CC2420 Low Power Communications Design Considerations
The CC2420 radio stack low power listening implementation relies on clear channel assessments to determine if there is a transmitter nearby. This allows the receiver to turn on and determine there are no transmitters in a shorter amount of time than leaving the radio on long enough to pick up a full packet.
The transmitters perform a message delivery by transmitting the full packet over and over again for twice the duration of the receiver's duty cycle period. Transmitting for twice as long increases the probability that the message will be detected by the receiver, and allows the receiver to shave off a small amount of time it needs to keep its radio on.
Typically, the transmission of a single packet takes on the following form over time:
+----------------------+-----------+--------------------+ | Backoff | Packet Tx | Ack Wait | +----------------------+-----------+--------------------+
To decrease the amount of time required for a receive check, the channel must be modulated by the transmitter as continuously as possible. The only period where the channel is modulated is during the Packet Transmission phase. The receiver must continuosly sample the CCA pin a moment longer than the backoff period and ack wait period combined to overlap the Packet Transmission period. By making the backoff period as short as possible, we can decrease the amount of time a receiver's radio must be turned on when performing a receive check. However, this also increases the probability of collision or channel capture.
If two transmitters attempt to transmit using low power listening, one transmitter may capture the channel if its backoff period is set too short. Both nodes transmitting at the same time will cause interference and prevent each other from successfully delivering their messages to the intended recipient.
To allow multiple transmitters to transmit low power listening packets at the same time, the backoff period needed to be increased greater than the desired minimum. This increases the amount of time receiver radios need to be on to perform a receive check because the channel is no longer being modulated as continuously as possible. In other words, the channel is allowed to be shared amongst multiple transmitters at the expense of higher power consumption.
