## 1 Introduction  
The high peak-to-average power ratio (PAPR) is a typical characteristic of orthogonal frequency division multiplexing (OFDM) signals. Prior to transmission through the channel, OFDM signals are amplified, and to achieve high efficiency, the power amplifier is driven near its saturation region. This, however, causes undesired non-linear amplification effects including signal distortion and out-of-band (OoB) radiation, especially if the signal input to the amplifier has high PAPR. Therefore, it is imperative to tackle the high PAPR problem inherent in OFDM signals. Although the PAPR reduction is not the ultimate goal of the communication system, it provides an instrument for maintaining the bit error rate (BER) while keeping the spectral mask at the transmitter output according to standard requirements. To address the PAPR issue, many classical solutions have been proposed over the years, including partial transmit sequence (PTS) [1], tone reservation (TR) [2], companding [3], selected mapping (SLM) [4], and clipping and filtering (CF) techniques [5, 6, 7, 8, 9, 10, 11]. CF is a simple but powerful approach that does not generally require the use of additional resource blocks or the transmission of side information, which make it of particular interest in the literature. In CF, the time-domain OFDM signal amplitude is limited to a predefined threshold level to keep the resulting PAPR within acceptable ranges. However, clipping the signal amplitude causes OoB power leakage which may not be tolerable by adjacent channels. The signal is therefore filtered to eliminate the OoB emissions. Nonetheless, filtering is invariably coupled with amplitude regrowth of the time-domain OFDM signal and this in turn increases the PAPR beyond the acceptable threshold. Consequently, iterative-based clipping and filtering (ICF) operation is needed to repeatedly clip the signal amplitude and filter the leaked emissions. However, ICF techniques are computationally expensive in general, especially when the number of sub-carriers is large. Simplified clipping and filtering (SCF) was introduced to achieve similar PAPR reduction performance in a single iteration of modified processing at a relatively cheaper cost. However, the required computational complexity is still high even for SCF. To solve this problem, the authors in [12] proposed an alternative SCF-based method which uses asynchronous neural network (ANN) modules at the transmitter side to achieve PAPR reduction performance comparable to existing SCF methods while significantly reducing the required complexity. However, the method in [12] is asynchronous in nature due to the separate training of two independent NN modules using correlated real and imaginary components, respectively. This can potentially cause convergence mismatch between the two NN modules leading, thereby, to one module under or overfitting with respect to the other. This ultimately results in increased OoB radiations and degrading BER performance for higher order modulations. In particular, OoB radiation is very sensitive to non-linear distortion due to peak limiting. The above inaccurate convergence in conventional ANN may result in unacceptable OoB emissions, which needs a novel solution. In this paper, we propose a synchronous neural network CF-based (SNN-CF) approach where the two modules of ANN are replaced by a single NN module which is simultaneously fed by both real and imaginary components to take the effect of their interdependence into account at the training stage. The main contribution of this work can be summarized as follows:  
* As an alternative NN-based PAPR reduction approach, we propose a synchronous neural network solution surpassing conventional CF-based approaches. The proposed method accurately generates CF-like output signals by synchronously learning both real and imaginary parts of raw OFDM input signals and mapping them to the desired target output. Hence, higher PAPR reduction performance and lower OoB power leakage can be achieved compared to conventional ANN solutions.