Patent ID: 7804917

Claim:
A clear channel assessment system, comprising: a double-energy window module capable of receiving a digital signal at a first frequency, the digital signal having a plurality of symbols, and detecting a difference in energy that occurs at the start of each symbol and using the difference to determine if a channel is clear; the module comprising: a first shift register capable of receiving the digital signal; a second shift register, coupled to the first shift register, capable of receiving output of the first shift register; a first energy calculator, coupled to the first shift register, capable of calculating an energy of the first register; a second energy calculator, coupled to the second shift register, capable of calculating an energy of the second shift register; an adder, coupled to the first energy calculator and the second energy calculator, capable of determining a difference in energy of the first and second registers; a latch, coupled to the adder, capable of latching the difference in energy; and an energy difference processor, coupled to the latch, capable of determining if a channel is busy based on the latched energy difference by comparing the latched energy difference to a threshold on each clock cycle of a plurality of clock cycles; repeating the comparing if the latched energy difference is less than the threshold; waiting and then repeating the comparing if the latched energy difference is greater than the threshold; returning to the comparing if the repeated comparing is not greater than the threshold; hopping to another frequency and comparing the latched energy difference to the threshold at the other frequency; and repeating the hopping and comparing; wherein a channel is busy if the energy difference is greater than the threshold across a plurality of frequencies.