The detailed difference between frequency modulation and amplitude modulation of power wireless microwave transmission technology

An overview of wireless communication technology

At present, mainstream wireless transmission technologies can be divided into: high-power, high-speed WAN transmission technologies (2G/3G/4G/5G cellular communication technology, microwave modulation transmission, etc.); low-power, low-speed WAN transmission technologies (Lora, Sigfox, NB -Internet of Things, etc.); high-power, high-speed short-range transmission technology (WIFI, Bluetooth, etc.); low-power, low-speed short-range transmission technology (Zigbee).

In the case where video backhaul of power lines in unmanned areas is the main business requirement, narrowband and short-range IoT wireless technologies are not suitable for this scenario. At present, the mainstream wireless video surveillance technologies include WLAN (Wireless Local Area Network), analog microwave modulation technology, 4G/5G mobile Internet of Things technology and satellite communication technology. The characteristics of each technology are analyzed as follows:

(1) WLAN (Wireless Local Area Network)

WLAN (Wireless Local Area Network) is not much different from the traditional Ethernet concept. It only converts the line transmission part of Ethernet (ordinary network card - five types of lines - ordinary HUB) into a wireless transmission form (wireless network card - microwave - AP, AP can It is understood as the digital microwave communication of wireless HUB) or two-way communication.

(2) Analog microwave modulation technology

The analog microwave modulation technology is to directly modulate the video signal to the microwave channel and transmit it through the antenna. The monitoring center receives the microwave signal through the antenna, and then demodulates the original video signal through the microwave receiver. This monitoring method has no compression loss, almost no delay, and can guarantee video quality, but it is only suitable for point-to-point single-channel transmission, and is not suitable for large-scale deployment. In addition, because there is no modulation calibration process, the anti-interference performance is poor, and it can hardly be used in complex wireless signal environments.

(3) 4G/5G mobile IoT technology

The 4G/5G wireless mobile network provided by the operator can achieve high-quality transmission of video images.

(4) Satellite communication technology

Video terminals rely on traditional communication satellite or high-throughput satellite technology to achieve point-to-point communication through satellite transmission channels.

The advantages and disadvantages of various wireless video surveillance technologies can be summarized as follows:

WLAN has high bandwidth, long distance, flexible networking mode, easy to be interfered, transmission needs to be unobstructed, and jump delay is large

Analog microwave modulation Simple networking, cheap price Not suitable for large-scale deployment, poor anti-interference

4G/5G is easy to deploy, the signal coverage and strength are different, and the communication cost is high

Satellite communication Wide coverage, stable transmission, high communication costs, and the existence of "Nanshan effect"

2. Technical analysis

WLAN (Wireless Local Area Network), satellite communication technology, etc. can be used to realize the back transmission of business information such as video monitoring and online monitoring of power transmission lines in unmanned areas.

(1) WLAN (Wireless Local Area Network)

At present, Mesh network and WDS network can realize wireless link communication between two wireless access nodes, realize the expansion of wireless network, and can be widely used in wireless video surveillance and backhaul network. The characteristics of each network are analyzed as follows:

1) WDS networking

The WDS network connects two separate LAN segments through a wireless bridge. The WDS network structure includes point-to-point and point-to-multipoint. At present, the wireless bridge equipment can realize point-to-point long-distance transmission of more than 10km, the actual data throughput is not less than 200mbps, and the power of the whole machine is less than 20W. In the entire network, the wireless bridge can implement different working modes according to different node functions: AP (base station) working mode is supported in the coverage scenario, CPE (client) working mode is supported in the access scenario, and supported in the return scenario. WDS working mode.

2) Mesh networking

In a mesh network, if a node's application fails, it can choose another application to communicate with. Data can still reach its destination at high speed, effectively avoiding a single point of failure. Therefore, Mesh networks are more stable than WDS networks. Although Mesh networking is convenient and flexible, the overall link bandwidth is low, the cost is high, and the normal transmission of data cannot be guaranteed. The link is long and there are many jumpers.

3) Comparison between Mesh networking and WDS networking

Reliability Automatically discover the best path between two nodes, no single point of failure, high reliability The networking link is a static path, there is a single point of failure, and the reliability is low

Multi-hop bandwidth loss High overhead, relatively low overall link bandwidth, large multi-hop bandwidth loss, small bandwidth loss

Single-hop delay Mesh devices work at the network layer, and the delay is relatively large. Wireless bridge devices work at the data link layer, and the delay is relatively small.

High and low cost

high availability

Power Supply Wind-solar complementation Wind-solar complementarity

Scenario Application The network topology is complex (mesh network or multi-link), the signal is interrupted by external factors (vehicle passing, construction, etc.), and the network topology and network nodes are not fixed. The number of network nodes is small, and the network topology is single (clear layers), no redundant links, no intermittent interruption of signal transmission links caused by external factors (vehicle passing, construction, etc.), and a relatively fixed network scale

(2) Satellite communication technology

Domestic satellite communications mainly use traditional KU satellites and high-throughput communication satellites, of which the high-throughput communication satellites are mainly Zhongxing 16 satellites and Asia-Pacific 6D satellites located in geosynchronous orbits. At present, the Zhongxing 16 satellite has been commercialized, and the Asia-Pacific 6D satellite is still in the orbital trial operation stage. The single-station download and return rates of Zhongxing 16 satellites can reach up to 150mbps and 12mbps, and the power of the single-station machine is about 40W.

Due to the low maximum backhaul rate of satellite remote stations, the Nanshan effect and high power consumption limit its wide application in transmission line video backhaul services. However, satellite remote stations can be used as sporadic supplementary means for wireless backhaul network monitoring points, and can also be combined with Wi-Fi bridging technology to provide short-range communication coverage in transmission line or substation inspection and emergency rescue. It can be equipped with COFDM image transmission equipment. Video imagery for drone independent inspection via satellite channels.

3. Application scenarios

The design of the monitoring signal backhaul scheme can be divided into the following two scenarios:

Scenario 1: The area without network coverage of the entire transmission line is scattered, and the coverage area without network is short. In the area without network coverage, the business information can be collected into the power tower with the operator's signal through the wireless link constructed by the Mesh network or the WDS network, and connected to the operator's power wireless network APN channel through the 4GCPE device, and returned to the monitoring center.

Scenario 2: The transmission line does not have a wide network coverage. In areas without network coverage, the wireless link constructed by Mesh network or WDS network directly returns service information to nearby substations (nearby substations refer to substations near transmission lines). However, the achievable network coverage distance will be limited by the device bandwidth and the number of hops on the main network link. The business model is further analyzed according to the distance between the two stations of the actual substation and the number of observation points.

The choice of Mesh network or WDS network architecture should be deployed according to the number and location of observation points along the actual transmission line. The overall network topology is the main link (between aggregation nodes) multi-hop relay (bridging), and the aggregation nodes use point-to-multipoint to achieve short-range coverage. Due to delay or bandwidth limitation, there are still monitoring points that cannot be back-transmitted. Satellite communication technology can be used as a supplementary point to realize back-transmission of monitoring information from monitoring points in areas without network coverage of transmission lines.

Four, wireless transmission topology

In the absence of shielding between outdoor power towers, transmission links can be constructed by multi-hop bridge connections between bridges and transmit various video signals. Wireless devices are used for the front end. The middle tower adopts equipment (three-mode equipment), two of which can receive front-end signals and transmit signals, and the third module can be used for wireless coverage. During maintenance, on-site maintenance personnel can establish a communication network through wireless devices and maintenance workshops. Vehicles can backhaul data collected by front-end workers via wireless devices and networks or satellites on nearby towers.

In the absence of shielding between outdoor power towers, two or more towers at the front can transmit the collected information to one tower through point-to-point, and then build the transmission through multi-hop bridge connections between network bridges. The link transmits the aggregated information back. Wireless devices are used for the front end. The middle tower adopts equipment (three-mode equipment), of which two modules can receive front-end signals and transmit signals, and the third module can be used for wireless coverage repair. On-site maintenance personnel can establish a communication network through wireless equipment and maintenance workshops. Vehicles can return data collected by front-end workers via wireless devices and networks or satellites on nearby towers.

In the absence of shielding between outdoor power towers, transmission links can be constructed by multi-hop bridge connections between bridges and transmit various video signals. ST58T8G devices are used for the front end. The middle tower adopts wireless equipment (three-mode equipment). The device can receive front-end signals and transmit signals separately, and the third module can be used for wireless coverage. During maintenance, on-site maintenance personnel can establish a communication network through wireless devices and maintenance workshops. Vehicles can backhaul data collected by front-end workers via wireless devices and networks or satellites on nearby towers. When there is no shielding between three or more towers, mesh networking can be set up to enhance the damage resistance of the link and ensure the reliability of the link.

When there is obstruction between outdoor power towers, some unobstructed towers can build transmission links through multi-hop bridge connections between bridges to transmit various video signals. When the blocked tower cannot directly transmit back, other nearby towers can be selected for back transmission according to the on-site situation. ST58T8G devices are used for the front end. The middle tower uses wireless equipment (three-mode equipment), two of which can receive front-end signals and transmit signals. The third module can be used for wireless coverage. During maintenance, on-site maintenance personnel can establish a communication network through wireless devices and maintenance workshops. Vehicles can backhaul data collected by front-end workers via wireless devices and networks or satellites on nearby towers.

Communication knowledge The difference between FM and AM! Frequency modulation (FM), the full name of frequency modulation. According to the change law of the required transmission signal, the instantaneous frequency of the carrier is changed. It is a modulation method that varies with the modulated signal. The circuit of this method is generally called a frequency modulator, which is widely used in various communication and broadcasting equipment. Generally speaking, it can judge its quality according to the large shift of FM frequency and the characteristics of FM. The amplitude of the modulated wave carrier generated by FM remains unchanged, and the vibration frequency changes with the modulated signal.

Amplitude modulation is a modulation method that changes the carrier amplitude according to the changing law of the desired transmission signal, but the frequency remains unchanged. It is mainly used for cable and radio communication broadcasting. Amplitude modulation generally refers to medium wave, and the range is 530-1600kHz. The transmission distance is long, and it is easily affected by the weather. At the same time, early ultra-shortwave broadcasting will generate additional amplitude modulation, resulting in distortion, and at the same time, the transmission security is poor, and the current use range is small. The frequency of the high-frequency carrier frequency modulation is not a single constant, but varies with the limited range of the modulated signal, and the amplitude is a single constant. The corresponding amplitude modulation is that the carrier frequency remains unchanged, and its amplitude varies with the modulating signal. The performance of FM and AM is that the former has strong anti-interference ability, wider frequency band hopping, larger FM power utilization and smaller distortion, but the service radius is relatively small.

The detailed difference between FM and AM:

(1) FM anti-interference ability is stronger than AM

The influence of external interference, processing industry and natural electricity interference on the modulation wave is manifested as parasitic amplitude modulation and noise. The difference between FM and AM is that spurious AM can be eliminated by FM's unique clipping method, whereas clipping cannot be used because the AM signal changes. At the same time, the larger the signal-to-noise ratio, the stronger the anti-interference. The signal-to-noise ratio obtained after demodulation is related to the modulation coefficient. The larger the modulation coefficient, the larger the signal-to-noise ratio, and the frequency modulation coefficient itself is much larger than the amplitude modulation, so the amplitude modulation is poor.

(2) The frequency bandwidth of FM wave is wider than that of AM wave

The bandwidth is related to the modulation factor. In general, the frequency modulation factor is usually greater than 1, while the amplitude modulation factor is less than 1. The bandwidth between the two can be seen.

(3) The power utilization rate of the frequency modulation system is relatively high

In the total transmit power, the side frequency power is the effective power for transmitting the modulated signal, and the side frequency power is related to the modulation coefficient. The larger the modulation coefficient is, the larger the side frequency power is. The characteristic of FM is that the frequency width is narrow, which also means that the penetration is weak, but the transmission distance is long. Generally, it is used for mobile phones and paging machines that require long-distance (more than 5 kilometers) transmission products. On the contrary, but relatively strong penetration. Therefore, it is mostly used in the fields of building wireless security, alarm, etc. These fields are generally not distance, but penetrating ability.