Noise or Barrage Jamming against Radio Communications
The noise jamming essentially consists in generating into the target’s receiver the increasing of the background (thermal) noise level, then reducing the receiver’s Signal to Noise Ratio (SNR).
Noise is the nemesis for any communication system and if the noise level can be increased, it makes it more difficult for the communication system to operate.
At the very least it decreases the range over which the communication system is effective.
In some cases, this effect is adequate to accomplish the goals of ECM.
In other cases, communication can be denied totally.
The first general concept to follow designing an effective Noise jammer is the balance between the frequency coverage against the target’s signal bandwidth and the available jammer transmitted power.
As for any jamming technique, the effectiveness of this jammer depends on the part of its own IBW entering the target’s receiver (Figure 1).
Of course, the optimal performance is when the jamming signal’s IBW matches perfectly the target signal’s IBW, featuring the highest J/S ratio for the specific condition (Jammer power over Signal power ratio rules the effectiveness of the jamming).
In this condition, the available jamming power totally enters the target’s receiver bandwidth.
The above condition is generated against frequency fixed target signals that are signals without any TRANsmission SECurity (TRANSEC) capability.
As known, TRANSEC aims to generate radio signal having a highest level of robustness against the jamming signals, thus performing either spectrum spreading or frequency hopping or sometimes both.
In order to engage effectively a target signal, a jamming system coordinates its operation with a previous sensing aiming to detect the signal and identify its spectrum characteristics, mainly its bandwidth, centre frequency and hopping rate, if any.
This knowledge provides the jammer with the best information necessary to tune its spectrum and timing behaviour.
Broadband noise, sometimes called as Barrage Noise (BBN), is a jamming signal transmitting noise energy across the entire, or partial, width of the frequency spectrum used by the target communication systems.
This type of jamming is useful against all forms of AJ communications when the target signal performs a hopping so fast to make ineffective narrow band jamming.
The J/S ratio constrains the channel capacity of a communication system.
Shannon first investigated this capacity, assuming the noise has a Gaussian probability distribution, in 1948 .
It expresses the maximum data rate that the channel can carry with an arbitrarily small error rate. If an attempt is made to transmit a digital signal through the channel with a higher bit rate than that given by the capacity, then errors are assured in the received signal.
The capacity of a channel corrupted by such noise is given by the following function (1)
where B is the bandwidth of the target signal (IBW), S is the average power of the signal, and N is the total average noise present given by N = (N0 + J0)/B, where N0 is the thermal noise and J0 is the jamming total transmitted power.
Clearly, as the noise level is raised by intentionally inserting Gaussian noise into the channel, the SNR decreases, thus decreasing the channel capacity.
Coloured Gaussian noise is Gaussian noise that has been subject to filtering and is the appropriate type to use in situations where the filtering effects are important.