Digital communications
Figure 10:
π
/4 DQPSK uses differential encoding and a rotating constellation.
Like DQPSK,
π
/4 DQPSK employs differential encoding, which means
information is encoded in the change in phase, rather than the phase itself.
However, the constellation for
π
/4 DQPSK rotates by 45° or
π
/4 radians after each
symbol transmission. See Figure 56. Unprimed constellation points may only
transition to primed constellation points and vice versa. The allowed transitions
are indicated in the
fi
gure. Notice that none of the transitions pass through the
origin, thus, solving the problem.
The 3
π
/8 8 PSK constellation is similar in design to the
π
/4 DQPSK. In this
case, data is not differentially encoded, but the constellation rotates to prevent
transitions through the origin. In this case, the basic constellation is that of 8 PSK,
except that the constellation rotates by 67.5° or 3
π
/8 radians after each symbol
transmission. A version of this constellation with a Gray code mapping is used in
the GSM EDGE mobile communication protocol. (See Figure 11.)
Figure 11: 3
π
/8 8 PSK follows standard 8 PSK, but the constellation rotates by
3
π
/8 after each symbol
Due to the rotation of the constellation, unprimed constellation points may only
transition to primed constellation points and vice versa. The allowed transitions
are indicated in the
fi
gure. Notice that none of the transitions pass through the
origin, again solving the problem. The exclusion from the origin is smaller than
for the
π
/4 DQPSK constellation, however. This constellation, therefore, places
more stringent demands on the linearity of the transmitter.
54
TSG4100A Series RF Signal Generators User Manual
