Several Factors Affecting the Distance of COFDM Wireless Communication
For COFDM wireless communication and wireless systems, there are four main performance indicators that affect the communication distance:
1. the power emitted by the transmitter;
2. the sensitivity of the receiver;
3. the whole system anti-jamming capability;
4 . the type and gain of the transmit / receive antenna.
In these four main indicators, the national electromagnetic compatibility standards (such as the North American FCC, the European EN specification) are only limited to the transmission power, as long as the acceptance sensitivity system of anti-jamming ability to optimize the two indicators, FCC or CE standards under the premise of expanding the communication distance of the system.
First, the impact of wireless communication distance of many factors
1, geographical environment
The farthest distance of communication is the straight and open space where sea level and land are accessible. This is also the geographical condition commonly used to assess the communication distance of a wireless communication device. Followed by the suburbs of rural areas, hills, riverbed and other semi-obstacle, semi-open environment, the communication distance is the nearest urban buildings or mountains, in short, the more intensive obstacles, the greater the impact of wireless communication distance, especially metal objects The greatest impact. Some common environments for wireless signal loss are shown in the following table:
According to the path loss formula: Ld = 32.4 + 20logf + 20logd
f = MHZ d = Km know the signal loss of 6dB, the communication distance will be reduced by half!
Another factor is the multi-path effect, so if the wireless module near the obstacles will also affect the communication distance and reliability.
2, the electromagnetic environment
DC motor, high voltage power grid, switching power supply, welding machine, high frequency electronic equipment, computer, microcontroller and other equipment on the wireless communication equipment communication distance has different degrees of impact.
3, climate conditions
When the air is dry, the communication distance is far and the air is wet (especially the rain and snow weather). The communication distance is close, and the increase of the temperature and the decrease of the receiving sensitivity are reduced in the temperature range of the permissible environmental working temperature. The communication distance.
4, the transmission power
The greater the transmission power, the greater the communication distance; theoretically, the transmit power can be increased without limitation, but in fact due to the cost or legal restrictions, the transmitter output power is limited, especially for low-power systems The request.
5, the receiver sensitivity
Receiver sensitivity reflects the receiver to capture the weak signal function, the higher the receiving sensitivity, the longer the communication distance. However, due to natural electromagnetic noise and industrial pollution, the inherent noise of electronic components, -123dBm (that is, 0.158uv) is generally considered to be modern wireless communications in pure hardware to achieve the acceptance sensitivity limit, it is difficult to break even Software error correction can only be improved by 1-3dB, if the communication system has been close to the sensitivity of this limit has no potential to be dug, to improve the communication distance can only proceed from other aspects.
6, the system anti-jamming capability
The actual communication environment always has a variety of interference sources, in the same transmit power and the same receiving sensitivity under the premise of the system's stronger anti-interference ability, the actual communication distance farther. Many high-frequency engineers have this experience: in the laboratory (shielded network room) test, amplitude modulation and FM transmitter transmission power and receiver sensitivity are the same, but in the actual environment test, the frequency of the FM communication distance is often Modulator several times, and even the modulator simply can not work, and FM machine still have a far distance from the communication, because the anti-interference ability of the FM machine is much stronger than the AM.
There are many factors that affect the anti-jamming capability of wireless communication system. It is mainly related to the modulation / demodulation mode, working bandwidth, circuit layout PCB layout and decoupling and shielding measures. In general, the anti-interference ability of FM system is superior In the amplitude modulation system, while the narrowband system has better anti-interference ability than the broadband system.
Therefore, the narrower the bandwidth, the stronger the anti-jamming capability, the farther the communication distance under the same transmit power and receiving sensitivity.
7, the antenna type and its gain
The higher the gain of the antenna, the farther the communication distance. When the transmitter uses a high-gain directional antenna, it can significantly improve the power density (field strength) in the communication direction, and the receiver can significantly improve the signal / noise ratio when using the high gain directional antenna and improve the field strength Increase the communication distance. There are several types of antennas that are currently suitable for use in ISM / SRD wireless communication devices that use frequency bands:
Whip antenna (helical antenna, rod antenna): gain 0 ~ 3.5dB, suitable for portable mobile handheld use.
In the gain sucker antenna: gain 5.5 ~ 7dB, suitable for fixed and on-board use.
High gain omnidirectional antenna: gain 8.5 ~ 10dB, need outdoor installation, suitable for fixed machine network.
High gain directional antenna: gain 10 ~ 12dB, need outdoor installation, suitable for long-distance fixed machine.
It should be noted that the higher the gain the antenna its geometric size is also greater, especially the high-gain directional or omnidirectional antenna requires outdoor installation to play its best efficiency, so the option to buy the antenna is also considered to use and install whether Convenience.
8, the antenna effective height
Under the same conditions, the higher the height of the antenna from the ground plane, the farther the communication distance, especially in the urban environment, to improve the height of the antenna than to increase the transmission power on the communication distance is much greater.
Second, the estimation of wireless communication distance
From the above analysis we can see: many factors will affect the communication distance of wireless communication equipment, and the geographical environment, electromagnetic environment, climate conditions on the wireless communication distance is determined by the user's use conditions, difficult to change, it is difficult Described with a mathematical expression, only those quantitative factors can be described by a mathematical expression. For the wireless communication equipment with operating frequency range between 300MHZ and 3GHZ, the transmit power Ptx, the reception sensitivity Prx ,, the transmit antenna gain G1 and the receiving antenna gain G2 (in units of all) are known in the line-of-sight range (also known as free space) (M), the communication distance R (m) can be calculated from the following formula: Why is there a big gap between the actual distance and the theoretical value?
There are three reasons:
First of all, the result of the formula is a pure theoretical value, only considering the electromagnetic wave propagation in the free space with the distance increases, the energy spread to a larger space after the energy density decreases after the signal field strength results, Without considering the air and the ground and other media on the absorption and attenuation of electromagnetic waves;
Secondly, the influence of the transmission efficiency of the transmitting antenna and the receiving efficiency of the receiving antenna (theoretically 100% but not substantially) is not taken into account;
Finally, there is no consideration of various electromagnetic interference and the receiver's anti-jamming capability, resulting in the actual results and theoretical calculations have a greater error.
There is always a wide range of sources of interference in the actual communication environment. Sometimes, although the signal strength sensed on the receiving antenna is much higher than the receiving sensitivity, the useful signal remains when the electromagnetic interference of the receiver is in the environment The receiver can not effectively suppress the interference, the same can not be effective radio communication, if the receiver's anti-interference ability is very strong, this effect is relatively small. In the transmission power and receiver sensitivity are the same premise, the system's anti-interference ability is stronger, the actual communication distance farther. Many high-frequency engineers have the experience: in the laboratory (shielded network room) test, AM system and FM system transmission power and receiver sensitivity are the same, in the actual environment test, the FM system communication distance is often AM system, especially when the environmental interference is serious, the AM system simply can not communicate, while the FM system can still maintain a far distance communication distance, because the FM system anti-jamming capability is much stronger than the AM system.
In contrast, the anti-jamming capability of the FM system is better than that of the AM system, and the anti-interference ability of the narrowband system is better than that of the broadband system. Therefore, under the premise of the same transmit power and receiving sensitivity, the bandwidth is narrower and the communication distance is farther.
Through the above analysis, we can conclude that in the actual communication environment, the communication distance of the micro power wireless communication system mainly depends on the anti-jamming ability of the system.
Third, to improve the communication distance measures
When the geographical environment and electromagnetic environment is certain, in order to ensure the stability and reliability of wireless communication, and fully tap the potential of low-voltage micro-power wireless communication equipment, but also to be economical and practical, in the engineering design can consider the following measures:
1, as much as possible to improve the effective height of the antenna
Increasing the effective height of the antenna can significantly increase the communication distance, especially in the urban environment, when the antenna is located on the roof to avoid many obstacles and away from the interference source, thus expanding the communication range.
2, the use of high-gain antenna
Antenna is a passive device, which itself can not amplify the signal, but the high gain antenna can significantly improve the energy density in the communication direction, improve the signal / noise ratio, thus expanding the communication range, its role with the flashlight or searchlight on the condenser mirror similar. But the high cost of high-gain antenna, geometric size and weight are relatively large, only suitable for fixed installation.
Therefore, in a little-to-multipoint or multipoint point of the wireless communication network can be considered in the host with high gain omnidirectional antenna, the extension is based on the distance from the host of different gain of different antennas, for fixed installation and distance from the host Far from the extension can also use high-gain directional antenna, and from the host closer to the optional low-cost ordinary whip antenna to reduce system costs. The transmitter uses a high gain directional antenna to significantly increase the signal strength in the communication direction, while the receiver uses a high gain directional antenna to significantly increase the received signal field strength and signal / noise ratio in the communication direction, thereby significantly expanding the communication distance , But only suitable for communication in the same direction, and the cost is also higher, the antenna geometry, weight is also heavier, only suitable for fixed installation.
3, to minimize the length of the RF cable
The RF coaxial cable used to connect the wireless transmitter and the outdoor antenna also has some loss to the RF signal. For example, the loss of the 50-3 cable is 0.2dB / m, the loss of the 50-7 type cable is 0.1dB / 50-9 type cable loss is 0.07dB / m,
The longer the cable, the greater the loss, the loss of the transmitted RF signal will lead to the decline in communication distance, so if necessary, the RF components can be installed directly in the bottom of the outdoor antenna, and RF components and user systems Of the connection is the use of multi-core shielded cable connection.
3, as far as possible away from a variety of sources of interference
The closer the interference source is, the lower the signal / noise ratio, and the lower the communication distance. If necessary, respectively, the number of transmission modules and electromagnetic interference will be taken to take shielding measures, and with a characteristic impedance of 50? RF coaxial cable to the antenna away from the source of interference, while the RF module connected to the power cord, signal The line also uses shielded cables and increases the necessary filtering network to suppress interference with maximum degradation, giving full play to the advantages of receiver high sensitivity.
4, priority to use anti-interference ability of wireless communications products
When the wireless communication receiver in the electromagnetic interference in the larger environment, if the anti-jamming capability can not keep up, the receiving sensitivity will become meaningless, this time should be preferred to use strong anti-interference products, if it is Digital communication system should also give priority to the use of software error correction products.
6, mine, waterproof, moisture
For the use of outdoor antenna system, you must take lightning protection, lightning protection measures, such as the installation of lightning rods, arrester, at the same time, for the overhead power supply line, the signal transmission line should also take lightning protection measures to prevent lightning from the cable machine. Waterproof measures should also be taken for open-air RF components to prevent rain from entering the machine when rain is applied. If the equipment is not used for long periods of time or is not frequently used and the air is humid, moisture protection measures should also be taken, for example, Place the place and regularly replace the desiccant, in short, to prevent the machine into the water and moisture, so that the circuit components moldy, rust and failure.
For COFDM wireless communication and wireless systems, there are four main performance indicators that affect the communication distance:
1. the power emitted by the transmitter;
2. the sensitivity of the receiver;
3. the whole system anti-jamming capability;
4 . the type and gain of the transmit / receive antenna.
In these four main indicators, the national electromagnetic compatibility standards (such as the North American FCC, the European EN specification) are only limited to the transmission power, as long as the acceptance sensitivity system of anti-jamming ability to optimize the two indicators, FCC or CE standards under the premise of expanding the communication distance of the system.
First, the impact of wireless communication distance of many factors
1, geographical environment
The farthest distance of communication is the straight and open space where sea level and land are accessible. This is also the geographical condition commonly used to assess the communication distance of a wireless communication device. Followed by the suburbs of rural areas, hills, riverbed and other semi-obstacle, semi-open environment, the communication distance is the nearest urban buildings or mountains, in short, the more intensive obstacles, the greater the impact of wireless communication distance, especially metal objects The greatest impact. Some common environments for wireless signal loss are shown in the following table:
According to the path loss formula: Ld = 32.4 + 20logf + 20logd
f = MHZ d = Km know the signal loss of 6dB, the communication distance will be reduced by half!
Another factor is the multi-path effect, so if the wireless module near the obstacles will also affect the communication distance and reliability.
2, the electromagnetic environment
DC motor, high voltage power grid, switching power supply, welding machine, high frequency electronic equipment, computer, microcontroller and other equipment on the wireless communication equipment communication distance has different degrees of impact.
3, climate conditions
When the air is dry, the communication distance is far and the air is wet (especially the rain and snow weather). The communication distance is close, and the increase of the temperature and the decrease of the receiving sensitivity are reduced in the temperature range of the permissible environmental working temperature. The communication distance.
4, the transmission power
The greater the transmission power, the greater the communication distance; theoretically, the transmit power can be increased without limitation, but in fact due to the cost or legal restrictions, the transmitter output power is limited, especially for low-power systems The request.
5, the receiver sensitivity
Receiver sensitivity reflects the receiver to capture the weak signal function, the higher the receiving sensitivity, the longer the communication distance. However, due to natural electromagnetic noise and industrial pollution, the inherent noise of electronic components, -123dBm (that is, 0.158uv) is generally considered to be modern wireless communications in pure hardware to achieve the acceptance sensitivity limit, it is difficult to break even Software error correction can only be improved by 1-3dB, if the communication system has been close to the sensitivity of this limit has no potential to be dug, to improve the communication distance can only proceed from other aspects.
6, the system anti-jamming capability
The actual communication environment always has a variety of interference sources, in the same transmit power and the same receiving sensitivity under the premise of the system's stronger anti-interference ability, the actual communication distance farther. Many high-frequency engineers have this experience: in the laboratory (shielded network room) test, amplitude modulation and FM transmitter transmission power and receiver sensitivity are the same, but in the actual environment test, the frequency of the FM communication distance is often Modulator several times, and even the modulator simply can not work, and FM machine still have a far distance from the communication, because the anti-interference ability of the FM machine is much stronger than the AM.
There are many factors that affect the anti-jamming capability of wireless communication system. It is mainly related to the modulation / demodulation mode, working bandwidth, circuit layout PCB layout and decoupling and shielding measures. In general, the anti-interference ability of FM system is superior In the amplitude modulation system, while the narrowband system has better anti-interference ability than the broadband system.
Therefore, the narrower the bandwidth, the stronger the anti-jamming capability, the farther the communication distance under the same transmit power and receiving sensitivity.
7, the antenna type and its gain
The higher the gain of the antenna, the farther the communication distance. When the transmitter uses a high-gain directional antenna, it can significantly improve the power density (field strength) in the communication direction, and the receiver can significantly improve the signal / noise ratio when using the high gain directional antenna and improve the field strength Increase the communication distance. There are several types of antennas that are currently suitable for use in ISM / SRD wireless communication devices that use frequency bands:
Whip antenna (helical antenna, rod antenna): gain 0 ~ 3.5dB, suitable for portable mobile handheld use.
In the gain sucker antenna: gain 5.5 ~ 7dB, suitable for fixed and on-board use.
High gain omnidirectional antenna: gain 8.5 ~ 10dB, need outdoor installation, suitable for fixed machine network.
High gain directional antenna: gain 10 ~ 12dB, need outdoor installation, suitable for long-distance fixed machine.
It should be noted that the higher the gain the antenna its geometric size is also greater, especially the high-gain directional or omnidirectional antenna requires outdoor installation to play its best efficiency, so the option to buy the antenna is also considered to use and install whether Convenience.
8, the antenna effective height
Under the same conditions, the higher the height of the antenna from the ground plane, the farther the communication distance, especially in the urban environment, to improve the height of the antenna than to increase the transmission power on the communication distance is much greater.
Second, the estimation of wireless communication distance
From the above analysis we can see: many factors will affect the communication distance of wireless communication equipment, and the geographical environment, electromagnetic environment, climate conditions on the wireless communication distance is determined by the user's use conditions, difficult to change, it is difficult Described with a mathematical expression, only those quantitative factors can be described by a mathematical expression. For the wireless communication equipment with operating frequency range between 300MHZ and 3GHZ, the transmit power Ptx, the reception sensitivity Prx ,, the transmit antenna gain G1 and the receiving antenna gain G2 (in units of all) are known in the line-of-sight range (also known as free space) (M), the communication distance R (m) can be calculated from the following formula: Why is there a big gap between the actual distance and the theoretical value?
There are three reasons:
First of all, the result of the formula is a pure theoretical value, only considering the electromagnetic wave propagation in the free space with the distance increases, the energy spread to a larger space after the energy density decreases after the signal field strength results, Without considering the air and the ground and other media on the absorption and attenuation of electromagnetic waves;
Secondly, the influence of the transmission efficiency of the transmitting antenna and the receiving efficiency of the receiving antenna (theoretically 100% but not substantially) is not taken into account;
Finally, there is no consideration of various electromagnetic interference and the receiver's anti-jamming capability, resulting in the actual results and theoretical calculations have a greater error.
There is always a wide range of sources of interference in the actual communication environment. Sometimes, although the signal strength sensed on the receiving antenna is much higher than the receiving sensitivity, the useful signal remains when the electromagnetic interference of the receiver is in the environment The receiver can not effectively suppress the interference, the same can not be effective radio communication, if the receiver's anti-interference ability is very strong, this effect is relatively small. In the transmission power and receiver sensitivity are the same premise, the system's anti-interference ability is stronger, the actual communication distance farther. Many high-frequency engineers have the experience: in the laboratory (shielded network room) test, AM system and FM system transmission power and receiver sensitivity are the same, in the actual environment test, the FM system communication distance is often AM system, especially when the environmental interference is serious, the AM system simply can not communicate, while the FM system can still maintain a far distance communication distance, because the FM system anti-jamming capability is much stronger than the AM system.
In contrast, the anti-jamming capability of the FM system is better than that of the AM system, and the anti-interference ability of the narrowband system is better than that of the broadband system. Therefore, under the premise of the same transmit power and receiving sensitivity, the bandwidth is narrower and the communication distance is farther.
Through the above analysis, we can conclude that in the actual communication environment, the communication distance of the micro power wireless communication system mainly depends on the anti-jamming ability of the system.
Third, to improve the communication distance measures
When the geographical environment and electromagnetic environment is certain, in order to ensure the stability and reliability of wireless communication, and fully tap the potential of low-voltage micro-power wireless communication equipment, but also to be economical and practical, in the engineering design can consider the following measures:
1, as much as possible to improve the effective height of the antenna
Increasing the effective height of the antenna can significantly increase the communication distance, especially in the urban environment, when the antenna is located on the roof to avoid many obstacles and away from the interference source, thus expanding the communication range.
2, the use of high-gain antenna
Antenna is a passive device, which itself can not amplify the signal, but the high gain antenna can significantly improve the energy density in the communication direction, improve the signal / noise ratio, thus expanding the communication range, its role with the flashlight or searchlight on the condenser mirror similar. But the high cost of high-gain antenna, geometric size and weight are relatively large, only suitable for fixed installation.
Therefore, in a little-to-multipoint or multipoint point of the wireless communication network can be considered in the host with high gain omnidirectional antenna, the extension is based on the distance from the host of different gain of different antennas, for fixed installation and distance from the host Far from the extension can also use high-gain directional antenna, and from the host closer to the optional low-cost ordinary whip antenna to reduce system costs. The transmitter uses a high gain directional antenna to significantly increase the signal strength in the communication direction, while the receiver uses a high gain directional antenna to significantly increase the received signal field strength and signal / noise ratio in the communication direction, thereby significantly expanding the communication distance , But only suitable for communication in the same direction, and the cost is also higher, the antenna geometry, weight is also heavier, only suitable for fixed installation.
3, to minimize the length of the RF cable
The RF coaxial cable used to connect the wireless transmitter and the outdoor antenna also has some loss to the RF signal. For example, the loss of the 50-3 cable is 0.2dB / m, the loss of the 50-7 type cable is 0.1dB / 50-9 type cable loss is 0.07dB / m,
The longer the cable, the greater the loss, the loss of the transmitted RF signal will lead to the decline in communication distance, so if necessary, the RF components can be installed directly in the bottom of the outdoor antenna, and RF components and user systems Of the connection is the use of multi-core shielded cable connection.
3, as far as possible away from a variety of sources of interference
The closer the interference source is, the lower the signal / noise ratio, and the lower the communication distance. If necessary, respectively, the number of transmission modules and electromagnetic interference will be taken to take shielding measures, and with a characteristic impedance of 50? RF coaxial cable to the antenna away from the source of interference, while the RF module connected to the power cord, signal The line also uses shielded cables and increases the necessary filtering network to suppress interference with maximum degradation, giving full play to the advantages of receiver high sensitivity.
4, priority to use anti-interference ability of wireless communications products
When the wireless communication receiver in the electromagnetic interference in the larger environment, if the anti-jamming capability can not keep up, the receiving sensitivity will become meaningless, this time should be preferred to use strong anti-interference products, if it is Digital communication system should also give priority to the use of software error correction products.
6, mine, waterproof, moisture
For the use of outdoor antenna system, you must take lightning protection, lightning protection measures, such as the installation of lightning rods, arrester, at the same time, for the overhead power supply line, the signal transmission line should also take lightning protection measures to prevent lightning from the cable machine. Waterproof measures should also be taken for open-air RF components to prevent rain from entering the machine when rain is applied. If the equipment is not used for long periods of time or is not frequently used and the air is humid, moisture protection measures should also be taken, for example, Place the place and regularly replace the desiccant, in short, to prevent the machine into the water and moisture, so that the circuit components moldy, rust and failure.
