[communication principle] Chapter 1 -- Introduction

Chapter one The introduction

Basic concepts of communication

news 、 Information and signals

• news ： The carrier of information
• Information ： The valid content contained in the message
• The signal ： The transmission carrier of the message
  • analog signal ： The signal parameter of load message refers to continuous ( It's uncountable 、 Infinitely many ) Of
  • digital signal ： The signal of the load message can Lin has only a limited number of fingers

Connection and difference

• Messages are the physical form of information
• Information is the valid content of a message
• Signal is the transmission carrier of message

Communication system model

General model of communication system

Function description

1. Source
   • Information sources , Information source for short
   • effect ： Convert various messages into original signals
   • Depending on the type of message , Information sources can be divided into analog sources and digital sources
2. Sending device
   • effect ： Generate a signal suitable for transmission in the channel , Make the characteristics of the transmitted signal match the characteristics of the channel , It has the ability to resist channel interference , And it has enough power to meet the needs of long-distance transmission
3. channel
   • A physical medium
   • effect ： It is used to send the signal from the sending device to the receiving end
   • Channel and channel the signal , It will also produce all kinds of interference and noise to the signal
   • The inherent characteristics of the channel and the interference and noise introduced are directly related to the quality of communication
4. Receiving equipment
   • effect ： Amplify and inverse transform the signal ( decoding , Demodulation, etc ), Recover the original electrical signal correctly from the received reduced signal
5. Shinjuku
   • The destination of the message
   • effect ： Restore the original electrical signal to the corresponding message

Analog communication system model

• The modulated signal is called Modulated signal
  • basic feature ：
    1. Carry information
    2. The spectrum usually has a bandpass form , also called Band pass signal

Digital communication system model

Function description

1. Source coding and decoding
   • Source code
     • Improve the effectiveness of information transmission , That is, try to reduce the number of symbols to reduce the symbol rate through some compression coding technology
     • Finish the mold / Number conversion
   • Source decoding
     • The reverse process of source coding
2. Channel coding and decoding
   • Error control , Improve the reliability of communication system
3. encryption and decryption
   • Ensure the security of the transmitted information
4. Digital modulation and demodulation
   • modulation ： Put the information on the carrier
   • demodulation ： Unload information from the modulated signal
5. Sync
   • Make the signals at both ends of the transceiver keep pace in time , It is to ensure the order of the digital communication system 、 accuracy 、 Prerequisites for scientific research

The characteristics of digital communication

• advantage
  1. Strong anti-interference ability , Noise does not accumulate
  2. Transport forklift hole
  3. The object uses credit digital signal processing technology to process digital information 、 Transformation 、 Storage
  4. Easy to inherit , It is the miniaturization of communication equipment , Weight reduction
  5. Easy to encrypt , And good confidentiality
• shortcoming
  1. Larger transmission bandwidth may be required
  2. Because digital communication requires high synchronization , Therefore, the system equipment is complex

Classification of communication system and communication mode

Classification of communication systems

aspect	give an example
Communication services	Telephone communication system 、 Data communication system 、 Image communication system
modulation ( transmission ) The way	Baseband transmission system 、 Bandpass transmission system
Signal characteristics	Analog communication system 、 Digital communication system
Transmission media	Wired communication system 、 Wireless communication system
Operating band	long-wave communication 、 Medium wave communication 、 Short wave communication 、 Far infrared communication
Signal multiplexing mode	Frequency division multiplexing 、 Code division multiplexing 、 Time division multiplexing

communication mode

1. Simplex 、 Half duplex and full duplex communication ( Point to point communication )
   1. Simplex communication ： The way messages can only be transmitted in one direction
   2. Half duplex communication ： Both sides of the communication can send and receive messages , But you can't do it at the same time
   3. Full duplex communication ： The working mode that both sides of communication can send and receive messages at the same time
2. Parallel transmission and serial transmission ( According to different transmission modes of data symbols )
   1. Parallel transmission ： The sequence of digital symbols representing information is transmitted simultaneously on two or more parallel channels in a group way
      • advantage ： Save transmission time , Fast
      • shortcoming ： need n Communication lines , The high cost
      • Suitable for short distance transmission
   2. Serial transmission ： The sequence of digital symbols is transmitted on a channel one by one in a serial way
      • advantage ： Only one channel is required , The required line laying cost is low
      • shortcoming ： Slow speed , Additional synchronization facilities are needed to solve the problem of collection 、 Synchronization of code groups or characters between sender and sender

Information and its metrics

• The amount of information ： How much information is transmitted

  • Only the uncertain content in the message constitutes information
  • The amount of information is related to the surprise of the receiver when he receives the message
  • The less likely the time the message is expressed , The more unpredictable , The more information there is

• hypothesis P(x) Indicates the probability of message occurrence ,I Indicates the amount of information contained in the message ,H Is the entropy of the information source ( Company b/ Symbol )

$I=lo{g}_a\frac{1}{P(x)}=-lo{g}_{a}P(x)$

$H(x)=P(x_1)[-lo{g}_{2}P(x_1)]+P(x_2)[-lo{g}_{2}P(x_2)]+...+P(x_M)[-lo{g}_{2}P(x_M)]$
$=-{\sum }_{i=0}^{M}P(x_i)lo{g}_{2}P(x_i)(b/Symbol)$

$H(x)=-{\int }_{-\infty }^{\infty }f(x)lo{g}_{a}f(x)dx(b/Symbol)$

Main performance index of communication system

• effectiveness ： The bandwidth occupied by transmitting a certain amount of information , That is, frequency band utilization
• reliability ： The accuracy of information transmission

effectiveness

• For digital communication systems , Its bandwidth utilization is defined as unit bandwidth ( Per Hertz ) Transmission rate in

• R_B Is the symbol transmission rate , Bit rate for short , also called R_B For baud rate

• R_b For information transmission rate , Message rate for short , Also called bit rate , It is defined as the average amount of information transmitted per unit time , The unit is bit / second (b/s)
  $\eta =\frac{R_B}{B}(Baud/Hz)\eta$ or ${\eta }_b=\frac{R_b}{B}(b/(s.Hz))$

• Let the length of each symbol be T_B(s), Then there are
  $R_B=\frac{1}{T_B}(Baud)$
  $R_b=R_{B}lo{g}_{2}M(b/s)$
  $R_B=\frac{R_b}{lo{g}_{2}M}(Baud)$

reliability

• Bit error rate P_e, It refers to the proportion of the number of incorrectly received symbols in the total number of transmitted symbols , Bit error rate is the probability of being transmitted incorrectly in the process of symbol transmission , namely

$P_e=\frac{Number\; of\; error\; symbols}{Total\; number\; of\; symbols\; transmitted}$

• Bit error rate P_b, Also known as bit error rate , It refers to the proportion of the number of erroneously received bits in the total number of transmitted bits , namely

$P_b=\frac{Number\; of\; error\; bits}{Total\; number\; of\; bits\; transmitted}$

• In binary P_e = P_b