## 4 Results and Discussions
### System under Test  
To demonstrate the effectiveness of the developed testing platform in testing complex communication protocols, an OFDM transmission system with transceivers that have adaptive modulation and coding capabilities was used. This system is complex enough to exhibit the test platform's capabilities fully. Specifically, two OFDM systems were tested. The first system implements a simplex transmission with a reverse channel to convey channel state information from the receiver to the transmitter. The second system is a full-duplex transmission system where the channel state information acquired by each receiver is multiplexed with the data flows to be sent to the corresponding transmitter. Figure 19 provides a simplified schematic of the simplex OFDM transmission system. For additional information regarding the architecture of this system with adaptive modulations but without adaptive coding, please refer to [59]. The implementation of the OFDM modem under test is available in [53].  
The OFDM transmission system that was tested and evaluated consists of several complex signal processing blocks, including the OFDM clock and carrier synchronization block, the channel transfer function estimator, the OFDM equalizer, the forward error correction (FEC) encoder and decoder (using LDPC codes), the SNR estimation block, the transmission control block, and the framing block. To perform a thorough dynamic white box testing of these signal processing blocks, acquiring and analyzing a large amount of data is necessary. This requires a fast and efficient monitoring system that can handle real-time signal acquisition and processing of a large amount of data. The need to acquire a large amount of monitoring data in real-time is a common characteristic of any platform used for testing and evaluating complex PHY-layer algorithms [38].  
Figure 19: The simplified architecture of the OFDM transmission system tested using the developed testing platform.  
Evaluation of the System under Test  
The panels in Figure 20 show the changes in key parameters of an OFDM transmission system being tested over time. The parameters include the number of iterations of the LDPC decoder, the error rate of the transport block/frame, the CPU usage of the application, the estimated SNR, the bits/symbol of the modulation scheme used, and the one-way travel time of the IP packets loaded in the transport blocks. It is worth noting that this paper is not focused on detailed testing and evaluation of an OFDM transmission system with adaptive coded modulation. Instead, the system in Figure 20 is used to showcase the capabilities of the developed testing platform. The parameters in Figure 20 are evaluated frame by frame or packet by packet. However, other parameters, such as the equalization coefficients and the decision error of the QAM symbols composing the OFDM symbol, need to be evaluated at each OFDM symbol, generating a more significant amount of monitoring data. The traces representing the variation over time of the considered parameters were generated with the Grafana utility, which was also used to query the database, storing the parameter values and time stamps. The results were obtained in the simulated duplex transmission system, where both OFDM transceiver applications run on the same physical machine. In the simulated transmission system, the parameters of the wireless link can be controlled, and more relevant results can be obtained. If the SDR IO modules are integrated into the GNU radio applications, the two OFDM transceivers can be connected by a real wireless link with no other changes necessary.