Wi-Fi Riddles: ‘Too good’ signal is bad signal?

TooMuchPowerWhile I was working on the next part of the “Unobvious and overlooked Wi-Fi” (which is about channels), I got an interesting knowledge nugget from our engineering. We all know that there is a lower limit to receiver sensitivity, we all know that there must be some upper limit, after which the Rx signal is so powerful, it simply oversaturates the radio. But that is it? Now I know it, even though I did not ask for it explicitly – I merely happened to run into a situation where it matters.. Read on…


Wi-Fi Riddles: Faster speeds require weaker signals?

A discussion of my previous post in the 802.11 Wireless professionals group lead to a discovery of a new bit of overlooked knowledge! Many thanks to Eduard Garcia-Villegas and Mike Rex, who turned my attention to this one.

Ever wondered why faster rates have smaller effective distance? It was always attributed to signal fading – higher rates require more complex modulation, which, in turn, requires higher SNR (Signal-to-Noise Ratio) value. So at greater distances, with lower SNR, one has to use lower rates. Simple.

Turns out, in addition to that the transmitters may as well lower their power when transmitting at higher rates! This is done because at higher power values transmitters are more likely to introduce errors in the signal they create due to non-linearity in their behaviour (if you want more details – just go to the discussion). Here’s an example from a datasheet that Eduardo provided (the numbers are Tx Power, dBm):

Tx Power: HT20, 2.4GHzTx Power: HT20, 5GHz

Note how the power falls as MCS number increases. Also note how this is different in 5GHz depending on channel number.

Mike Rex added, that the users of alternative consumer WLAN router firmwares (such as DD-WRT and Tomato) sometimes suffer from strange performance and link reliability issues, as these firmwares allow varying radio power levels (and other power settings) beyond what manufacturer has intended, resulting in poor Tx signal quality. This mainly is just a result of the shortcomings and compromises in hardware design (even if the APs are built on the same chipset), but certainly not something a non RF engineer would suspect first! 😉

Well, I learned something – this blog already pays off. 🙂 Many thanks to Eduard, Mike and all others, who tool part in the discussion. This bit has been rolled into the original post to keep all things together.

Wi-Fi, the unobvious and overlooked: Power.

Everyone nowadays buys ‘super-fast’ 802.11n or 802.11ac APs, but not everyone manages to get super-fast connection with them. In this article we’ll discuss the not-so-obvious or often overlooked nuances that can substantially improve (or impair) the Wi-Fi connection. Everything below is applicable both to small home networks with off-the-shelf consumer-grade routers using stock and advanced (DD-WRT, etc) firmware, as well as to enterprise-grade gear and networks. Thus, to keep it simple, I will be using the home WLAN as an example, since even the most advanced admins and most proficient engineers still live in residential areas with high neighbour AP density, and everyone wants fast and reliable Wi-Fi. (more…)