Whenever blocking interference is mentioned in this article, it refers to interference that is located outside the communication channel and exceeds the capacity of the receiver circuit, which can cause a decrease in the receiver's ability to process normal signals.
The use of spread spectrum and frequency hopping technology is beneficial for noise interference, but it cannot improve the blocking level of the receiver. On the contrary, due to the need for a wider front stage, blockage is more likely to occur. Here, the definition of blocking level is the interference level required to compress the receiving sensitivity by 6dB, which is located outside the instantaneous passband of the receiver.
In order to improve sensitivity, civilian receivers usually enter low-noise amplifiers and mixers after simple filtering of antenna signals. From the perspective of power saving, these circuits cannot use high-power devices as their dynamic range is relatively small. Usually, they only need to provide interference signals of around -20dBm. Even if there is a small deviation between the interference frequency and the receiving frequency, it can reduce the receiving sensitivity by 6dB. At this point, -20dBm is the blocking level of the receiver. If the interference is further enhanced, the receiver will not receive any useful signals at all. If there is no appropriate limiting circuit in the front-end of the receiver, stronger interference can burn it out.
The number of bits in ADC is usually only 12 or 14, which limits its dynamic range. In order to adopt high-speed frequency hopping schemes, the bandpass of the intermediate frequency filter is usually large, and interference on non receiving frequencies can also reach the ADC. A slight increase in interference can overload the ADC, or if AGC is used to prevent the ADC from overloading, the normal signal will be weak to less than 1 bit when it reaches the ADC
Taking the common transceiver chip AD9361 as an example, it can hardly resist out of band interference higher than -24dBm. It is not complicated to induce a power of -24dBm on the receiver. Taking a distance of 100 meters and a gain of 3dB for both the transmitting and receiving antennas as an example, the required power is -24+32.45+68-20-6=50.45dBm，100W。
Blocking interference, due to its simplicity and effectiveness, is currently the most commonly used refusal method by "relevant departments" who do not have to worry about legal responsibility for interfering with other communication services. Due to high radiation, it is generally not possible to continuously turn on the drone for defense, and it is required to only turn on the drone when it is seen.
The aiming interference described in this article is a targeted interference applied based on the instantaneous frequency and startup time of the interfered signal. Although regular drones have their approved frequency range, some drones may use any frequency to engage in worrisome activities. If all interference is required, the required power is high, the operating range is short, and the impact on normal communication is difficult to eliminate. Narrowband data transmission or frequency hopping signals have a fixed frequency at any moment, and if only targeting these frequencies, interference power can be greatly saved. For simple direct sequence spread spectrum, aiming interference is usually not defined
A typical aiming interference scenario is shown in the following figure. The reconnaissance receiver continuously monitors possible communication frequency bands and sends data to the computer. When the computer detects the signal from the remote control, it immediately informs the interfering transmitter of the parameters that need to be interfered with, causing the interfering transmitter to start transmitting. After a period of time (e.g. 1 millisecond), the interference is paused and the reconnaissance receiver continues to search for the remote control signal. If the remote control signal continues to exist or changes frequency, the new parameters are notified to the transmitter and the interference is restarted. If the remote control signal disappears, stop interfering. Separating the receiver and transmitter allows for simultaneous reconnaissance and interference.
The advantage of this type of interference is that it does not emit interference without a signal, and the interference level is very low, making it highly environmentally friendly. If the remote control signal is not spread spectrum, it is usually sufficient to make the receiving level equal or slightly higher. If it is a spread spectrum signal, due to the low spread spectrum gain, it usually only needs to be within 20dB. The power setting can be determined based on the instantaneous bandwidth of the remote control signal, and can be appropriately increased when the bandwidth is wide. Regardless of frequency or bandwidth, it can be measured by reconnaissance receivers. If technology allows, modulation methods can also be determined, and certain signals (such as WIFI signals near defenders) can be detected.
The main challenge of targeting interference is response speed. If the hopping speed is 1000 hops per second, the dwell time of a single frequency point is only 1ms. Based on half of the interference, there is only 500 μ s of time for reconnaissance, analysis, judgment, command, and transmitter activation. Now this indicator can be easily achieved. If specific identification of signal types is not required and only FFT and spectral type judgment are performed, the entire process can be completed within a few microseconds. However, the transmitter requires special design to be tuned so quickly and achieve sufficient power. Fortunately, the jumping speed of the remote control is not fast now.
In addition, the defense situation of the reconnaissance receiver should also be considered. The height of the drone is relatively high, and it is possible that the drone can receive remote control signals, while the reconnaissance receiver on the ground cannot receive them. At this point, it is necessary to raise the antenna and increase the gain. But it can also lead to receiving many non remote control signals, especially when the fortified area is inside the city. This will pose high requirements for signal recognition. If the remote control simulates common urban signals such as WIFI signals or uses WIFI technology, the difficulty will be relatively high.
The entire equipment is relatively expensive. If the frequency hopping range is further widened or other UWB technologies are used, the cost of reconnaissance and jamming equipment will further increase
Hot News2024-08-15
2024-08-15
2024-08-15
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