1.The significance of using wirelessfrequency hopping
The robustnessof wireless communication comes from two challenges: external interference andmultipath fading.
In the ISMcommon frequency band, frequency is a very valuable resource. As shown in thefigure below, the 2.4GHz frequency band includes WiFi, Bluetooth and ZigBee, aswell as cordless phones, microwave ovens, etc., so it is necessary to avoid thesame frequency interference.
In the actualcommunication environment, walls, doors, people, trees and buildings may causethe reflection of wireless signals. As shown in the figure below, in additionto the straight-line path PD, signals of other reflection paths (PM1 and PM2)will be superimposed. These mixed signals may make the receiving device unableto decode, which is called multipath fading.
Multipath decline is a more complex problem, because it isalmost impossible to analyze all its influencing factors. The following figureis a typical experiment: install a receiver and transmitter respectively inx-axis length of 500px and y-axis length of 875px, move one device 25px each time,and count the success rate of communication.
It can be seenfrom the above results that even if only one cm is moved at the same frequency,the multipath fading may cause the communication success rate to drop from 100%to 0%; and the communication success rate at the same location may also improvefrom 0% to 100% after changing the frequency, which is the benefit of frequencyhopping communication.
The technologyto solve "external interference" and "multipath fading" is"frequency hopping communication", which means that the frequency ischanged every time. As shown in the figure below, there is noise interferencein fb.17-fb.20, because frequency hopping technology can avoid interferencechannel to continue communication.
Antiinterference methods in common wireless communication
2.4G ZigBee canuse a total of 16 channels, and the frequency is from 2405Mhz to 2480Mhz.ZigBee usually uses a fixed channel (the frequency is constant). If ZigBee isinterfered by other 2.4G signals (Bluetooth, WiFi, etc.), it will automaticallyselect another channel with less interference to use.
ZigBee supportstwo channel access modes, one is beacon mode and the other is non-beacon mode.
In beacon mode,a "superframe" format is specified. At the beginning of superframe,beacon frame is sent, which contains some time sequence and networkinformation, followed by competitive access period. During this period, eachnode accesses the channel in a competitive way, followed by a non competitiveaccess period. The node accesses the channel in a time division multiplexingway, followed by a non active period, followed by a node Enter the sleep state,wait for the next superframe cycle to start and send the beacon frame.
The non beaconmode is more flexible. All nodes access the channel in a competitive way and donot need to send beacon frames periodically.
Obviously, allnodes in the whole network can synchronize because of periodic beacons in thebeacon mode, but the scale of this synchronization network will not be verylarge. In fact, ZigBee is more likely to use non-beacon mode.
Bluetooth adoptsa series of unique measures such as AFH (adaptive frequency hopping), LBT(listen before talk), power control to overcome interference and avoidconflict.
In the processof FH communication, AFH frequency adaptive control refuses to use thefrequency points that have been used but the transmission is not successful, sothat FH communication can be carried out on the available frequency pointswithout interference, so as to greatly improve the quality of received signalsin FH communication.
Bluetooth adoptsFHSS technology, using 79 channels, each channel occupies 1Mhz, and the signalcontinuously jumps randomly between 79 FM points at the rate of 1600hz. Infact, the Bluetooth signal occupies 79Mhz frequency band.
WiFi uses DSSS,and the bandwidth of each channel is 22Mhz. It uses random backoff to scramblefor the channel.
GSM airinterface adopts TDMA technology. GSM is based on narrow-band TDMA system,which allows eight groups of calls at one RF. At present, the frequency hoppingmode adopted by GSM is characterized by changing the use frequency of a channelat each burst interval, but the frequency remains unchanged during thetransmission of a complete burst, the frequency hopping is about 217 hops / s,and the interval is 4.615ms for each TDMA frame.
CDMA system is acommunication system based on code division technology (spread spectrumtechnology) and multiple access technology. The system assigns each user aspecific address code. The address codes are quasi orthogonal to each other, sothey can overlap in time, space and frequency. For example, think of bandwidthas a big house, and everyone will enter the only big house. If they use acompletely different language, they can hear their partner's voice clearly andonly be disturbed by others' conversation.
Frequencyhopping algorithm of lorawan
Lorawan uses8-channel frequency hopping technology to improve communication reliability andavoid "external interference" and "multipath fading". Eachdata frame randomly uses different frequencies, and the frequency range is asfollows:
The above is allabout this article. I hope it will help you.
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