What Is Received Signal Strength Indication?

1. Introduction

In this tutorial, we’ll explore the received signal strength indication (RSSI) and learn why it matters so much in wireless communication.

2. What Is RSSI?

Simply put, RSSI measures the strength of the signal that our device receives from a wireless source, such as a Wi-Fi router or a cell tower. So, RSSI shows how much power from the signal reaches our device.

When our device connects to a network, it constantly checks the RSSI to gauge the strength of the signal. The stronger the signal, the better our connection, and the less likely we are to experience slow speeds or drop-offs.

3. Estimation of RSSI Using the Path Loss Model

RSSI is usually measured in dBm (decibels relative to a milliwatt).

RSSI shows how strong a signal is when it reaches a receiver (like when our phone picks up Wi-Fi). Many devices calculate this for us, but if we ever need to estimate it ourselves, the formula is:

   

Here’s what each part means:

• is the transmitter’s power in dBm
• is the path loss, which tells us how much signal is “lost” on its journey from the transmitter to the receiver, especially as distance increases

3.1. Path Loss

Path loss is affected by obstacles, reflections, and absorption along the way. The signal often weakens as we move away from the source or when there’s an obstacle between it and the receiver. We can model this weakening effect with the log-distance path loss:

   

This formula helps us predict the signal loss at any distance:

• is the path loss at a reference distance . Often, we use 1 meter for in calculations
• is the path loss exponent, which varies based on the environment. It’s usually between 2 and 4. For example, in free space, might be close to 2, but in an office with walls and furniture, it might go up to 3 or 4
• is the distance between the transmitter and receiver

3.2. Why Are the RSSI Values Often Negative?

When calculating RSSI, we subtract a positive path loss value from the transmit power . Since   tends to be large at longer distances, the result is often a negative RSSI value.

For example, let’s say a router transmits at 20 dBm. At 1 meter, the path loss is 30 dB. We want to find the RSSI at 5 meters, so first, we need to compute :

   

Now, we calculate RSSI:

   

As we see, we get a negative value because the path loss is greater than the transmitter’s power. The further we move from the transmitter, the larger the path loss, and the more negative the RSSI.

3.3. Can RSSI Be Positive?

We don’t usually calculate RSSI for distances less than  , but if were somehow smaller than the reference distance , we’d get a lower path loss value, potentially making RSSI positive.

This means our signal would appear very strong, possibly even surpassing the transmitter’s power if we were extremely close! However, for most practical cases, especially in RF engineering, we deal with distances greater than the reference distance.

4. How to Interpret the Value of RSSI?

So, while this formula gives us a useful way to predict signal strength, it’s still an estimate. Real-world factors, like interference from other signals or environmental changes, can impact RSSI, and hardware differences between devices can also play a role. However, using this approach, we can get a good sense of how strong a signal might be at various distances.

Here’s how the numbers break down:

So, the larger the RSSI value, the stronger the signal!

5. How Do Our Devices Measure RSSI?

Whenever our smartphone, laptop, or other wireless gadget connects to a Wi-Fi network or cell tower, it immediately starts measuring RSSI. This helps the device determine how strong the connection is and whether it needs to adjust for better performance (like switching to another network or boosting the power output).

However, RSSI values depend on the hardware inside our devices. Different phones or laptops might show slightly different RSSI readings even when connected to the same network, as the chipsets and antennas used for wireless communication can vary. This is why two people standing in the same spot might experience different Wi-Fi or cellular signal strength.

If we want to measure RSSI ourselves, there are plenty of tools and apps out there. Tools like Wi-Fi analyzers or network scanning apps allow us to see the signal strength in real time. They’re great for troubleshooting weak signals or figuring out where the Wi-Fi dead zones are in our house.

6. Factors Affecting RSSI

Several factors can weaken our signal strength.

First, the farther we are from a Wi-Fi router or cell tower, the weaker our signal becomes. This is due to the inverse-square law: as distance doubles, the signal strength drops to a quarter. So, staying close to the transmitter gives us a better RSSI.

Physical obstacles (such as walls, floors, and furniture) can also interfere. Concrete and metal surfaces are especially tough on signals, so Wi-Fi often struggles in certain rooms or behind thick walls.

Other wireless devices around us can cause electromagnetic interference (EMI). For example, microwaves and baby monitors can create a “noise” that competes with our signal, lowering the RSSI.

Even the direction and alignment of antennas on the device and router can make a difference. When these aren’t aligned well, the connection weakens, reducing our RSSI.

Finally, frequency plays a big role. For example, 2.4 GHz signals travel farther and handle obstacles better, often giving a stronger RSSI over longer distances. However, the 5 GHz frequency provides faster speeds at shorter ranges, but its RSSI drops off quicker over the same distance.

7. Applications and Use Cases of RSSI

RSSI has many practical uses, especially when optimizing and troubleshooting wireless networks. For instance, in Wi-Fi troubleshooting, RSSI can help us determine why our signal is weak in some areas of our house. We can then adjust the position of our routers or antennas to improve coverage.

In cellular networks, RSSI is critical for mobile phones to evaluate signal strength. It also helps with handover decisions when we move from one cell tower’s coverage to another without losing connection.

RSSI estimates the distance between Bluetooth and IoT devices. Bluetooth beacons, for example, use RSSI to trigger actions based on proximity, like when a smartphone detects a nearby device.

Lastly, RSSI-based localization is a big deal in indoor positioning systems. It can help us track our location in buildings like shopping malls or airports by measuring signal strength from multiple access points.

8. Limitations and Challenges of RSSI

While RSSI is super useful, it does have its share of limitations.

First, environmental variability plays a big role. Factors such as weather, interference from other devices, or even people moving around can cause RSSI to fluctuate. This makes it less reliable in dynamic environments.

Another challenge is non-linearity. RSSI doesn’t always drop off in a straight line with distance, especially in cluttered environments with walls, furniture, or other obstacles.

Lastly, inconsistent standards can be a problem. Different devices and manufacturers use slightly different methods to measure RSSI, so comparing results across hardware can be tricky.

9. Conclusion

In this article, we explained the received signal strength indication (RSSI).

RSSI is a key metric that tells us how strong a wireless signal is. We can use it to optimize our Wi-Fi, cellular, and Bluetooth connections. Though not always perfect, it gives valuable insight into network performance.