RSSI och SNR

För att kunna mäta signal styrka och signal brus nivå. Används RSSI (Received Signal Strength Indication) och SNR (Signal-to-noise ratio).

SNR

Är förhållandet mellan den mottagna effektsignalen och bullernivåns effekt. Om SNR är för dåligt sker omsändningar av meddelanden vilket påverkar batteritiden av enheter. Generellt sätt kan man tänka att om SNR är över 0 ligger signalen över bullernivån ligger det under 0 är signalen under bullernivån. Vanliga värden för SNR inom lora är -20dB till +10dB.

RSSI

Är den mottagna signalens kraft i milliwatt och mäts i dBm. Värdet används hur väl en mottagare kan "höra" en signal från en enhet. RSSI förväntas vara ett negativt värde. Närmare RSSI ligger 0 desto bättre är signalen. Om RSSI värdet är -120dBm är signalen svag och är minimum gränsen för LoraWan. En bra signal har ett RSSI värde på -30dBm.

Fälttestaren i sensor online använder färgkoder i kartapplikationen för att snabbt se om signal värdet är okej eller ligger under gränsen. Nedan följer färgkoderna för fälttestar verktyget i SensorOnline:

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Adaptive Data Rate

Tthe Adaptive Data Rate (ADR) mechanism. ADR allows us to exploit the advantages of the LoRa® physical layer. Adapting the data rate in a LoRaWAN® network allows us to easily scale the network, simply by adding gateways. Additionally, using ADR can drastically increase the capacity of such a etwork.

Let’s begin by looking at a legacy system. Consider a M2M network using FSK-based communications, for example, MBUS. For this exercise, we will consider the 2.4 kbps incarnation of MBUS operating on a single channel. For our scenario, imagine that we are monitoring water use in an urban area, using the metering industry standard of 2500 water meters per square kilometer. In such a case, using typical MBUS settings, assume that we get an urban coverage range of about 500 meters. At this density, figure that this corresponds to 2000 water meters.

Assuming that we transmit at 2.4 kbps and have 64 bytes of payload data, if we run the numbers, we end up only being able to transmit twice a day before we exceed a 1% packet error rate, due to collisions.

A LoRaWAN network is more robust than this. To understand why, we will look into the behavior of the LoRa modem and the sensitivity available for each data rate (spreading factor or SF). For this exercise, assume that the LoRa bandwidth is fixed at 125 kHz.

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Given the time on air, it is clear that end devices (nodes) closer to the gateway do not need the high linkbudget that goes along with SF12; nor do they need to stay on air as long. This being the case, using ADRcan optimize the SF used for each meter, and minimize the subsequent Time on Air. ADR is a very simple mechanism that changes the data rate based on simple rules:

  • If the link budget is high, the data rate can be increased (i.e. the SF is increased)
  • If the link budget is low, the data rate can be lowered (i.e. the SF is reduced)
  • It tailors the node’s data rate to the available link budget.
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Determining the Data Rate How does the network server determine the appropriate data rate? Let’s take a look: The end device’s application sends a message up through the gateway, which simply passes the message along without acting on the data. The gateway in a LoRaWAN network is a simple, low-cost device that converts LoRaWAN packets into IP packets which can be sent over a secure backhaul to the network server. These IP packets include a small amount of metadata about the reception time and signal strength. Based upon the strength of the received signal, the network server determines what the optimal node data rate should be (that is, the spreading factor) What is the influence of this ADR mechanism when there is only one gateway? Figure above illustrates th

What is the influence of this ADR mechanism when there is only one gateway? Figure above illustrates the result of a simplified case, in which we consider a free space path loss model to estimate the attenuation between both antennas (that of the gateway, and that of the device). Nodes close to the gateway use a high data rate (e.g. SF7). Therefore, they spend less time-on-air and exploit the low link budget that they need. For more distant nodes, the data rate is lower (e.g. SF12) and the link budget is higher. In reality of course, the path loss picture is more complicated. It will depend on the specific environment around the gateway, as well as where and how the nodes are deployed. The important point to keep in mind is that, because the communications are orthogonal to each other: multiple data rates on the same channel can be received simultaneously. Moreover, the amount of airtime required for a device to send its payload is optimized, which radically reduces the device’s energy consumption.