Power Matching: Transmit Power, or EIRP?

Power Matching: Transmit Power, or EIRP?

This question has come up on Twitter in the past, and is something I have needed to have clarified for myself previously. I am referring to power matching between access points and clients. For proper WLAN design, the AP must hear the client at the same level that the client hears the AP (or higher). Consider this analogy: Have you ever tried to have a conversation with someone whispering at you, but they are so far away that you must shout at them?

Client specs are something we can’t typically control, but we can find out (roughly) what they are. We want to have the received signal levels matching at both ends of the link. How should we do this? The only variable we have is the AP’s transmit power. I did some basic link budgets (5 GHz only) to help visualize the effects of matching transmit power levels verses matching effective isotropic radiated power (EIRP) levels.

Here are a couple quick definitions for terms in the link budgets you’ll see here.

transmit power – the conducted power level measured at the intentional radiator of the radio (typically the antenna connector on an AP)

equivalent isotropically radiated power (EIRP) – the power level of the signal that is radiated into the air by the antenna; can be equated to transmit power plus antenna gain.

loss – total drop in signal power from transmitting antenna to receiving antenna; this can include free-space path loss and obstruction loss from walls, water bags, potato sacks, etc.

system gain – a term used to describe the sum of all gains and losses in a radio system; in these examples, this means the transmitting antenna gain minus the loss plus the receiving antenna gain.

received signal level (RSL) – the power level of the RF signal that is present at the receiver port of the radio, to be demodulated.

Note: I use RSL rather than RSSI for theoretical examples, because RSSI is a completely proprietary interpretation of RSL based on a value of 0-255 and the vendor’s code or driver.

First we need to find client transmit power and antenna gain values. I’ll use an iPad Mini 3 as an example, since they are pretty common in the K-12 space that I work in. I found the transmit power levels on the FCC website by searching with the FCC ID here. The Grantee Code for Apple is BCG. You can find the model number for any Apple device, which should also be the Product Code for the search page, on this Apple support page. Make sure you note if the device is WiFi only, or cellular as well, as the model number will be different.

FCC ID search page

FCC ID Search Form

The results page looks like the next image. You can find the actual power levels by clicking on the Display Grant link in any row which has a frequency in the band you are curious about.

Search results for iPad Mini 3 (WiFi only)

Search results for iPad Mini 3 (WiFi only)

You’ll then see a page with the following information.

Conducted power levels for iPad Mini 3 (WiFi only) in 5 GHz

Conducted power levels for iPad Mini 3 (WiFi only) in 5 GHz

Note that the term conducted power is synonymous with transmit power. Now, we see that the maximum transmit power level that is the lowest out of all the bands is in UNII-2e (5500-5700 MHz), and is 0.07W, or 70 mW. I’ll give you a minute to convert that to dBm… Roughly 18 dBm? Now, Cisco’s recent Best Practices for Apple Devices white paper says that most iOS devices will transmit between 9 and 14 dBm. A good WLAN pro should understand that client devices will almost never transmit at their maximum power. In this example, we’ll use 10 dBm (10 mW) as our client transmit power level. Even when we try to be as precise as possible with client specs, we are still stifled by a lack of good documentation direct from the manufacturers.

You can find the antenna gain for many common devices in the Clients tab of the Revolution Wi-Fi Capacity Planner. I’d love to know where he got that information from! From there, I found that the iPad Mini 3 has a gain of 2.8 dBi for 5 GHz. I’m going to use the Juniper WLA532 as the example AP, which is a 3×3:3 802.11n dual-band access point with a 5 dBi antenna for 5 GHz. I’ve also done an example using an 8 dBi patch antenna on the AP. For this example we’re going to assume free space and obstruction loss of -80 dBm. The client might be 40-50 feet away from the AP, with one cinder-block wall between them; a pretty realistic scenario in my K-12 environment. Let’s have a look at the link budgets.

Example link budgets

Example link budgets

So, the received signal levels match when the AP transmit power is matched to the client. This is because, while antenna gain affects both directions of the link, the AP transmit power affects only one direction: the AP-to-client downlink. Put another way, changing the AP antenna gain will change the RSL in both directions, but changing the AP transmit power affects the RSL at the client only. If the transmit power is matched, changing the AP antenna gain will alter the RSL by the same amount in both directions. You can see that the greater the difference in transmit power is, the greater the difference in received signal levels will be.

As another example, we can look at a ten-mile point-to-point system. Often, physical mounting locations differ, so that one side must use a smaller, lighter, lower-gain antenna, and the other side can use a much larger, heavier antenna with higher gain. Imagine a 5 GHz link from the rooftop of a business to a guyed tower. I might use a 1′ square, 23 dBi panel antenna on a small non-penetrating tripod on the rooftop, and a 6′, 37 dBi dish mounted with 2″ angle steel brackets on the tower. The total system gain is enough to provide a proper fade margin, but if you alter the transmit power on one end or the other, you end up with a lower RSL on one side. This is an extreme example, but it does further illustrate the point. Here is a standard link budget with matched transmit power,  different antennas, and different EIRPs.

Link budget for 10 mile point-to-point link

Link budget for 10 mile PtP link with matching transmit power

Now here is the same link with one side’s transmit power level lowered so that the EIRPs match.

Link budget for 10 mile point-to-point link with EIRP matching

Link budget for 10 mile PtP link with EIRP matching

So, we can see that matching the transmit power from client to AP will give us a matching receive signal level because of the synchronous effect of antenna gain. To ensure equal receive levels, a WLAN pro must ensure all other things are equal! System gain is always equal in both directions, so that leaves the transmit power as the only variable. WiFi is a two-way street! Match your transmit power levels to ensure your clients are heard by your APs.

Thanks for reading!