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The paper actually deals with $\pi/4$-QPSK, not with plain DQPSK. In $\pi/4$-QPSK, the symbols are always rotated by a minimum of $\pm \pi/4$. Rappaport's Wireless Communications textbook has this table: \begin{align*} 11 \rightarrow & \, \pi/4 \\ 01 \rightarrow & \, 3\pi/4 \\ 00 \rightarrow & \, -3\pi/4 \\ 10 \rightarrow & \, -\pi/4 \end{...


The diagram from the textbook in the opening post appears to have some mistakes. For this IFFT method, the box that says 'QAM modulator' should just be a complex number generator (such as a digital QAM generator that produces a complex number from 'x' binary bits at a time - that is meant to represent a vector on a QAM grid). And there should be two D/A ...

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