## 4 Practical Model, Experimental Tests and Results  
This section describes the practical implementation of the proposed system, shown in Figure 3, and presents the test results. The GSM network infrastructure was developed on the GNU Radio software (open source project) [51], version 3.7.10.1, running in a computer with Ubuntu OS (Operational System), according to the OpenBTS project, and it was connected to a BladeRF SDR board model x115, as shown in Figure 3 (at the top right corner). To address legal issues, the network is configured to operate on the Absolute Radio Frequency Channel Number (ARFCN) 975 (downlink frequency of 925.2 MHz), a valid GSM frequency within the ISM (Instrumentation, Scientific, and Medical) band. Figure 3: Practical implementation of the proposed system. This infrastructure can be located at the control center of the grid operator. From it, SMS messages with \(\hat{P}_{ref}\) and \(\hat{Q}_{ref}\) power references information can be sent. In the head of the messages, it is defined whether the reference information is \(\hat{P}_{ref}\) or \(\hat{Q}_{ref}\). Each SMS message is composed of eight symbols (each one having eight bits, totaling 256 possible different levels), each one representing the power value in a 100 ms time interval. A full SMS structure is shown in Figure 4.  
During the transmission process, the SMS messages containing the power references are created directly by a script running in OpenBTS (open source project) [52; 53], version 4.0. After each message is sent, a sleep command is executed, resulting in a guard interval between messages. Therefore, this system is very efficient, as it is necessary to send a new message only when the power reference values change, avoiding the need for continuous transmissions. On the other hand, the receiver system is placed at the wind generator. It is composed of a PIC18F45K22 microcontroller development board, in which a Teli G865 QUAD GSM module is connected. A smart card from a cellular service operator (not enabled in the test region) is employed, which avoids any legal or testability problems for the proposed system. During the reception process, the GSM signal is firstly demodulated and decoded. Particularly, in the GMSK demodulation process, it used four symbols and a retrace length of 16 symbols. Then, the recovered bits are grouped to determine the power reference levels transmitted, to be followed by the DFIG power control. After that, the power reference levels are proportionally converted to analog voltage levels by the internal Digital-to-Analog Converter (DAC) of the microcontroller. The DAC has 5-bit resolution with a 2.048 V reference value. These quantized levels of power reference are sent to the power control, implemented in a TMS320F28335 DSP board. The Figure 5 shows a block diagram summarizing the implemented communication system. On the wind generator side, the Direct Torque Control (DTC) is applied to a DFIG mechanically coupled to a DC motor, which emulates the wind effect. The DFIG parameters are presented in Table 1. The DTC was implemented with a power electronic converter using Insulated-Gate Bipolar Transistors (IGBTs). The rotor speed is measured by an encoder (with 3600 pulses per revolution), and the rotor voltage commands are modulated by using symmetrical Space Vector Pulse Width Modulation (SV-PWM) with a switching frequency of 10 kHz. Figure 4: SMS information structure. Figure 5: Block diagram of the communication system.