Spread spectrum technology has evolved from a military technology to one of the basic components of current and next-generation wireless systems. From cellular systems to cordless systems to wireless LAN systems, spectrum is an important component in the system design process, and spread spectrum communication is an indispensable part
- How does spread spectrum work?
Spread spectrum uses wideband, noise-like signals that are difficult to detect, intercept, or demodulate. In addition, spread spectrum signals are more difficult to jam than narrowband signals. These low probability of intercept and anti-jamming characteristics are why the military has used spread spectrum for many years. The frequency band of the spread spectrum signal is intentionally designed to be much wider than the information it carries to make it more like noise.
- Direct Sequence DSSS
The carrier of a direct sequence radio remains at a fixed frequency. The narrowband information is spread into a larger bandwidth using a pseudo-random chip sequence, and the power density of the signal is much lower than that of a narrowband signal. Therefore, it is more difficult to detect the presence of a spread spectrum signal because power density is the amount of power at a specific frequency. At the receiving end of a direct sequence system, the spread spectrum signal is despread to generate the original narrowband signal. If there is an interfering interferer in the same frequency band, the interferer will be diffused during the despreading process. As a result, the impact of the interferer will be greatly reduced, which is how direct sequence spread spectrum fights interference. Assuming a spreading factor of at least 10 times, the amplitude of the interferer will be greatly reduced by at least 90%.
- Frequency Hopping (FHSS)
Frequency hopping technology does not spread the signal, and the receiver and transmitter must be synchronized in time and frequency to ensure the correct transmission and reception of the signal. It will take more time to search for the signal and lock it. Therefore, the delay time is usually longer. The direct sequence radio only needs a few bits to lock the code chip sequence. In order to achieve initial synchronization, the frequency hopper usually stays on a fixed frequency before the frequency hopping or communication begins. If the jammer happens to be on the same frequency as the stop frequency, the frequency hopper cannot hop at all. Once the frequency hopping, if the receiver loses synchronization, resynchronization will be very difficult or even impossible. However, when dealing with multipath, the frequency hopper is better than the direct sequence radio. Because the frequency hopper does not stay at the same frequency, and if one frequency is not too close to the original frequency, the zero point of one frequency will not usually be zero on another frequency. Therefore, the frequency hopper can usually handle multipath fading better than the direct sequence radio.
- Advantages of spread spectrum
In the spread spectrum system, the impact of crosstalk interference can be basically eliminated. Due to its inherent frequency diversity characteristics (thanks to the wide spectrum spread), it is less susceptible to multipath fading. Due to the pseudo-random nature of the PN sequence, the over-the-air signal has been "randomized." Only a receiver with the exact same pseudo-random sequence and synchronized timing can despread and retrieve the original signal. Therefore, spread spectrum systems provide signal security that traditional analog wireless systems cannot. In addition, there is the advantage of longer communication distances.