## 4 LoRa Traffic Generator Development and Validation
The second advantage of the traffic generator is to generate a very large number of LoRa packets so that it can allow for the emulation of LoRa networks of a high-density large-scale type and by doing so effectively opening new research directions within the field of wireless sensor networks. Thus, the architecture that has been initially presented (e.g., by means of a single SDR platform) has been developed and perfected with the introduction of additional multiplexers with a large data volume of generated traffic as a result. Figure 11 depicts the packet spectrum of various power levels taken from within the semi-anechoic chamber. The LoRa traffic generator is able to generate data from thousands of sensors, which makes it a powerful instrument for performance assessment and scalability evaluation. Due to its low cost it can be easily implemented by researchers in a low effort manner. Another advantage is its scalability, by simply integrating other SDR devices the traffic generation capacity may be proportionally increased. Figure 12 depicts the spectrogram of a LoRa packet by using the architecture mentioned in Figure 5. The central communication channel frequency has been of 868.3 MHz. This implementation variant ensures a high level of performance. As seen, all the packet characteristics respect the LoRaWAN specifications. Figure 11: LoRa traffic generator, spectrum. It can be observed that the LoRa sensors communicate on the same channel as the LoRa traffic generator. Thus, the LoRa traffic generator can obtain LoRa concurrent transmissions so that a high number (e.g., thousands) of nodes can be emulated. The validation of the proposed traffic generator has been done in an evaluation scenario, where the spreading factor has been modified from SF7 to SF12, alongside the channel bandwidth, from 125 kHz to 250 kHz, only for SF7. Figure 13 presents the performed evaluation scenario setup in the anechoic chamber. As previously stated, this includes the Hope RF LoRa node, the developed LoRa gateway that is connected to the TNN platform and the implemented SDR LoRa traffic generator. It has to be mentioned, that all of the packets are correctly received by the GW when the LoRa traffic generator is not active. The LoRa traffic generator and the sensors use the same transmission power level. Thus, when the traffic generator is used, there is a collision that occurs between the packets sent by the LoRa generator and the packets sent on the same channel by the sensor. In the performed tests, the LoRa traffic generator acts as a source of interference. The monitored communication link from a performance standpoint belongs to the LoRa sensor node that uses an Atmega328 micro-controller integrating an RFM95 radio transceiver. The sensor communicates with the GW module by means of different SFs, on 868.3 MHz communication channel and generates one packet every two seconds. The received packets are sent by the GW module to the TTN platform. The LoRa sensor node can use one SF at a time, which can be modified by changing the software. Figure 12: LoRa packet spectrum. The packet created by the LoRa traffic generator respects the LoRaWAN specifications and is received correctly by the gateway module. Figure 13: LoRa packet spectrum. The SDR LoRa traffic generator varies the SF parameter of the transmitted packet generating packets at random time moments using the same 868.3 MHz communication channel. These packets are received by the GW module and are reported to the TTN platform. The MIMO structure of the SDR platform allows the architecture to send different packets with different modulated SFs. Each transmitter can send packets modulated with a different SF. In the performed tests the payload was varied from 1 byte to 243 bytes. As the payload increases, the performance level is not affected, like the results shown in Section 3, obtained in an empirical simulation level. The LoRa traffic generator results are depicted in Figure 14. A total number of 6043 packets were sent on 868.3 MHz communication channel.