mNB-IoT: Concurrent Uplink Transmission for Multiple User Equipments in NB-IoT

The Narrow Band-Internet of Things (NB-IoT) is a wireless protocol with low-power, low-cost, and wide coverage, which has a broad application prospect in fields like smart city or manufacturing. In order to expand the network coverage, the repetition mechanism is designed in NB-IoT, where the same information is required to be transmitted repeatedly for the accurate decoding of received signals in low signal-to-noise ratio (SNR) environments. However, such repetition mechanism will cause a dramatic decrease in the spectrum utilization, which will be greatly reduced by up to 1/128. Literature has explored efficient coding methods, such as Non-Orthogonal Multiple Access (NOMA), but they can only achieve 55% maximum improvement on spectral efficiency. To further increase the spectrum utilization, this paper propose a novel uplink repetition mechanism design for NB-IoT, namely mNB-IoT. It can enable multiple user equipments (UEs) to transmit data to base stations in parallel using the same subcarriers. Based on the timing synchronization of NB-IoT, the original repetition mechanism is changed to the repetition based on orthogonal sequences to realize concurrent transmission among multiple UEs. The repetition intervals and times are then unified among UEs to maximize spectral efficiency. Experiments built upon 3 USRP devices and simulations on 128 UEs have proven the efficiency of mNB-IoT in realizing the repetition mechanism without spectral efficiency reduction.