Clear channel assessment method and system for ultra wideband OFDM

1. 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.

2. The system of claim 1 , wherein the plurality of frequencies includes 4 frequencies when energies are compared in 6 frequencies.

3. The system of claim 1 , wherein the threshold is adaptively adjusted based on energy in the first shift register.

4. The system of claim 3 , wherein the threshold is equal to half the energy in the first shift register when the half is between a minimum and maximum threshold.

5. The system of claim 1 , further comprising:

a digital receiver capable of using a preamble correlation method to determine if a channel is busy; and

an OR gate coupled to the receiver and the module.

6. A UWB transceiver incorporating the system of claim 1 .

7. A method, comprising:

using a double-energy window to receive a digital signal at a first frequency, the digital signal having a plurality of symbols, and detect a difference in energy that occurs at the start of each symbol; and

determining if a channel is clear based on the difference in energy, the using and determining comprising

storing the digital signal in a first shift register;

storing output of the first shift register in a second shift register;

calculating the energy of the first register and the second register;

determining the difference in energy in the registers;

latching the difference in energy; and

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 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 latched energy difference is greater than the threshold across a plurality of frequencies.

8. The method of claim 7 , wherein the plurality of frequencies includes 4 frequencies when energies are compared in 6 frequencies.

9. The method of claim 7 , further comprising adaptively adjusting the threshold based on energy in the first shift register.

10. The method of claim 9 , wherein the threshold is equal to half the energy in the first shift register when the half is between a minimum and maximum threshold.

11. The method of claim 7 , further comprising: determining if a channel is busy using a preamble correlation method.

12. The method of claim 7 , wherein the method is performed in a UWB transceiver.

13. A system, comprising: means for using a double-energy window to receive a digital signal at a first frequency, the digital signal having a plurality of symbols, and detect a difference in energy that occurs at the start of each symbol; and

means for determining if a channel is clear based on the difference in energy, the means for using and determining comprising

means for storing the digital signal in a first shift register;

means for storing output of the first shift register in a second shift register;

means for calculating the energy of the first shift register and the second shift register;

means for determining the difference in energy in the registers;

means for latching the difference in energy; and

means for 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 latched energy difference is greater than the threshold across a plurality of frequencies.