1. A threshold adjustment method employed by a communications apparatus, the method comprising: receiving an analog signal; adjusting a threshold of the received analog signal in accordance with a threshold adjust signal to generate a threshold adjusted signal; sampling the threshold adjusted signal to generate a sampled signal; generating a histogram from the sampled signal; processing the histogram to generate an error signal, wherein processing includes calculating a difference between y-intersects of at least two substantially straight curves defined by log values from the histogram, wherein the y-axis represents the distribution of the received signal; and generating the threshold adjust signal in accordance with the error signal.