Telecom Techniques Guide


Slow and Fast Frequency Hopping in GSM

Slow and Fast Frequency Hopping in GSM

Frequency hopping can be briefly defined as a sequential change of carrier frequency on the radio link between the mobile and the base station.

In GSM, one carrier frequency is divided into eight time slots. Each time slot provides one physical channel, which can be assigned to one link between a mobile and a base station.

The communication between the mobile and the base station occurs in bursts inside the assigned time slot. Each burst lasts about 577 ms.

When frequency hopping is used, the carrier frequency may be changed between each consecutive TDMA frame.

This means that for each connection the change of the frequency may happen between every burst.

This is called Slow Frequency Hopping (SFH), because more than one bit is transmitted using the same frequency.

In Fast Frequency Hopping (FFH), the carrier frequency is allowed to change more than once during a bit duration, but this is not implemented in GSM At first, the frequency hopping was used in military applications in order to improve the secrecy and to make the system more robust against jamming.

In cellular network, the frequency hopping also provides some additional benefits such as frequency diversity and interference diversity.

gsm base frequency hopping

The basic principle of frequency hopping is presented in Upper Figure.

Slow Frequency Hopping in GSM:

Slow Frequency Hopping is an essential feature in GSM (Global System for Mobile Communications) that enhances the system’s resistance to interference and eavesdropping. In slow frequency hopping, a mobile device or base station periodically changes its frequency within the allocated frequency band, typically with a slow hopping rate. This deliberate and gradual frequency change is synchronized between the transmitter and receiver, ensuring that both parties are aware of the hopping pattern.

One primary advantage of slow frequency hopping is its ability to combat frequency-selective interference. By changing frequencies slowly, the system can mitigate the impact of narrowband interference sources. This is particularly valuable in GSM, where multiple users share the same frequency band, and interference from adjacent channels can be a concern. Slow frequency hopping helps ensure that the system maintains a reliable and interference-free connection, improving the overall quality of voice and data transmission.

Fast Frequency Hopping in GSM:

Fast Frequency Hopping is another frequency management technique employed in GSM, but it operates at a considerably higher speed compared to slow frequency hopping. In fast frequency hopping, the transmitter rapidly changes frequencies within the allocated band, often on a per-burst or per-slot basis. The hopping pattern is predefined and synchronized between the transmitter and receiver.

Fast frequency hopping offers several benefits in GSM, primarily related to security and interference resistance. Its rapid frequency changes make it challenging for eavesdroppers and interferers to track and disrupt the communication. Additionally, fast frequency hopping can help combat frequency-flat interference, where a wideband jamming signal affects the entire frequency band simultaneously. This technique is especially valuable in military or secure communication scenarios, where confidentiality and robustness against jamming are paramount.

In summary, both slow and fast frequency hopping techniques play essential roles in GSM networks. Slow frequency hopping enhances resistance to narrowband interference, ensuring high-quality communication. Fast frequency hopping, on the other hand, provides superior security and interference resistance, making it suitable for secure or mission-critical communication applications. The choice between these techniques depends on the specific requirements of the communication system.

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