Abstract:
For PAR reduction in OFDM systems, the clippingbased Active Constellation Extension (ACE) technique is simple and attractive for practical implementation. However, we observe it cannot achieve the minimum PAR when the target clipping level is set below an initially unknown optimum value. To overcome this low clipping ratio problem, we propose a novel ACE algorithm with adaptive clipping control. Simulation results demonstrate that our proposed algorithm can reach the minimum PAR for severely low clipping ratios. In addition, we present the tradeoff between PAR and the loss in πΈπ/ππ over an AWGN channel in terms of the clipping ratio.
Among various peak-to-average ratio (PAR) reduction techniques, the active constellation extension (ACE) technique is attractive for use in the down-link. The reason is that ACE allows the reduction of high-peak signals by extending some modulation constellation points toward the outside of the constellation without any loss of data rate. This advantage, however, comes at the cost of a slight power penalty. For practical implementation, low complexity ACE algorithms based on clipping were proposed in [1], [2]. The basic principle of clipping-based ACE (CB-ACE) algorithms involves switching between the time domain and the frequency domain [3]. Filtering and applying the ACE constraint in the frequency domain, after clipping in the time domain, both require iterative processing to suppress the subsequent regrowth of the peak power. The CB-ACE algorithms provide a suboptimal solution to the given clipping ratio, because the clipping ratio is predetermined at the initial stage. However, the CB-ACE algorithms have a low clipping ratio problem in that they can not achieve the minimum PAR when the target clipping level is set below an initially unknown optimum value. Moreover, it is difficult to determine the optimal clipping level at the initial stage, because many factors, such as the initial PAR and signal constellation, have an impact on the optimal clipping level determination [1]. To the best of our knowledge, a practical CB-ACE cannot predetermine the optimal target clipping level. In this paper, to solve the low clipping ratio problem of CBACE, we introduce a new method of ACE for PAR reduction. Our approach combines a clipping-based algorithm with an adaptive clipping control, which allows us to find the optimal clipping level. Simulation results show that our proposed algorithm can achieve the minimum PAR regardless of the low target clipping level. We also discuss the tradeoff relationship between PAR and BER over an AWGN channel in terms of the target clipping ratio.