Time-hopping spread spectrum communication system
Time hopping is also a kind of spread spectrum technology. Time-hopping spread spectrum communication system (TIme Hopping Spread Spectrum CommunicaTIon Systems, TH-SS) is the abbreviation of time hopping spread spectrum communication system, which is mainly used in time division multiple access (TDMA) communication. . Similar to the frequency hopping system, time hopping is to make the transmitted signal jump discretely on the time axis. We first divide the time axis into many time slots. These time slots are usually called time slices in time-hopping spread spectrum communication, and several time slices form a time-hopping time frame. In a frame, which time slot transmits the signal is controlled by the spreading code sequence. Therefore, time hopping can be understood as: multi-slot time-shift keying with a pseudo-random code sequence. Since a much narrower time slot is used to transmit the signal, the frequency spectrum of the signal is relatively broadened. Figure 1-9 is the principle block diagram of the time-hopping system.
Figure 1-9 Simplified block diagram of a time-hopping spread spectrum communication system
(a) transmitting system; (b) receiving system
At the sending end, the input data is stored first, the spreading code sequence generated by the spreading code generator to control the on-off switch, after two-phase or four-phase modulation, and then transmitted after radio frequency modulation. At the receiving end, when the receiver's pseudo-code generator is synchronized with the sending end, the desired signal can enter the demodulator through the switch every time. The demodulated data also passes through a buffer memory in order to restore the original transmission rate, transmit the data uninterrupted, and provide users with a uniform data stream. As long as the sending and receiving ends are strictly synchronized in time, the original data can be restored correctly.
The time-hopping spread spectrum system can also be regarded as a time-division system, the difference is that it is not fixedly allocated a certain time slot in a frame, but is controlled by the spreading code sequence to change the position according to a certain law Time slot. Time-hopping systems can use reasonable allocation of time to avoid strong interference from nearby transmitters, which is an ideal multiple access technology. However, when there are many time-hopping signals in the same channel, there may be several signals overlapping in a certain time slot. Therefore, the time-hopping system is the same as the frequency-hopping system, and error correction coding must be used, or the time-sharing system must be coordinated site. Because the simple time-hopping spread spectrum system is not strong against interference, it is rarely used alone. Time-hopping spread-spectrum systems are usually used in combination with other methods of spread-spectrum systems to form various hybrid methods.
From the perspective of interference suppression, the time-hopping system benefits very little, and its advantage is that it reduces the duty cycle of working time. An interfering transmitter must continuously transmit in order to obtain the interference effect, because the jammer is not easy to detect the pseudo-code parameters used by the system when jumping.
The main disadvantage of time-hopping systems is that the timing requirements are too strict.
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