As the receiver scans through a number of “empty” channels in auto frequency hop mode until it finally finds the desired signal it is apparent that it spends most of its time on determining the absence of a valid signal. (Let’s call this mechanism no signal detection!)
The characteristics of determining the absence of a signal therefore has a significant impact on overall system performance. The two most important parameters are how fast and how accurately a no signal condition is detected. Typically these two requirements go against each other.
This article goes through the mechanisms through which no signal detection is acquired and discusses the characteristics of each one.
(1) No signal detection with the lack of preamble detection (LoP detection). This method simply asserts a no signal condition if a valid preamble is NOT detected with a given time period from the start of the receive process. Form the nature of method it is self-evident that the receiver has to stay at least as much time on any one channel as it needs for preamble detection. As an example if the demodulator has to see at least 40 bits of preamble for a successful detection and the observation time period expires after 30 bit times it may happen that decision prematurely yields a no signal condition.
The minimum time the receiver needs for preamble detection depends on the configuration of preamble detection threshold and the configuration of the PLL feedback AFC (Automatic Frequency Control). With the recommended 20 bit preamble detection threshold the minimum required preamble length is 32 bits with the PLL feedback AFC being disabled and 40 bits with the PLL feedback AFC being enabled. It follows than that the receiver has to stay at least this much time on each channel to be able to acquire no signal detection. The accuracy of this method matches the accuracy of signal (preamble) detection therefore there will not be any sensitivity degradation with this method.
(2) No signal detection with RSSI measurement. This method asserts a no signal condition if the current RSSI measurement does not exceed the RSSI threshold value before an RSSI timeout period. Compared to the 1st method this one clearly trades off detection accuracy (ultimately sensitivity and immunity) to detection time. Dependent on the averaging configuration (MODEM_RSSI_CONTROL) of the current RSSI measurement the demodulator needs to observe the signal from a fraction of a bit time (only revC2/A2) to several bit times for a valid RSSI measurement. The more averaging is applied in the measurement path the more accurate the measurement becomes. The most crucial point of this method is selecting an appropriate RSSI threshold level as it will ultimately limit overall sensitivity (i.e., valid signals that measure less RSSI will go unnoticed) and will have a great effect on the false signal detection performance (i.e., noise detected as a valid signal). This latter one could ultimately lead to packet losses as the receiver may “waste” too much time on a “false detect” channel to arrive to the wanted channel in time for a detection.
So here is the ultimate trade-off: the higher the threshold the less the sensitivity, the lower the threshold the worse the false detect performance. Both might not be achieved at the same time. One has to assess the cost of false detections and degraded sensitivity to adjust the threshold to one’s specific needs. This process may as well take some experimenting on a lab bench. As a starting point have a look at the following article that elaborates on RSSI reading accuracy: http://community.silabs.com/t5/Wireless-Knowledge-Base/RSSI-accuarcy-on-SI4x6x/ta-p/127172.
Another rather important aspect of this method is that the RSSI measurement cannot distinguish between desired and undesired signals in the channel. This means that detection will occur also on signals that are not meant for the receiver but are in the same channel. The implication of such detection is similar to a “false detection’s” on noise, i.e., the receiver will stay on the channel looking for preamble and may waste too much time to arrive to the wanted channel in time. Ultimately this behavior can cause a PER floor in the link. This behavior constitutes to poor co-channel rejection performance.
With this method signal detection is still done via preamble detection.
(3) No signal detection with the DSA (Digital Signal Arrival) detector. This option is only available on C2/A2 versions of the ICs and can only be applied to (G)FSK modulated signals (i.e., not OOK). The DSA is a feature block in the demodulator that measures some physical parameters of the signal being received and asserts valid signal / no signal conditions based on the results. This block as opposed to working on the demodulated data stream (i.e., at the bit rate as method (1) does) is part of the demodulator itself so is inherently much faster. Typically the DSA only has to observe the signal for a period of two bit times to assert a no signal condition which is a huge improvement on the 32/40 bit times method (1) is capable of. Upon the assertion of a no signal condition the DSA can trigger the receiver’s state machine for hopping to the next channel automatically. How this is done technically is that whenever the DSA block is enabled the PREAMBLE_TIMEOT signal is replaced by the DSA’s no signal detection signal internally. As you can see there is a massive improvement in no signal detection time compared to method (1). How about the accuracy? Engaging the DSA block may result in some sensitivity loss (from a faction of a dB with low FSK modulation indices (<=1) to a few dB with higher modulation indices) compared to method (1). The AFC tracking range of this method matches method (1) and (2) when the PLL feedback AFC is not engaged there. All in all this method nearly matches the performance of method (1), the speed of method (2) at a price of some restrictions on the modulation format (only (G)FSK with preferably a modulation index <= 2).
This article is part of a series that discusses various aspects of auto frequency hopping. Find the links to the other articles below.