Near-far problem

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

Technical explanation

The near-far problem is a condition in which a receiver captures a strong signal and thereby makes it impossible for the receiver to detect a weaker signal.^[1]

The near-far problem is particularly difficult in CDMA systems, where transmitters share transmission frequencies and transmission time. By contrast, FDMA and TDMA systems are less vulnerable. There is a long-standing issue that the dynamic range of one or more stages of a receiver can limit that receiver’s ability to detect a weak signal in the presence of strong signal. The near-far problem usually refers to a specific case of this in which ADC resolution limits the range of signals a receiver can detect in a direct sequence spread spectrum (DSSS) system such as CDMA. The receiver’s AGC must reduce its gain to prevent ADC saturation, which causes the weaker signal to fall into the noise of the ADC. This is different from a condition of one signal interfering with another because if the ADC had sufficient resolution, it would be possible to recover both signals.

DSSS allows multiple transmitters to use the same bandwidth at the same time. One price of such a system is that the dynamic range of the system is limited by the dynamic range of the receiver ADC.

Analogies

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. Since one transmission's signal is the other's noise, the signal-to-noise ratio (SNR) for the farther transmitter is much lower. This makes the farther transmitter more difficult, if not impossible, to understand. 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 short, the near-far problem is one of detecting or filtering out a weaker signal amongst stronger signals.

To place this problem in more common terms, imagine you are talking to someone 6 meters away. If the two of you are in a quiet, empty room then a conversation is quite easy to hold at normal voice levels. In a loud, crowded bar, it would be impossible to hear the same voice level, and the only solution (for that distance) is for both you and your friend to speak louder. Of course, this increases the overall noise level in the bar, and every other patron has to talk louder too (this is equivalent to power control runaway). Eventually, everyone has to shout to make themselves heard by a person standing right beside them, and it is impossible to communicate with anyone more than half a meter away. (In general, however, a human is very capable of filtering out loud sounds; 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.)^[2]

Taking this analogy back to wireless communications, the far transmitter would have to drastically increase transmission power which simply may not be possible.

Solutions

In CDMA systems and similar cellular phone-like networks, the problem 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.

Other possible solutions to the near-far problem:

1. Increased receiver dynamic range - Use a higher resolution ADC. Increase the dynamic range of receiver stages that are saturating.
2. Dynamic output power control – Nearby transmitters decrease their output power so that all signals arrive at the receiver with similar signal strengths.
3. TDMA – Transmitters use some scheme to avoid transmitting at the same time.
4. FDMA – Decrease bandwidth so fewer stations are required to share the same bandwidth at the same time.

See also

• Direct-sequence spread spectrum
• Signal-to-noise ratio
• Capture effect

References