near-far problem

The near-far problem is a situation that is common in wireless communication systems (in particular, CDMA). The near-far problem can also be called the hearability problem. In some signal jamming techniques the near-far problem is exploited to disrupt communications.

Technical explanation

The problem is this: consider a receiver and two transmitters (one close to the receiver; the other far away). If both transmitters transmit simultaneously and at equal powers then the receiver will receive more power from the nearer transmitter. This makes the farther transmitter more difficult, if not impossible, to "understand." Since one transmission is the other's noise the Signal-to-noise ratio (SNR) for the farther trasmitter is much higher. If the nearer transmitter transmits a signal in magnitudes of order over the farther transmitter then the SNR for the farther transmitter may be below detectability and the farther transmitter may just as well not transmit. This effectively jams the communication channel. In CDMA systems or other cellular phone-like networks, this is commonly solved by dynamic output power adjustment of the transmitters. That is the closer transmitters use less power so that the SNR for all transmitters at the receiver is roughly the same.

Analogies

To place this problem in more common terms, imagine you are talking to someone 20 feet away. If the two of you are in an anechoic chamber then a conversation is quite easy to hold at normal voice levels. Instead, now, imagine two people are yelling into your friend's ears. Your friend will not be able to understand anything you say. In order for your friend to hear you you would have to use loudspeakers and turn the volume way up such that you overpower the other two yelling in your friend's ears. Taking this analogy back to wireless communications, the far transmitter would have to drastically increase transmission power which simply may not be possible. This analogy could also be extended to headlights on an automobile, though much more loosely. On a dark night and facing directly into the headlights, it is very difficult to read the license plate, the hood ornament, or any other detail that is dimly-lit. In this case, the brightness of the headlights is many times brighter than the light reflecting off other parts of the car. In short, the near-far/hearability problem is one of detecting and receiving a weaker signal amongst stronger signals.

Near-far Problem

Technical explanation

Analogies

See also