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Estimating and Concurrent Suppressing OFDM Major Impairments: Residual CFO and High PAPR

Two major impairments of the Orthogonal Frequency- Division Multiplexing (OFDM) are: the shift between the carrier frequencies at the transmitter and at the receiver, which is commonly referred to as Carrier Frequency Offset (CFO), and the (high) Peak-to-Average Power Ratio (PAPR) that is inherent to the transmitted OFDM signal [1]. However, although both have been subject to extensive investigations in the last two decades, specifically with wireless networks such as the Long-Term Evolution (LTE) in particular, significant consequent irreducible performance degradation has remained in focus, such as e.g. the Inter-Carrier Interference (ICI). Specifically, although utmost care is given to CFO estimation and compensation in OFDM wireless systems, still some residual CFO remains and can degrade the OFDM transmission performance. Furthermore, high OFDM PAPR implies large peaks of the signal that is therefore not appropriate to pass through the nonlinear High-Power Amplifier (HPA) at the transmitter operating close to the saturation region, and so introducing in-band distortion and adjacent channel interference. With this regard, among several methods for PAPR reduction in OFDM systems, clipping the OFDM signal prior to entering the HPA is quite simple to implement, but it is a highly nonlinear operation, and so introduces in-band and out-band interference, as well as degrades the subcarriers’ orthogonality. Moreover, so far the CFO and the PAPR have been treated independently, but recently the Error Vector Magnitude (EVM) based analytical model for the CFO-induced constellation symbol phase distortion, is developed [2-6], which essentially reveals that the maximal CFO-caused squared phase deviation is linear with the instantaneous (per-OFDM-symbol) PAPR. This implies that any PAPR reduction technique, such as e.g. clipping or coding, indirectly suppresses the CFO- induced phase deviation, too. The analytically achieved results and conclusions are tested and successfully verified by conducted Monte-Carlo simulations.

OFDM ; residual CFO ; PAPR suppression

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