Improving Out-of-Band Power Emissions in OFDM Systems using Double-length Symbols

A method that improves the out-of-band power emissions in wireless systems that use orthogonal frequency division multiplexing (OFDM) in their air interface is proposed in this paper. The key idea behind this approach is to obtain the output signal using an inverse discrete Fourier transform (IDFT) calculation with double length, when compared to the single-length IDFT computation that takes place in a conventional OFDM system. Double-length symbols provide implicit continuity in half of the original union points of the whole transmission, and can be used in conjunction with other well-known techniques already proposed in the literature (e.g., spectral precoding). The use of cyclic prefix and suffix strategies is also presented for time-varying channels, enabling the accommodation of the output signal in the same original bandwidth (i.e., without affecting the sampling rate). Obtained results show important reductions in the out-of-band power emissions, which can be even further improved when combined with rate-one spectral precoding techniques. Since the output signal is completely obtained in a digital way, the proposed approach is suitable for future applications that are foreseen in the area of software defined radio (SDR) and cognitive radio (CR).