Waveform design for 5G telecommunication systems

Abstract

본 논문은 차세대 이동통신 시스템을 위한 DFT-spread OFDM 신호 디자인 문제를 다룬다. 먼저 성상회전이 포함된 BPSK 심볼을 DFT-spread OFDM 시스템으로 전송할 경우에 최대전력 대 평균전력 비를 최소화하는 신호 최적화 문제를 다룬다. 최대전력 대 평균전력 비의 이론적인 상한 값을 유도하여 계산 복잡도를 현저히 낮추어 기존 알고리으로 수치최적화를 가능하도록 만들었다. 또한 수신 성능 확보를 위해 신호 대 잡음 간섭 비를 유도하여 제약조건으로 사용하였다. 제안하는 방법으로 최적화된 파형들은 최대전력 대 평균전력 비 성능 뿐만 아니라 비선형 증폭기를 통과한 후의 신호 품질 성능에서도 기존의 파형들보다 보다 높은 성능을 보였다. 마지막 장에서는 BPSK의 결과를 확장하여 QPSK 심볼 및 높은 차수의 QAM 심볼을 전송할 경우에도 적용하였다. QPSK 심볼을 전송할 경우의 최대전력 대 평균전력 비의 이론적인 상한값을 유도 하였으며 또한 신호 대 잡음 간섭 비 역시 QPSK 심볼에 맞게 확장하여 제약조건에 도입하였다. 이 경우 역시 제안한 최적화된 파형값들이 기존의 파형들보다 보다 낮은 최대전력 대 평균전력 비를 갖음을 확인 할 수 있다.

In this thesis, we discuss the Discrete Fourier transform (DFT)-spread orthogonal frequency-division multiplexing (OFDM) signal design for next generation mobile communication system. Specifically, we propose a joint optimization technique of constellation-rotation and pulse-shaping that minimizes peak-to-average power ratio (PAPR) for DFT-spread OFDM system.

DFT-spread OFDM system has already been used in 4G Long-Term Evolution (LTE) uplink system, and it is decided to use it in 5G New Radio (NR) uplink system also due to its low PAPR characteristic. In addition, 5G NR supports constellation rotated pi/2-BPSK symbols that was not supported in 4G LTE. The pi/2-BPSK modulation, as a special constellation-rotated BPSK modulation, exhibits a much smaller PAPR of the transmitted signal than the QPSK and higher-order modulations do. However, constellation rotations other than pi/2 and joint optimization of constellation rotation and pulse shaping have not been investigated.

To further reduce the PAPR, we consider the optimization problem of reducing the PAPR of a DFT-spread OFDM signal when constellation-rotated BPSK symbols are transmitted. In particular, a joint optimization of the constellation-rotation angle and the pulse-shaping vector is conducted with or without the allocation of additional sub-carriers. To avoid the exponential complexity required to evaluate the PAPR, a computationally efficient upper bound on the worst-case PAPR is derived and adopted as the objective function. Unlike conventional upper bounds, the proposed upper bound exploits the structure in the relative phase among symbol waveforms and incorporates the constellation-rotation effect. The signal-to-interference-plus-noise ratio (SINR) at the output of a constellation-derotating matched filter is also derived and adopted as the inequality constraint function. Then, joint optimizations are conducted through numerical search for various signal parameters and target SINRs. Surprisingly, the optimal rotation angle is neither 0 nor pi/2. It is shown that the proposed combinations significantly outperform the existing combinations of the rotation angle and the shaping vector when the signal is amplified by a nonlinear power amplifier.

And then we consider the problem of reducing the PAPR of a DFT-spread OFDM when QPSK symbols or higher-order QAM symbols are transmitted. Similar to previous chapter, the upper bound which reduces the computational complexity and the received SINR for the transmission of QPSK symbols are derived. Then, joint optimizations are conducted through numerical search for various signal parameters and target SINRs. Surprisingly, the optimal rotation angle is neither 0 nor 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.