《Communication principle》

communication principle

What does modulation mean? effect? Why?

In communication systems, the purpose of modulating analog baseband signals is to allow multiple baseband signals to be modulated and transmitted simultaneously on the wired channel. It is also suitable for transmitting frequency band signals in wireless channels; and it can also enhance the signal Anti-noise ability. Therefore, the role of modulation can be summarized as reducing interference, improving the system's anti-interference ability, and at the same time realizing the interchange between transmission bandwidth and signal-to-noise ratio.

Control：

Convert various digital baseband signals into digital modulated signals (modulated signals or frequency band signals) suitable for channel transmission/use a certain parameter of the carrier to represent the baseband signal.

Purpose of modulation:

Converting the analog signal or digital signal to be transmitted into a signal suitable for channel transmission means converting the baseband signal (source) into a bandpass signal with a very high frequency relative to the baseband frequency. This signal is called a modulated signal, and the baseband signal is called a modulated signal. Modulation can be achieved by changing the amplitude, phase, or frequency of a high-frequency carrier wave as the signal amplitude changes.

Cause :

The signals we usually hear and see are not suitable for direct transmission by antennas due to reasons such as frequency, bandwidth, and susceptibility to interference. Therefore, a high-frequency signal is used as a carrier, and the signal to be transmitted is mixed into the carrier and passed through the antenna. emission

① Modulation moves the spectrum of the baseband signal so that the spectrum characteristics of the modulated wave are suitable for the spectrum characteristics of the channel.

②Modulation moves low-frequency signals to high frequencies, making the signal energy easier to radiate through the antenna

③Modulation moves multiple baseband signals to different carriers, completes frequency allocation of signals, and enables multiple signals to be transmitted on the same channel without interfering with each other, thereby achieving frequency division multiplexing and improving channel utilization.

④ Modulation can reduce noise and interference and improve the anti-interference ability of the system.

⑤ Modulation can improve the utilization of frequency resources.

Action:

Wireless communication uses space radiation to transmit signals. According to electromagnetic wave theory, it can be known that the antenna size is about one-tenth or larger of the signal wavelength, so that the signal can be effectively radiated. If the signal is sent directly without modulation, the required antenna size is at least several kilometers! It is virtually impossible to build such an antenna. Modulation can move the signal to any desired higher frequency range, where it can easily be radiated as electromagnetic waves.

What is linear modulation? What is nonlinear modulation?

Linear modulation is a linear shift in the frequency spectrum. Common ones include amplitude modulation, double sideband modulation, single sideband modulation and residual sideband modulation.

Nonlinear modulation is no longer a linear shift of the original modulated signal, but a nonlinear transformation of the spectrum.

What is frequency modulation? What is phase modulation? How to convert between the two?

1. Frequency modulation means that the instantaneous frequency offset changes proportionally with the modulating signal,
2. Phase modulation means that the instantaneous phase offset changes linearly with the modulating signal.

Phase-modulated signals can be obtained by first differentiating the baseband signal and then frequency-modulating it; by integrating the baseband signal first and then phase-modulating it, frequency-modulated waves can be obtained.

Why can FM systems trade bandwidth and signal-to-noise ratio, but AM systems cannot?

Because the bandwidth of AM is fixed.

What is the relationship between the modulation gain of an FM system and the signal bandwidth?

The larger the signal bandwidth, the higher the modulation gain.

What is Casson's formula? What is its significance?

Casson's formula means that the bandwidth of an FM signal is equal to twice the maximum frequency deviation plus the frequency of the modulating signal;

It shows that the bandwidth of the FM signal depends on the maximum frequency deviation and the frequency of the modulation signal.

What are the characteristics of the waveform and spectrum of AM signals?

The waveform envelope of the AM signal is the same as the envelope of the modulated signal. The AM signal spectrum includes the carrier frequency component, upper sideband and lower sideband. The spectrum structure of the upper sideband is the same as that of the original modulated signal, and the lower sideband is the mirror image of the upper sideband. The total power of the AM signal includes carrier power and sideband power,

What is overmodulation of a modulated signal?

The amplitude modulation coefficient of the AM signal is greater than 1, causing the envelope of the AM signal to no longer reflect the changing pattern of the baseband signal.

Why suppress carriers?

To improve modulation efficiency, reduce carrier power. For the double sideband that suppresses the carrier, the modulation efficiency can be increased from one-third to one.

What are the methods for generating single sideband modulated signals? What are the technical difficulties in each?

Phase shifting method and filtering method. The filtering method is to first generate a double-sideband modulated signal and then use a sideband filter to filter out one of the sidebands. The main difficulty is the production of the sideband filter because the frequency interval between the upper and lower sidebands is very narrow.

The phase shift method uses Hilbert transform to form a single sideband modulation signal. The difficulty lies in the production of a broadband phase shift network.

What conditions do the transmission characteristics of a vestigial sideband filter satisfy? Why?

It must meet the characteristics of complementary odd symmetry at the carrier frequency. In this way, the required modulation information can be recovered from the residual sidebands without distortion during coherent modulation.

Why is the anti-noise performance of DSB and SSB modulation systems the same?

Because under the same input signal power and noise conditions, the output signal-to-noise ratio of the two is the same.

The respective roles of modulation, channel coding and source coding, and their relationship

connect:

The source coding is converted into channel coding after signal modulation.

Source code:

The statistical characteristics of the source are used to solve the correlation of the source, improve the effectiveness of the information, remove the redundant information of the source, and increase the information rate output by the source. For analog sources, advanced analog-to-digital conversion is also required.

Channel coding:

In order to ensure the reliability of the communication system, the noise and interference in the channel must be overcome. It adds some necessary supervision symbols to the information symbols to be sent, and uses supervision rules between the receiving end and the sending end to detect and correct errors, trying to improve reliability to correct errors in the information transmission process.

How does a signal pass from sending to receiving?

Forcommunication systems, from a broad category, they can be divided into analog communication systems and digital communication systems. In contrast, the analog communication system is a relatively old communication system with a relatively simple structure. The analog communication system can be divided into several modules: the transmission of analog signals, the analog signals are generated by the source and modulated. That is to say, the analog signal is moved, and after the move, the signal is sent to the channel. Then, the receiving end performs demodulation and finally obtains the demodulated signal. The entire process, from generation and modulation to demodulation and reception of analog signals, has a relatively simple structure. Analog signal--modulation--channel--demodulation--receiving end.

Fordigital communication system, its modules are more detailed and complex. After coming out of the source, it enters the source encoding module, and the signal output by the source encoding module may go to the encryption module. This is to ensure the privacy of communication, and there is usually an encryption and decryption module. Then this digital information sequence will enter channel coding to improve reliability. Then modulation is performed. After modulation, some transformations may be performed and then put on the channel. Then demodulation, channel decoding, decryption, source decoding and signal sink are performed at the receiving end. The front part of the digital communication system, the sending device, is divided into many modules, and the receiving device is also divided into many modules. Therefore, compared with the analog communication system, the digital communication system obviously has more modules and is more complex.

What is digital communication? Describe the main advantages and disadvantages of digital communication systems?

Digital communication streams are communication systems that use digital signals to transmit information.

Theadvantagesof digital communication systems are:

1. Errors are controllable;
2. Strong anti-interference ability;
3. Since digital signal transmission uses binary codes, computers can be used to store, process and exchange digital signals;
4. Easy to integrate and encrypt, so digital communications are highly confidential;
5. The equipment is easy to integrate and computerize;
6. Digital communication systems can transmit various information, making the communication system flexible and versatile, and thus can constitute a modern digital communication network for signal processing, transmission and exchange.

ConclusionCorrection:

1. The frequency band utilization rate is not high, the frequency bandwidth is occupied, and the system equipment is relatively complex.

Advantages and Disadvantages of Analog Communication

Excellent score：

1. The signal spectrum passing through the channel is relatively narrow, so the channel utilization is high.

Finish point：

1. The transmitted signal is continuous, and it is difficult to remove noise interference after being mixed in, that is, the anti-interference ability is poor.
2. Not easy to keep secret
3. Equipment is not easy to integrate on a large scale

What are the measures of effectiveness and reliability of analog and digital systems?

The effectiveness of an analog system is measured by effective bandwidth, and reliability is measured by signal-to-noise ratio.

The effectiveness of a digital system is measured by frequency band utilization, and reliability is the bit error rate or error rate.

whereeffective bandwidth refers to the maximum frequency bandwidth that can effectively pass the channel signal;signal-noise Ratio is the ratio of signal power to noise power; Frequency band utilization is the transmission rate of symbols within the unit frequency band; The bit error rate refers to the probability that a symbol is transmitted incorrectly in the transmission system, that is, the proportion of the number of erroneous symbols in the total number of transmitted symbols.

Supplement: The anti-noise indicators of the analog system are the output signal-to-noise ratio and system gain.

The role of the two filters before and after in the digitization process

The purpose of passing an LPF (Low Pass Filter) before A/D conversion is to limit the maximum frequency of the signal so that it meets the condition that when the sampling frequency is certain, the sampling frequency should be greater than or equal to 2 times the maximum frequency of the signal. This filter is also called an "anti-folding" filter

After the D/A conversion, an LPF is passed to filter out the high-frequency continuation spectrum in order to smooth the sample-and-hold staircase output wave, so it is also called a "smoothing" filter.

What are the communication methods classified by signal flow and time?

Simplex, duplex, half-duplex

What are symbol rate and information rate? What is their relationship like?

The symbol rate is the number of symbols transmitted per unit time, and the information rate is the average amount of information (bits) transmitted per unit time. The information rate is equal to the symbol rate times the information entropy.

Describe the characteristics of energy signals and power signals

The energy signal has limited energy, but its power is 0; the power signal has limited average power, but its energy is infinite.

What is additive interference? What is multiplicative interference?

Regardless of whether there is a signal or not, there will be interference that changes with the size of the signal, which is called multiplicative interference; the noise in the channel is called additive.

Channel classification

According to different classification methods and channel composition, channels can be divided intonarrow channels (referring to the transmission medium of the channel) and < /span>discrete channels (digital channel). According to the physical properties of the channel, channels can be divided into wireless channels, wired channels, optical channels, etc. (analog channels) and continuous channels. According to the type of signal input and output terminals, channels can be divided into modulated channel , and the generalized channel composed of transmitter, transmission medium and receiver is calledcoded channel (In addition to the transmission medium of the channel, it also includes certain equipment of the communication system), which consists of a modulator, a transmission medium, and a demodulator. The channel is calledGeneralized channel

Generally classified according to input and output methods.

Continuous channel:

can be divided into constant parameter channel and random parameter channel, the properties (parameters) of constant parameter channels do not change with time, or change very slowly, which can also be considered as constant parameter channels; the properties (parameters) of variable parameter channels change with time. Some wireless channels can be regarded as constant parameter channels, and the other part can be regarded as random parameter channels, while wired channels are generally considered as constant parameter channels.

constant parameter channel

include:

1. cable channel
2. Fiber Channel
3. Radio distance relay channel
4. Satellite channel.

accompanying channel

include:

1. Shortwave ionospheric reflection channel. (Since the ionosphere is not a plane but has a certain thickness, and has two to four layers (D, E, Fi, F2) with different heights, the signal emitted by the transmitting antenna is reflected by the ionosphere at different heights and transmitted from different heights. The signal reflected by the ionosphere and reaching the receiving end is the sum of many signals from different directions, different path lengths and losses. This signal is called multipath signal, this phenomenon is called multipath propagation)
2. Tropospheric scatter channel 
3. Mobile communication channels also belong to multipath propagation parametric channels.

Main properties of continuous channels:

1. Having a pair (or multiple pairs) of input and output terminals;
2. Most channels are linear, that is, they satisfy the superposition principle;
3. The signal will be delayed when passing through the channel, and will also be subject to fixed or time-varying losses;
4. When there is no input signal, there is still noise output at the output end of the channel. According to the above properties, we can use a two-terminal (or multi-terminal) time-varying linear network to represent the continuous channel.

What is a constant parameter channel? What is a parametric channel? What is the multipath effect?

Constant parameter channel: A channel whose channel transfer function does not change with time;

Variable parameter channel: A channel whose channel transmission function changes randomly and rapidly with time parameters.

Multipath effect: The impact on the signal due to multipath transmission. It means that after electromagnetic waves propagate through different paths, each component field arrives at the receiving end at different times, and superposes on each other according to their respective phases, causing interference, causing the original signal to be distorted or causing errors.

The impact of constant parameter channels on signal transmission can be completely eliminated, while the impact of variable parameter channels on signal transmission can only be eliminated in the sense of statistical averages.

Generally speaking, the common characteristics of various random parameter channels are:

1. The transmission attenuation of a signal changes with time
2. The transmission delay of the signal changes with time
3. The signal reaches the receiving end through several paths, and the length (delay) and attenuation of each path change with time, that is, multipath transmission exists.

The main impacts of multipath transmission on signal transmission are:

1. Rayleigh fading
2. Frequency dispersion (frequency spread)
3. frequency selective fading

Overcoming multipath effects:

It is mainly solved by reducing the symbol transmission rate. For example, OFDM technology changes serial transmission into parallel transmission in order to reduce the symbol rate. MIMO systems specialize in using different antennas to send copies of the signal, and complex receiving systems combine different chips for processing to improve system performance. Time domain equalization, orthogonal frequency division multiplexing (OFDM) and Rake receivers can all be used to combat interference caused by multipath. Improve the distance measurement accuracy of the receiver, such as narrow correlation code tracking loop, phase ranging, smooth pseudo-range, etc.; anti-multipath antenna

Wireless channel fading

Scale is relative to wavelength. The large scale means that the distance is much larger than the wavelength, which is fading in the macro sense; the small scale is several times the wavelength, short distance and short time, and the study is fading in the micro sense.

Path loss:

It refers to the loss caused by the propagation of radio waves in space. It is caused by the radiation diffusion of the transmit power and the propagation characteristics of the channel. It reflects the change of the average received signal power in the macro range. Theoretically, for the same sending and receiving distance, the path loss is also the same. However, in practice, it is often found that the received power at different receiving points with the same sending and receiving distance varies greatly, and even the received power at the same receiving point fluctuates greatly at different time points. The longer the wavelength, the smaller the attenuation, and the greater the received power Pr; the longer the distance, the greater the attenuation, and the smaller Pr. The attenuation of Pr is caused by path loss.

Shadow effect:

In wireless communication systems, when the mobile station is in motion, a semi-blind area is formed in the propagation and reception area due to the obstruction of the radio wave transmission path by large buildings and other objects, thus forming an electromagnetic field shadow. This changes with the position of the mobile station. The fluctuations in the median field strength at the receiving point caused by constant changes are called shadow effects. Generally obeys the lognormal distribution.

Multipath effect:

means that after electromagnetic waves propagate through different paths, each component field arrives at the receiving end at different times, superimposing each other according to their respective phases, causing interference, causing distortion of the original signal or generating errors. The multipath effect produces delay expansion (the time difference between the fastest and slowest signals arriving at the receiving end), resulting in multipath fading after the superposition of multiple signals. Models for this fading include Rayleigh fading, which refers to the amplitude of the receiving end signal. The one-dimensional distribution of V(t) obeys the Rayleigh distribution (that is, at any time ti, V(ti) is a random value obeying the Rayleigh distribution). Due to the multipath effect, the signal arrival time will change. Due to changes in arrival time, symbols within the same carrier will interfere with each other, forming inter-symbol interference (ISI). In order to eliminate inter-symbol interference (ISl), guard intervals should be inserted between symbols.

Doppler effect:

The Doppler effect brings Doppler spread (i.e. Doppler frequency shift). The coherence time of the channel is inversely proportional to the maximum Doppler frequency shift. If the symbol period of the transmitted signal is less than the coherence time, then the received signal is considered to have experienced It is slow fading (time non-selective fading), otherwise it is considered that the received signal experiences fast fading (time selective fading). (Coherence time is the time the channel remains unchanged). Due to the influence of the Doppler effect, the center frequency of the signal will shift. Due to the frequency shift of the center frequency, the signal will cause interference to other carriers, forming inter-carrier interference (Inter Channel Interference, ICI). In OFDM, in order to eliminate ICI, the guard interval is usually served by the cyclic prefix CP (this can eliminate both ICI and ISI).

Definition of channel capacity

Channel capacity refers to the maximum average information rate at which a signal can transmit data without errors in the channel.

The channel capacity can be measured by the Nyquist criterion under the ideal condition ofno noise. In the absence of noise, the data rate The limitation only comes from the bandwidth of the signal. If the bandwidth is B, then the maximum signal rate that can be transmitted is 2B; in the case of noise Measured and calculated using Shannon's formula.

What is Shannon's formula? How to understand?

C=Blog2(1+S/N), where C is the channel capacity, B is the channel bandwidth (Hertz), S is the signal power (Watts), and N is the noise power (Watts).

Shannon's theorem states:

Under ideal circumstances, the theoretical limit of information transmission rate that a communication system can achieve. That is to say, if the information rate R of the information source is less than or equal to the channel capacity C, then in theory there is a method (channel coding technology) that can enable the output of the information source to be transmitted through the channel with an arbitrarily small error probability. (To achieve error-free transmission of information)

Three ideas for increasing channel capacity:

1. Increase signal transmission power
2. Reduce noise power
3. Increase the channel bandwidth: This is not an unlimited increase, because increasing the bandwidth will increase the output noise power, resulting in a decrease in the input signal-to-noise ratio, and in severe cases, a threshold effect may occur. Bandwidth can be exchanged for signal-to-noise ratio. If the channel capacity C is given, the bandwidth can be increased to reduce the requirement for signal-to-noise ratio; or the signal-to-noise ratio can be increased to exchange for a narrower frequency band and save spectrum resources.

What does envelope detection consist of?

Differential rectifier and low pass filter. In order to ensure the quality of detection, the amplitude of the inserted carrier should be much larger than the amplitude of the signal.

What is the threshold effect? What is the threshold value? Why does coherent demodulation not have threshold effect? What is the optimal decision threshold level?

Threshold effect is essentially caused by the nonlinearity of the envelope detector. When the useful signal is overwhelmed by noise, the input signal-to-noise ratio is reduced to a certain extent, resulting in the output signal The noise ratio deteriorates drastically. The input signal-to-noise ratio at which the threshold effect begins to appear is the threshold value. The decision level that minimizes the bit error rate is the optimal threshold level.

Since the coherent demodulated signal and noise can be demodulated separately, a valid signal is always present at the output of the demodulator.

What is the method to lower the threshold?

Phase locked loop frequency discriminator and negative feedback demodulator. There are also pre-emphasis and de-emphasis techniques