Prevent Jamming With Fast Frequency Hopping

Key Takeaways

• An overview of the spread spectrum technique. 

• An overview of the frequency hopping spread spectrum technique.

• Comparing slow and fast frequency hopping.

Fast frequency hopping is mainly employed in communication systems where anti-jamming is required

To prevent the hacking and jamming of wireless communication systems, it is common to inject carrier signals of varying frequencies into information signals. The technique of increasing the bandwidth of the signal transmitted using pseudo-random noise is called the spread spectrum technique. 

There are two ways to implement spread spectrum communication techniques: direct sequence spread spectrum and frequency hopping spread spectrum. WiFi connectivity and Bluetooth data sharing often use the frequency hopping spread spectrum technique. The types of frequency spread spectrum techniques are the slow frequency spread spectrum technique and the fast frequency spread spectrum technique. Fast frequency hopping is most suitable with non-coherent modulations to combat jamming in communication systems. In this article, we will look at these techniques as well as how to compare them.

Shannon’s Theorem and the Spread Spectrum Technique

According to Shannon’s theorem, when the bandwidth of a signal is wider, the signal is received correctly, even in noisy environments. There is a direct relationship between the channel bandwidth and the signal-to-noise ratio (SNR). The idea of increasing the bandwidth of narrowband information signals to increase the SNR at the receiving end after conversion is the fundamental principle of the spread spectrum technique. With this widening bandwidth concept, the spread spectrum technique is classified into four types based on the point at which the pseudo-random noise is injected into the communication channel:

1. Time hopping spread spectrum technique (THSS) - Pseudo-random noise is injected as an on/off gate to the signal to be transmitted.
2. Chirp spread spectrum technique (CSS) - When the carrier frequency is swept linearly in time.
3. Direct sequence spread spectrum technique (DSSS) - Pseudo-random noise is injected at the data level.
4. Frequency hopping spread spectrum technique (FHSS) - Pseudo-random noise is injected at the carrier frequency level.

The Frequency Hopping Spread Spectrum Technique 

In the frequency hopping technique, the transmitter injects the carrier frequency according to a specific hopping pattern known only to the sender and receiver. The signal transmitted sees several channels and various sets of interfering signals during each frequency hopping event. There is no distinctive frequency corresponding to the signal transmitted to interfere or jam it. This prevents communication failures due to signal fading or jamming.

Let’s focus on one method of frequency hopping: the fast frequency hopping method. 

Fast Frequency Hopping 

In fast frequency hopping (FFH), multiple frequency hops are utilized for transmitting one data bit. When the fast frequency hopping spread spectrum technique is employed, the ratio TS/TH is greater than unity, where TS is the duration of the information signal or symbol and TH is the frequency hopping time. A ratio greater than unity indicates that the carrier frequency changes repeatedly during the duration of the signal to be transmitted. 

Applications

Fast frequency hopping is mainly employed in communication systems where anti-jamming is required, as it is the best spread spectrum technique to use along with non-coherent modulations to combat jamming. Along with fast frequency hopping, the modulations commonly used are frequency shift keying (FSK) and multiple frequency shift keying (MFSK).  

Signal Detection

Coherent signal detection is difficult when fast frequency hopping is utilized. Usually, non-coherent detection is used and the orthogonality between the FSK tones is ensured by keeping a minimum distance between them, which is equal to the multiples of fH, where fH is the hop frequency . In this way, the crosstalk between the hop frequency bands is eliminated in fast frequency hopping.

A Comparison of Slow and Fast Frequency Hopping

A comparison of slow and fast frequency hopping

Whenever a signal transmission is facing the threat of jamming or hacking, fast frequency hopping is a suitable spread spectrum technique to use as a countermeasure. Cadence software offers assistance in designing wireless communication systems utilizing spread spectrum modulation.  

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