Frequently Asked Questions
If gateways are using GPS for precision timing, do you have any guidelines for the GPS installation?
There is a lot of data to confirm that GPS timing performance is significantly affected by (1) the constellation of satellites in view, (2) interference or multipath. The recommendation is that the GPS antenna location is carefully considered to give the maximum ‘view’ of the open sky and if possible to reduce the effects of signal reflections from other sources. Semtech can provide a short study on the effects of GPS antennas and chipset configuration on the timing accuracy of the recovered 1pps signal. For deployment, the main influence is the antenna installation.
Results show that location uncertainty reduces with the number of packet repetitions, but the effect diminishes after 6 to 8 packets, why is that? Would increasing the number of channels help?
Evidence suggests that increasing the number of channels would make a positive impact on accuracy since it would increase frequency diversity. Any kind of diversity aids the algorithms in the solver to make better choices and weight the data presented in a more effective manner. Until now, the field data has only been collected for 8 channel European LoRaWAN deployment. Once more data is available this assumption can be validated and more detail added to this.
Results show that location uncertainty reduces with the number of packet repetitions, but the effect diminishes after 6 to 8 packets. Why is that, if I need better precision can I just send more packets?
To some extent the answer is yes, however, that is not the whole story. The main benefit of packet repetition is obtained due to the fact that the different packets are sent on different frequencies. Since the channel selection is random, there is no guarantee that 8 packets sent will cover all the channels in an 8 channel system, however, one would expect that, statistically most of the channels would have been used. The effect of changing the frequency is to change the multipath effects experienced by the signal bouncing off the different buildings and other objects. Once the algorithm has received a packet on each available frequency then the improvement becomes rather small. There is some variance of the multipath due to time but field data suggests that it is rather less than the frequency related and would, therefore, give minimal benefit. So, for an N channel LoRaWAN deployment, the benefit is mostly brought with approximately N packets transmitted.
Until June 2016 the only non-urban data that is available is a rural near-line-of-sight trial. The trial is detailed in a separate report but the summary is that in a near-line-of-sight trial, the mean uncertainty measured was 20 to 50 metres as shown below:
The LoRa gateways are designed to be used in long range star network architectures and are utilized in a LoRaWAN system. They are multi-channel, multi-modem transceivers, can demodulate on multiple channels simultaneously and even demodulate multiple signals on the same channel simultaneously due to the properties of LoRa. The gateways use different RF components than the end-point to enable high capacity and serve as a transparent bridge relaying messages between end-devices and a central network server in the backend. Gateways are connected to the network server via standard IP connections while end-devices use single-hop wireless communication to one or many gateways. All end-point communication is generally bidirectional, but also supports operation such as multicast enabling, software upgrade, over the air or other mass distribution messages to reduce the on air communication time. There are different gateway versions depending the desired capacity and installation location (home versus tower).
A LoRa Gateway is buit using the SX1301 whereas the final product operates as a base station in a LoRa Network. The base stations sends/receives all data to and from the sensors (end nodes) and is connected (backhaul) to a network server (OSS/BSS) for further processing of the data.
Capacity is, first and foremost, a consequence of the number of packets that can be received in a given time. A single SX1301 with8 channels can receive approximately 1.5 million packets per day using LoRaWAN protocol.
So, if your application sends one packet per hour, then a single SX1301 gateway can handle about 62,500 end devices.
The license is free of charge for the 2nd generation"location enabled” gateway, as are the 1st generation gateway reference designs.
In order to get as close to NLOS as possible it is important to observe the rules of propagation and try to get enough height in the gateway antenna to avoid the first Fresnel zone as much as possible. The mathematics defining the Fresnel zone are beyond the scope of this FAQ, however, as guidance, for an ISD of 4km, the antenna height should be 16m above the ground (and trees). With enough antenna height and an open landscape performance in the region of 10-30m have been observed.
With the early trial data, Semtech is recommending a 1.5km ISD in most urban cases as a guideline. There is, however, no substitute for detailed network planning and the coverage target should be four gateways. Targeting a configuration giving a four gateway redundancy means that in most cases, the sensor will be in coverage of all four, and in almost all cases will have the required minimum of three.