> 3 | Analyzing Radio Link Stability

> Employing the "lsta" URCs described in Chapter 2 it is possible to analyze the stability of the radio link in order to detect possible jamming by an RF transmitter emitting a strong jamming signal evenly distributed accross the GSM bands. This jamming leads to the module loosing its radio link even though the number of channels as well as the mean of the received signal strength across frequencies are high.
> The first type of "lsta"-URC (<indValue>=0) indicates the stability of the radio link. If <lstaEdv>=0, the radio link has been lost. In combination with a rising received signal strength (<lstaRssi>) this could be a first indication for a jammer. However, this type of URC is issued as a warning only during IDLE or SLEEP mode.
> The second type of "lsta"-URC (<indValue>=1) indicates a loss of radio link. It may confirm the first jamming indication and allows assumptions about the reasons for the network loss. This URC is also issued during a voice or data call connection.
> The following assumptions can be made based on a full band scan and its appropriate URC:

> • | If <lstaNo>, i.e., the number of registered ARFNCs, is less than 40, the module may be out of network coverage.

> • | If <lstaVar>, i.e., the signal strength variance across frequencies, is high (~>10), there could be industrial interferences of the radio link. Industrial (non-jamming) interference signifies unintentional radio link disturbances by strong industrial radio sources.

> • | If the <lstaVar> is low and the <lstaMean> is high (~>40), this could be an indication for jamming. Depending on the distance of the jammer, the <lstaMean> could also be medium.

> The terms "high", "medium" and "low" are used in the above assumptions for <lstaMean> and <lstaVar> since it is not possible to state exact value ranges for these parameters that could indicate a jammer. The parameter values are "high" or "low" if compared to the values in a normal transmission situation.
> The following Figure 2 illustrates how the issued URCs might be analyzed to determine whether a radio link is corrupted by a jammer or not.
> Section 3.1 lists URC examples for radio link monitoring and jamming indications.
> Please note that in order to monitor the module’s network registration status and to note whether a possible jammer might be on or off again while the module is registered the +CREG URC as well as the +CIEV URCs ("service", "rssi" and "lsta" indicators) should be enabled and checked. For details on how to enable these URCs please see [1].
> Figure 2: Radio link stability analysis