ScienTECH ST2115, Руководство пользователя

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ST2115
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Since c
2
( t ) = 1 for all t. The resulting signal A
c
u ( t ) cos 2 pf
c
t occupies a bandwidth
(approximately) of R Hz, which is the bandwidth of the information-bearing signal.
Therefore, the demodulator for the dispread signal is simply the conventional cross
correlator or matched filter. Since the demodulator has a bandwidth that is identical to
the bandwidth of the dispread signal, the only additive noise that corrupts the signal at
the demodulator is the noise that falls within the information-bandwidth of the
received signal.
PN Sequences and Generators
Definition: A Pseudo-random Noise (PN) sequence is a sequence of binary numbers,
e.g. ±1, which appears to be random; but is in fact perfectly deterministic. The
sequence appears to be random in the sense that the binary values and groups or runs
of the same binary value occur in the sequence in the same proportion they would if
the sequence were being generated based on a fair "coin tossing" experiment. In the
experiment, each head could result in one binary value and a tail the other value. The
PN sequence appears to have been generated from such an experiment. A software or
hardware device designed to produce a PN sequence is called a PN generator.
A pseudorandom number sequence generator comprises a bidirectional shift register
arranged to be loaded with a multi-bit sequence. The shift register is responsive to an
i
th
clock pulse and an i
th
direction control bit for shifting the multi-bit sequence in one
of two directions, delivering an i
th
output bit and receiving an i
th
input bit. The multi-
bit sequence successively defines one of nodes of an Eulerian graph connected by
branches. A feedback circuit is connected to the shift register for converting a set of
input data to a set of output data. The input data comprises a multi-bit sequence stored
in the shift register in response to an (i+1)th clock pulse, the i
th
output bit and the i
th
direction control bit and the output data comprises an (i+1)th input bit and an (i+1)th
direction control bit. The output data is supplied to the shift register so that multi-bit
sequences produced by the shift register move around the graph following every
branch exactly once, and outputting the (i+1)th input bit to form a pseudorandom
number sequence.
Example:
PN generator is typically made of N cascaded flip-flop circuits and a specially
selected feedback arrangement as shown below.
PN Sequence Generator Figure 9
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