Communication principle
Reference Course: "Concise Course of Communication Principles"-Huang Baohua and other mechanical industry press
Chapter One Introduction (1.1~1.3)
1. The composition of the communication system
Source and input converter -> sending equipment -> channel (+ noise dB/km) -> receiving equipment -> output converter and sink
Communication: the transmission of data (the carrier of information) from the sender to the receiver.
Signal: the carrier during data transmission.
Signal types: sound, light, and electricity (electromagnetic waves)
The difference between digital signal and analog signal
source network invasion and deletion
2. Information and metrics
(1) Information: I(Si)=loga(1/P(Si))=-loga(P(Si))——>The information carried by a certain signal
a=2 or 0 or e, corresponding to bit, ha Knight
Example: Among 26 English letters, the probability of e is 0.105 and the probability of x is 0.002. Find the amount of information carried by e and x.
I(e)=-log2(0.105)bit
I(x)=-log2(0.002)bit
(2) Source entropy (bit/sym): The average amount of information of all characters emitted by a certain source.
There are two ways to find it: source entropy method and independent average method
H(s)=ΣP(Si)I( Si)=-ΣP(Si)log2(P(Si))
Maximum source entropy: independent equal probability, M types of symbols
Hmax(S)=ΣP(Si)I(Si)=Σ1/Mlog2M=log2M
Example questions in the book: Examples 1-3
Note: Find the average information content of each symbol is H(s), and the information content of the symbol column is the total I(Si)
3. Performance indicators
(1) Effectiveness index: speed or data amount transmitted per unit channel In
analog system: bandwidth B In
digital system: bandwidth B, symbol rate Rs, information rate Rb, frequency band utilization rate η
(2) Reliability index: correct transmission Data
simulation: signal-to-noise ratio S/N
digital: bit error rate Pe, bit error rate Pb
dB=10lgS/N
**Symbol rate Rs: **The number of transmitted symbols per second, and the relationship between the symbol width Ts: Rs=1/Ts(Baud)
Information rate Rb(bit/s) : Rb=H(s)Rs
Rb= I/T
Rb=Rslog2M
When M increases, Rb also increases the
frequency band utilization rate η : Symbol
frequency band utilization rate ηs=Rs/B (Baud/Hz)
Information frequency band utilization rate ηb=Rb/B (bit/s. Hz)
Examples in the book 1-4, 1-5, 1-6
Bit error rate Pe = number of error symbols / total number of transmitted symbols
Bit error rate Pb = number of error bits / total number of bits transmitted
Pb=Pe/log2M
Chapter 2 Preliminary Knowledge
1. Classification of signals and systems
Digital, analog, discrete, continuous
2. System and its classification:
Non-linearity, linearity (uniformity, superposition)
, time-varying, time-invariant (output is only affected by input and not affected by time)
ideal, causal (physically achievable) *refers to input as e(t) and output as r(t)
3. Ascertained signal analysis:
The Fourier transform of the periodic signal, the
non-periodic Fourier transform
of the signal x(t) of period T0. Example: The frequency spectrum of cos10t is abbreviated
4. Fourier transform characteristics:
Symmetry (dual), linear superposition, time shift, frequency shift, convolution
5. Paseval's Theorem
Find the book yourself, we didn’t focus on this part
6. Signal bandwidth: learn to look at the picture, the first spectrum zero bandwidth, etc.
7. Expectation, variance, covariance
We took a question about expectation and variance, remember to learn probability theory
8. System noise:
From the perspective of probability distribution: Gaussian (narrow band), non-Gaussian
Power spectrum angle: white noise, colored noise
and input x(t) relationship: additive, multiplicative
9. Shindo
Twisted pair-telephone system, coaxial cable-long distance
10. Channel capacity (very important, all concepts and examples should be mastered)
Channel capacity refers to the maximum rate of error-free transmission of information on the channel
N: noise power w
S: signal power w
B: bandwidth hz
C: channel capacity bit/s
analog: C=Blog2 (1+S/N)
digital: C=2Blog2M
Shannon Formula-when C>=Rb, error-free transmission can be achieved
Examples in the book 2-21, 2-22
Chapter 3 Analog Modulation
1. Amplitude modulation
Full amplitude modulation 
image source network infringement deletion
expression SAM(t)=[A0+m(t)]cos2Πfct
amplitude modulation index m=(A(t)max-A(t)min/(A(t)max+A(t) )min)
when m=1, full amplitude modulation
A(t)=A0+m(t)
Example 1: Find the value range of A0 m(t)max=3V m(t)min=-1V
m=(A(t)max-A(t)min/(A(t)max+A(t) min)=(A0+3-A0+1)/(2A0+2)
A(t)=A0+m(t)
A(t)max=A0+3
A(t)min=A0-1
when m> 1:
①A0+1<0, no solution
②A0+1>0, -1<A0<1
when m=1, A0=1
when m<1, 2/(A0+1)<1
if A0+ 1<0 then A0<-1
if A0+1>0 then A0>1
Example 2: Find the modulation degree when m(t)=Amcoswt and bipolar square wave
2. Spectrum and bandwidth of SAM signal
3. Demodulation of SAM signal
4. Suppress carrier double sideband amplitude modulation DSB-SC
Chapter 4 Digital Baseband Transmission
1. Code type: Represents the voltage combination form of the digital signal
Single and bipolar, full half-duty, differential code, AMI code, HDB3 code (learn the encoding and decoding methods of these codes by yourself, there are more tutorials on csdn)
2. Inter-code crosstalk
Learn to judge the crosstalk between codes and find the roll-off coefficient according to the image
3. No crosstalk between codes
One type of question type is a comprehensive analysis question, analyze which type of system is better.
The answer should be compared from two aspects, one is effectiveness-to see the frequency band utilization, the other is reliability-to see how fast the waveform trails change over time
Regarding the attenuation speed of waveform tailing, raised cosine t^3>triangular, trapezoid>rectangular t, so the raised cosine that decays quickly with time is generally selected.
Chapter 5 Digital Modulation
Messy bandwidth:
BMASK=BMPSK=BMDPSK=BMQAM=2B
BMFSK=2B+fmax-fmin
BMSK=0.5B
B=(1+α)Rs
B2FSK=|f1-f2|+2fs
Chapter 6 Digital Transmission of Analog Signals
Sampling -> Quantization -> Coding
Sampling Theorem: fs>=2fh
The purpose of using non-uniform quantization for the
13-fold line of A-law: to improve the small signal.To master the coding of the 13-fold line of A-law, and find its quantization error
. Two channels in the reference book example.
Chapter 7 Channel Coding
The principle of channel coding : The channel encoder at the
sending end adds redundant symbols (supervisory symbols) to the original signal according to a certain rule, so that the original digital information becomes digital information with a certain rule, and the receiving end uses this rule to receive codes Yuan to discover or correct errors.
Purpose : Check and correct digital information
Go through each code in the book
The master was too lazy to knock, and directly photographed the handwritten draft of the class. The sprayers from all walks of life who disliked the ugliness of the master, quit the upper left corner. Thank you! Answering your questions shows that he is a good person. If you don't answer your questions, it means that he only treats the article as his memoir and has no obligation.
Some nonsense:
It is too inconvenient to post pictures on csdn. Lord is a slacker, don't knock your hands~ If the article is useful to you, please be sure to like it. After all, my work in organizing notes is also very hard. Why don't I go to sleep.