Phase Noise Resilient Three-Level Continuous-Phase Modulation for DFT-Spread OFDM

Abstract

In this paper, a novel waveform with low peak-to-average power ratio (PAPR) and high robustness against phase noise (PN) is presented. It follows the discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM) signal model. This scheme, called 3MSK, is inspired by continuous-phase frequency shift keying (CPFSK), but it uses three frequencies in the baseband model – specifically, 0 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm f_{symbol}/4$ </tex-math></inline-formula> , where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{symbol}$ </tex-math></inline-formula> is the symbol rate – which effectively constrains the phase transitions between consecutive symbols to 0 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm \pi /2$ </tex-math></inline-formula> rad. Motivated by the phase controlled model of modulation, different degrees of phase continuity can be achieved, allowing to reduce the out-of-band (OOB) emissions of the transmitted signal, while supporting receiver processing with low complexity. Furthermore, the signal characteristics are improved by generating an initial time-domain constant envelope signal at higher than the symbol rate. This helps to reach smooth phase transitions between 3MSK symbols, while the information is encoded in the phase transitions. Also the possibility of using excess bandwidth is investigated by transmitting additional non-zero frequency bins outside the active frequency bins of the basic DFT-s-OFDM model, which provides the capability to greatly reduce the PAPR. The most critical tradeoffs of the oversampled schemes are that improved PAPR is achieved with the cost of somewhat reduced link performance and, in case of excess band, also the spectrum efficiency is reduced. Due to the fact that the information is encoded in the phase transitions, a receiver model that tracks the phase variations without needing reference signals is developed. To this end, it is shown that this new modulation is well-suited for non-coherent receivers, even under strong phase noise (PN) conditions, thus allowing to reduce the overhead of reference signals. Evaluations of this physical-layer modulation and waveform scheme are performed in terms of transmitter metrics such as PAPR, OOB emissions and achievable output power after the power amplifier (PA), using a practical PA model. Finally, coded radio link evaluations are also provided, demonstrating that 3MSK has a similar bit error rate (BER) performance as that of traditional quadrature phase-shift keying (QPSK), but with significantly lower PAPR, higher achievable output power, and the possibility of using non-coherent receivers.