Résumé
Among the design aspects that limit the network capacity of low power wide area networks (LPWAN), the adoption of an appropriate random access (RA) medium access control (MAC) protocol plays a fundamental role. Recently, ultra narrow band (UNB) networks, which operate over Time- and Frequency-Asynchronous ALOHA (TFAA), have emerged as a novel alternative to spread spectrum communications for LPWAN applications. While the UNB TFAA approach fulfills with the primary requirements of LPWANs, namely low power consumption, low terminal cost and long range communication capabilities, its throughput and packet error rate (PER) performance is rather poor. In this paper, we propose Contention Resolution Time- and Frequency-Asynchronous ALOHA (CR-TFAA), an enhancement of TFAA which is in line with recent advances on asynchronous RA schemes, such as Asynchronous Contention Resolution ALOHA (ACRDA). Numerical results show that the proposed technique provides a remarkable throughput-PER performance. Moreover, compared to ACRDA under similar conditions, CR-TFAA provides better PER performance over practical channel loads.