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Tuesday, 28 December 2021

Fast Frequency Hopping Spread Spectrum

Fast Frequency Hopping Spread Spectrum Block Diagram:


Fast-hopped signals occur in the FH system when there are multiple hops per symbol. As a result, the hop rate Rh is an integer multiple of the MFSK symbol rate in fast-frequency hopping. During the transmission of a single symbol, the carrier frequency will hop or shift numerous times. As a result, each hop is a chip in a fast FH-MFSK system.


                                                     Figure 1: Fast FH-MFSK demodulator


In general, fast frequency hopping is used to overcome a smart jammer's technique, which entails two steps: measuring the spectral composition of the broadcast signal and returning the interfering signal to the same frequency range.


Before the jammer can execute these two operations, the transmitted signal must be jumped to a new carrier frequency.


Non-coherent detection is used to recover data at the receiver. The detecting process, on the other hand, differs from that utilized in a slow FH-MFSK receiver. A typical rapid FH-MFSK demodulator is seen in Figure 1.


The signal is first de-hopped using a PN generator that is identical to the one used in the transmitter. After that, a low pass filter with a bandwidth equal to the data bandwidth is used to filter the signal. A bank of 'M' envelope detectors is used to demodulate the filtered signal.


A clipping circuit and an accumulator follow each envelope detector. In the event of a deliberate jammer or other powerful unanticipated interference, the clipping circuit plays a crucial role. On a chip-by-chip basis, the demodulator does not make symbol judgments. The energy from the N chips is gathered and stored. After adding the energy from the Nth chip to the energy from the N-1 preceding ones, the demodulator chooses the symbol that corresponds to the accumulator with the most energy.

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