Frequency hopping spread spectrum communication system Figure 1-6 Simplified block diagram of frequency hopping spread spectrum communication system Compared with the conventional communication system, the frequency hopping spread spectrum communication system has the biggest difference between the carrier generator of the transmitter and the local oscillator in the receiver. In the conventional communication system, the frequency of the two output signals is fixed, but in the frequency hopping communication system, the frequency of the two output signals is hopping. In the frequency hopping communication system, the carrier generator of the transmitter and the local oscillator in the receiver are mainly composed of a pseudo-random code generator and a frequency synthesizer. A fast-response frequency synthesizer is a key component of a frequency-hopping communication system. In Figure 1-7,,,,. The data modulator selects one of the two frequencies based on the binary data signal, that is, the data modulator selects one of the two frequencies every second. The frequency synthesizer has 8 frequencies {,,,,,,,} available for hopping, hopping to a new frequency after every 3 bits are transmitted. The frequency hopping signal is down-converted with the local reference oscillation signal in the receiver, the set of reference local oscillator frequencies is {,,,,,,,,}, and the intermediate frequency signal after down-conversion is concentrated at a frequency of and a width of In the band.
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Frequency-hopping spread-spectrum communication system is the abbreviation of Frequecy Hopping Spread Spectrum CommunicaTIon Systems (FH-SS), or more simply, frequency-hopping communication system, which should be called "multi-frequency, selective Code and frequency shift keying communication system ". It uses a binary pseudo-random code sequence to discretely control the output frequency of the RF carrier oscillator, so that the frequency of the transmitted signal jumps with the change of the pseudo-random code. The number of frequencies that can be randomly selected in a frequency-hopping system is usually a few thousand to discrete frequencies. Among so many discrete frequencies, which one is output is determined by a pseudo-random code each time. See Figure 1-6 for a simplified block diagram of a frequency hopping spread spectrum communication system. 
(a) transmitting system; (b) receiving system
The transmission frequency of the transmitter of the frequency hopping communication system is controlled by a pseudo-random code sequence to control the frequency synthesizer (pseudo) randomly from one to another within a predetermined frequency set. The frequency synthesizer in the receiver also hops in the same order to generate a reference local oscillator signal that differs from the received signal frequency by only one intermediate frequency. After mixing, a fixed frequency intermediate frequency signal is obtained. This process is called De-hopping of frequency-hopping signals. The de-jumped IF signal is amplified and sent to the demodulator for demodulation to recover the transmitted information.
In frequency-hopping communication systems, the rate of instruction codes (pseudo-random codes) that control frequency hopping is not as high as the pseudo-random codes in direct-sequence spread-spectrum communication systems, generally tens of b / s to kb / s. Since the rate of change of the output frequency in the frequency hopping system is the rate of the spreading pseudorandom code, the rate of the spreading pseudorandom code is also called the frequency hopping rate. According to different frequency hopping rates, frequency hopping systems can be divided into two types: slow frequency hopping systems and fast frequency hopping systems.
Assuming that data modulation uses binary frequency shift keying modulation, Tb is an information symbol bit width, and the data modulator outputs one of two frequencies per second. Every second the RF frequency of the system output signal jumps to a new frequency. If>, such a frequency hopping system is called a slow frequency hopping system. Here is an example to illustrate the working process of the slow frequency hopping system, see Figure 1-7. 
Figure 1-8 Schematic diagram of frequency hopping of frequency fast hopping system
In a slow frequency hopping system, the frequency hopping speed is slower than the rate of change of the output symbol of the data modulator. If the frequency of the RF output signal jumps multiple times in each data symbol, such a frequency hopping system is called a frequency fast hopping system. Figure 1-8 shows the frequency of the RF signal output by the fast frequency jump system.
In Figure 1-8, the frequency synthesizer has 16 frequencies {,,,,,,,,,,,,,,,},,.
