Low PAPR preamble for improved channel estimation

Orthogonal Frequency Division Modulation (OFDM) is an efficient multi-carrier modulation scheme. It is used in many wireless communication systems due to its robustness towards fading channel behavior and a relative ease of implementation coming from computationally efficient Inverse Fast Fourier Transforms (IFFT) on the transmitter and Fast Fourier Transform (FFT) on the receiver. However, the OFDM waveform has a large Peak-to-Average Power Ratio (PAPR). To avoid clipping and nonlinear distortion, the transmitter's Power Amplifier (PA) needs to be operated at a significantly lower power level than its peak rating (i.e. "backed off"). This paper proposes a novel low PAPR preamble for channel estimation. This solution yields a preamble with PAPR of approximately 2.6 dB, regardless of the number of subcarriers. The subcarriers are evenly spaced and equal amplitude, ideal for channel sounding. The result is a preamble construction that operates over a variable number of subcarriers, improves SNR, and is easy to implement. The theoretical basis for the technique is presented, and a particular implementation of the technique in FPGA hardware is discussed. Both simulations and measurements demonstrate significant benefits of the proposed technique in long-range wireless communications applications that use an OFDM waveform.