Inthe design of wireless sensor networks, most terminal nodes require air wake-upto complete the design requirements of low-power design. So how does the LoRamodule achieve wake-up in the air? This article will reveal to you。
Inwireless sensor network design, most of the lora module transceiver nodesrequire low power processing. In order to reduce power consumption, the nodeneeds to reduce useless working hours. In most IoT applications, the radio partis in the receiving state most of the time during wireless communication, andit is also the main energy consumption. The node can not be in active state atall time because the battery capacity is limited . The ideal state is that whenthere is data to be received, the node is in the receiving state, and when noinformation is received, the lora node is in a sleep state, which requires anair wake-up technique.
Thephenomenon that wake_up over the air seems to be that the transmitter wakes upthe receiver from sleep. In fact, the receiver wakes up periodically and checkswhether there is a valid signal in the air. If not, it continues to sleep; ifthere is, then Waken into the receiving state. Therefore, when there is no validsignal in the air, the average power consumption of the receiver is low.
However,how to find the call signal is traditionally determined by setting the RSSIvalue. Only when the signal strength is sufficient, it is considered valid, andthe node is awakened, otherwise it is the opposite. The use of a spreadspectrum modulation technique presents challenges in determining whether thechannel is already inuse by a signal that may be below the noise floor of thereceiver. The use of the RSSI in this situation would clearly beimpracticable.To this end the channel activity detector is used to detect the presence ofother LoRa® signals.To solve this problem, CAD channel activity detectiontechnology can be used to detect other LoRa signals.
TheLoRa packet consists of a preamble, an optional header, and a data payload, asshown in the following figure:
Thechannel activity detection mode is designed to detect the LoRa preamble on thewireless channel with the highest possible power consumption efficiency. In CADmode, the lora module will quickly scan the band to detect the LoRa packetpreamble.
The followingoperations are performed during the CAD process：
ThePLL is locked.
Theradio receiver captures LoRa preamble symbol of data from the channel. Theradio current consumption during thatphase corresponds to the specified Rx modecurrent.
Thewireless receiver and PLL are turned off and modem digital processing begins.
The modem searches for a correlation betweenthe radio captured samples and the ideal preamble waveform. Thiscorrelationprocess takes a little bit less than a symbol period to perform. The radiocurrent consumption during thatphase is greatly reduced.
Afterthe calculation is completed, the modem generates a CadDone interrupt signal.If the association is successful, a CadDetected signal is generated at the sametime.
Thechip goes back to standby mode.
Ifthe preamble is found, the interrupt is cleared, and then the chip is set to Rxsingle or continuous mode to begin receiving data.
The time takenfor the channel activity detection is dependent upon the LoRa modulationsettings used. For a givenconfiguration the typical CAD detection time is shownin the graph below, expressed as a multiple of the LoRa symbolperiod. Of thisperiod the radio is in receiver mode for (2 SF + 32) / BW seconds. For theremainder of the CAD cycle theradio is in a reduced consumption state.
TheCAD program is broken down into multiple events in sequence to betterillustrate the program and power consumption, as shown in the following figure：
IDDR_L:full Rx mode
IDDC_L:Low power processing
Duringthe channel activity detection process, the receiving end is in the fullreceiving mode only for the first half of the time, and then enters the lowpower processing phase, during which the CAD power consumption changes with theLoRa bandwidth.
Sincethe CAD detects the preamble part of the data packet, in order to achieve theair wake-up, combined with the node periodically detecting the time, it isnecessary to set the appropriate preamble transmission time to ensure thepreamble transmission time > the node periodic detection time, then acertain preamble needs to be set. The code length can be achieved byconfiguring the RegPreambleMsb and RegPreambleLsb registers. As shown in thefigure below, you can change the length of the transmit preamble by setting thepreamble register length between 6-65536.。
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