11 - Noise Floor and SNR

When discussing wireless interference, it's important to consider the potential sources of disruption. This includes non-Wi-Fi devices as well as signals from distant origins that may have originally been Wi-Fi but have become unrecognizable by the time, they reach us.

These distant signals no longer adhere to our Wi-Fi protocol and essentially become part of the background noise. Understanding this concept is crucial because it's not just about clear communication between two parties: it's about maintaining communication clarity amidst surrounding noise.

Just as excessive ambient noise can make it challenging to hear someone speaking, the wireless environment can also be filled with interference. To overcome this, we use tools like the Signal-to-Noise Ratio (SNR) for analysis.

Background noise is omnipresent, both in the auditory and radio frequency realms. For instance, while writing this blog in my office, I hear the humming of my PC, the sound of the AC unit, and even kids in the background. However, most of this audio noise doesn't disrupt my conversation with my wife without her being bothered by the PC's hum.

In the wireless context, we also contend with a certain level of background RF noise originating from various sources, including natural ones.

We should also consider RF signals from various sources, including non-Wi-Fi traffic when discussing interference. However, this interference is typically in the background, originating from distant systems or Wi-Fi devices broadcasting from far away locations. By the time these signals reach our immediate vicinity, they become challenging for our wireless receivers to detect.

When addressing RF noise, we're concerned with signals that typically fall within the range of approximately negative 90 to negative 100 decibels in terms of the Received Signal Strength Indicator (RSSI). This range represents our noise floor, a critical factor to monitor.

To illustrate, imagine us standing at opposite ends of a ballroom, having a conversation without needing to raise our voices. The acoustics are favorable, and communication is clear. However, as more people join the room and engage in their own conversations, the noise level increases, making it progressively harder for us to hear each other.

Imagine a crowded ballroom where constant background conversations create a challenging environment for communication. In such a scenario, the odds of us effectively communicating from a distance are low. There are a couple of approaches we could take to overcome this situation.

One option is to raise our voices, essentially yelling or shouting at each other, which increases our power level. However, the more practical solution is for us to come closer together. As we move closer, our proximity decreases, allowing us to communicate more effectively once again.

In this illustration, we have two situations: one without noise and the other with noise. In the noise-free environment, our voices stand out as there is minimal background noise. However, even in a slightly noisy room with a distant AC system, the noise level remains low, not significantly affecting our conversation.

In contrast, when there's considerable noise in the room, speaking at the same level becomes challenging, as the noise level is high relative to our voices. The greater the noise level, the harder it is to bridge the communication gap. If the noise level surpasses our communication efforts, effective communication becomes nearly impossible.

This concept of comparing our signal to the background noise is crucial in both audio and radio frequency communication. It highlights the importance of the Signal-to-Noise Ratio (SNR), where we aim to speak above the noise floor. As mentioned, the Received Signal Strength Indicator plays a vital role in this context.

The Received Signal Strength Indicator (RSSI) measures the signal strength of wireless devices, often considered a quality signal, especially for non-real-time applications. A negative 67 dBm is typically the threshold for satisfactory performance.

However, Signal-to-Noise Ratio (SNR) is equally important. An RSSI of negative 67 dBm can represent two scenarios: one with minimal environmental noise and another with high noise levels relative to the signal strength. Companies like Aruba Networks, Cisco Systems, specializing in wireless and real-time applications, recommend a positive 25 decibel SNR for voice and video applications.

To understand this number, let's delve into some math. SNR involves dividing the signal by the noise, and since decibels are logarithmic, we manipulate the exponents. For example, dividing 10⁵ by 10³ becomes subtracting the exponents: 10^(5-3) equals 10², which is 100.

In the context of RSSI, negative 67 dBm is considered good, but assessing SNR is crucial for determining communication quality, especially in real-time applications.

To achieve a Signal-to-Noise Ratio (SNR) of 25 dB, we can simplify it into a subtraction problem. Starting with a signal strength of negative 67 dB, we need to subtract the noise level to reach 25 dB. Thus, 25 dB equals negative 67 minus noise, which leads us to find that the noise should be less than or equal to negative 92 dB relative to the signal.

In practical terms, if the RSSI is as low as negative 67 dB (our threshold), and the noise remains at negative 92 dB or lower, we maintain a 25 dB difference, which is favorable. However, if the noise worsens, such as reaching negative 88 dB, we'd require the RSSI to be no lower than negative 63 dB to maintain that 25 dB difference.

This emphasizes that while RSSI is essential, it's not the sole factor in signal quality determination. When noise levels rise significantly, alternative strategies like increasing transmission power or reducing the distance from the access point may be necessary to maintain an adequate SNR, especially for reliable voice-over wireless communication.

In wireless site surveys, considering both RSSI and SNR is crucial to establish signal strength thresholds for quality assessment, with SNR being a key takeaway for optimal wireless performance.

The Signal-to-Noise Ratio (SNR) is a crucial metric that considers both signal strength and background noise. SNR is a mathematical operation, typically a division of signal power by noise power in raw milliwatts. In the context of decibels, we can directly subtract signal strength from noise strength to calculate the SNR, which should always yield a positive decibel value. A negative SNR implies that the noise is higher than the signal, indicating poor signal quality.

When noise levels exceed the norm, it's essential to adjust our expectations for what constitutes a good Received Signal Strength Indicator (RSSI). For instance, an RSSI of negative 65 or even negative 62 may be excellent in a typical noise range of negative 90 to negative 100 dBm. However, if noise levels rise, such as reaching the negative 80s, we must reconsider our RSSI standards since they assume low noise thresholds.

Understanding SNR is vital for accurate wireless site surveys and determining whether a signal strength is adequate for various applications.

Happy Learning!!