16 - Loss and Link Budgets

In the previous blog, we introduced the concept of power loss in wireless systems. We discussed how various factors, such as connectors, cables, and other components, can result in a reduction of the power that ultimately reaches the antenna. In this blog, we will delve deeper into this topic and also explore the concept of the link budget, which provides a comprehensive view of power considerations across the entire system.

While we often emphasize the gains achieved in a wireless system, typically through the addition of antennas to boost the signal, it's equally important to address losses, as we did in our last blog.

To recap, the fundamental idea is that the presence of connectors and longer cables contributes to power losses. This means that not all the transmitted power makes it to the antenna intact.

In the context of cables, their length plays a significant role in power loss. Longer cables introduce more electrical resistance, causing a greater power loss during transmission. Additionally, we mentioned the use of lightning arrestors. Lightning arrestors are devices designed to safeguard access points from electrical surges resulting from lightning strikes, whether they are direct hits or occur in proximity.

When discussing power losses, we typically quantify them in decibels (dB). The losses from these components are typically less than one dBm (decibel-milliwatt) per device. For instance, connectors may lead to a loss of around half a dB, while cables might result in a quarter of a dB loss. The specific loss also depends on the type and quality of the devices being used. To understand the expected decibel loss for a particular component, you can refer to its specification sheet when making a purchase decision.

This brings us to the important concept of the link budget. The link budget is a way to account for the power losses and gains that occur as a wireless signal travels through the environment. It's essentially a comprehensive analysis that covers the entire path from the transmitter to the receiver.

In this end-to-end conversation, we consider the journey of the transmitting radio as it traverses the wireless system, passes through the airspace, and reaches the receiving end. To illustrate this, we can break it down into several key components:

Let's consider a scenario in which we have an intentional radiator power of 18.5 dBm, and our antenna provides a gain of 5 dBi. To calculate the effective isotropic radiated power (EIRP), we add these values, resulting in a total EIRP of 23.5 dBm.

The next factor to account for is the free space path loss, which depends on several variables. First, it is influenced by the frequency used, as higher frequencies experience faster attenuation in free space, as previously discussed. Secondly, it depends on the distance between the transmitter and receiver. For our example, let's assume a free space path loss of 90 dB, which means we lose 90 dBm across the airspace.

Subtracting this loss from our total EIRP, we get a received power level of -66.5 dBm (23.5 dBm - 90 dB). However, we must also consider the gain on the receiver's side. Assuming both the transmitter and receiver use antennas with a 5 dBi gain, the signal is boosted by the same amount on the receiving end, resulting in a new power level of -61.5 dBm (-66.5 dBm + 5 dB).

Nonetheless, there will be additional losses in the system. Let's assume we experience a loss of 1.5 dBm during transmission. This brings our total received power level to -63 dBm (-61.5 dBm - 1.5 dB).

We can represent this power variation over distance in a graph, with positive dBm on one side and negative dBm on the other. As the signal travels, we'll observe fluctuations in power. For instance, the transmitter itself might initially emit a power of 20 dBm, but due to losses in cables and connectors, the power decreases gradually along its path.

This explains the fluctuations in power levels as the wireless signal propagates through the environment, helping us understand the signal's strength at various points along the way. It's a critical aspect of designing and optimizing wireless communication systems.

What can we practically gather from all of this? Well, one key takeaway is that when it comes to outdoor wireless systems, it's imperative to pay closer attention to link budgets, although this concept isn't irrelevant for indoor systems either.

The distinction lies in the complexity of the components involved. In indoor setups, we usually have fewer components to contend with. For instance, we typically don't require lightning arrestors, and our cable lengths, if we're using cables at all, are minimal. Additionally, indoor environments may utilize internal antennas, which essentially involve negligible cable lengths and minimal connector losses, even though there are technically conductors involved.

Conversely, outdoor wireless systems often necessitate more extensive considerations. This is partly due to the need for weatherproofing and the various elements that outdoor systems must endure. In outdoor setups, we commonly use larger connector types, which can introduce higher losses.

Moreover, outdoor systems frequently require longer cables. This is because outdoor access points and antennas may need to be positioned at greater distances from each other. This flexibility is essential for accommodating various environmental factors. Outdoor setups also demand the inclusion of lightning arrestors to protect against electrical surges.

All these factors contribute to greater losses in outdoor wireless systems when compared to indoor systems. When selecting and ordering components for these systems, it's crucial to refer to the technical specifications of the devices being installed. These specifications provide valuable information about factors such as cable lengths, connector types, and other losses, helping you design and configure effective outdoor wireless systems.

To ensure the success of our wireless designs, we must thoroughly understand and quantify the potential losses within our system. This means we need to be able to predict and calculate what these losses will be. For example, if we initially plan for a three-dBi antenna but find that our anticipated losses are one dB higher than expected, we should be prepared to consider using a more powerful four-dBi antenna to compensate for these losses. This decision-making process is an integral part of wireless system design.

Another crucial consideration is the impact of frequency. As mentioned earlier, higher frequencies lead to greater signal degradation in terms of free space path loss. However, it's important to note that this principle also applies to the electrical components of the system. Even when the signal is in the form of an electrical current traveling through cables and other components, higher frequencies will result in more significant losses. This highlights the importance of selecting the appropriate frequency for a given wireless application.

In summary, it's essential to recognize that wireless system design isn't just about adding antennas and focusing on gains. Every component within the system plays a role in the overall signal loss. The goal is not to eliminate all losses but to understand and manage them effectively. By doing so, we can optimize our wireless systems for the best possible performance.