Digital Communication ABC

The purpose of communication is to transmit information from one place to another. How can we quantify the information? Shannon information theory defines the information entropy (entropy) H =-Σ PI log2 (PI), that is, the average information contained in each source output. Pi indicates the probability of each possible value. The unit of entropy is bit ). For example, I tell you: I am a boy. This information contains 1 bit, because gender only has two possible values: male and female, with a probability of 0.5.

Digital Communication Systems (hereinafter referred to as DCS) have many advantages, which are more reliable and flexible than analog communication. encryption algorithms can be used to improve security. The first task of DCS is to format all kinds of information and convert real-world information into digital signals through sampling. The next step is the source code, that is, the digital signal is encoded into an information bit stream in a certain way. Judging the amount of information in a general thing is not as simple as the Gender judgment mentioned above. Over the years, we have used various methods, such as probability analysis and research on human sensory characteristics, to express things with the least amount of information possible. Various image compression and video compression algorithms aim to reduce the number of transmitted bits.

Anyone who has played single-chip microcomputer knows UART (general asynchronous serial port). Once it is received, three lines in one location can implement simple communication. Due to the attenuation of the line, the waveform of the receiving end will produce distortion, and the difficulty of correct determination is increased (that is, the error rate increases ). It can be slightly improved. For example, RS232 increases the level to +/-12 V, which leads to a more relaxed decision threshold. A little more complicated. waveform coding technologies such as NRZ, Ami, and HDB3 can be used for improvement. A non-modulated transmission method like RS232 is called direct baseband transmission. Ethernet LAN (802.3) is the most widely used baseband transmission system. However, this direct transmission of the base band (baseband) signal cannot be transferred far. to transmit the signal remotely, carrier modulation is required.

It is transmitted after carrier modulation and then demodulated at the receiving end to restore the bit stream. This process is the modulation/Demodulation (modem ). the typical digital modulation methods include PSK, ask, and PSK. However, physical channels always have various kinds of interference, and modulation and demodulation itself cannot guarantee information reliability. Therefore, channel encoding is generally required between source encoding and modulation. Channel encoding adds redundant information codes so that the receiver can detect or even correct errors. Common terms include: hamming code, Gray code, cyclic code, BCH Code, reed-soloman (RS code) (the above are grouping codes), convolution codes, and Turbo codes.

We designed a communication system to have the following options: maximum bit rate (speed), minimum bit error rate (correctness), minimum power, and minimum bandwidth (Frequency-controlled ). We constantly improve the channel coding and modulation technologies to reach the above objectives. However, there is no free lunch in the world. To improve the performance of one aspect, you have to sacrifice the other. For example, if the redundant information of channel encoding is added too much, the effective transmission rate must be reduced, and the redundant information is too little to reach the low bit error rate, so we must weigh it. Shannon proof that the Shannon limit Eb/N0 =-1.6db, bit energy/noise power is less than-1.6db, cannot design a reliable transmission communication system. In the system design, the error rate is generally fixed, and the transfer power and bandwidth are weighed. A bandwidth-limited system (with limited spectrum resources) uses a high bandwidth efficiency modulation method to sacrifice Eb/N0 (such as increasing the transmission power) to achieve the maximum bit rate in available bandwidth, for example, GMSK technology is used in the GSM mobile communication system. In a power-constrained system, high-energy-efficiency modulation is used to sacrifice bandwidth resources.

In summary, the basic diagram of the communication system can be summarized:

Information formatting --> source code --> channel code --> modulation --> physical channel ￥ )))

Information restoration <-- source decoding <-- Channel Decoding <-- demodulation <-- physical channel ￥ (((

With the rapid development of digital communication technology, new technologies are emerging, especially access technologies. Such as wireless WiMax, CDMA (the main technology of 3G mobile communication), and wired GPON. In the future, more reliable and high-speed technologies will bring convenience and rich communication methods to humans.