21 BPSK modulation transmission block diagram.
As presented in the BPSK modulation block diagram in Fig. 2.21, there is a dimension of the complex
envelope domain that was not being utilized. Sending additional information on the Q-channel
will double the spectral efficiency of this modulation scheme.
2.2.2 Quaternary Phase Shift Keying (QPSK)
QPSK has four allowable phase changes; prior to considering the associated phase state table where
we need to group adjacent bits of information to make a di-bit or a symbol, we would like to present
the QPSK modulator block diagram. In Fig. 2.22, 4 bits are drawn that are converted to 2 symbols.
The symbols are grouped in terms of the filled and unfilled circles and boxes used.
The information signal m(t) enters a serial-to-parallel (S/P) converter which generates a complexvalued
symbol defined as a(t) jb(t). The S/P converter will take the bits corresponding to even time
intervals and place them into a(t), while the bits corresponding to odd time intervals will be placed
into b(t). The time duration of a(t) and b(t) is equal to two bit times, 2Tb, or a symbol time Ts.
If we assume that the information-bearing signal m(t) is a random sequence, then a(t) and b(t) will
also appear random.
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