CMX7163 QAM Modem
CMX7163
7.4.9 Carrier Sense Mode
Carrier sense mode is a receive mode, pending a transmission. A carrier sense period, averaging window
length and threshold must be defined in the Program Blocks prior to entering this mode. When the
CMX7163 is in I/Q receive mode the signal strength is calculated internally – as the I/Q signal contains
amplitude information.
On entry to Carrier Sense mode, reception will begin (or continue if the previous mode was receive) with
an attempt to search for a frame sync. During the defined carrier sense period average RSSI will be
computed over a moving window. Three outcomes are possible:
1. If during the carrier sense period the average RSSI is above the carrier sense threshold then
transmission will be aborted, and search for frame sync will continue. The device reverts to
receive.
2. There is a possibility that a valid frame sync will be detected during the carrier sense period. If this
is the case, the transmission will be aborted immediately and the device reverts to receive.
3. If the RSSI average remains below the carrier sense threshold then transmission will proceed.
In each of the three possible cases, status bits will be used to indicate the result of the carrier sense
period.
If the carrier sense mechanism is used in conjunction with GPIOA as a Tx trigger, operation is as follows:
The device is put in receive, searching for a frame sync. If frame sync is found during this period then it is
indicated to the host via the status bits and normal reception resumes. No carrier sense happens until
GPIOA is used to start the transmit process, at which point carrier sense begins and operation is as
described above.
Note: The Command FIFO and Command Buffer will automatically be flushed when a carrier sense
attempt to transmit results in the CMX7163 reverting to receive mode. This is to avoid accidentally
processing transmit commands pre-loaded by the host as receive commands. This is the only situation in
which the FIFOs or buffers will be flushed other than by direct host instruction.
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