PAPR Reduction by Constellation Rotation and Pulse Shaping for DFT-Spread OFDM with QPSK Symbols

In this paper, the problem of reducing the peakto-average power ratio (PAPR) of a discrete Fourier transform-spread orthogonal frequency-division multiplexing signal is considered for the transmission of constellation-rotated QPSK symbols. In particular, a joint optimization of the constellation-rotation angle and the pulse-shaping vector is conducted, where the number of allocated subcarriers is larger than the number of QPSK symbols. To avoid the exponential complexity required to evaluate the exact PAPR, a computationally efficient upper bound on the worst-case PAPR is presented and adopted as the objective function. The signal-to-interference-plus-noise ratio (SINR) at the output of a constellation-derotating matched filter is also presented. Then, joint optimizations are conducted through numerical search for various target SINRs. Surprisingly, the optimal rotation angle is close to but not exactly equal to pi/4. Numerical results show that the proposed technique is not only applicable to QPSK symbols but also to QAM symbols to significantly reduce the PAPR.