Clear channel assessment
A clear channel assessment procedure is described. The procedure includes setting a threshold value of signal strength. When a message is ready to transmit, the received signal strength intensity (RSSI) is measured, and compared with the threshold. If the received signal strength is less than the threshold value, the message is transmitted, otherwise the system waits before trying again. After a predetermined number of failed attempts the threshold value is increased. The threshold value may be reduced if no acknowledgement is received.
1. A method of operating a radio, the method including:
(a) setting a threshold value of signal strength to be an initial threshold value;
(b) waiting for the radio to have a message to transmit on a channel;
(c) attempting to transmit the message by:
(d) measuring the received signal strength on the channel;
(e) comparing the received signal strength with the threshold value, and
(f) if the received signal strength in the channel is less than the threshold value, transmitting the message and reducing the threshold value to the received signal strength plus a predetermined amount; or
(g) if the received signal strength is at least the threshold value, waiting for a period of time and repeating from step (c); and
(h) increasing the threshold value if the comparison in step (e) determines that the received signal strength is at least the threshold value for a number n of attempts to transmit the message, where n is a positive integer.
2. The method according to claim 1 , wherein step (a) of setting the initial threshold value includes:
(a1) measuring the signal strength on the channel a plurality of times over a time period;
(a2) determining the lowest measured value;
(a3) setting the initial threshold value to be the lowest measured value added to a predetermined margin.
3. The method according to claim 1 , wherein each attempt to transmit the message includes:
if in step (e) the received signal strength is less than the threshold value repeating steps (d) and (e) of measuring the received signal strength in at least one channel and comparing the received signal strength with the threshold;
and transmitting the message and reducing the threshold value to the received signal strength plus a predetermined amount only if the received signal strength remains less than the threshold.
4. The method according to claim 1 , wherein the number n of attempts is predetermined to be 2, 3 or 4.
5. The method according to claim 1 , wherein steps (c) to (g) are carried out by a medium access layer.
6. The method according to claim 1 , wherein the transmitting step uses half duplex transmission.
7. A radio, comprising:
a transmitter and receiver for transmitting and receiving in one or more channels;
a control processor for controlling the transmitter and receiver, wherein the control processor includes code arranged to cause the radio to carry out the steps of:
(a) setting a threshold value of signal strength to be an initial threshold value;
(b) waiting for the radio to have a message to transmit;
(c) attempting to transmit the message by:
(d) measuring the received signal strength on at least one channel;
(e) comparing the received signal strength with the threshold value, and
(f) if the received signal strength on the at least one channel is less than the threshold value, transmitting the message and reducing the threshold value to the received signal strength plus a predetermined amount; or
(g) if the received signal strength is at least the threshold value, waiting for a period of time and repeating from step (c); and
(h) increasing the threshold value if the comparison in step (e) determines that the received signal strength is at least the threshold value for a number n of attempts to transmit the message, where n is a positive integer.
8. The radio according to claim 7 , wherein the radio is an ISM band radio.
9. The radio according to claim 7 , wherein the code is arranged to cause the radio to carry out the step (a) of setting the initial threshold value using the sub-steps of:
(a1) measuring the signal strength on the at least one channel using the receiver a plurality of times over a time period;
(a2) determining the lowest measured value;
(a3) setting the initial threshold value to be the lowest measured value added to a predetermined margin.
10. The radio according to claim 7 , wherein the code is arranged to cause the radio in each attempt to transmit the message to carry out the steps of:
if in step (e) the received signal strength is less than the threshold value repeating steps (d) and (e) of measuring the received signal strength on the at least one channel and comparing the received signal strength with the threshold;
and transmitting the message and reducing the threshold value to the received signal strength plus a predetermined amount only if the received signal strength remains less than the threshold.
11. The radio according to claim 7 , wherein the transmitter and receiver are half duplex.
12. A computer program product comprising a computer readable storage structure embodying computer program code thereon for execution by a processor in a radio that enables the radio to perform the steps of:
(a) setting a threshold value of signal strength to be an initial threshold value;
(b) waiting for the radio to have a message to transmit on a channel;
(c) attempting to transmit the message by:
(d) measuring the received signal strength on the channel;
(e) comparing the received signal strength with the threshold value, and
(f) if the received signal strength in the channel is less than the threshold value, transmitting the message and reducing the threshold value to the received signal strength plus a predetermined amount; or
(g) if the received signal strength is at least the threshold value, waiting for a period of time and repeating from step (c); and
(h) increasing the threshold value if the comparison in step (e) determines that the received signal strength is at least the threshold value for a number n of attempts to transmit the message, where n is a positive integer.