INTRODUCTION
ORTHOGONAL Frequency Division Multiplexing (OFDM) has been widely accepted and is being considered for future high data rate wireless access. Despite its popularity, OFDM does have its own peculiar problems,of which high PAPR happens to be one. In order to reduce PAPR of OFDM signals, two schools of method have been suggested. A good account of these techniques is given in and references cited therein. One group intends to reduce the occurrence of large peaks before multicarrier modulation by breaking the independence among signals on different subchannels. The methods include coding,selective mapping and partial transmitting . However, in these methods, either redundancy or computational complexity is relatively high, which may result in large delay or overhead in practical systems. The second group processes the OFDM signals directly. Deliberate clipping before D/A conversion is seen as the simplest way to achieve low PAPR. Nevertheless, digital clipping suffers from three problems: in-band distortion (IBR), which causes significant performance penalty; out-of-band radiation (OBR), which reduces the spectral efficiency; and peak regrowth after D/A conversion. Recently, companding transforms have been proposed by several authors to reduce PAPR .
It has been claimed that the companding transforms outshine the performance of the clipping by a fair margin. However, in these works, the impact of filtering out OBR has not been considered in detail. In this letter, we explore the impact of filtering out OBR on the performance of companded OFDM systems. Using computer simulations, we show that filtering does deteriorate
the performance significantly. We outline the reasons for this performance degradation and further propose a method to compensate for the degradation. This method is based on the use of curve fitting method to find out a suitable polynomial to be used for decompanding at the receiver. As a result, the companding and decompanding transforms are no longer exact inverse of each other. To the best knowledge of the authors, this non-symmetric companding and decompanding method has not been considered in the literature yet. We will show through simulations that this method indeed improves the performance in comparison to existing symmetric methods when filtering is necessary for bandlimited conditions.
ORTHOGONAL Frequency Division Multiplexing (OFDM) has been widely accepted and is being considered for future high data rate wireless access. Despite its popularity, OFDM does have its own peculiar problems,of which high PAPR happens to be one. In order to reduce PAPR of OFDM signals, two schools of method have been suggested. A good account of these techniques is given in and references cited therein. One group intends to reduce the occurrence of large peaks before multicarrier modulation by breaking the independence among signals on different subchannels. The methods include coding,selective mapping and partial transmitting . However, in these methods, either redundancy or computational complexity is relatively high, which may result in large delay or overhead in practical systems. The second group processes the OFDM signals directly. Deliberate clipping before D/A conversion is seen as the simplest way to achieve low PAPR. Nevertheless, digital clipping suffers from three problems: in-band distortion (IBR), which causes significant performance penalty; out-of-band radiation (OBR), which reduces the spectral efficiency; and peak regrowth after D/A conversion. Recently, companding transforms have been proposed by several authors to reduce PAPR .
It has been claimed that the companding transforms outshine the performance of the clipping by a fair margin. However, in these works, the impact of filtering out OBR has not been considered in detail. In this letter, we explore the impact of filtering out OBR on the performance of companded OFDM systems. Using computer simulations, we show that filtering does deteriorate
the performance significantly. We outline the reasons for this performance degradation and further propose a method to compensate for the degradation. This method is based on the use of curve fitting method to find out a suitable polynomial to be used for decompanding at the receiver. As a result, the companding and decompanding transforms are no longer exact inverse of each other. To the best knowledge of the authors, this non-symmetric companding and decompanding method has not been considered in the literature yet. We will show through simulations that this method indeed improves the performance in comparison to existing symmetric methods when filtering is necessary for bandlimited conditions.
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