hjr-Communication principle (i): Communication fundamentals

The internet is used to convey information, to understand the network programming, first to learn some basic knowledge of communication

From the initial letter to telegram, telephone, to video communication, from radio, radio, to digital television, is the credit of the communication system, text, voice, images and so on are information, so it is the information age

Transfer rate

The transfer rate can be measured by either (code transfer rate) or (information transfer rate)

(1) code-element transfer rate (BAUD/S)

(2) Information transfer rate (BIT/S)

A unit code element can be any binary number, a unit of information is a bit, that is, a 2 binary number

Error rate

(1) (BER): Error code/transmission total number of code

(2) (into trusting rate): Error information/Total information

How to describe the data transfer capability.

A. Noise-free case, based on (bandwidth or frequency width determination)

Digital system: Bandwidth (bps)

Analog System: Bandwidth (HZ)

The following collectively referred to as bandwidth

According to the first criterion of Neiguist, the information transmission rate is equal to twice times of the bandwidth to ensure the non-interference between the front and rear code elements.

Rmax (maximum information transfer rate) = B*2

B: Bandwidth

B. Noise (bandwidth and signal-to-noise ratio)

Calculate with Shannon formula

C: (channel capacity) channel error-free maximum information transfer rate, that is, rmax (information transfer rate maximum)

B: Bandwidth

S/N: Signal-to-noise ratio (db)

The most common is the signal to noise ratio 30db, can get s/n=1000

Electrical signals can be divided into analog and digital signals according to signal characteristics.

The transmission mode mainly has the following several

1, the direct transmission of the signal (analog signal can not be directly transmitted directly, digital signal may be transmitted directly), the study called Baseband modulation

2, signal + carrier transmission (signal is also called baseband signal is low-frequency signal, carrier is high-frequency signal), called carrier modulation or band-pass modulation

Why to use carrier:

The carrier is the equivalent of a high-speed truck, the signal equivalent to a low-speed bicycle, the bicycle can not run, you could use the high-speed truck with it to run

Professional point is

Low frequency wave antenna size is too long difficult to lay, signal bandwidth is narrow, the emission efficiency and system anti-interference ability is weak, anti-fading ability is weak, can not channel multiplexing, etc., so use high-frequency carrier to run with it

The carrier is called modulation of the analog signal, and the carrier is the call code of the digital signal,

So, summing it up, there are 5 types of baseband transmissions.

(1) Digital signal baseband transmission

(2) Analog signal analog carrier modulation

(3) Analog signal digital carrier code

(4) Digital signal analog carrier modulation

(5) Digital signal digital carrier code

First (1)

Digital baseband signal is the use of high and low levels or pulses to represent information, (such as positive and negative pulses 1, 0, high and low level represents 1,0) There are many kinds of representations, each has advantages and disadvantages

These signals are generally encoded first, such as Ami code, is 0 unchanged 1 alternating to +-1, encoding is to improve the original signal some characteristics

After encoding to pass through the channel signal generator (compression of the input signal band, so that the input signal is more suitable for the channel) * Band is the maximum frequency allowed to transmit the signal and the minimum frequency of the allowed signal to the frequency range between this

Then through the channel (divided into wired and wireless, cable fiber electromagnetic waves, etc.), the channel should be considered when the noise

Then passes through the receiving filter (filtering noise and other interference)

Finally, a sample judgment device (to recover or regenerate the baseband signal is processed for the passed signal)

The signal that goes through all the steps also needs to undergo the last time pulse synchronous extraction, which relies on the synchronous extraction circuit to extract the original pulse information from the derived class baseband signal.

In this way transmission process mainly need to pay attention to the inter-code crosstalk:

is due to the overall characteristics of the system transmission is not ideal, leading to the waveform distortion, widening, and the front waveform appears a long trailing, spread to the current sampling time of the code element, thus the current code element of the decision caused interference

Simple point is that the code of the preceding moment affects the current moment of the code, resulting in a judgment error