Effect of Controlling Parameters of Tone Reservation Based on Null Subcarriers (TRNS) on the Performance of OFDM Systems

Effect of Controlling Parameters of Tone Reservation Based on Null Subcarriers (TRNS) on the Performance of OFDM Systems

Mohamed Mounir (El-Gazeera High Institute (EGI) for Engineering and Technology, Egypt) and Mohamed Bakry El Mashade (Al Azhar University, Cairo, Egypt)
Copyright: © 2020 |Pages: 18
DOI: 10.4018/IJSKD.2020040103
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High data rate communication systems usually implement Orthogonal Frequency Division Multiplexing (OFDM) to face frequency selectivity. High Peak to Average Power Ratio (PAPR) is an OFDM disadvantage that causes Bit Error Rate (BER) degradation and out-of-band (OOB) radiation when OFDM signal pass through nonlinear Power Amplifier (PA). In order to overcome this problem larger Input Back-Off (IBO) is required. However, large IBO decreases the PA efficiency. PAPR reduction techniques are used to reduce the required IBO, so that PA efficiency is saved. Several PAPR reduction methods are introduced in literature, among them Tone Reservation based on Null Subcarriers (TRNS) is downward compatible version of Tone Reservation (TR) with small excess in the average power and low computational complexity compared to others. As will be shown, TRNS is the best practical one of the four downward compatible techniques. Performance of TRNS is controlled by three parameters; number of peak reduction tones (PRTs), predefined threshold (Amax), and number of iterations (Itr). In order to increase PAPR reduction gain, enhance BER performance, and reduce the required IBO to follow the given power spectral density (PSD), we have to choose the values of these parameters adequately. Results showed that, we have to reduce the threshold value to the average (i.e. Amax =0 dB). Also, we have to increase number of PRTs. However, we have to maintain the spectrum shape. Finally, we have to choose moderate number of iterations (e.g. Itr ≈50), as excessive increase in number of iterations is not useful, especially at high PAPR values.
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1. Introduction

Orthogonal frequency division multiplexing (OFDM) is widely used in recent high data rate communication systems. As, OFDM effectively mitigate frequency selectivity. However, OFDM suffers from high PAPR. Unfortunately, high Peak to average power ratio (PAPR) degrades power amplifier (PA) efficiency, or worse degrades BER performance and increases out-of-band (OOB) radiation. In order to preserve PA efficiency, enhance BER performance, and reduces the OOB radiation. PAPR reduction techniques have to be used (Mounir et al., 2017). In literature, many PAPR reduction techniques were presented, These PAPR reduction techniques may be classified into three broad categories; coding techniques, multiple signaling and probabilistic techniques, and Signal distortion techniques (Rahmatallah, & Mohan, 2013; Mahmood, & Mohan, 2019). In coding techniques redundancy are not used for error correction as usual. Instead the codeword that minimize or reduce the PAPR is selected for transmission. Of course, not all codewords result in a bad PAPR (Sandoval et al., 2017). The second category is the Multiple Signal Representation (MSR) techniques. MSR techniques include interleaving (Jayalath, & Tellambura, 2000), partial transmit sequence (PTS) (Jawhar et al., 2019), Selective mapping (SLM) (Zahra et al., 2014), Dummy Sequence Insertion (DSI) (Maivan & Nguyentrong, 2019), tone injection (TI) (Sandoval et al.,2017), and active constellation extension (ACE) (Youssef et al., 2016), in MSR several candidate for the same OFDM symbol are generated and the one with the minimum PAPR is selected for transmission. Signal distortion techniques include Clipping and filtering (Mounir et al., 2017), companding (Mounir & El Mashade, 2019), tone reservation (TR) (Mounir & El Mashade, 2018) peak cancellation (Song, & Ochiai, 2015), and windowing (Chen et al., 2009). Signal distortion techniques reduce the PAPR by clipping the peaks of the OFDM symbol or adding a well-designed peak reduction signal to the OFDM symbol. However, not all of the signal distortion techniques degrade the Bit Error Rate (BER). In literature there are different classifications for PAPR reduction techniques. One of them is the classification according to downward compatibility, in which PAPR reduction techniques is classified to downward compatible and incompatible techniques (Louet, & Palicot, 2008). In downward incompatible techniques (also called non-blind techniques) receiver must be modified to do some complement processes to that done on the transmitter. On the other hand, receivers of downward-compatible techniques (also called blind techniques) have not to be modified (Kozhakhmetov, 2008).

Downward-compatible techniques include clipping and Filtering (CF) (Mounir et al., 2017), Tone Reservation based on Null Subcarriers (TRNS) (Wu et al., 2007), and Active Constellation Extension (ACE) (Ciochina et al., 2006; Youssef et al., 2016), and Partial Transmit Sequence (PTS) (Youssef et al., 2016).

CF is not a practical downward compatible technique as illustrated in (Mounir et al., 2017).

PTS divides OFDM symbol into subblocks, then rotate each subblock with an optimized phase coefficient before collection and transmission. Depending on the used system PTS can works as downward compatible/incompatible technique. PTS can be used as downward compatible technique (i.e. no Side Information (SI) is needed), if it portioned using adjacent Subblock Partitioning Scheme (SPS) with at least one pilot contained in each subblock. Thus, PTS decoding will be done during equalization process, as the phase rotation equivalent to the channel effect. PTS is a highly complex technique (Youssef et al., 2016).

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