## 7 Conclusions  
The major scientific contribution of the given research work is in the multiple experimental results which provided real-world validation of FM-ACSK modulation scheme. Moreover, a novel symbol timing synchronization scheme and novel symbol detector were proposed.  
The experiments have shown that the developed Adalm Pluto SDR-based FM-ACSK transceiver can provide real data communication with a speed of about 10 bps over a distance of 20 m using average transmit power about 3 mW. Further improvement of performance at low SNR can be achieved by boosting the processing gain via increasing the spreading factor of the spread-spectrum modulator. In accordance with Figure 4, the current implementation uses a relatively low spreading factor as there are just approximately 9 chaotic oscillations per one data symbol. However, a further increase in spreading factor can be achieved at the expense of data transfer speed or an increase of chaos generators' sample rate. Employment of FEC can decrease BER to arbitrary low values at the cost of the transmission speed.  
The proposed solution suits very well wireless sensor network (WSN) applications which are characterized by relatively low transfer speed and low SNR due to low-power transmission or large distance. However, WSN applications require compact and cost-efficient transceivers due to large-scale deployments of the sensor nodes. Transitioning from the MATLAB-based prototype into a compact and cost-efficient solution can be carried out in several ways.  
Firstly, SDR unit can be kept whereas the software part can be rebuilt using open-source tools such as GNU Radio and Python which can run on a single-board computer such as a Raspberry PI. This, along with the size and cost reduction, can provide also a performance boost.  
If further reduction of cost and size is necessary, Simulink code can be converted into very high-Speed integrated circuit hardware description language (VHDL) using Simulink CoderTM[40] and run on FPGA connected to the radio frequency (RF) transceiver integrated circuit (IC), for example, AD9363, which used within Adalm Pluto. Compared to SDR unit-based solutions, FPGA can run significantly faster and provide much larger spreading factors or data transfer rates. Availability of Simulink CoderTM was one of the major motivations for the selection of MATLAB versus GNU Radio in our research.  
For even larger cost and size reduction, VHDL code can be converted into application-specific integrated circuit (ASIC) layout and integrated with a custom RF part. Furthermore, the transceiver and especially chaos generators can be built partly analog, providing a significant reduction of power consumption.  
Conceptualization, A.A.; methodology, A.A.; software, N.T.; validation, A.A.; formal analysis, A.A.; investigation, A.A.; resources, A.A.; data curation, N.T.; writing--original draft preparation, N.T.; writing--review and editing, A.A.; visualization, N.T.; supervision, A.A.; project administration, A.A.; funding acquisition, A.A. All authors have read and agreed to the published version of the manuscript.  
This work has been supported by the European Regional Development Fund within Activity 1.1.1.2 \"Post-doctoral Research Aid\" of the Specific Aid Objective 1.1.1 \"To increase the research and innovative capacity of scientific institutions of Latvia and the ability to attract external financing, investing in human resources and infrastructure\" of the Operational Programme \"Growth and Employment\" (No.1.1.1.2/VIAA/2/18/345).  
The authors declare no conflict of interest.