Packet-Level Index Modulation Under Duty Cycle Constraints

Low power wide area (LPWA) is an important technology in the field of the Internet-of-Things (IoT) because of its low cost and long-distance transmission capability. In LPWA, including LoRa, which is one of its standards, the transmission time per channel is limited by the duty cycle (DC) at each terminal. Thus, packet-level index modulation (PLIM) has been proposed to increase the number of bits. PLIM can transmit additional data by mapping information bits on the time and frequency axes in addition to LoRa transmission. However, if the transmit bit sequence is not i.i.d, a transmitter may continue to choose particular frequency channels, making the transmission on those frequency channels impossible due to DC constraints. To avoid the intensive use of certain frequency channels in PLIM, this paper proposes to adaptively change indexes on the frequency axis. Each LoRa terminal decides the index exchange at regular intervals. A LoRa terminal predicts its transmission occupancy time on each frequency channel based on the transmission history up to the decision period. If the difference between the predicted maximum and minimum transmission time on frequency channels exceeds a predetermined threshold value, the LoRa terminal exchange indexes between the two frequency channels. The simulation results show that the proposed method can avoid the intensive use of specific channels and reduce the untransmittable packets due to DC constraints.