Chapter 3: Link Budgets

This chapter discusses two topics that will come up time and again while planning your WISP: path loss (how far can my signal go?), and link budget (what devices can I use for this link?).

A “Link Budget” describes the basic elements of a radio link, and provides a prediction of the signal assuming ideal conditions (we’ll start working on realistic conditions later!). This is important to understand if a given radio link can work, how well it will work, and the extent to which it will survive inclement weather. A solid understanding of link budgets is essential for a WISP who wishes to install equipment with a minimum of trial-and-error.

A basic link budget requires the following numbers:

To calculate a link budget, add the transmit power, the transmitter antenna gain, and the receiver gain, and subtract the free-space path loss. The result is a predicted RSSI [Receiver Signal Strength Indication]. If it is less than the “receiver sensitivity”, then in ideal conditions the link will function. The difference between the two numbers is the fade margin. This is how many decibels you can afford to lose before the link will stop functioning correctly; loss can come from obstructions, atmospheric events (such as storms and temperature gradients) and from misaligned antennas.

Example: Two NanoBridge M5-22 systems at 10 kilometers

For example, let’s assume that we have two Ubiquiti NanoBridge M5-22 systems pointing at one another, at a frequency of 5800 mhz and a distance of 10 kilometers.

We start by calculating the free-space path loss (FSPL), as described in the preceding section:

FSPL (dB)=32.44+20 log10⁡f+20 log10⁡d
d is the distance of the receiver to the transmitter in kilometers (km).
f is the signal frequency in Megahertz (Mhz).

Plugging in the numbers to calculate FSPL, we learn:

FSPL (dB) = 20 * log10 (10.0) + 20 * log10 (5800) + 32.44
FSPL (dB) = 20 * 10 + 20 * 3.763 + 32.44
FSPL (dB) = 20 + 75.26 + 32.44
FSPL (dB) = 127.7 dB

Next, we check the Ubiquiti data-sheets for the NanoBridge M5, and learn that it has a maximum transmit power of 23 dB, and an antenna gain of 22 dB. For this example, we'll use a “receive sensitivity” of -70 dB – we aren't looking for a super-high capacity link (this number is fully explained in “Modulation”, below). We’re also assuming that we are using the maximum possible power output; in many cases, this won’t be true and doing so will prevent us from achieving the maximum possible data rates.

Our initial link-budget now looks like this:

Item Value
Transmitter Power 23
Transmitter Antenna Gain 22
Receiver Antenna Gain 22
Free Space Path Loss -128 (round up)
Receive Sensitivity -75 dB
Fade Margin 14 dB (-61 + -75)

We can learn from this that over 10 kilometers at 5800 mhz, a pair of NanoBridge M5-22 should be able to achieve a signal level of -61 dB. For Ubiquti equipment, -55 dB is considered the ideal signal, but anything into the -75 range will work (see “Modulation”, below) – so this should be a usable link, capable of nearly full speed. The “fade margin” is calculated as the difference between the receive sensitivity and the RSSI. In this case, we are 14 dB stronger than the minimum required signal to connect – which provides us with plenty of room for error (see “Fade Margin”, below).

Example: Rocket M5, 20dB Antenna Transmitter, 6km distance to a NanoStation M5

Since this is a rather complicated calculation, lets calculate a second link. This time, we will simulate a Ubiquiti Rocket M5, equipped with a 20 dB sector antenna. Our pretend customer is 6 kilometers away (3.7 miles), and has a Ubiquiti NanoStation M5 on their roof. We are operating at 5805 Mhz.

Once again, we calculate the Free Space Path Loss:

FSPL (dB) = 20 * log10 (6.0) + 20 * log10 (5805) + 32.44
FSPL (dB) = 20 * 0.78 + 20 * 3.76 + 32.44
FSPL (dB) = 123.3 dB

We also go to Ubiquiti’s website, and obtain the antenna gain, transmit power and receive sensitivities for both the Rocket M5, the 20 dB sector antenna we are modeling, and for the NanoStation M5.

This lets us build the following link budget:

Item Value
Transmitter Power 27
Transmitter Antenna Gain 20
Receiver Antenna Gain 16
Free Space Path Loss -123 (round up)
Receive Sensitivity -75 dB
Fade Margin 15 dB (-60 + -75)

Good news! This link will work in ideal conditions, with 15 dB of extra power to correct for weather, noise or alignment – or allow us to lower our power levels.

Example: Two NanoStation M5s, 30 kilometer distance

Let's calculate one more link budget, this time for a horribly optimistic link. We'll try and link a pair of NanoStation M5s together at 5805 Mhz, at a distance of 30 kilometers! Once again, we start by calculating Free Space Path Loss:

FSPL (dB) = 20 * log10 (30.0) + 20 * log10 (5805) + 32.44
FSPL (dB) = 20 * 1.477 + 20 * 3.76 + 32.44
FSPL (dB) = 29.54 + 75.27 + 32.44
FSPL (dB) = 137.25
Item Value
Transmitter Power 27
Transmitter Antenna Gain 16
Receiver Antenna Gain 16
Free Space Path Loss -137 (round up)
Receive Sensitivity -75 dB
Fade Margin -3 dB (-78 + -75)

This link really isn't going to work. At thirty kilometers, the path loss between the two radios is just too much – so even in ideal conditions, you can't achieve the minimum of -75 dB for a (poor) link between the systems!

Performing a calculation like this before you go and try a link can save you a lot of time and effort; otherwise, you might be sitting on a roof-top for an hour trying to make a link work, blissfully unaware that the laws of physics are ensuring that you will not achieve a satisfactory outcome!

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