NB-IoT Physical Random Access Channels (NPRACHs) With Intercarrier Interference (ICI) Reduction

This article proposes a novel technique to effectively detect superimposed preambles transmitted via narrowband Internet of Things (NB-IoT) physical random access channels (NPRACHs). Because the quasi-stationarity of these channels conserves local phase coherence over a few symbols, a symbol-level matched filter (SLMF) is used to effectively reduce out-band noise and interference without excessively accumulating phase increments occurring with the cross-correlation operations. The squared norms of the SLMF outputs are accumulated to test for the presence of a user equipment (UE). The false alarm and miss probabilities in the presence of frequency selectivity, time selectivity, and different channel gains through which the preambles of multiple UEs propagate are rigorously derived. To reduce intercarrier interference (ICI), which is mainly caused by different carrier frequency offsets (CFOs), a forward error reduction method based on interference regeneration and cancellation (IRnC) is studied. The IRnC method effectively reduces the miss probability but is not able to reduce the false alarm probability because the ICI cannot be perfectly regenerated and completely canceled. In addition, a closed-loop CFO compensation method is proposed to effectively maintain low false alarm and miss probabilities by sending quantized normalized CFO (NCFO) estimates back to the corresponding UEs via random access response (RAR) and reducing the CFO accordingly at the transmission end. Computer simulations confirm the theoretical analyses and show that the proposed technique achieves low false alarm and miss probabilities.