Practical BER-Based Estimation of Residual OFDM CFO by Reducing Noise Margin

Abstract

The main OFDM drawbacks are Carrier Frequency Offset (CFO) and large Peak-to-Average Power Ratio (PAPR), which both degrade the Bit Error Rate (BER). Specifically, we consider here clipping or any other PAPR reduction method sufficient to prevent the nonlinear high-power amplifier from generating errors. Moreover, in small cells, the signal-to-noise ratio is large, while the small time dispersion allows the OFDM symbol cyclic prefix to prevent intersymbol interference. This retains the CFO to solely determine the BER and vice versa, enabling indirect estimation of CFO-induced phase distortion by simple BER testing. However, a particular problem is measuring very low BER values (generated by alike residual CFO), which could last a long time in order to acquire statistically enough errors. The test time can be drastically reduced if the noise margin is reduced in a controllable way, by adding the interfering signal to each subcarrier at the receiver. This approach is shown to enable efficient and accurate short-term BER (and so CFO phase error) testing.

Closed-Form Enhanced Detection of Clipped OFDM Symbol

“…We implemented the common OFDM transmission architecture presented in Figure 1, by means of MATLAB simulation tools, where we adopted 256 subcarriers with 16 QAM modulation, and, specifically, the mean transmitter power of 0 dBm (with no loss of generality, though the mobile station and the base station power means of less than 1% out of 23 dBm or 42 dBm maximal output power, respectively, can be expected e.g. in 4G/5G networks [4]).…”

“…With this regard, CFO hinders the carrier frequency synchronization between transmitter and receiver, which degrades mutual orthogonality of OFDM subcarriers, and thus causes inter-carrier interference (ICI) [2][3][4][5]. On the other hand, when the overly high PAPR of the OFDM signal surpasses the dynamic range of the transmitter output high-power output amplifier (HPA), the latter is thus forced into saturation operating regime which causes clipping of peak signal values [6][7][8][9][10][11][12].…”

“…Various techniques have been proposed for CFO and high-PAPR mitigation [2][3][4][5][6][7][8][9][10][11][12][13], which, for the latter, range from HPA linearization by inserting a digital predistorter (DPD) whose expansion characteristics is inverse to the compression one of the HPA [10], through neural network based approach [13], to peak amplitude clipping of the OFDM signal to be transmitted, as a simple and frequently applied method for PAPR reduction [9], [11].…”