Pagina 138 - TELE-satellite-1207

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— 06-07-08/2012
in eight possible phase shifts in H-
8PSK instead of four in QPSK - see
Figure 7. In practice, θ should be
something between 7-12°.
3 bits are assigned to every possi-
ble phase shift in a H-8PSK constella-
tion. Except for the 2 bits that create
a 2-bit symbol like in QPSK, there is
one additional bit shown in blue ink
in Figure 7. The HP stream is still
coded like in a QPSK modulation. For
example, if we need to send a sym-
bol 00 we use 45° but we additionally
take into account one bit from the LP
stream and depending on its value,
we either increase or decrease 45°
by θ°. As you can see, for every 2
bits of the HP stream, we send one
bit of the LP stream.
Let’s examine an example in which
our original QPSK stream is enhanced
with LP stream. In our example, we
increased the QPSK shifts by θ° for
the first 2 symbols and decreased it
by θ° for the successive 2 symbols.
The result is shown in Figure 8. Fig-
ure 9 and 10 show the HP and LP bit
streams resulting from demodula-
Should we wanted to present our
H-8PSK example with a series of con-
the signal as in Figure 2 would look
like the picture in Figure 4.
Almost always, the QPSK modula-
tion is explained with a constellation
diagram as the one shown in Figure 5.
The phase shift changes shown in
Figure 3 could be represented as four
vector positions presented in Figure 6.
Now H-8PSK is a modification of
QPSK. Each of the phase shifts de-
fined for QPSK are replaced with two
new values: greater by θ° and less
by θ°. For example instead of 45°, we
get 45°- θ° and 45°- θ°. This results
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Low priority bit stream after H-8PSK
High priority bit stream after H-8PSK
H-8PSK phase shift