The robustness of wireless communications comes from two challenges: external interference and multipath degradation.
External interference
In the ISM common frequency band, frequency is a very precious resource. As shown in the figure below, the 2.4 GHz band includes WiFi, Bluetooth, and ZigBee, as well as cordless phones, microwave ovens, etc., which need to avoid the same frequency interference.
Multipath decline
In an actual communications environment, walls, doors, moving people, trees, and buildings can all cause reflections of wireless signals. As shown in the figure below, in addition to the straight path Pd, signals of other reflection paths (Pm1 and Pm2) are superimposed. These mixed signals may make the receiving device unable to decode. This is called multipath fading.
Multipath decline is a more complex issue because it is almost impossible to analyze all of its influencing factors. The following figure is a typical experiment: A receiver and transmitter are installed on the X axis with a length of 20cm and the Y axis with a length of 35cm. Each time one of the devices is moved by 1cm, the statistical communication success rate is calculated.
From the above results, it can be seen that even if only 1cm is moved at the same frequency, the multipath fading may cause the communication success rate to suddenly drop from 100% to 0%; and after the same location is replaced, the communication success rate may also improve from 0% to 100%, this is the benefit of frequency hopping communication.
Frequency hopping communication
The technology to solve "external interference" and "multi-path degradation" is "frequency-hopping communication," which means that each communication is replaced with a frequency. As shown in the figure below, there is noise interference at fb.17~fb.20, because the frequency hopping technique can be used to avoid the interference channel and continue communication.
1. ZigBee
The 2.4G zigbee can use a total of 16 channels at frequencies from 2405 MHz to 2480 MHz, and zigbee usually uses a fixed channel (no change in frequency). If zigbee is interfered with by other 2.4G signals (Bluetooth, WIFI, etc.), it will automatically select another channel with less interference to use.
ZigBee supports two channel access modes, one is a beacon mode and the other is a non-beacon mode.
The beacon mode specifies a "superframe" format in which a beacon frame is transmitted at the beginning of a superframe, which contains some timing and network information, followed by a contention access period, during which time the nodes The competition mode accesses the channel, followed by the non-contention access period. The node accesses the channel in time division multiplexing mode, and then in the non-active period, the node enters the dormant state and waits for the next superframe cycle to start sending the beacon frame again.
The non-beacon mode is more flexible. Nodes access the channel in a competitive manner and do not need to periodically send beacon frames.
Obviously, due to periodic beacons in the beacon mode, all nodes of the entire network can be synchronized, but the size of this synchronous network will not be large. In fact, it may be non-beacon mode that is used more in ZigBee.
2. Bluetooth
Bluetooth adopts AFH (AdapTIve Frequency Hopping), LBT (Listen Before Talk), power control and a series of unique measures to overcome interference and avoid conflicts.
AFH frequency self-adaptive control is to reject those frequency points used in the frequency hopping communication process that have been used but the transmission is unsuccessful, so that the frequency hopping communication can be performed at the frequency point where no interference is available, thereby greatly improving the frequency hopping communication. The quality of the received signal.
Bluetooth uses frequency hopping spread spectrum (FHSS) technology, uses 79 channels, each channel occupies 1MHz, the signal continuously hops at a rate of 1600Hz among 79 frequency modulation points randomly, and the Bluetooth signal actually occupies 79MHz band.
3. WiFi
WiFi uses DSSS. Each channel has a bandwidth of 22 MHz. Random backoff is used to compete for channels.
4. GSM
The air interface of GSM adopts time division multiple access technology. GSM is based on a narrowband TDMA system that allows eight groups of calls to be performed simultaneously on one radio. Currently, the frequency hopping method adopted by GSM is characterized in that the frequency of use of one channel is changed in each burst interval, but the frequency remains unchanged during transmission of a complete burst, and the frequency hopping thereof is approximately 217 hops/s. The interval is 4.615ms for each TDMA frame.
5. CDMA
The CDMA system is a communication system based on a code division technique (spread spectrum technique) and a multiple access technique. The system assigns each user a specific address code. The mutual quasi-orthogonality between address codes can overlap in time, space, and frequency. For example, imagine the bandwidth as a big house, and all people will enter the only big house. If they use completely different languages, they can clearly hear the voice of their peers and only be disturbed by some conversations from others.
Third, OpenWSN frequency hopping algorithmOpenWSN uses 16-channel frequency hopping to improve communication reliability and avoid "external interference" and "multipath degradation." Each data frame uses a different frequency in the transmission time slot, and its frequency is calculated as follows:
Frequency = (ASN + channelsOffset) % 16
ASN (Absolute Slot Number) is an absolute slot number. Each slot is incremented by one and shared by all nodes. Its role is to ensure that after one frame fails, the retransmission of the next frame uses a different frequency (because ASN increases by one).
The channelsOffset is the "appointment" channel for both parties of the communication (eg, A and B are agreed by 12, D and F by 7...). After every 100 time slots, both parties need to reapply channelsOffset.
A typical communication diagram of OpenWSN is as follows. On the left is the slot and frequency matrix, and on the right is the network topology.
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