period is increased. Additionally, adjacent peaks (grating lobes) become
farther apart.
L
0
L
-L
-1
Figure 4.19. Typical autocorrelation of an MLS code of length L.
Linear Shift Register Generators
There are numerous ways to generate MLS codes. The most common is to
use linear shift registers. When the binary sequence generated using a shift reg-
ister implementation is periodic and has maximal length it is referred to as an
MLS binary sequence with period
L
, where
2
n
L
=
1
(4.51)
n
is the number of stages in the shift register generator.
A linear shift register generator basically consists of a shift register with
modulo-two adders added to it. The adders can be connected to various stages
of the register, as illustrated in Fig. 4.20 for
n
=
4
(i.e.,
L
=
15
). Note that
the shift register initial state cannot be Ðzero.Ñ
Σ
output
1
2
3
4
shift register
Figure 4.20. Circuit for generating an MLS sequence of length
L
=
15
.
Search WWH ::
Custom Search