Signal is a tool for carrying messages and a carrier of messages. Broadly speaking, it includes optical signals, acoustic signals and electrical signals. For example, the ancient people used the billowing smoke generated by lighting the beacon to transmit the news of the enemy invasion to the distant army. This is a light signal; when we speak, sound waves are transmitted to the ears of others, so that others understand our intentions. This belongs to sound. Signals; all kinds of radio waves traveling in space, electric currents in the telephone network extending in all directions, etc., can be used to express various messages to a distant place. This is an electrical signal. People only know what the other party wants to express by receiving light, sound, and electrical signals.
Analog signal is a form of propagating energy. It refers to the signal that is continuous in time (uninterrupted), and the magnitude of the value is also continuous and uninterrupted (traditional audio signal, video signal). ). If a sound wave vibrates the medium it passes through, the sound wave can be measured in frequency (in cycles per second or hertz (Hz)). Digital signals are transmitted through the media by representing binary numbers as electrical pulses (where each pulse is a signal element). The voltage on the line changes between high and low states. For example, a high level can be used to transmit a binary one, and a low level can be used to transmit a binary zero. Bandwidth is a term that refers to the number of bits transmitted through a link per second.
Figure s-1 describes analog and digital signals, where analog signals are equivalent to digital signals.
In long-distance transmission, the signal is degraded due to attenuation, noise, and interference from other wires in the wire bundle. The analog signal can be amplified periodically, but if the signal is corrupted by noise, it is a distorted signal that is amplified. In contrast, since the digital signal can be easily extracted from the noise and retransmitted, it is more reliable to transmit the digital signal over a long distance.
1. Co-frequency interference
Co-frequency interference refers to the overlapping area of the coverage of two or several adjacent base stations. The field strength of the receiving point is the sum of the field strengths of the signals from each base station. Because each base station signal propagation path, medium and used transmitting equipment are different. Therefore, the time for the signals sent by each base station to reach the overlapping area is also different, that is, there is a relative time delay difference between the signals, which results in a relative phase difference of the signals. Due to the existence of the phase difference, the signals in the overlapping area interfere with each other, which directly affects the normal reception of the BP machine. Of course, the same frequency interference also has a certain relationship with the modulation degree and frequency deviation. It is the key to solve the problem of co-frequency interference to adjust the time for the signals sent by each base station to reach the overlapping area in a certain way. According to the CCIR report, the non-return-to-zero direct FSK modulated POCSAG code commonly used in my country's wireless paging. When the relative delay difference between the modulated signals of the base stations is less than 1/4bit period, the BP machine in the overlapping area can get a satisfactory receiving effect. When the modulation signal rate is 1200bit/s. The relative delay difference should be less than 208μs. The time delay adjustment range of our common MOTOR0LALT transmitter is 180-220μs. Since the transmission medium between the splitter and each base is different, the specific adjustment of the transmitter time delay is generally based on the base station farthest from the central station (time delay 180μs). Calculate the delay of other bases with a delay of 1μs per kilometer. It is necessary to make many adjustments in actual work to achieve the required effect. should pay attention. According to the requirements of "wireless paging technology system", the sensitivity of the digital pager should not be less than 5μV/M, and the sensitivity of the Chinese character pager should not be less than 10μV/m. According to theoretical calculations, when the height of the transmitting antenna is doubled, the signal field strength is also doubled; and when the effective transmission power is doubled, the signal field strength is increased by 40%. To increase the communication distance by 1 times, the antenna must be increased by 4 times, or the power must be increased by 16 times. It can be seen that the higher the antenna is. The easier it is to cause co-channel interference. The power is not adjusted as much as possible, it depends on the specific situation; when necessary, a directional antenna can be used. When adjusting, the delay should also take into account the influence of different base station antenna heights.2. Intermodulation interference This is caused by the nonlinear effect of two or more radio frequency signals of different frequencies at the end of a certain transmitter's power amplifier, which produces a new frequency component equal to another frequency point. The third-order intermodulation interference is divided into two types, two types and three types. When the four frequencies F1-F4 satisfy F1+F2-F3=F4, and F1-F3 is the transmitting frequency, and F4 is the receiving frequency. F4 will be interfered. This kind of interference is called type three third-order intermodulation interference; when the three frequencies F1'-F3' satisfy 2F1'-F2'=F3', and F1' and F2' are the transmitting frequencies. When F3' is the receiving frequency, F3' will be interfered. This interference is type 2 and 3 order intermodulation interference. There are three ways to eliminate intermodulation interference: One is to use the spatial isolation of antennas to reduce the coupling between transmitters. The magnitude of the spatial isolation attenuation between antennas is related to the relative positions of the two antennas. When erected horizontally, the distance should be greater than 1.5-2 times of the larger wavelength; when erected vertically. The spacing should be greater than the larger wavelength. The second is to install a unidirectional device at the output end of the final stage of the transmitter. If the frequency of the interference source differs from the interfered frequency by more than 3MH2, a cavity filter can be used. The third is the combined use of the above two methods.3. Spurious interference spurious interference mainly refers to that due to the poor filtering characteristics of the transmitter frequency multiplier, some second and third harmonic components are output at the transmitter output stage, resulting in spurious radiation signals. In addition, the transmitter’s unqualified technical indicators will also cause the carrier-centric noise distribution to be quite wide, causing interference in a frequency band of several MHz. A more effective way to eliminate spurious interference is to insert a selective filter at the output of the transmitter to reduce the interference signal. When the transmitter carrier frequency power is greater than 25W, the radiated power skin of any discrete frequency is lower than the transmitter carrier frequency power by 70dB, so it will not interfere with normal communication. Transmitters that are seriously inconsistent with technical indicators should be resolutely eliminated.4. Adjacent channel interference "Adjacent channel interference" refers to the interference between adjacent or adjacent channels. Currently, VHF and UHF radio stations widely used in mobile communication systems have a channel interval of 25KHz. As we all know, the frequency spectrum of the FM signal is very wide. Some of the harmonic components fall into the passband of the adjacent channel receiver, which will cause adjacent channel interference. This kind of interference is mainly caused by the serious disqualification of the transmitter's technical indicators. Generally, when working with multiple base stations, the frequency stability of the transmitter is required to be 5×10-6; the maximum allowable frequency deviation of modulation is 5KHz. The paging system in my country is set to 4.5KHz; the adjacent channel radiated power to the adjacent channel receiver should be 70dB lower than the carrier power.
Signal coding scheme
The synchronization method uses PCM digital channel to transmit data. If the data signal is synchronized with the clock of the digital end office, at this time, the data signal output by the data terminal is affected. The PCM channel clock is controlled, so only the data signal needs to be multiplexed. Here the data terminal equipment is in a controlled subordinate position, so the flexibility is poor.
If the data signal and the data end office clock are asynchronous, then the data signal can be multiplexed into a 64kbit/s aggregate signal by filling mode, which is the asynchronous mode.
For digital devices, the receiver must have some way to know the beginning and end of bytes in the data stream. In asynchronous communication, byte boundaries are indicated by start and stop bits. In synchronous communication, the timing mechanism helps the transmitter and receiver to be in a synchronized state. The synchronization signal can occupy a separate channel, but more often it is directly integrated into the signal.
The unipolar unipolar code has voltage for 1, and no voltage for O. There is no special coding. Unipolar codes accumulate DC components.
In the bipolar bipolar code, a positive voltage means 1, and a negative voltage means 0. This solution reduces power requirements and reduces high-level attenuation. The DC component of the bipolar code is greatly reduced, which is conducive to transmission. The voltage state of the RZ (return to zero system) return to zero code returns to zero after a certain signal state. The pulse of the return-to-zero code is narrow. According to the inverse relationship between the pulse width and the transmission bandwidth, the return-to-zero code occupies a wider frequency band on the channel.
NRZI (press 1 inverted non-return-to-zero system) No matter the level is high or low in NRZI code, it does not represent binary 1 and 0. Instead, the voltage change represents a binary one. If there is no voltage change, the next bit is 0; if there is a voltage change, the next bit is 1. It is difficult to determine the end of one bit and the beginning of another bit in the transmission of non-return-to-zero codes. It is necessary to use some method to make timing or synchronization between the transmitter and the receiver. NRZI is used for slower RS-232 serial communication and data storage on hard drives. On synchronous links, long strings of consecutive bits (possibly thousands of zeros) can cause problems. The receiver may lose synchronization and cannot detect the correct number of 0s in the continuous string. Another problem is that the long string of 0s is a direct current, which cannot pass through certain electrical components. Manchester encoding and other solutions solve these problems by adding clock signals. Manchester (Manchester) In Manchester encoding, there is a transition in the middle of each bit, and the transition in the middle of the bit is used as both a clock signal and a data signal; a transition from high to low means "1", and a transition from low to high Represents "0". This provides the receiver with a timing signal that can be synchronized with it. Manchester encoding is commonly used on LAN.
Division according to frequency band
is defined as The interference signal is in the uplink frequency band of the mobile network, and the external radio frequency interference source interferes with the base station. Uplink interference will reduce the coverage of the base station. In the absence of uplink interference, the base station can receive mobile phone signals from far away. When uplink interference occurs, the mobile phone signal must be stronger than the interference signal to communicate with the base station. Therefore, the mobile phone must be closer to the base station.
It refers to the interference signal sent by the interference source in the downlink frequency band of the mobile network, and the mobile phone receives the interference signal. It is impossible to distinguish between normal base station signals, which interrupts the communication between the mobile phone and the base station, resulting in dropped calls or failure to register.
According to frequency points
In-band interference of unwanted signals with the same frequency as the useful signal is also called co-channel interference. These unwanted signals are the same as useful signals. They are amplified in the receiver and converted into the IF passband. Therefore, as long as there is co-frequency interference at the input of the receiver, the receiving system cannot filter and suppress it.
Mainly including adjacent channel interference, intermodulation interference, blocking interference, spurious interferencep>
1. Adjacent channel interference: "Adjacent channel interference" refers to signal interference from adjacent frequencies of the signal frequency used. Adjacent frequency interference is caused by the imperfect receiving filter, which causes signals of adjacent frequencies to leak into the transmission bandwidth. Adjacent channel interference can be minimized through precise filtering and channel allocation. 2. The near-far effect:
If the base station of the adjacent channel transmits within a very close range from the user receiver, and the receiver uses the base station signal of the preset channel, this problem will become very serious. This is called the near-far effect. When the channel used by the mobile station very close to the base station and the channel used by a weak signal mobile station are adjacent channels, the near-far effect will also occur. (In the UMTS system, since all mobile stations use the same frequency band, the near-far effect is more obvious, but the UMTS system uses good power control to eliminate the influence of the near-far effect). 3. Intermodulation interference When two or more different frequency signals act on a non-linear circuit, they will modulate each other to produce a new frequency signal output. If the frequency falls within the working channel bandwidth of the receiver, it constitutes interference to the receiver. , It becomes intermodulation interference. 4. Blocking interference Any receiver has a certain receiving dynamic range. When the out-of-band interference signal is strong to a certain extent, and the received power exceeds the maximum power level allowed by the receiving dynamics, it will cause the receiver to be saturated and blocked, thereby affecting the receiving performance of the system. , This type of interference is called blocking interference. Blocking will cause the receiver to not work normally, and long-term blocking may also cause permanent performance degradation of the receiver.5. Spurious interference Due to the roll-off characteristics of the emission filter (any filter cannot be an ideal step method), there is always a certain amount of out-of-band radiation, which is what we usually call emission spurious. The interference due to emission spurs is called spurious interference.
Interference between mobile communication systems
In-band interference: CDMA transmission signals are directly or indirectly used by intermodulation and other methods In-band noise acts on the GSM receiver, causing the sensitivity of the GSM receiver to decrease. This type of interference is divided into emission spurious interference and intermodulation interference;
Out-of-band interference: When the out-of-band interference is strong to a certain level, it will cause the receiver to be saturated and blocked, thereby affecting the receiving performance of the GSM system. , This type of interference is also called blocking interference
Interference signal source
In the collection, processing, and transmission of multimedia audio and video signals, anti-interference has always been the main focus of many integrators and developers. The target of breach. When using a video capture card to capture video signals, after the video is captured and compressed, it needs to be transmitted to a designated host. Generally, it is sufficient to use the cable that comes with the device. However, in some specific industries, the distance of video transmission is long, and some signal interference phenomena are often encountered in video transmission and collection, causing the transmitted signal to be fluctuated, interfered, etc., and irregularities will be seen on the monitor. The thin line scrolls from top to bottom, causing the captured video to appear frame loss and blur. The reasons and solutions for the interference of video signals are briefly introduced by the technology of the same three-dimensional video network. This phenomenon basically does not occur in short-distance transmission, but long-distance transmission is susceptible to interference sources.
According to different sources of interference, interference signal sources can be divided into three sources:
Interference caused by front-end equipment
Interference from the power supply of the front-end camera, The interference caused by the quality of the camera itself can be judged by directly connecting the monitor to the front end. If the interference is caused by the power supply, it can be solved by replacing the power supply, using a switching power supply, and adding an AC filter in the 220V AC loop.
Power supply interference, mainly in the following situations:
1)50Hz power supply interference, due to the different ground potentials at both ends and the existence of cable sheath resistance , Causing a 50Hz ground potential difference between the two grounds, thereby generating interference signal voltage. When the interference signal is superimposed on the video signal, a wide horizontal dark band appears on the normal image.
2)Unclean power supply interference, the power supply referred to here is not "clean", which means that there is an interference signal superimposed on the normal power supply (50-cycle sine wave). And the interference signals on this kind of power supply mostly come from the devices using SCRs in the power grid, especially the high-current, high-voltage SCR devices, which pollute the power grid very seriously, which leads to power supplies in the same power grid. Not "clean".
3) The second harmonic and third harmonic interference of 50Hz power frequency: Harmonic interference is mainly manifested around high-current or high-voltage power lines. It is the radiation signal of the power cable to the surroundings. Its frequency is 2500Hz and 125000Hz, mainly interfere with the low frequency band of the video signal.
When it comes to harmonic interference, one has to talk about the inevitable broadcast signal interference during transmission. The interference of broadcast signals is very strong and very common. Due to the needs of practical applications, when the cable must be erected in the air, the cable itself is equivalent to a very long antenna. As a result of the antenna effect, a broadcast interference signal voltage will be generated on the terminal load, causing the interference signal to be mixed into the video signal. This kind of interference signal appears on the image as a dense oblique mesh, which may even overwhelm the image in severe cases.
Interference in the transmission process
It is mainly the interference caused by the damage of the transmission cable, electromagnetic radiation interference and ground wire interference (ground potential difference). For the transmission cable, you can replace the cable Or add anti-jamming equipment to solve it.
Terminal equipment interference
Mainly the power supply of the monitoring room, the interference generated by the equipment itself, the interference caused by the grounding, the interference caused by the connection of the equipment and the equipment, etc. The simple way to judge is to monitor The room is directly connected to the camera for observation.
Ground potential difference interference
Causes and elimination methods of ground potential difference interference during signal transmission:
Ground potential difference interference is the interference that often occurs in the system. The reason for the ground potential difference interference is that there are more than two conflicting grounds in the system, and there is a certain voltage difference between the ground and the ground. , This voltage forms interference current through the outer shielding net of the signal cable, which causes interference to the image. The main component of the ground current is the interference pulse generated by 50 Hz alternating current and electrical equipment. The image appears as horizontal black stripes, distortion, and horizontal clutter, and it may move slowly in the vertical direction.
Due to the interference caused by the damage of the video cable, it is the best way to replace the cable. It is impossible to replace it. If the interference is snowflake or net interference, you can choose an anti-jamming device with amplifying principle.
The solution is:
a, Isolate the front-end equipment from the ground, but avoid the possible danger of lightning or electric shock.
b、 Use anti-jamming equipment with isolation function, such as anti-jamming device, video isolator, etc.
Electromagnetic radiation interference
a,The formation and solution of electromagnetic radiation interference during signal transmission
For video interference, the discussion is mainly based on the interference method; the audio signal has a larger wavelength, and the shielding effect of the communication building is more obvious. In contrast, the radiation interference is negligible.
b,The principle of transmission line to eliminate external electromagnetic interference
If the cable is buried in the ground, or the lead cable, Balanced and symmetrical cables, etc. can better overcome this interference.
The coaxial cable is shielded against electromagnetic interference. A coaxial cable is composed of an outer conductor and an inner conductor, and an insulating material is used as a filler between the inner and outer conductors. The outer conductor is usually a net braided by copper wires, which has a good shielding effect on external electromagnetic interference. The inner conductor is under the tight protection of the outer conductor, so the coaxial cable has good anti-interference ability.
The shielding effect of the cable shielding layer on the lower the frequency of the signal, the worse the shielding effect, due to this reason, the introduction of interference signals such as carrier phone, radio signal and so on. They cause the interference of horizontal stripes on the image.
c,The interference and elimination of strong electromagnetic radiation on the line
The transmission line has the ability to resist external electromagnetic interference and can effectively Transmission signal. However, when the interference source is too strong, it will interfere with the image signal. There are two main types of these strong electromagnetic interferences:
1) There are strong electromagnetic radiation sources nearby.
2) Improper line design, interference caused by strong current lines to the transmission line.
Strong electromagnetic radiation sources are generated by high-power radio stations or electrical equipment with electromagnetic radiation. The interference caused by strong electromagnetic radiation appears on the image as net-like ripple interference. For this kind of interference, the following methods can be used to eliminate the interference.
1) Avoid interference sources as much as possible, and the system equipment and lines should be away from the radiation source at a certain distance.
2) Choose a cable with good shielding performance. The braiding density of the outer shielding net of the coaxial cable directly affects the anti-interference performance of the cable. The greater the braiding density, the stronger the anti-interference ability.
3) Add anti-jamming equipment.
The main reason for the interference caused by the wiring is that the transmission cable and the strong electric line are wired in parallel with a long distance and close size, and electromagnetic coupling occurs with each other. The anti-interference ability of the coaxial cable is low in the low frequency band, and the strong electric interference component is mainly the 50 Hz alternating current and its harmonics, so the threat to the coaxial cable is greater. Therefore, it is necessary to avoid long-distance and close parallel wiring of signal lines and strong current lines. Strong electric lines and signal transmission lines should be laid in separate ducts, and a certain distance between the ducts should be kept; of course, short-distance parallel laying of transmission cables and strong electric lines will not cause great interference. At both ends of the system and in the equipment cabinets, it is inevitable that strong current lines and signal lines are wired in parallel at a short distance, which will not cause major interference.