Spread spectrum communication

< p> Spread spectrum communication

Spread spectrum technology, also known as spread spectrum technology, is a technology that has developed rapidly in recent years. Not only has it played an irreplaceable advantage in military communications, but it has also penetrated into all aspects of communications, such as satellite communications, mobile communications, microwave communications, Wireless positioning system, wireless local area network, global personal communication, etc. The so-called spread spectrum technology refers to a technology in which the transmitted information is spread to a frequency band much wider than the information bandwidth, and the receiving end restores it to the information bandwidth through related reception. Such a system is called a spread spectrum system or spread spectrum system.

Spread spectrum technology has the following characteristics:

Very strong anti-interference ability

Since the signal is extended to a very wide frequency band, correlation processing is performed on the spread-spectrum signal at the receiving end to compress the bandwidth and restore it to a narrow-band signal. For the interference signal, due to the correlation with the pseudo-random code for spreading, it is expanded to a very wide frequency band, so that the interference power into the signal pass band is greatly reduced, and the output signal of the correlator is correspondingly increased. / Interference ratio, so it has a strong anti-interference ability. Its anti-interference ability is proportional to the spreading factor of its frequency band. The wider the spectrum spread, the stronger the anti-interference ability.

Multi-access communication

Spread spectrum communication itself is a multiple access communication method, which becomes spread spectrum multiple access. It is actually a type of code division multiple access (CDMA), which uses different spreading codes to form different networks. Although the spread spectrum system occupies a very wide frequency band, but because each network can share the same frequency band at the same time, its spectrum utilization rate is even higher than that of a single channel single carrier system. CDMA is a major communication method for personal communications in the future.


Because the spread spectrum system spreads the transmitted information to a very wide frequency band, its power density decreases with the spread of the frequency spectrum, and can even drown the signal in noise. Therefore, its confidentiality is very strong, and it is very difficult to intercept or eavesdrop and detect such signals, unless the spreading code used by the sending end is used and synchronized with the relevant detection, whether the spread spectrum signal is powerless . Due to the low power spectral density of the spread spectrum signal, in many countries, such as the United States, Japan, Europe and other countries, for dedicated frequency bands, such as the ISM frequency band, as long as the power spectral density meets certain requirements, you can use the frequency band without approval. In China, in principle, the National Radio Regulatory Commission does not allow free frequency bands. The use of any radio frequency band must be applied for. However, in the 2.4G ~ 2.483G frequency band, the National Radio Regulatory Commission allows civilian products to be used. Generally, the application will agree , And basically no fees.

Spread spectrum technology includes the following methods:

Direct sequence spread spectrum, referred to as direct spread, is recorded as DS (Direct Sequence)
Frequency hopping, recorded as FH (Frequency Hopping)
When jumping, recorded as TH (TIme Hopping)
Linear frequency hopping, denoted as Chirp

In addition to the above four basic spread spectrum methods, there are also combinations of these spread spectrum methods, such as FH / DS, TH / DS, FH / TH, etc. The main applications in communication are DS, FH and FH / DS.

Basic modulation and demodulation principle of spread spectrum system

Direct sequence spread spectrum

The direct-spreading technology uses a pseudo-random code (PN CODE) to add modulo 2 to the information bits to obtain a spreading sequence, and then transmits the spreading sequence modulated carrier wave into the air. At this time, the power spectral density occupied by the system is also greatly reduced. The PN code is generated by a pseudo-random sequence generator. Its code rate is much higher than the original information code rate. The length of each PN code (ie, the width of the CHIP CHIP) is very small.

The reception of the direct-spreading system generally uses related reception, which is divided into two steps, namely, despreading and demodulation. At the receiving end, after the received signal is amplified and mixed, the same and synchronous pseudo-random code used at the transmitting end de-spreads the intermediate frequency signal to restore the spread signal to a narrow-band signal, and then demodulate to restore the original information sequence. For interference and noise, since they are not related to pseudo-random codes, the relevant despreading of the receiver is equivalent to one-time spreading, which spreads the spectrum of interference and noise, reduces the interference power into the frequency band, and makes the input signal of the demodulator The noise ratio and the carrier-to-interference ratio are improved, which improves the anti-interference ability of the system. In addition, it is difficult for receivers using different PN codes, that is, uncorrelated, to find and solve the information in the spreading sequence. Because the correlation between PN codes of different structures is very low, code division multiple access CDMA uses the same principle to distinguish different users .

For direct-spread systems, it is best to despread and then demodulate, because the wireless signal will have a large signal attenuation in space transmission. The signal-to-noise ratio before despreading is very low, and the signal is even submerged in noise. It is difficult for a general demodulator to demodulate normally at a very low signal-to-noise ratio, resulting in high bit errors. However, under indoor communication conditions, due to the higher strength of the signal, it can be demodulated before despreading. When the signal reaches a certain level, the simple demodulator can work normally. It can demodulate the signal into a data stream (without despreading), and then use a common integrated circuit to despread the digital related signals. This method is generally used for wireless LAN cards with direct expansion. The processing of the radio frequency unit is greatly simplified, the volume can be reduced a lot, and the cost is significantly reduced.

In terms of performance, despreading first and then demodulating are significantly better than demodulating and then despreading. Despreading first can obtain the spreading gain (the ratio of the spread spectrum bandwidth to the bandwidth of the original information) through the despreading process to improve the signal-to-noise ratio of the received signal. Outdoor long-distance (over 2 and 3 kilometers) spread spectrum communications must use this method to ensure communication quality and reliability.

Synchronization of DS systems

When the direct-spreading system adopts the first despreading, it is only possible to use the same code sequence to correlate the despreading of the spread spectrum signal after the synchronization of the pseudo-random code (PN code) is completed. The rate and phase of the local PN code of the receiver must be consistent with the received high-speed spreading sequence. Even if the phase difference between the transmitting and receiving ends is greater than one CHIP chip, their correlation does not exist. The first step in despreading is to capture a phase state consistent with the local PN code in the received signal. The phase acquisition in the spreading sequence is generally implemented using a matched filter or a phase search circuit. During the search synchronization process, the receiver changes the clock rate of the local PN code to make the PN code phase in the received signal correlate with the local PN code phase. The device slides relatively. During the sliding process, when the correlation peak exceeds the acquisition threshold, the flag completes the synchronous acquisition. At this time, the phase error of the PN codes of the sending and receiving parties is already less than a chip width (Tc). The acquisition enters the tracking state, the phase difference is further reduced, the correlation is increased, and a high signal-to-noise ratio of the despread signal is obtained, which meets the requirements of the subsequent demodulation threshold.

There is also a more advanced receiving technology in the DS technology called RAKE receiving technology. RAKE receiving technology can achieve multipath diversity. Due to the influence of various combinations of atmospheric conditions, geographic location and other factors, the signal transmission in space is very different from that of only direct waves. The signal passes through multiple paths (direct, reflection, refraction, atmospheric waveguide) and reaches the receiving end after different delays. The arrival time of each signal is different, and the phases are inconsistent, which causes the amplitude of the final signal to cancel each other, causing a large signal fading.

The direct-spreading system that despreads first and then demodulates has the ability to resist multipath, and separates the relevant peaks on the main channel (maximum peak) in time. Thereby reducing multipath interference. The RAKE receiving technology implements multipath diversity technology, which can combine the received multipath signals to obtain a weighted gain and convert it into a synthesized signal to achieve higher anti-fading performance. However, due to the complicated and expensive implementation of RAKE technology's receive weighted merge, only a few companies in the United States have implemented this technology in their spread spectrum systems.

"Soft Spread Spectrum" in DS

For broadband methods such as wireless local area networks, when the rate reaches several megabits per second in a short range, if the general direct sequence spread spectrum technology is used, the spectrum bandwidth of the system expansion will even exceed the frequency range specified by the open ISM band. Unlike the "true spread spectrum" method, soft spread spectrum actually uses coding to complete frequency expansion. Soft spreading is a type of (N, K) coding, where K is the information code represented by a pseudorandom sequence of N bits. With a few bits of information corresponding to a pseudo-random code, the expansion multiple is not large, and it is not necessarily an integer. In contrast to "True Spread Spectrum", each information code is added modulo 2 to multiple integer bit PN codes. Under the condition of indoor distance communication, soft spread spectrum not only meets the system requirements of the open frequency band, but also can achieve a high rate and low cost.

Frequency hopping technology

Frequency-hopping technology is completely different from direct-sequence spread-spectrum technology, and is another kind of spread spectrum. The carrier frequency of frequency hopping is controlled by a pseudo-random code. Within its working bandwidth, its frequency synthesizer continuously changes the frequency according to the random law of PN code. At the receiving end, the receiver frequency synthesizer is controlled by a pseudo-random code and keeps the same change law as the transmitting end.

Frequency hopping is the spreading of the carrier frequency in the sense of continuous hopping within a certain range, instead of spreading the transmitted information, the processing gain of direct sequence spreading will not be obtained. Frequency hopping is equivalent to the instantaneous narrow-band communication system, which is basically equivalent to the conventional communication system. Since it cannot resist multipath and has low transmission efficiency, the frequency transmission system with the same transmission power is smaller than the direct-spread system in the effective transmission distance. The advantage of frequency hopping is anti-interference, and fixed-frequency interference will only interfere with some frequency points. Used for the transmission of voice information, when fixed frequency interference only accounts for a part, it will not cause a great impact on voice communication.

The level of hopping speed directly reflects the performance of the frequency hopping system. The higher the hopping speed, the better the anti-interference performance. The military frequency hopping system can reach tens of thousands of hops per second. In fact, the mobile communication GSM system is also a frequency hopping system, and its specified hopping speed is 217 hops per second. For cost considerations, the hopping speed of commercial frequency hopping systems is very slow, generally below 50 hops / sec. Because the slow-hop frequency-hopping system can be easily implemented, low-speed wireless LAN products often use this technology.

Different Spread Spectrum System Performance The above describes the principles and performance of various spread spectrum communication systems. The implementation method and difficulty of spread spectrum communication equipment directly determine its final performance and cost. Generally speaking, the slow-hop frequency-hopping system is the simplest to implement and has the lowest cost, but the worst performance. The coding technology using soft spread spectrum can achieve high rates, but it is limited to indoor close range applications. In a direct-spreading system that demodulates and then despreads, an integrated circuit can be used to perform digital processing on the spreading sequence directly, but only if the signal strength is high enough. The best performance in the spread spectrum system is the technique of first despreading and then demodulating in direct sequence spread spectrum. This spread spectrum system correlates the spread spectrum signal first, and then correlates the demodulation. Recovery greatly increases the complexity of the system. For example, a direct-spreading system with a rate of 64K has a pseudo-random code rate of more than 5 Mbit / s, and its implementation method is much more complicated than a frequency-hopping system with a 3M rate. Therefore, the cost of a high-performance direct sequence spread spectrum system is high.

Influence of channel characteristics on spread spectrum system

Channel characteristics are critical to the transmission of wireless signals. The signals undergo different distortions and distortions through different channels. The transceiver device of the communication system must be designed according to the channel characteristics, and the application positioning of the wireless spread spectrum system using different technologies is also different. In wireless communication, due to the influence of climate, environment, distance and other factors, the amplitude and phase of the received signal fluctuate randomly. The main considerations are slow fading, fast fading, flat fading, and frequency selective fading. The indoor channel's time fading characteristic is slow fading, and at the same time, the delay spread factor is small, so it is relatively simple to reach the communication rate of Mbps or more. The characteristics of outdoor wireless transmission channels are very different. Various factors such as fast fading, deep flat fading, and long spread delay must be considered. When the communication rate is high (occupies a large bandwidth), various uncertain factors such as frequency selective fading must also be considered. In addition, its receiving sensitivity must ensure signal pickup when the signal attenuation is hundreds of dB. To ensure communication quality and communication reliability (expressed in availability). In order to ensure sufficient performance indexes (bit error indexes), conventional microwave frequency band communication systems generally reserve 30 ~ 50dB link margin (or fading reserve) in the link design in advance. However, for errors caused by multipath transmission and deep fading, in addition to the use of fast automatic gain control AGC and other means. Anti-multipath fading technology must be used. As described above, the high-performance implementation of direct-spreading technology (despreading and demodulation) can well offset the adverse effects of multipath fading. Better RAKE reception technology can even achieve multipath diversity reception, thereby offsetting the serious deterioration of performance in outdoor wireless fading channel systems. In addition, due to the wide spectrum of DS technology, the selective fading of some frequency bands will not affect the overall reception. Generally, indoor spread-spectrum equipment should not be used outdoors. For example, even with high towers and good transmission line space, wireless LAN spread-spectrum products cannot actually solve the rapid decline in communication quality due to channel characteristics. General wireless LAN spread spectrum equipment uses high-gain antennas. After the transmission distance exceeds 3, 4 kilometers, the bit error rate will still rise rapidly, and the availability is very low with the change of climate and environment. At this time, it is unrealistic to adopt measures such as adding a power amplifier to increase the transmission power. Considering the deep fading and the non-linear distortion of the device, the bit error rate will appear flat. At the same time, the parasitic radiation generated by matching the non-linear shape may even affect the conventional microwave Other devices in the frequency band communicate normally.

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