EIS is, to a first-order approximation:

• the receiver sensitivity
• plus feedline losses
• minus antenna gain

So if the receiver sensitivity is -90 dBm, and the feedline losses are 1 dB, and the antenna has a gain of 4 dBi, EIS is -93 dBm (-90 + 1 - 4).

Note the similarity to EIRP, which is basically the same thing with the sign flipped:

• the transmit power
• minus feedline losses
• plus antenna gain

I say this is an approximation because real-world effects can cause deviation from this ideal. Namely, the antenna is in some orientation to the device, that device will generate noise, and the antenna will receive some of that noise which will degrade the receiver's sensitivity. A simple measurement of receiver sensitivity, which involves applying a test signal directly to the receiver module's terminals, would not account for this effect because there is no antenna to receive noise from the receiver (and whatever device it may be a part of).

To measure EIS, a test signal source is set up to produce a known signal. The field intensity is measured at the position where the device under test (DUT, which will be a receiver and its antenna) will be located. From this field intensity, the power that would be received by an ideal isotropic receiver is calculated.

The EIS is the minimum such power where the BER is still above the threshold.

A link budget might look like this:

1. transmitter power
2. +/- adjustment due to antenna mismatch
3. - transmitter feedline loss
4. + transmitter antenna gain
5. - path loss
6. + margin for interference, fading
7. + receiver antenna gain
8. - receiver feedline loss
9. + margin for self-interference, antenna efficiency, and other practical details
10. >= receiver sensitivity

The objective is to ensure the final sum (item 10) is at least the receiver's sensitivity, otherwise the BER will be too high and communication unreliable.

But all this detail in the link budget can be complicated, so EIRP and EIS exist to simplify the parts specific to the radios and the antennas.

EIRP encapsulates items 1 through 4. Essentially, EIRP tells you how much power is actually radiated from the transmitter, including all the details of the antenna and how that antenna interacts with the transmitter.

Similarly, EIS encapsulates items 7 through 10, including everything about the receiver's antenna, the receiver's sensitivity, and all the details about how that antenna actually interfaces with that receiver.

So if you know EIRP and EIS, the link budget calculation simplifies to:

1. EIRP
2. - path loss
3. + margin for interference, fading
4. >= EIS

EIS might seem a little silly, if it's just sensitivity + antenna gain - feedline loss. But it also incorporates subtle details that might otherwise escape attention.

For example, the receiver generates some amount of noise which will be picked up by its own antenna, raising the noise floor and degrading sensitivity. A simple measurement of receiver sensitivity, which involves applying a test signal directly to the receiver module's terminals, would not account for this effect because there is no antenna to receive noise from the receiver (and whatever device it may be a part of).

Or, the presence of the receiver module or the feed arrangement may unintentionally alter the antenna's radiation pattern.

In other words, receiver sensitivity and antenna gain are idealized numbers, whereas EIS represents the actual performance of the combination which may be less.

To measure EIS, a test signal source is set up to produce a known signal. The field intensity is measured at the position where the device under test (DUT, which will be a receiver and its antenna) will be located. From this field intensity, the power that would be received by an ideal isotropic receiver is calculated.

The EIS is the minimum such power where the BER is still above the threshold.

Because the EIS is measured with an actual receiver and antenna combination in an anechoic chamber, it will represent the full and actual performance of that particular combination of devices.