All posts by firas.shaari

The 4th video in the series of the 802.11ax training videos. In this video I explain the difference between OFDM and OFDMA and introduce the concept of RUs in the 802.11ax standard

The 3rd video in the series of the 802.11ax training videos. In this video I explain the different types of PPDUs in the 802.11ax standard.

The 2nd installment in the series of the 802.11ax training videos. In this video I highlight most of the important differences between 802.11ac and 802.11ax.

Looking at the S11 and S21 parameters of dual band ANTs shows acceptable performance up to 6.7GHz. In the photos below 4 markers on the S21 parameter graph can be seen. MK1 was set to the beginning of the 2.4GHz band, MK2 was set at the beginning of the 5GHz band, MK3 was set at the last frequency in the 6GHz where the dual ANTs still gave acceptable levels of performance and finally MK4 was set at the end of the 6GHz band (7.2GHz) where the performance degraded by a factor of 6dB below the performance measured in the 5GHz band.

Note: For more info on the S-Parameters and what do they mean you. this page https://www.antenna-theory.com/definitions/sparameters.php gives a simple and clear explanation of the concept.

On the 23/02/2021 at around 10:00am I took on a 4 hours exam an attempt to achieve the iNARTE Spectrum Management Certification. The exam was challenging, I wouldn’t say it was difficult but, definitely it wasn’t an easy one. For anyone who might be interested in taking on the exam a good background in Electrical Engineering principles and a fair knowledge of HAM radio rules regulations and best practices will definitely be a big a help to clear this exam. Finally, you can check my certification by clicking on the link below.

The Hackrf is a small affordable SDR (software Defined Radio) that can operate in frequencies up to 6GHz with a channel bandwidth of 20MHz. This makes it an excellent choice when it comes to experimenting with WiFi. One of the few drawback of a Hckrf is the lack of amplification on the Tx side. In this experiment I was successful in amplifying the transmit signal of a Hackrf by 30dB using a Skyworks FEM ( Front-End Module).

GNU radio is used to generate an 64 tone OFDM signal which is forwarded to the Hackrf. I will be writing a detailed post on how to replicate this experiment in the future. For now I would like to share with my readers short videos showing the difference in Tx power with and without the a FEM.

I have noticed that many of our colleagues in the WiFi community have been expressing their disappointment with the 802.11ax OFDMA performance. I remember having a similar feeling at one-point in time during the early stages of experimenting with OFDMA. At first, I was excited to find some cases where OFDMA was superior in performance to CSMA/CA but as testing progressed, I also found many other cases where CSMA/CA performance was equal to or, in many cases superior to OFDMA. I felt disappointed as well. In my head OFDMA was supposed to bring order to a random and chaotic WiFi medium. 

When OFDMA was announced as part of the 802.11ax standard everyone was thrilled. Many thought it would become the silver bullet to all our WiFi problems. Since the early days of WiFi, we have been trying to control the medium and how and when clients have access to it. PCF (Point coordination function) is a good example on our early attempts to control the medium.

OFDMA as an access method might be new to WiFi but, it has been a well-established method of access in the cellular world for at least 10-15 years now. I believe the idea that OFDMA was supposed to transform WiFi networks into a cellular-like synced networks is the source of which the disappointment stems from. 

After almost 2 years of testing OFDMA as part of the new 802.11ax standard, I can fairly say that it works. I have forced APs in OFDMA mode only and gone through numerous test case scenarios, whereas I mentioned above, I found cases where it was superior to CSMA/CA but also many cases where it was not. I have felt the disappointment that many of my colleagues are expressing now. Then it hit me! It wasn’t OFDMA that was flawed, it was my unrealistic expectation of it.

OFDMA as a part of the 802.11ax standard was not meant to replace CSMA/CA, nor was it meant to transform WiFi into a cellular-like network. It was meant to complement CSMA/CA. To fill the gaps where CSMA/CA falls short. The real advantage of an 802.11ax AP is not its ability to use OFDMA on the UL or the DL nor its ability to use MU-MIMO. The real advantage is to know when to use them.

An 802.11ax AP’s scheduler is the real MVP. It is the brain that decides when using OFDMA over CSMA/CA is beneficial and when it is not. Most of the shortage in OFDMA performance that many observe is not due to flaws within the OFDMA itself as an access method. It is mostly due to the failure of the scheduler in applying OFDMA to right cases. Knowing when to use OFDMA or not is a complicated process. Many factors come into play. To name a few, packet sizes, SNR, throughput rates, clients’ configuration, and number of clients. A scheduler’s job is to take into account all these factors and many more, and make decisions on a packet per packet basis on whether to use SU-MIMO, OFDMA, MU-MIMO, or a combination of OFDMA and MU-MIMO. If this sounds like a very difficult and highly sophisticated thing to do, because it is. What differentiate 802.11ax APs from each other’s in performance is not their ability to perform OFDMA or MU-MIMO, it is their schedulers ability to choose the right technique, or the right combination of techniques for a certain traffic flow.

The thing that I hated the most turned out to be the best part about the whole CWNE journey. I learned so much writing these essays. I hope they inspire at least one person to follow the lead of all the people who took on this journey and became CWNEs before them.