1. Principle of serial spread spectrum communication
Direct sequence spread spectrum (DSSS), (Direct seqcuence spread spectrdm) is the direct use of a spreading code series with a high code rate using various modulation methods at the sending end and spreading the signal spectrum, and at the receiving end, the same spreading code To decode sequentially, restore the spread spectrum signal to the original information. It is a digital modulation method. Specifically, it is to add two sources with a certain PN code (pseudo noise code). For example, at the transmitting end, replace "1" with 11000100110 and replace "0" with 00110010110. This process achieves spreading, and at the receiver, as long as the received sequence is 11000100110, it returns to "1". 00110010110 will return to "0", which is despreading. In this way, the source rate is increased by 11 times, and the processing gain is more than 10dB, which effectively improves the overall noise ratio.
The advantages of direct sequence spread spectrum:
The RF bandwidth of the DS system is very wide. A small part of the spectrum fading will not make the signal spectrum fading seriously
Multipath interference is caused by encountering various reflectors (mountains, buildings) during the propagation of radio waves, which distorts the received signal at the receiving end, leads to crosstalk between codes, and causes increased noise. The direct-spread system can use these interference energy to improve the performance of the system.
In addition to the synchronization required by the general communication system, the direct-spreading system must also complete the synchronization of the pseudo-random code, so that the receiver uses the synchronized pseudo-random code to despread the received signal. With the lengthening of the pseudo-random codeword of the direct-spreading system, the required synchronization accuracy is also high, so the synchronization time is long.
Both direct-spread and frequency-hopping systems have strong security performance. For direct-spread systems, the RF bandwidth is very wide, the spectral density is very low, and even submerged in noise, it is difficult to check the presence of the signal. Because the spectral density of the DS signal is very low, the DS system has little effect on other systems.
Direct-spreading systems generally use coherent demodulation and despreading, and their modulation methods mostly use BPSK, DPSK, QPSK, MPSK and other modulation methods. In the frequency hopping mode, since the frequency is constantly changing and the carrier synchronization must be completed once during the dwell time of the frequency, as the frequency hopping frequency increases, the shorter the synchronization time required. Therefore, frequency hopping mostly uses non-coherent demodulation, and the demodulation methods used are mostly FSK or ASK. From the performance point of view, the direct-spread system uses frequency and phase information, and its performance is better than frequency hopping.
2. Technical features of direct sequence spread spectrum communication:
Direct sequence spread spectrum (Direct Sequence Spread Spectrum) system is to spread the information to be transmitted with a pseudo-random code (PN code) to a wide frequency band, at the receiving end, the same pseudo-random code used for spreading at the receiving end is used to receive The received spread spectrum signal undergoes related processing to recover the transmitted information.
Direct-sequence spread-spectrum communication began to appear in the Second World War and is an important wireless secret communication technology for the US military. Now DS technology is widely used in many fields including computer wireless network.
Strong anti-interference
Anti-interference is one of the main characteristics of spread spectrum communication. For example, the signal spreading width is 100 times, narrow-band interference basically does not work, and the intensity of broadband interference is reduced by 100 times. If you want to maintain the original interference intensity, you need to increase 100 Double the total power, which is essentially difficult to achieve. Signal reception requires spread spectrum coding for correlation despreading processing, so even if the same type of signal is used for interference, without knowing the signal's spreading code, due to the different correlation between different spreading codes, the interference Does not work either. Because of the strong anti-interference ability of spread spectrum technology, the US military has widely used wireless bridges with spread spectrum technology to connect computer networks distributed in different areas in the Gulf War.
Good concealment
Because the signal is spread over a wide frequency band, the power per unit bandwidth is very small, that is, the signal power spectral density is very low, and the signal is submerged in white noise. Pick up useful signals, and the extremely low power spectral density rarely interferes with other telecommunications equipment.
Easy to implement code division multiple access (CDMA)
Direct-spread communication occupies broadband spectrum resource communication, which improves anti-interference ability. Is it a waste of frequency band? In fact, on the contrary, spread spectrum communication improves the frequency band utilization. It is precisely because the direct-spread communication uses spread-spectrum coding for spread-spectrum modulation transmission, and the signal reception needs to be obtained by using the same spread-spectrum coding for related despreading, which provides the basis for frequency multiplexing and multiple-access communication. Make full use of the correlation characteristics between spreading codes of different code types. Different spreading codes assigned to different users can distinguish the signals of different users. As long as many users use their own spreading codes in pairs, they can Interferingly use the same frequency communication at the same time, thereby achieving frequency reuse and making full use of the crowded spectrum. The sender can use different spreading codes to send data to different receivers; similarly, the receivers can use different spreading codes to receive the data sent by different senders and realize multiple-access communication. A technical report from the National Aeronautics and Space Administration (NASA) states that the use of spread-spectrum communications has increased spectrum utilization. In addition, spread spectrum code division multiple access is also easy to solve the problem of adding new users at any time.
Anti-multipath interference
Anti-multipath interference in wireless communication has always been a difficult problem to solve. Taking advantage of the correlation characteristics between spread spectrum codes, the strongest useful signal can be extracted and separated from the multipath signal at the receiving end using related techniques, or multiple The waveforms of the same code sequence from the path are added to make it strengthened, so as to achieve effective anti-multipath interference.
High direct communication rate
The direct communication rate can reach 2M, 8M, 11M, no need to apply for frequency resources, simple network construction, and good network performance.
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