Long range (LoRa), a low power and wide area modulation scheme based on a chirp spread spectrum, is the most popular and widely adopted Internet of Things (IoT) technique. In LoRa, acquiring channel state information (CSI) is imperative for improving the system performance, but is a challenging task. In this article, we tackle the problem of preamble design for the CSI acquisition with LoRa modulation in a multiple access scenario with multiple transmitters and a receiver. The main contributions of this article are three-fold. First, we propose two novel and effective preamble transmission strategies: 1) single-chirp preamble transmission and 2) multi-chirp preamble transmission, both employing modulated up chirps. Second, we present how to optimize such preamble signals jointly with transmission power (for the former strategy) and combining coefficients (for the latter strategy) in the sense of minimizing total mean square error (MSE) in estimating the CSI of the whole links between the transmitters and the receiver under practical power constraints, adopting the linear minimum MSE (LMMSE) criterion. In addition, to gain more insights and further lower the design complexity, we investigate several special, yet important, scenarios. Lastly, we present extensive numerical results simulated in realistic propagation environments to substantiate the superiority and effectiveness of the proposed schemes. Also, we reveal new tradeoffs between the proposed single- and multi-chirp preamble transmission strategies in terms of performance and complexity.
