9 - Free Space Path Loss and RSSI

As we know, the goal of a wireless signal is to ensure that it can be received by the other side. So, if an access point is communicating, the signal should eventually reach the intended client. At this point, we need to ask: Is the signal strong enough to be received effectively? As much as I'd love to say that a weak signal is sufficient, it's not. It needs to be strong enough to be reliably received.

Think of it like a whispered conversation. If I'm whispering to you from a distance, I might be able to make myself heard, but if I whisper too quietly, you won't understand me. The same concept applies here.

A significant aspect of this discussion is the concept of free space path loss (FSPL). FSPL represents the idea that as a signal propagates through free space, it loses energy. Eventually, the energy level drops so low that it becomes unusable. Additionally, signals can fade due to obstacles such as walls, metal doors, and interference, which we will delve into later blogs. However, even without considering these factors, we still encounter energy loss as the wave propagates.

The primary reason for this phenomenon relates to the other balloon analogy we've mentioned in previous blogs. Recall that we discussed how balloons are made from latex. When I hold a deflated balloon in my hand, the latex is highly concentrated, which means that all the latex can fit within the palm of my hand.

However, when I inflate the balloon, all this latex is stretched out completely. So when we consider the amount of latex per square inch or perhaps per square centimeter, it becomes significantly thinner than when it was concentrated.

We're dealing with the same concept here. When the signal was close to the access point, it closely matched the transmitted power level. Let's assume it was transmitted at 20 dBm. So, right around the access point, it might drop somewhat quickly, but it remains at around +10 dBm.

However, as we move just a few inches away from the access point, the signal weakens further. It might drop to, let's say, around 1 dBm. And if we move a few inches more, it's already in negative territory, maybe around -10 dBm. These distances can be quite short as we transition from +20 dBm to -10 dBm and beyond. The rapid signal weakening occurs due to a combination of factors, including the principle illustrated below and the fact that the signal radiates out into three-dimensional space.

This phenomenon is referred to as beam divergence, and it's crucial in the context of Wi-Fi because as our signal strength decreases, our overall throughput also decreases.

The stronger the signal I have, the more information I can encode into the waveform; that's the fundamental principle. This is of great significance in the context of Wi-Fi, which is why achieving good signal strength is a top priority. When we assess whether a signal is strong enough, we rely on the Received Signal Strength Indicator or RSSI.

The RSSI provides information about how much of the original 20 dBm signal remains when it reaches the receiver. This measurement is expressed in decibels (dBm). For instance, I might receive a signal on my wireless tablet, and the RSSI reading could be -62 dBm.

When we refer to RSSI, we're essentially describing the signal strength as measured by wireless site survey software or any tool used to assess signal quality.

Typically, we refer to 'signal strength,' which is essentially the abbreviated form of RSSI (Received Signal Strength Indicator). Both terms pertain to the strength of the signal received by the receiver. This concept plays a significant role in our discussions about network surveys.

Earlier, we mentioned conducting a site survey, where one key objective is to determine whether the signal coverage meets our specific requirements. We establish different thresholds for signal strength. For instance, in a data-focused survey without the need for real-time applications, we might set the minimum acceptable signal strength at -72 dBm.

In contrast, if the survey is intended to support real-time applications like voice and video, we could set a higher target, such as -67 dBm for RSSI. This represents a different RSSI value we aim to achieve during the survey.

It's important to note that while RSSI is a critical factor in our surveys, it's not the sole consideration. It serves as the initial step, helping us understand the signal strength received from the access point. Our survey assessment also involves factors like background noise, Signal-to-Noise Ratio (SNR), and other related concepts, which we will delve into in upcoming blogs. In general, a higher RSSI corresponds to greater bandwidth and improved overall throughput.

Conversely, as we might expect, the lower the RSSI, the poorer the overall throughput becomes. This reduction in throughput manifests in a couple of ways. First, it leads to a decrease in raw bandwidth, as we mentioned earlier when we observed a drop from 300 to 24 megabits per second due to moving too far away from the access point.

Additionally, we may encounter a higher rate of re-transmissions. Re-transmissions occur when a signal is sent to the other side but isn't received. For instance, if an access point sends a signal to a client, and for some reason, the client doesn't receive it, the same packet must be sent again. Each time we re-transmit a packet, it directly impacts the throughput. In essence, re-transmissions can significantly reduce throughput, potentially cutting it in half if every frame requires re-transmission. This underscores the importance of having a higher RSSI, as it contributes to better wireless performance.

Happy Learning!!