H - Electricity – 04 – L
Patent
H - Electricity
04
L
H04L 12/56 (2006.01) G01S 1/68 (2006.01) G08B 1/08 (2006.01) G08B 21/02 (2006.01)
Patent
CA 2244583
A method and apparatus for accessing a communication channel in a communications system having a transmitter and a receiver. Frequency-hopping spread-spectrum techniques are utilized in which the transmitter of the message assigns a message a priority. Once the message is assigned the priority, the transmitter then selects one of the frequencies from a set of pre-determined randomly-ordered set of frequencies utilized by the spread- spectrum system. Once the frequency is selected, a timer is set to a hop interval and started. The transmitter then senses the channel. If the channel is not clear by the time the timer expires, the transmitter returns to the step of selecting a frequency and resetting the timer. Once an available frequency is discovered, the method waits an interval based on the priority of the message. If the channel is still available the message is then transmitted. If the channel is not available, the timer is then checked, if the timer has expired, then the transmitter returns to the step of selecting a frequency and resetting the timer. Otherwise the transmitter continues to sense the channel. The transmitter waits for an acknowledgment after transmitting the message. If an acknowledgment is not received before the timer expires, then the sequence re-starts. Another aspect of the invention is a fast scanning algorithm in which a next frequency is selected from a pre-determined set of frequencies. The Received Signal Strength Indication (RSSI) at that frequency is then measured. When the measured RSSI is above a threshold, a first timer is started. A second timer is then started when bit synch has been achieved. A data packet is then read and validated. The sequence is repeated continuously. When a frequency is scanned and the RSSI is below a threshold, the method returns to selecting the next frequency. If the first timer expires and bit synch has not been detected, the method stops and returns to select the next frequency. The sequence also aborts and another frequency is selected if the data cannot be validated or there is no more data at the selected frequency. The invention provides apparatus and computer-readable instruction code for carrying out the fast-scanning method, and a system with aspects of both the transmitter and receiver of this invention.
Marks & Clerk
Pronet Tracking Systems Inc.
Spectrum Tracking Systems Inc.
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