Signal Design Technology
Coding Technology:
Source coding refers to removing redundant components in the signal source through compression coding, in order to compress the symbol rate and bandwidth, and achieve the purpose of effective signal transmission. (A/D conversion, information compression)
Channel coding refers to the method of rearranging the signal symbols according to certain rules or adding auxiliary codes to prevent errors in the symbols during transmission, and to perform error detection and error correction to ensure reliable signal transmission. (Correction and error detection have a very good effect in digital satellite communication, and it is an important technology to realize the transmission quality of communication system)
source coding technology
Encoding the voice signal:
(1) Waveform coding
Directly convert time domain signals into digital codes.
PCM: 300-3400 Hz speech signal is converted into 64kb/s (8 kHz sampling, 8-bit quantization) digital signal
There are also coding methods such as differential PCM (DPCM), adaptive DPCM (ADPCM) and adaptive predictive coding (APC) with higher compression rates.
Its characteristic is that under the condition of high-speed code (16-64 kb/s), high-quality voice signal can be obtained; however, when the encoding transmission rate is lower than 16 kb/s, the voice quality drops rapidly.
(2) Parameter encoding
It is based on a model of the articulation mechanism. The pronunciation mechanism model is described by a set of filter parameters that simulate the spectral characteristics of the vocal cords and a number of sound source parameters, and parameter coding is a coding method that transforms it into a digital code.
The compression ratio is very high, the amount of calculation is large, and the voice quality can only reach a medium level.
For example, the linear predictive coding (LPC) and its improved version used in the satellite mobile communication system, the transmission rate can be compressed to 2-4.8 kb/s, or even lower.
(3) Hybrid coding
It is a comprehensive coding method, which absorbs the advantages of waveform coding and parameter coding.
Such as multi-pulse excitation linear prediction coding (MPLP) system, regular pulse excitation coding (RPE) system, code excitation linear prediction coding system (CELP) and so on.
Hybrid coding can compress the rate to 4-16 kb/s, and within this range, a good voice effect can be obtained.
channel coding technique
Channel coding is essential when channel quality is poor, satellite power is limited, and high signal transmission rates and low bit error rates are required.
The purpose of channel coding is to improve the reliability of signal transmission
Means: add extra bits
If the amount of information transmitted per unit time is constant, the increased redundant symbols are reflected as an increase in bandwidth
Under the same bit error rate requirement, the increase of bandwidth can be exchanged for the reduction of the normalized signal-to-noise ratio Eb/n0 value
Under a given bit error rate, the difference between the signal-to-noise ratio Eb/n0 of uncoded and coded transmission is called the coding gain (unit is dB)
Mainly using block codes and convolutional codes for coding
linear block code
Supervision symbols: At the transmitting end, some error control symbols need to be added to the information symbol sequence, which are called supervision symbols.
Redundancy: refers to the number of added supervision symbols. For example, if a supervisory symbol is added to the encoded sequence for every two information symbols on average, the redundancy of this encoding is 1/3
Coding efficiency (referred to as code rate): Let the number of information symbols in the coding sequence be k and the total number of symbols be n, then the ratio k/n is the code rate redundancy: the number of supervisory symbols (nk) and the number of information symbols ratio of k
Code distance: The number of digits with different numbers on the corresponding bits in the two code groups is called the distance of the code group, referred to as the code distance. The yardage distance is also known as the Hamming distance.
The structure of the block code: the code that groups the information codes and adds several supervisory codes to each group of information codes is called the block code.
In block codes, the supervisory symbols only supervise the information symbols in the code group
Hamming code
