RFSlow vs. Fast Frequency Hopping in GSM
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This article explains the difference between slow frequency hopping and fast frequency hopping as used in GSM (Global System for Mobile communications). Frequency hopping involves the RF carrier frequency used for transmission “hopping” between available channels. This creates a frequency-hopped spread spectrum signal.
The sequence of carrier frequencies is determined by a Pseudo-Random (PN) sequence.
The primary goal of frequency hopping is to provide secure communication between a GSM Mobile Station (MS) and a Base Station (BS).
Depending on the relationship between the hopping rate and the symbol rate, frequency hopping can be categorized into two main types: slow and fast.
Slow Frequency Hopping
In slow frequency hopping, several symbols are transmitted between frequency hops. In other words, the time duration between two hops (Th or Tc in the diagram) is greater than the data bit duration (Tb).
As shown in Figure 1, the frequency remains constant for a duration long enough to transmit multiple data bits. There are two sub-types of slow hopping: synthesized and baseband hopping.
Fast Frequency Hopping
In fast frequency hopping, one complete data symbol, or even a fraction of a data symbol, is transmitted between frequency hops.
This means the frequency hopping rate can often exceed the data bit rate in a binary sequence. In fast frequency hopping, the time duration between two hops (Th or Tc in the diagram) is less than or equal to the data bit duration (Tb).
As illustrated in Figure 2, the frequency changes very rapidly, even within a single data bit’s duration.
Benefits of Each Approach
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Fast Frequency Hopping: Provides frequency diversity at the symbol level. This makes the transmitted signal more resilient against fading and narrowband jamming.
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Slow Frequency Hopping: Can achieve similar benefits by using error correction coding techniques. This allows the system to recover from errors introduced by fading or interference even though the frequency doesn’t change as rapidly.
