Improving LoRaWAN downlink performance in the EU868 spectrum

Abstract

LoRaWAN is one of the most widely used Internet of Things protocols. It is developed driven by the easiness of the deployment and the support of any type of application. However, LoRaWAN has been made mostly for uplink transmissions rather than downlink traffic. As it has been shown by many studies in the literature, it suffers from very poor performance even with moderate downlink traffic. This is mainly due to the radio duty cycle restrictions applied on gateways but also due to the half-duplex nature of LoRa transceivers. To mitigate the problem of the reduced downlink performance, this paper proposes different channel, band, and downlink window schemes taking into account the recently announced Band 47b in the EU868 spectrum which adds four extra downlink channels with a total radio duty cycle of 10%. The main issue that is tackled is how the additional downlink time can be used effectively in the new schemes. The advantages and disadvantages of each scheme are discussed and ranked based on the easiness of their integration into the native LoRaWAN and the number of modifications they require. Extensive simulation results are presented and are compared to the baseline. The results reveal that schemes which require more changes to the protocol exhibit higher performance gains. More specifically, if a 10% duty cycle channel is applied in the first receive window, a higher than 200% performance gain in terms of packet delivery ratio and energy consumption can be achieved.

Introduction

LoRaWAN is an open standard developed by the LoRa Alliance which provides LoRa-enabled IoT devices with the necessary MAC and link layer mechanisms. LoRa is a proprietary spread spectrum modulation which can achieve long ranges (over 10 km with line of sight) and presents remarkable resistance to interference and Doppler effects. LoRa and LoRaWAN are used in a wide range of applications such as the asset tracking, the monitoring of soil, air pollution, water quality etc. [1], [2].

LoRaWAN creates a star of stars topology usually consisting of several end-nodes and one or more gateways which are connected to a group of servers through a cloud-based system. The Network server, the Join server, and the Application server, are responsible for a number of operations in the network such as the encryption key management, the transmission of the acknowledgments, and the interconnection with the user applications. The gateways forward uplink/downlink data from/to the end-devices to/from the servers. The communication between the gateway(s) and the end-nodes is done through LoRaWAN while the communication between the gateway(s) and the servers is done through non-LoRaWAN protocols (see Fig. 1).

LoRaWAN distinguishes three modes of operation for the end-devices; Class A, B, and C. In Class A, the end-devices are considered as energy constrained devices which need to be on only when they are transmitting or receiving data (even if no data is actually transmitted by a gateway). The MAC layer of Class A is Aloha-based, which means that transmissions are performed without employing any collision avoidance mechanism. Downlink transmissions (i.e., acknowledgments and commands) are sent using two receive windows, one and two seconds (default values) after the end of the uplink transmission. More details about the downlink mechanism of LoRaWAN are given in the next section. In Class B, synchronization beacons are employed by the gateways to allow a number of end-devices to open more receive windows at specified times. Finally, in Class C, the end-devices have constantly their radio on, thus, it is assumed they have unlimited power resources. The last two classes of devices are not often used and most commercial devices implement only the first mode of operation.

Due to the Aloha-based MAC of LoRaWAN, many collisions may occur which leads to a high number of retransmissions and channel saturation [3]. Downlink traffic worsens the performance because, first, it increases the amount of traffic and, second, it blocks uplink transmissions for the duration of the downlink since LoRa transceivers are half-duplex. As it is stated by many studies [4], LoRaWAN is mainly made for unconfirmed traffic. Nevertheless, there are applications where the acknowledge of the reception is vital for the nature of the measurements (e.g., healthcare applications). Apart from that, LoRa networks operating in the sub-GHz spectrum have to obey radio duty cycle regulations which limit the total transmission time for both end-devices and gateways. LoRaWAN relies on the deployment of additional gateways to solve the problem of limited downlink time, however, this increases the cost of the deployment.

In this work, the limited downlink capabilities of LoRaWAN are discussed and a number of radio channels, bands, and receive window arrangement schemes are proposed taking into account the newly introduced band for access points in the EU868 spectrum¹. This new band, which can also be used by RFID devices with 2 W transmit power, provides 4 additional channels in the EU868 spectrum with a total duty cycle of 10% for access points (gateways) [5]. Adaptive power control is also required which can be easily achieved in LoRaWAN through the adaptive data rate mechanism. Even though the new band was introduced in mid 2017, the present work is so far the only study which explores its potential in enhancing LoRaWAN’s downlink performance. This paper describes how the new available channels can be used in LoRaWAN proposing a number of channel, band, and window arrangement schemes. The proposed schemes are ranked in ascending order starting from the scheme which requires the least amount of changes in the protocol. Extensive simulation results are presented using variable number of nodes, number of gateways, node placement distributions, packet retransmission attempts, and percentages of confirmed traffic.

The contributions of the paper are summarized as follows:

• An analysis of the current downlink status of LoRaWAN in the EU868 spectrum is given emphasizing at the limitations.

• The potential of the new recently introduced band for downlink is discussed.

• Four new frequency and window arrangement plan schemes to take advantage of the available spectrum that the newly introduced band offers are proposed.

• A discussion of the advantages and disadvantages of the proposed schemes is made focusing on their “implementability” and ranking them based on the easiness of their integration in the native LoRaWAN.

The rest of the paper is organized as follows: Section 2 gives the essential technical background around LoRa and LoRaWAN, and presents the related work. Section 3, discusses the limitations of the current downlink status of the protocol while Section 4 presents the characteristics of the new band in detail. Section 5 discusses the proposed channel/band and receive window schemes stressing their advantages and disadvantages. The evaluation setup and the results are presented in Section 6, while special evaluation cases are presented in the Appendix A Gaussian node placement, Appendix B Less retransmissions. Finally, conclusions and ideas for future work are presented in Section 7.