The previous articles in this series explained the various mechanisms and their performances auto frequency hopping can be achieved with.
This article elaborates on the configuration of the various no signal detection schemes discussing the Rx state machine options and the timeout parameters for the LoP (Lack of Preamble) and RSSI measurement schemes.
(1) Rx state machine in auto frequency hopping mode
On the Frequency hop tab in WDS in the Auto frequency hop Rx project there are 5 options for hopping (even though we only discussed 3 before) and the DSA is not even mentioned so some explanation is due.
The reason that we have more options than no signal detection mechanisms is that we provide for “backup” no signal detection mechanism in case the primary algorithm decides falsely on detection. With such backup mechanisms we can make the receiver “hop on” as opposed to checking (or getting stuck on) the same channel again. (These mechanisms are readily available in the form of preamble and sync word detections; we can simply opt for hopping if they are not detected.)
The backup no signal detections that can optionally make the receiver hop for the various primary no signal detection schemes are listed below:
Lack of preamble detection: (1) invalid sync word detection
RSSI measurement: (1) lack of preamble detection and (2) invalid sync word detection
DSA detection: (1) invalid sync word detection
The combinations of these events make up the list in WDS.
DSA detection, however does not explicitly appear in the list. The reason for this is that when the DSA is enabled it’s no signal detection event will automatically replace the preamble timeout event internally. From the hopping mechanism's point of view it does not matter what exactly the source of the preamble timeout event is, which implies that the schemes where the primary no signal detection is preamble timeout will be “reused” if the DSA is enabled (shown below).
For the sake of clarity find the Rx state machine operation for all combinations below:
(1) Hop on preamble timeout
(2) Hop on RSSI timeout or preamble timeout
(3) Hop on preamble timeout or invalid sync word
(4) Hop on RSSI timeout or preamble timeout or invalid sync word
(2) Timeout configurations for the LoP and RSSI measurement schemes
Both the LoP and RSSI measurement no signal detection algorithms need a timeout parameter. Both can be set in the red highlighted region on below WDS screenshot.
For both of these schemes there is a maximum time needed for the receiver to make a good decision. Obviously the timeout interval must be long enough to incorporate this time period. The important consideration here is that these maximum times (captured in the tables in the Timing article) are valid from the demodulator’s perspective. As discussed in the Timing chapter by the time the signal reaches the demodulator it suffers a propagation delay through the Rx chain (T_prop_del). So to ensure the demodulator can indeed “see” the signal for the defined maximum time period the receiver must dwell T_prop_delay + T no_signal amount of time on any one channel. Now, the timeout parameters (both preamble and RSSI) do measure the whole time the receiver dwells on any one channel therefore they shall be set to T_TO = T_PropDelay + T_NoSignal, where T_TO stands for T_TimeOut.
For example with the LoP detection scheme (when the PLL FB AFC is enabled) the demodulator has to see 40 bits of preamble for a good decision therefore the preamble timeout parameter shall be set to T_TO_pr = 40 Tb + 4 Tb = 44 Tb (11 nibble Tb).
Similarly with RSSI measurement detection with averaging the result over 4Tb the RSSI timeout parameter shall be set to T_TO_rssi = 4Tb + 4Tb = 8 Tb (2 nibble Tb).
This article is part of a series that discusses various aspects of auto frequency hopping. Find the links to the other articles below.