Near-far problem
From Wikipedia, the free encyclopedia
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.
[edit] 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 due to the inverse square law 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's signal is the other's noise the signal-to-noise ratio (SNR) for the farther transmitter is much lower. If the nearer transmitter transmits a signal that is orders of magnitude higher than 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. This sometimes can have a noticeable impact on battery life, which can be dramatically different depending on distance from the base station. In high-noise situations, however, closer transmitters may boost their output power, which forces distant transmitters to boost their output to maintain a good SNR. Other transmitters react to the rising noise floor by increasing their output. This process continues, and eventually distant transmitters lose their ability to maintain a usable SNR and drop from the network. This process is called power control runaway. This principle may be used to explain why an area with low signal is perfectly usable when the cell isn't heavily loaded, but when load is higher, service quality degrades significantly, sometimes to the point of unusability.
[edit] 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 radically increase the volume such that you overpower the other two yelling in your friend's ears. This is as said just an analogy, in fact a human is very capable of filtering out loud sounds from other sounds, and sometimes the solution to not hearing your friend would be to actually turn up the stereo, results may vary. Precise citation needed [Swe,Mag,"Illustrerad Vetenskap" 1980-1990 somewhere]. Similar techniques can be deployed in signal processing where suitable criteria for distinguishing between signals can be established, see signal processing and notably adaptive signal processing.
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. Seeing stars during the daytime is similar: The background sky light signal drowns out the weaker light of the star.
In short, the near-far/hearability problem is one of detecting and/or filtering out a weaker signal amongst stronger signals.
