Rozprestreté spektrum

Spread Spectrum
Theoretically, the basics of distributed spectrum were mastered as early as 1940 by C. E. Shannon. The most important results of this work are the basic relationships between the capacity of the transmission channel, signal power, noise power and bandwidth. This relationship says that for the same transmission capacity, there are essentially two transmission options - either a strong signal and a small bandwidth, or a large bandwidth, and then a weak signal is sufficient. However, increasing the bandwidth also brings with it an increase in thermal noise, which worsens the S/N ratio. Nevertheless, the use of a weak signal, despite the large bandwidth used, is tempting. From the previous formulas, we conclude that, for example, if we transmit data at a speed of 4 kb. s -1 and the S/N ratio will be 40 dB (noise will be 100x stronger), we need a bandwidth W = 4000.100 = 400 000 Hz = 400 kHz. Such possibilities are very tempting, especially if we realize that the interfering signal may not only be noise, but also other broadcasts, for example. We are actually hiding a useful signal.


The first users of this system were soldiers who always tried to ensure that the enemy did not even detect their communication links and thus could not locate communication points or effectively disrupt connections. The actual use of the spread spectrum was made possible only by the development of digital technology. Gradually, the technique of the spread spectrum also reached the civil spheres. It is used, for example, in unlicensed microwave links on common frequencies. Another use is GPS (Global Positioning System) where the signal on the ground is at a level of about -135 dBm and the noise power of receivers with a bandwidth of 9 MHz, is about -104.5 dBm, ie about 30.5 dBm above the received signal.


The spread spectrum technique currently uses two basic types of modulation: extended spectrum, Direct Sequence Spread Spectrum (DSSS) and frequency hopping, Frequency Hopping Spread Spectrum (FHSS).


In DSSS, pseudorandom bit sequence scattering of narrowband modulation is used to create a broad spectrum. Such a signal is similar to noise. The advantage of this system is that the strong interfering signal spreads around the useful signal during demodulation. This achieves that the previously weak useful signal with narrow-spectrum interference becomes strong narrow and the strong narrow interfering signal becomes wider and weaker. It is based on the theory of the "signal area" which remains constant. The simplified relation defines it as signal strength x bandwidth = constant.


Identical sequences are programmed into both the receiver and transmitter. Demodulation is only possible if the pseudo-random sequence is identical but also time-synchronized. It was this requirement that caused the biggest problems in implementing spread spectrum technology. Only integrated circuits have enabled a reliable and economical solution to this problem. The phasing of the pseudorandom sequences is performed by autocorrelation. Therefore, the clock frequency in the receiver is lower than in the transmitter. Using synchronization circuits, the receiver sequence is synchronized and demodulation takes place. At the same time, we know the initial and continuous synchronization. If a long pseudo-random sequence is used, shorter synchronization sequences are initially used to allow faster synchronization. During reception, synchronization is ensured by so-called tracking. At the output of the demodulator we receive a standard narrowband signal, which we further process in the usual ways.


Another method is the Frequency Hopping Spread Spectrum FHSS, in which the frequency of the narrowband carrier signal is changed in short periods of time, e.g. 50 ms, again by means of a pseudo-random bit sequence. In the receiver, the pseudo-random sequences are phased again and then the actual communication by carrier hopping takes place.


Others used are: time hopping Time Hopping, where the code specifies the moment of signal transmission and Chirpmodulation at which a certain frequency band is retuned during one pulse (used for radars).


Influence of interference on broadband signals.Because the signal in DSSS is spread over a wide spectrum, the narrowband signal cannot significantly affect the transmission. Even with a certain loss of the transmitted band, the signal is demodulatable. Intentional interference is quite difficult due to the hidden signal in the noise. The problem is if there is a foreign transmitter near the receiver with such a strong signal that the receiver is not able to correctly evaluate the useful signal. This problem is referred to in the American literature as the "near-far problem." Although FHSS is a narrowband signal, due to the fact that there is only a short moment on a certain, disturbed frequency, again in the event of a failure of this section, the information is repeated on another frequency after detection. Modern systems are able to behave adaptively and automatically skip a certain frequency in the event of longer interference.


The least favorable situation occurs when there are multiple broadband signals in a certain band. They must use different coding sequences to avoid interference. If the correlator can clearly distinguish them, there can be several broadband signals in one band without mutual interference.


Source: Wireless communication (I am the author)
Rozprestreté spektrum - DSSS

DSSS
Rozprestreté spektrum - FHSS

FHSS
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