A Unified Random Access Procedure for an NB-IoT Protocol in Nonterrestrial and Terrestrial Networks

For rapid deployment, Internet of Things (IoT) technology operating in nonterrestrial networks (NTNs) usually follows 3GPP specifications. The 3GPP aims to establish unified specifications for global narrowband IoT (NB-IoT) operations. This article investigates a unified random access (RA) process for an NB-IoT protocol seamlessly operating between NTNs and terrestrial networks without global navigation satellite system (GNSS) connectivity. For high accuracy and backward compatibility, the common Doppler shift is employed for pre- and post-compensations at a narrowband physical RA channel (NPRACH) receiver; meanwhile, a common propagation delay is employed for pretime advance (Pre-TA) and RA opportunity (RAO) postponement at the NPRACH receiver on the satellite. Derivations of joint maximum likelihood estimation result in a bank of matched filters (MFs), whose outputs are employed to test for the presence of user equipment (UE). The squared magnitudes of the symbol-level MF (SLMF) outputs are coherently accumulated to improve the signal-to-interference-plus-noise ratio performance of the proposed NPRACH detector, instead of the conventional extension of the length of the MF impulse response. The residual frequency error and excess timing delay are detected at half of the subcarrier spacing and half of the symbol duration, respectively, via a grid search. The false alarm and miss probabilities are derived in the presence of a high Doppler frequency, long differential delay, random initial phase errors, and different channel gains through which the preambles of multiple UEs propagate. Computer simulations confirm the theoretical analyses and show that the proposed technique performs effective RA tasks with low miss probabilities.