Myth Buster: WLAN’s and Radio Frequency

Have you ever wondered about certain myths about radio frequency that seem to pervade the WLAN world? Then this could be the blog for you. We certainly come across a lot of people who have a particular mindset when it comes to wireless networks and how they work. 

In this blog we’re going to take a look at some of the common beliefs people hold about radio frequency and dispel the myths. 

Do higher frequencies travel as far as lower frequencies? 

Yes. A common misconception is that higher frequencies don’t travel as far as lower frequencies, but this is not the case. Regardless of frequency, RF signals travel the same distance. The confusion comes from it seeming at a surface level that lower frequencies travel further.

But actually it’s due to attenuation. In space, a high frequency gamma ray with a frequency of approximately 1020 Hz (1 nanometer wavelength) can travel billions of miles, even lightyears, from neutron stars here to us on earth. Regardless of frequency, there is no attenuation of the radio wave so it travels an equal distance.

Here on earth, the distance RF goes (or is heard) is determined by how much is absorbed or attenuated. We need to take into account what the radio waves have to go through.

So when we’re thinking about longer wavelengths (lower frequency):
– There are less wavelengths to get through an object
– Less wavelengths are therefore absorbed
– The signal is less degraded

For these reasons, lower frequencies appear to go further.

Just to confuse things a bit further – Here’s an inconsistency with the above. In terms of 2-way radio, it’s a commonly known fact that UHF frequencies (450-470MHz) work better in an indoor environment than VHF (150MHz). But didn’t we just say that lower frequencies appear to travel further because less are absorbed? Yet here we are saying this type of higher frequency works better indoors.

In terms of size, the wavelength for VHF frequencies are about 6 feet and the wavelength of UHF frequencies are about 2 feet.

In an indoor environment, things like windows and doors are generally larger than the UHF wavelength and smaller than the VHF wavelength. This means that the attenuation for UHF is less.

What’s interesting is that if you increase the frequency, for example to 900MHz, the even shorter wavelength begins to have an increase in attenuation from things that are not windows and doors attenuating the signal more than the smaller frequency.

The UHF at 450MHz is the optimum frequency for an indoor environment.

For the tech geeks out there that are thinking – Hang on, what about UHF radios having higher gain antennas than VHF? Whilst that is indeed true, the dB gain difference isn’t enough to explain why the UHF does so much better indoors.

So let’s go back to where we started. It is a misconception to say that lower frequencies travel further than higher frequencies. However, it is true that higher frequencies (smaller wavelengths) are attenuated more than lower frequencies. It is also true that designing antenna systems that can receive higher frequencies at a greater distance is somewhat more challenging.

Does Free-Space Path Loss Increase with Frequency?

Not necessarily, despite common beliefs. If you are using a Free-Space Path Loss (or FSPL) calculator, it will assume that you are maintaining the same wavelength proportionally sized antenna as you lower the frequency. In other words, you are changing only the frequency so just one variable. You are not changing the antenna size.

When you lower the frequency, you aren’t seeing that the antenna is also getting bigger at the same time.

Decreasing the frequency whilst not increasing the size of the antenna would mean that the dB gain would decrease accordingly and the overall system gain would remain the same.

If you adjust both the frequency and the antenna size correctly then the FSPL stays the same.

However, there are scenarios where FSPL can increase with frequency. In the real world, buildings and other structures in free space will absorb more energy for higher frequencies as we’ve said above. Higher frequencies have smaller wavelengths and more attenuation. When buildings are in the path, path loss can increase with frequency.

Can increasing output power be a solution?

Many people think that increasing output power can never be a solution and that you must match your AP to client power.

Actually, an AP’s receiver us usually better than a client device. In addition to this, the MRC (maximal ratio combining) means that you should be able to allow for at least 3dB better radio receiver which can also mean 3dB more AP radio transmitter to balance things out.

Did you know that it’s actually okay to have asymmetrical MCS rates with downstream being higher than upstream? (the Modulation Coding Scheme helps us engineers to understand data rates and evaluate RF environments).

In a typical WLAN, users will be doing more downloading than uploading. Having a higher average MCS rate for users, they will be quicker to get on and off the wireless network, which increases the overall performance.

In most cases, it’s okay to have AP power at 15-17dBm.

In terms of Point-to-Point links, you could find that turning up the power is an easy way to boost performance. In addition, Mesh backhaul links between AP’s could benefit from higher power by increasing the average MCS rates between them. 

What is this not going to solve? Poor design and holes in coverage. You also need to bear in mind potential co-channel interference and being a good neighbour. We’re not saying turn up the power to crazy levels, but generally turning up the power by a few decibels could help your network. 

Does Wi-Fi 6E have more frequency space (1200MHz) than other unlicensed bands?

Let’s talk about the 60GHz band. One 802.11ad / 802.11ay channel has more RF bandwidth than combining all of the 900MHz, 2.4GHz, 5GHz, Wi-Fi 6E and 24GHz bands.

If you double the RF bandwidth will it double the throughput?

This is not always the case. You might find that the physical baseband layer (PHY) doubles but the actual throughput is less likely to do so. 

If you narrow the channel by half, then the Signal to Noise Ratio (SNR) increases by 3dB – Possibly even more if there are other sources of interference. An improvement of 3dB in SNR generally means an increase of 1 to 2 MCS rates.

It’s also true in reverse. If you double the RF bandwidth the SNR will be reduced by 3dB, generally reducing the MCS rate by 1 or 2. 

So, doubling the RF bandwidth doesn’t automatically mean you double the throughput but likely somewhere in-between. 

Throughput will only be doubled if full MCS rates are maintained either way and:

  • The link is strong
  • There is extra SNR beyond what is needed for the maximum MCS rate

Imagine an outdoor Point-to-Point scenario where the RF bandwidth was reduced from 40MHz down to 20MHz. By narrowing the RF bandwidth, the SNR increased and interference was reduced. The MCS rate increased which made up for the throughput that was lost from narrowing the RF bandwidth – In essence, increasing the throughput by reducing the RF bandwidth. Interesting!