IMPACT OF LINEAR POWER AMPLIFIER EFFICIENCY ON CAPACITY OF OFDM SYSTEMS WITH CLIPPING Gill R. Tsouri Rochester Institute of Technology Department of Electrical Engineering grteee@rit.edu Dov Wulich Ben-Gurion University of the Negev Department of Electrical and Computer Engineering dov@ee.bgu.ac.il ABSTRACT Capacity of clipped OFDM systems is analyzed. In contradistinction to previous work, we take into account intrinsic properties of the linear power amplifier efficiency - the dependence of power efficiency on clipping ratio. The following tradeoff emerges: a low clipping ratio causes signal distortion and decreases capacity. However, it also increases the efficiency of the power amplifier, which increases capacity. We resolve this tradeoff by evaluating the optimal clipping ratio for achieving maximal capacity. We show that increasing the clipping ratio doesn’t necessarily result in higher capacity. Instead there exists an optimal clipping ratio for given channel SNR. Increasing the clipping ratio beyond optimal significantly reduces capacity. Index Terms— OFDM, PAPR, Clipping, Capacity 1. INTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) is widely recognized as an attractive modulation method for high-speed communications. A main drawback of OFDM is its high Peak to Average Power Ratio (PAPR). High PAPR requires the use of a Linear Power Amplifier (LPA) with large linear dynamic range in order to avoid signal distortion and spectral spreading. Many methods for reducing the PAPR were suggested in the past [1,2,3], amongst them is clipping using a soft limiter coupled with out of band filtering [4]. Clipping was shown to be a simple and effective approach to PAPR reduction – see [3], [4] and references therein. Rather recently, the capacity of OFDM systems with clipping was analyzed [4]. Such analysis has practical meaning for current OFDM systems, due to advances in coding theory [5] which allow for marginally reaching capacity. In [4] a completely linear LPA with ideal power efficiency (lossless LPA) was considered. Capacity analysis of OFDM with clipping was carried out, accounting for in- band distortion, out of band filtering and peak re-growth. The capacity as function of channel Signal to Noise Ratio (SNR) was depicted for various clipping ratios and over- sampling ratios. It was shown that capacity as a function of the clipping ratio increases monotonically for any SNR and for any over-sampling ratio. In this work we analyze the capacity of OFDM systems with clipping, while taking into account the power loss due to the limited efficiency of practical and commonly used LPAs [6]. The motivation for this analysis stems from recent work on the effects of PAPR reduction coupled with LPA power efficiency considerations on overall system performance [7,8]. In [7] the optimal PAPR for achieving minimal Bit Error Rate (BER) was found while taking LPA power efficiency into account. It was shown that the optimal PAPR is not necessarily the lowest one and that decreasing the PAPR too much results in performance loss. LPA power efficiency was also considered in [8] for capacity analysis of distortion-less PAPR reduction methods, such as selected mapping and tone reservation. We are unaware of past work regarding the impact of practical LPA power efficiency on the capacity of clipped OFDM. An intrinsic property of any LPA is that its power efficiency decreases as its required linear dynamic range increases [6,9]. This translates to a decrease in channel SNR, and hence a decrease in capacity. In a clipped OFDM system, the required dynamic range is determined by the clipping ratio. The following tradeoff with regard to the clipping ratio emerges: a low clipping ratio causes signal distortion and decreases capacity [4]. However, it also increases the power efficiency of the power amplifier [6], which increases capacity. The main contribution of our work 1 is in evaluating the clipping ratio which resolves this tradeoff and the capacity lost incurred when using a sub-optimal clipping ratio. We demonstrate that there is an optimal clipping ratio which achieves maximal system capacity, and that increasing the clipping ratio beyond the optimal ratio reduces capacity. This result is different from the one obtained for an ideal lossless LPA [4], for which increasing the clipping ratio never decreases capacity. We assume a clipped OFDM system with out of band radiation filtering as is defined in the block scheme of [4] with an additional LPA block after the low pass filter. 1 The basis for the work presented in this paper was developed while being at Ben-Gurion University.