Reduction of Non-Orthogonal Interference in OFDM Receivers by Optimized Windowing Patrick Nickel † , Wolfgang Gerstacker † , Christof Jonietz † , Gerd Kilian ‡ , Albert Heuberger ‡ , and Wolfgang Koch † † Institute for Mobile Communications, University of Erlangen–Nuremberg, Cauerstr. 7, 91058 Erlangen, Germany, Email: {nickel, gersta, jonietz, koch}@LNT.de ‡ Communications Department, Fraunhofer IIS, Am Wolfsmantel 33, 91058 Erlangen, Germany, Email: {kil, heb}@iis.fraunhofer.de Abstract— For conventional orthogonal frequency- division multiplexing (OFDM) systems, the guard interval is disregarded in the receiver and rectangular shaped windows are used implicitly due to the discrete Fourier transform (DFT). We consider a Nyquist- shaped window in the receiver, so that the transmit signal does not need to be modified, and optimize the window shape in order to suppress interference from non-orthogonal single carrier interferer signals. For the window coefficients, a closed form solution is derived. This type of interference occurs for example in OFDM radio systems like Digital Radio Mondiale (DRM) that are operating in low frequency bands. (Disturbance is given for instance by harmonics of switched-mode power supplies.) Further for multiple interferers, the receiver with optimized window out- performs the conventional receiver. Index Terms— Multicarrier transmission, OFDM, Nyquist window, Interference. I. INTRODUCTION U SUALLY, orthogonal frequency-division mul- tiplexing (OFDM) systems apply a guard in- terval as pre- and/ or postfix in order to obtain flat fading subcarriers for low complexity equalization. Diversity of the channel is retained by interleaving and joint coding over all subchannels. In the Digital Radio Mondiale (DRM) system [1], multilevel cod- ing (MLC) is deployed. For decoding, e.g. iterative approaches are used [2], that need information on the signal–to–noise ratio (SNR) of each OFDM symbol in order to guarantee high performance of the transmission system. The guard interval is ig- nored in conventional OFDM receivers, as it may be affected by intersymbol interference (ISI), and the discrete Fourier transform (DFT) is directly used for demodulation of the subchannel symbols. As a part of the guard interval may not be affected by ISI due to a shorter impulse response of the channel, this unconsumed part may be used in order to apply a different window than the implicitly (due to the DFT) used rectangular window. Different windowing techniques have been published in [3]–[5]. To retain the orthogonality of the OFDM subcarriers, Nyquist windows have been introduced and optimized in [3]–[5] in order to maximize the subcarrier SNR after demodulation and to mitigate intercarrier inter- ference (ICI) occurring due to improper frequency synchronization of the receiver. For this scenario, a closed form solution for the optimum window coef- ficients is derived in [4], [5]. A polynomial-shaped window is proposed in [6], for which coefficients are determined by numerical optimization. In this paper, non-orthogonal interference from sinusoidal signals with frequencies unequal to the OFDM signal subcarrier frequencies is considered. In our approach, an optimum window in the sense of minimum interference leaking [3] into adjacent OFDM subcarriers is used. The window has to be optimized according to the current interference scenario. As the interference cannot be completely removed by windowing, and since, due to the code properties, it is better to reduce moderate interfer- ence on the less disturbed subcarriers than strong interference on nearly lost subcarriers, with respect to the optimum bit error rate performance, a trade- off between main-lobe width and side-lobe heights of the interference on the subcarriers is determined. An adaptive approach that minimizes the overall interference on the OFDM subcarriers is presented in [7] for digital subscriber line (DSL) systems, which has the tendency to focus mainly on the stronger interfered carriers with the disadvantage, that subcarriers with moderate and low interference may deteriorate. This causes a non-optimal decod- ing performance, as usually on the more interfered carriers not very much gain can be obtained. In contrast to this, our approach defines a region, where interference parts are not included in the optimiza- tion. Other solutions dealing with the sinusoidal interference problem [8], [9] use fixed windows like Hanning, Hamming and raised-cosine windows with different parameter sets. In difference to e.g. [8], no modifications at the transmitter side are needed for our solution. The paper is structured as follows. The system model is introduced in Section II, and the derivation of an optimum Nyquist-shaped window for the in- terference scenario is given in Section III. Finally, performance is presented in terms of simulation results and analyzed in Section IV. II. SYSTEM MODEL We adopt the transmission model of [5]. For the generation of the OFDM signal, a D u -point inverse DFT (IDFT) is applied. A ν denotes the complex am- plitude of the transmit symbol on subcarrier ν , where A ν is taken from the same zero-mean signal set for all subcarriers ν . The subcarrier spacing is given by Δf sub =1/(D u T ), where T denotes the modulation interval. A guard interval 1 of D g = D pre + D post samples in total is added with D pre samples as cyclic 1 A symmetric representation of the guard interval is used for clarification.