Digital Signal Processing Reference
In-Depth Information
modulation method was adopted, but in this case in conjunction with the
COFDM multicarrier method.
Q
I
Fig. 26.13.
Mapping of an “normal” QPSK or a “normal” DQPSK, with state tran-
sitions which also pass trough the zero point
Q
Q
Q
I
I
I
Symbol
i, i+2,i+4,…
/4shift
DQPSK
Symbol
i+1 i+3,i+5,…
Fig. 26.14.
Transition from DQPSK to Π/4-shift DQPSK
Considering now the transition from DQPSK to /4-shift DQPSK (Fig.
26.14.). On the left, the constellation pattern of simple QPSK is shown.
On the right, QPSK rotated by 45 degrees, i.e. by /4, is shown. /4-shift
DQPSK is composed of both. The carrier phase is shifted on by 45 degrees
from symbol to symbol. If only 2 bits per vector transition are to be repre-
sented, the 180-degree phase shifts can be avoided. It can be shown that
phase shifts of +/-45 degrees (+/-/4) and +/-135 degrees (+/-3/4) are suf-
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