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.
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.
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