Information processing, storage, data reception and transmission technologies have penetrated almost all spheres of our lives. A broad range of devices dealing with information demand for some specific requirements such as low power consumption, the possibility of off-line operations for a prolonged period, and information transmission across long distances. The article touches upon the method that can be used to decrease the IoT devices power consumption. It is demonstrated that the power consumption of the IoT device can be reduced with the help of transmission data amount reduction.
2.1
LoRa is a network technology and modulation method at the same time. LoRa modulation method is patented by Semtech company and based on spread spectrum modulation and chirp spread spectrum (SCC), when data is coding by wide band impulses with the frequency which increases or decreases in some time interval. This solution unlike usually applied direct sequence spread spectrum (DSSS) makes a receiver more frequency deviation resistant. LoRa is also used for forward error correction working in sub gigahertz frequency band.
LoRa allows signals with signal-to-noise ratio of 20dB to be demodulated, while the majority of systems with frequency shift keying (FSK) can work correctly with signal-to-noise ratio 8-10dB. LoRa modulation determines physical layer (PHY) which can be used in networks with various architecture such as mesh networks, star, dot-to-dot and others.
Due to high sensitivity (-148dbm) LoRa is ideal for devices which require low power consumption and high communication stability. 2.2
If LoRa is a physical layer, then LoRaWAN is a MAC protocol of canal level for long range low power consumption networks with many nodes. LoRaWAN network has a star-type architecture without retranslations and mesh connections. Network nodes are characterized by low power consumption (up to 10 years battery life, conventional batteries, size AA), low bit rate, long range (15 km in rural areas and 5 km in urban areas), and low cost of end-node devices.
LoRaWAN protocol is optimized for power-constrained devices and includes various device classes, providing the best compromise between the bit rate and battery life time. The protocol ensures full bi-directional communication. LoRa end-nodes fulfill various functions such as measurement, administration and control. End-nodes do not constantly transmit data, they switch on the transmission only for some time (as a rule, for 1-5 seconds). Outside these hours the end-nodes transceivers are kept in the sleeping or receiving mode. 2.3
The structure of LoRaWAN message contains two mandatory parts called preamble and payload. The message begins with a preamble sequence, which is expressed in the impulses sequence, which covers the complete bandwidth. Then devices transmit the synchronized messages to determine LoRaWAN networks using the same frequency band. A payload header can be found between the preamble and the payload. At the end of the message an excess information code can be disposed.
End-node devices can be divided into following categories [6]: Class А. End-node device is an initiator of data exchange with the base station. After every transmission, class A device opens 2 special reception windows to get a confirmation message from the base station. Class A devices have the smallest power consumption Class B. Class B devices have scheduled transmission of signals. The base station can also be the initiator of data exchange. Class С. Reception window is almost always open. It closes only to transmit data to the base. Class B devices have the biggest power consumption. 3
Monitoring device using LoRaWAN protocol was applied to investigate different algorithms and the modes of information exchange organization in LoRaWAN network 3.1
The investigated device is used to monitor heating systems. It is designed on the base of microcontroller MKM14Z64CHH5 [7]. The software that implements LoRaWAN protocol is integrated in the microcontroller. The device calculates the data of two temperature sensors, two pressure sensors, and two water consumption counters. The device also contains two channels for connecting to the alarm system. The physical layer of protocol is implemented by SX1276 module [8], which communicates with microcontroller by serial peripheral interface (SPI).
This device belongs to type A, and operates in a brief alternating mode to ensure a minimum current consumption. The questioning of pulse inputs condition is performed at 30 Hz.
The measurement of analogue temperature and pressure signals is performed every minute during 2 ms.
During the idle time, the device is in Deep Sleep mode. In this study, the calculations of consumption are made with the help of data taken from the device documentation modules [7,8], presented in the Tab. 1, 2.
The average current consumption on the quartz resonator equals 3 mA. The average current consumption of device in system monitoring mode, when it gets censors data, is 25.04 uА. 3.2
Temperature and pressure parameters are transmitted in floating format and have the length of 32 bit to provide required accuracy [9]. The structure of measurement is presented in the Tab. 3, where k is the length of message which contains 2 parameters of temperature, and 2 parameters of pressure.
k+8 Alarm 12..15 Pressure 1
Thus, the message length without sensors data accumulation equals to 24 bytes. If there is data accumulation, the messenger length equals to 16⋅ M+8 bytes, where M is the number of measurements.
The designed message format implies the caching of measured temperature and pressure parameters. Caching data size is 16 bytes. When using the cache, data is written into the ring buffer every minute depending on the frequency of their measurements. Maximum bytes number of information message (N) and configuration depends on a chosen transmission mode (Tab. 4). It is derived from the constraints of the physical layer, depending on the effective modulation rate used in the view of possible sealing repeater.
For further calculations we take the data message length in accordance with the condition that the length cannot be
bigger then allowed by protocol. E.g., for DR1 we take the data message length equals to 40 byte, which corresponds to
the data accumulation for 2 seconds. For transmission mode DR1 bit rate equals to 440 bps, therefore, 0.73 sec is
required for the transmission of our messages (
Then, the average current consumption of the transmission-reception session (
28.53∙26∙.6703∙1000 ≈ 1624 uА
Thus calculated average current consumption of the device for other transmission mode values is presented in Tab. 5,
where k is the number of measurements in the package.
(
The measured parameters are stored in the ring buffer. Each parameter is defined by the range of values that the user configures. The message contains the average value of each parameter for the period of time between the transmission and the 2-byte status, which, in turn, contain the information about the presence of threshold, exceeding by each parameter and the time when this parameter exceeds a threshold. In this case, the message will be composed of 26 bytes that can be transmitted in one communication session. The message is transmitted with a user-specified frequency. If the user needs to instantaneous values of the parameters for a specified period of time he may request them any time. In this case the informative remains the same as in the method described in Sec. 3-3.
The transmission of our messages with the length equals to 26 bytes requires 0.832 sec. in DR0 mode (
Assume that the sensors data is averaged for two hours. Suppose a user requests a full measurement data for the last
two hours once per day. With this information, one makes it possible to calculate the average value of the device current
consumption during a session data exchange (
This average current consumed by the device during a transmission interval 2 h. Average current consumption, when
we transmit instantaneous values of measurement parameters, equals to the calculated consumption in the Tab. V.
Sending a two-hour data archive using DR0 mode takes 1 hour as it is necessary to withstand the duty cycle of the
transmission time. Thus, we find a day averaged value of the current consumed by the device during this operation (
As can be seen, the average current consumption during operation of the device has been significantly decreased. This allows us to increase the battery life. Reducing the length of the transmitted message also allows us to send messages with the highest scattering factor. This will improve the reliability of data delivery.
The study has showed that the frequency of transmission session has a major impact on energy efficiency.
The proposed algorithm can reduce the consumption by at least two times and at the same time to transmit data at a maximum spread factor, which positively affects the reliability of data delivery. Since the algorithm seeks to reduce the time in the network, it also allows improving the coexistence of networks, and reducing the collisions within a network.