Near-far problem
Encyclopedia
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.
The near-far problem is particularly difficult in CDMA systems where transmitters share transmission frequencies and transmission time.
In contrast, FDMA and TDMA
systems are less vulnerable.
The issue of the dynamic range
of one or more stages of a receiver limiting that receiver’s ability to detect a weak signal in the presence of strong signal has been around for a long time. 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. This 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 the dynamic range of the system is limited by the dynamic range of the receiver ADC.
(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 20 feet 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 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 a couple of feet 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.)
Taking this analogy back to wireless communications, the far transmitter would have to drastically increase transmission power which simply may not be possible.
Other possible solutions to the near-far problem:
Radio jamming
Radio jamming is the transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency...
techniques, the near-far problem is exploited to disrupt communications.
Technical explanation
The near-far problem is a condition in which a strong signal captures a receiver making it impossible for the receiver to detect a weaker signal.The near-far problem is particularly difficult in CDMA systems where transmitters share transmission frequencies and transmission time.
In contrast, FDMA and TDMA
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
systems are less vulnerable.
The issue of the dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
of one or more stages of a receiver limiting that receiver’s ability to detect a weak signal in the presence of strong signal has been around for a long time. The near-far problem usually refers to a specific case of this in which ADC
Analog-to-digital converter
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
resolution limits the range of signals a receiver can detect in a direct sequence spread spectrum (DSSS
Direct-sequence spread spectrum
In telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...
) system such as CDMA. The receiver’s AGC
Automatic gain control
Automatic gain control is an adaptive system found in many electronic devices. The average output signal level is fed back to adjust the gain to an appropriate level for a range of input signal levels...
must reduce its gain to prevent ADC saturation. This 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 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 ratioSignal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
(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 20 feet 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 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 a couple of feet 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
Signal processing
Signal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
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.
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:
- Increased receiver dynamic range - Use a higher resolution ADC. Increase the dynamic range of receiver stages that are saturating.
- Dynamic output power control – Nearby transmitters decrease their output power so that all signals arrive at the receiver with similar signal strengths.
- TDMA – Transmitters use some scheme to avoid transmitting at the same time.
- FDMA – Decrease the number of users per signal bandwidth to fewer stations are required to share the same bandwidth at the same time.
See also
- Direct Sequence Spread SpectrumDirect-sequence spread spectrumIn telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...
- Signal-to-noise ratioSignal-to-noise ratioSignal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...
- Capture effectCapture effectIn telecommunication, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency will be demodulated....