Design of Noncoherent and Coherent Receivers for Chirp Spread Spectrum Systems

Abstract

LoRaWAN is a prominent communication standard to enable reliable low-power, long-range communications for the Internet of Things (IoT). The modulation technique used in LoRaWAN, commonly known as LoRa modulation, is based on the principle of chirp spread spectrum (CSS). The main objective of this article is to design a practical, low-complexity coherent receiver for a recently proposed CSS-based modulation scheme, called phase-shift keying CSS (PSK-CSS), that embeds extra information bits in the starting phases of conventional CSS symbols. To this end, a novel method is proposed to perform coarse timing and frequency synchronization that makes use of a preamble consisting of both up and down chirps and the shape of the pulse shaping and/or receive (matched) filters. Furthermore, to enable fine synchronization, timing, and phase loop filters are designed as simple first-order and second-order phase-locked loop (PLL) circuits with dynamic gain control. A natural outcome of our design is a practical noncoherent receiver that can be used for the conventional LoRa/CSS system and has better performance and/or lower computational complexity as compared to other existing designs. Extensive simulation results are presented to demonstrate the excellent performance and merits of the proposed design. In particular, the bit-error-rate (BER) performance of the higher-rate PSK-CSS system obtained with the proposed coherent receiver is only 0.25 dB worse than that obtained with the ideal co-coherent receiver, whereas it enjoys about 0.75-dB power gain over the noncoherent detection performance of the conventional CSS system. Thanks to the extra information bits carried by the phases of CSS symbols, the PSK-CSS system implemented with quadrature phase-shift keying (QPSK) delivers additional 23.44% and 18.75% data rate as compared to the conventional CSS system for the spreading factors (SFs) of 8 and 10, respectively.