Talk faster and not so loud…

… I’m in a hurry and my head is pounding.

Ever read something a dozen times only to notice something completely new the 13th time around? Not too long ago I was perusing some boring vendor specs (chances are it was ours) and noticed something on the transmit power specs:

As the data rate increases, the maximum transmit power decreases. I’ve actually observed this before but I never stopped to think of why. Why does the transmit power decrease?

I’m quite fortunate that I work for a company like Ruckus. I can walk into Bill Kish’s office (uber genius CTO) and ask him this very question. Instead of trying to put things in my own words and looking like I’m some smart guy, I’m going to start by quoting Bill:

“OFDM backoff due to high peak to average power ratios of higher OFDM modulations.

OFDM signals are more challenging to amplify since with all the subcarriers you sometimes get ‘unlucky’ combinations of the values on the different subcarriers which results in high peak signal strength. These high peaks would cause the amplifier to go non-linear, distorting the signals. Sorta like ‘clipping’ in an audio amplifier.

This effect gets worse with higher order modulations. So two things: for OFDM you need to turn down power compared to e.g. CCK and furthermore you need to turn down power even more for each higher order modulation. Good radio design and good PAs (power amplifiers) can reduce this effect but not really eliminate it.”

I ran this through the GT translator and this is what popped out:

Let’s imagine that you have an audio source (e.g. your phone) connected to an amplifier and speakers. During slow, soft music (e.g. 1 Mbps CCK) everything is fine. However, when you start playing faster music like techno and house mixed together (OFDM) the chances of “too much music” coming out of your source and overpowering your amplifier increases. As the music gets faster and chances of a peak increase, the input volume to the amp needs to be reduced to avoid clipping. This is the very reason why AP manufactures sets decreasing transmit powers for higher OFDM modulation rates.

When any manufacturer tells you their maximum Tx power and EIRP that is almost always for the lowest data rates. Sure, that helps you just fine if you are trying to gain more coverage but coverage isn’t the main concern for most enterprise installations. Throughput and capacity are paramount.

What’s the best way to compensate for this Tx power loss at higher speeds? Antennas.

Resisting… sales…. pitch….

🙂

GT Hill

Wi-Fi observations by a man without a real first name.