Understanding RSSI and LQI Metrics of IOT

 In the rapidly increasing adaption of Internet of Things (IoT), understanding  the basics network performance metrics is crucial for building reliable and efficient systems. Two fundamental metrics that play a n important role in the IOT Device communication are the Received Signal Strength Indicator (RSSI) and Link Quality Indication (LQI). Let's dive deep into what these metrics mean and why they matter for IoT implementations.


Received Signal Strength Indicator (RSSI)

RSSI serves as a fundamental measurement of RF power received by a wireless device. What makes RSSI particularly interesting is that it measures all RF power in a channel, regardless of the source. This means it captures:

  • Signals from IEEE802.15.4 transmitters
  • Interference from Bluetooth devices
  • WiFi signals
  • Background radiations

This comprehensive measurement makes RSSI an essential tool for Clear Channel Assessment (CCA), helping devices determine if a channel is free before transmitting. In simpler single-hop networks, RSSI often serves as the primary metric for determining link quality.

Link Quality Indication (LQI)

While RSSI gives us raw power measurements, LQI provides a more nuanced view of connection quality. In multi-hop networks like Zigbee, LQI acts as a cumulative metric that helps determine the overall cost of using a particular link. Think of LQI as a "quality score" that helps routing algorithms make smarter decisions about path selection.

Here is a screenshot from a RUCKUS IOT Controller showing the RSSI/LQI Metrics of a ZigBee/Assa Abloy Door lock:


A sniffer view using EFR32xG24 Dev Kit:


Understanding the Relationship between RSSI and LQI Metrics:

The relationship between RSSI and LQI isn't always straightforward. Here are some scenarios that demonstrate their complex interaction:

1. The Weak but Clear Signal

In this case, you might see low RSSI but high LQI. Despite the signal being weak, it's clear enough to maintain good communication quality. This scenario often occurs in long-distance communications with minimal interference.

2. The Weak and Noisy Signal

When a weak signal exists in an environment with virtually no noise, both RSSI and LQI will be low. This represents a poor communication scenario where both signal strength and quality are compromised.

3. The Strong Interference

An interesting case occurs when strong interference is present, resulting in high RSSI and high LQI. This can be misleading as the strong readings don't necessarily indicate good communication conditions.

4. The Clean Strong Signal

A strong signal in a low-noise environment produces high RSSI but low LQI. This represents ideal communication conditions where the signal is powerful and clear.

5. The Saturated Receiver

In cases of extremely strong signals that overwhelm the receiver, both RSSI and LQI readings will be high. However, this situation might indicate potential communication problems due to receiver saturation.

Here is a reference of the RSSI/LQI value that would typically be required for a reliable IOT Gateway and End Node communication:

When designing IoT networks, it's important to consider both metrics rather than relying on just one. While RSSI might indicate good signal strength, LQI provides crucial information about the actual quality of communication, helping ensure reliable data transmission across your IoT network. 

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