Will the signal be received?

For communication to take place, the receiver must accurately detect the transmitted signal. That is not as simple as it sounds. Here are three possible problems.

1. The signal may not have sufficient power

As the electromagnetic wave propagates away from the transmitter, it attenuates. The power drops off with distance and the signal can also be blocked by obstructions. As the power drops, error rates will increase. If the signal power is not above the sensitivity threshold of the receiving radio, it will not be detected.

If low power results in transmission errors, the radios may be programmed to switch to a modulation technique with a lower spectral efficiency. For example, the IEEE 802.11b (WiFi) protocol specifies four alternative modulation schemes. If too many errors occur at the highest speed, 11 Mbps, the radios step down to 5.5, 2 or 1 Mbps.

2. The receiver may not be tuned to the transmitting frequency

The receiving radio must be tuned to detect signals at the frequency of the transmission. Radios contain electronic devices called filters which enable them to detect waves in certain frequency bands while ignoring others.

We are surrounded by electromagnetic radiation. For example, radio and TV signals are probably coming into the room you are sitting in right now, but you are unaware of them. If there is a radio or TV set in the room with you, it can detect the signals. But, which of those many signals do you want to detect?

When you change the station on your car radio or change TV channels, you are changing the frequency it is detecting. When you tune your TV set to channel 2, you are telling it to filter out, to ignore, all signals but those between 54 and 60 MHz. When you switch to channel 4, you are telling it to tune its filter to detect only signals in the 66 to 72 MHz band.

Note that, since filters are not perfect, the FCC would not grant a license to use channel 3 in an area where channels 2 and 4 are in use.

3. Other transmissions may interfere with the signal

You only wish to receive one signal. Other radio waves in the same frequency range will interfere with the signal you are trying to detect. If this noise is sufficiently powerful, communication will degraded or stopped.

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