# A Software-Defined Radio for Future Wireless Communication Systems at 60 GHz
## 3 Results Discussion
### OTA 60 GHz Performance  
Now, the effect of RF impairments on the quality of TRX OFDM system, outlined in Section 3.2, is addressed with 60 GHz over-the-air measurements under different conditions of RX SNIR and employed modulations. With possible OFDM performance degradation due to multipath effect in mind, the testbed was firstly set inside an anechoic chamber under LOS condition. With such controlled environment, it is possible to accurately assess the RF front-end impact on the previously discussed OFDM communication system. The system QoS was assessed through EVM figure of merit and spectral efficiency per stream analysis. In the end, testbed results with 25 dBi antennas show a maximum distance range of 74.5 cm. A Bit Error Rate (BER) estimation was accomplished by using the relation between this metric and EVM, given in Table 6[30]. The EVM results were computed for each OFDM IF modulated signal under different SNIR in either the presence or absence of CFO. For example, Table 7 shows the measured average EVM values for an IF OFDM modulated signal at 312.5 MHz (IF4). The minimum average EVM value, \(-\)32.99 dB, is verified for an input power of \(-\)14.83 dBm when both TX/RX devices are clocked using different sources. Such value together with Table 6 indicates that the RF front-end can handle QAM modulations up to 256-QAM, meaning that 1 Gbps of data transmissions are possible with the proposed mmWave testbed. For quality assessment of the proposed testbed the EVM value, 4-, 16-, 64-, and 256-QAM received constellations, are depicted in Figure 10a-d, respectively. These results present a slight scattering distortion on the received symbols when compared with the B2B configuration (see constellations of Figure 19). However, even for 256-QAM, all constellations exhibit a well defined point scattering area, which indicates a low probability of decoding erroneous bits. \begin{table}
\begin{tabular}{c c c} \hline \hline
**BER** & **Modulation** & **EVM** \\ \hline \multirow{4}{*}{\(10^{-3}\)} & 4-QAM & \(-\)10 dB \\  & 16-QAM & \(-\)17 dB \\  & 64-QAM & \(-\)24 dB \\  & 256-QAM & \(-\)32 dB \\ \hline \multirow{4}{*}{\(10^{-4}\)} & 4-QAM & \(-\)12 dB \\  & 16-QAM & \(-\)19 dB \\ \cline{1-1}  & 64-QAM & \(-\)26 dB \\ \cline{1-1}  & 256-QAM & \(-\)34 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Relation between EVM, BER, and digital modulation, according to [PERSON] and [PERSON] [30]. Figure 19: Received scatter constellation plots for: (**a**) 4-QAM; and (**b**) 256-QAM. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Gain** & **TX Input** & **Analog RX** & \multicolumn{2}{c}{_EVM_\({}^{\ddagger}\)} & \multicolumn{2}{c}{_EVM_\({}^{\ddagger}\)} \\
**Sel.** & **Power [dBm]** & **SNIR [dB]** & **[\%]** & **[dB]** & **[\%]** & **[dB]** \\ \hline
0 & \(-25.63\) & \(28.70\) & \(6.83\) & \(-23.30\) & \(5.40\) & \(-25.34\) \\ \hline
1 & \(-19.76\) & \(33.79\) & \(3.67\) & \(-28.72\) & \(3.01\) & \(-30.42\) \\ \hline
2 & \(-17.83\) & \(36.00\) & \(3.11\) & \(-30.13\) & \(2.62\) & \(-31.60\) \\ \hline
3 & \(-16.21\) & \(36.67\) & \(2.74\) & \(-31.21\) & \(2.30\) & \(-32.52\) \\ \hline
4 & \(-14.83\) & \(37.81\) & \(2.35\) & \(-32.58\) & \(2.23\) & \(-32.99\) \\ \hline
5 & \(-13.64\) & \(38.48\) & \(2.50\) & \(-32.36\) & \(2.31\) & \(-32.70\) \\ \hline
6 & \(-12.62\) & \(38.08\) & \(2.69\) & \(-31.36\) & \(2.26\) & \(-32.87\) \\ \hline
7 & \(-11.72\) & \(39.00\) & \(2.55\) & \(-31.87\) & \(2.37\) & \(-32.46\) \\ \hline \hline \end{tabular} \({}^{\dagger}\)) Shared clock source configuration; (\(\ddagger\)) Independent clock source configuration.  
\end{table} 
Table 7: SNIR vs. EVM results using IF4 OFDM modulation for both a shared and independent clock. Figure 20: Received scatter constellation plots for: (**a**) 4-QAM; (**b**) 16-QAM; (**c**) 64-QAM; and (**d**) 256-QAM, considering an IF4 OFDM power transmission of \(-14.83\) dBm on the mmWave RF front-end. Furthermore, Table 8 shows the minimum average EVM and its degradation for the remaining IF OFDM modulations.