Narrowband Interference Mitigation in Turbo-Coded OFDM Systems

A method for the mitigation of the effect of narrowband interference (NBI) on the turbo decoder in OFDM systems is proposed. The presence of NBI leads to a contaminated Gaussian (CG) noise probability density function (pdf) which induces an outlier effect in the data detection problem. The outlier effect leads to significant degradation in the performance of turbo coded OFDM systems which use Gaussian noise pdf based log likelihood ratios (LLRs), with the degradation increasing as a function of the power of NBI and the number of subcarriers affected by NBI. We propose to use outlier detection theory to detect subcarriers affected by NBI, and then downweigh the corresponding LLRs before passing them to the turbo decoder. Extreme value theory (EVT) is used to define the weight function in this weighted-LLR (W-LLR) method. The method is easy to implement, is of modest computational complexity, and shows a significant improvement in the simulated error rate performance when compared with the simple unweighted turbo decoder in the presence of NBI. Furthermore, frequency selectivity and diversity mapping in OFDM systems such as IEEE 802.16 d/e WMAN standard causes the co-channel interference (CCI) to look like NBI within the FEC block. Therefore, the W-LLR method can also be applied for CCI mitigation in these systems. Since reuse-one cellular systems have a CCI limited performance, the proposed method provides a significant improvement over the normal turbo decoder.