Receiver Sensitivity

Value to Enter:

Enter the dBm value specified by your radio equipment vendor for the receive sensitivity at a particular bit rate. In the absence of vendor specifications, use the following sensitivity values:

Significance of This Value:

When the calculator makes an assessment regarding the suitability of your collective input values it weighs the power and gain in your system against the aggregate losses (Free Space Path Loss, cable attenuation, rain fade, and link fade.) If the power and gain is able to provide a received signal strength that is at, or above, your specified receiver sensitivity value the link is considered suitable. The following table shows some general receive sensitivity values for 802.11b devices. Note that model numbers are not included because this table is only intended to give a perspective on the fact that as signal drops below -80 dBm you approach the minimum receive sensitivity for most 802.11 gear.

For example, an 802.11b/g radio, operating with a 30mW output (like a typical PCMCIA or built-in Wi-Fi adapter for a notebook computer) may specify an 54Mbps bit rate at -70dBm dropping to a 1 Mbps bit rate (with 802.11b) at -90 dBm. A 800 MHz receiver (like a cell phone) might have a receive sensitivity closer to -115 dBm. This level of sensitivity (-115 dBm) is approximately the level at which background noise, caused by the noise temperature of the earth, begins to enter the equation. The 45 dB difference between -70dBm and -115dBm implies that the 800 MHz receiver is over 30,000 times more sensitive than the 802.11b radio. (That's right, over thirty thousand times more sensitive!).

Background and Technical Perspective:

Noise Floor

The circuitry in a radio receiver must be capable of electrically manipulating the current induced into the antenna by the radio signal. The induced signal is very weak. Radio circuitry is electronic in nature and therefore creates some amount of heat when electricity flows through it. This heat causes electronic noise in the receiver circuitry. An expensive piece of equipment is designed to produce less internal thermal noise. As might be imagined, signal energy can not be distinguished from thermal noise when the signal energy if very weak. This level is called the noise floor for a receiver.

Receive Sensitivity

The signal level required for proper operation of a receiver is called its receive sensitivity. In order to get signal to the receive circuitry at a level above the noise floor it's necessary to overcome several additional losses in the receiving radio unit. Antenna coupling loss is an example of what must be overcome to achieve a circuit signal level above the noise floor. The antenna element has energy induced into it by the fluctuating electromagnetic field coming from the transmitter. This energy must be transferred to the wires, circuit board traces, IC's, and discrete components making up the receiver circuitry. The energy transfer occurs through an antenna coupling. Often this takes the form of a pair of inter-circled wire loops in the form of an electrical transformer (called inductive coupling. Antenna coupling introduces loss.