# Improving Energy Efficiency in LoRaWAN Networks with Multiple Gateways
## 3 LoRa and LoRaWAN Overview  
This section presents the aspects of LoRa and LoRaWAN technologies that are relevant to our work. LoRa is a physical-layer technology developed by the Semtech corporation [2]. It modulates signals using a CSS technique that spreads a narrow-band signal over a wider channel bandwidth (BW). It supports multiple frequency channels (e.g., on the 433, 868, or 915 MHz ISM bands) depending on the region in which it is deployed [4]. This technique makes the signal robust to interference due to the processing gain of the spread-spectrum technique. As a result, the maximum power budget for LoRa operating in the 868 MHz band can exceed 150 dB (the receiver can decode transmissions 19.5 dB below the noise floor), thus enabling long communication ranges. LoRa is characterized by various configurable parameters affecting the throughput, power consumption, and communication range, as detailed below. * Carrier Frequency (CF): Represents the central frequency used in a band. LoRa operates in the unlicensed sub-GHZ ISM band, and the CF can be programmed within the range of 137 MHz to 1020 MHz in steps of 61 Hz. Specifically, the CF varies according to geographical regions, such as 868 MHz in Europe, 433 MHz in Asia, and 915 MHz in America. LoRa nodes typically support sixteen channels, while gateways usually support eight channels. In Europe, networks operate in the EU863-870 ISMband, which allows up to eight channels with a 125 kHz bandwidth each. Moreover, the three following channels (868.1, 868.3, and 868.5 MHz) are mandatory [4]. * Spreading Factor (SF): Refers to the number of bits encoded per symbol. This is the key parameter in LoRa because it affects the time of transmission, data rate, communication range, and power consumption. LoRa supports multiple SFs ranging from seven to 12. Table 2 gives the variation of the data rate, sensitivity, and SNR thresholds of different SFs for the 868 MHz band. Note that for SNR values lower than \(-20\) dB, a node is considered out of network coverage. Going from SF 7 to SF 12 decreases the receiver's sensitivity from \(-123\) dBm to \(-137\) dBm and, therefore, extends the coverage. However, this comes at the expense of lowering the data rate and increasing the transmission time and energy consumption. The throughput limitation in LoRa can be seen from Table 2, where the data rate varies between 0.3 (using the highest SF 12) and 5.4 kbps (using the lowest SF 7). We note also that the different SFs are quasi-orthogonal [3], which enables the simultaneous reception of packets with different SFs. * Bandwidth (BW): Defines the spectrum occupied by a symbol. It can be set to 125, 250, or 500 kHz. A larger bandwidth corresponds to higher data, at the expense of reducing sensitivity. * Coding Rate (CR): LoRa uses a forward error correction (FEC) scheme to perform error detection and correction by adding redundancy bits, improving the robustness of the transmitted signal. The coding rate can be set to \(\dfrac{4}{4+CR}\) with CR values ranging from 1 to 4. Increasing the CR value gives more protection against interference. However, this will increase the transmission time and power consumption. * Transmission Power (TP): It can be adjusted from \(-4\) dBm to 20 dBm in 1 dB steps. Due to hardware implementation limitations, the range frequently varies from 2 dBm to 20 dBm. The highest TP is subject to regional regularity constraints, e.g., 14 dBm in Europe [4]. Using higher TP values extends the communication range at the expense of increased power consumption. A LoRa symbol (chirp) is made up of \(2^{SF}\) chips in which the chip rate equals the BW. For example, SF 9 means that each chirp consists of \(2^{9}\) RF chips carrying nine data bits.