# A Software-Defined Radio for Future Wireless Communication Systems at 60 GHz
## 3 Results Discussion
### Back-to-Back Performance  
Following the ADC/DAC assessment, measurements with baseband integration were conducted. It is important to mention that, following the 5G pre-trials specification presented in [1], a multi-Gigabit/s real-time OFDM TRX engine was developed [29]. Such OFDM baseband engine architecture is extensively discussed in [29] and was based on the previous work reported in [30]. Such engine enhancements led to a 1 Gbps real-time transmission rate (considering 256-QAM), which fullfills next wireless generation requirements. That said, the considered OFDM main design operating parameters, particularly the FFT and data size block values are given in Table 3.  
Figure 16: ADC SFDR measurement results at: (**a**) the input; and (**b**) channel A output. Figure 15: FMC230 performance boundaries versus frequency tone: (**a**) SFDR; and (**b**) SINAD versus ENOB. Table 4 outlines the main SDR system specifications accomplished by integrating both OFDM modulator/demodulator, with the digital IF architecture using the disruptive modular fully pipelined hardware architecture illustrated in Figure 11.  
Firstly, the TRX performance of such SDR was evaluated in B2B configuration using EVM as QoS assessment metric. In Figure 17, an example of the transmitted OFDM signal spectrum was obtained considering a frequency centered at 312.5 MHz (IF4) and selected digital gain of 7. Additionally, numerous OFDM IF possibilities were considered, such as zero IF (DC), IF2 (156.25 MHz), IF4 (312.5 MHz), and IF6 (468.75 MHz). Figure 18 shows the output power transfer \begin{table}
\begin{tabular}{c c} \hline \hline
**Feature** & **Value** \\ \hline Processing BW & 1.25 GHz \\ Transmission BW & 150 MHz \\ ADC/DAC sampling rate & 1.25 GSPS \\ Modulation & OFDM with 4-/16-/64-/256-QAM \\ Max spectral eff. & 8 bits/s/Hz/user \\ IF freq. & DC-612 MHz \\ User configuration & Single \\ Antenna configuration & SISO \\ Number of DAC/ ADC channels & 2/3 \\ Resolution of DAC/ADC & 14/10 bits \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Main SDR system specifications. Figure 17: Frequency spectrum of the transmitted OFDM signal, for an IF4 and gain select of 7. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline FFT size block & 1024 \\ Data block size & 800 \\ Sub-carrier bandwidth & 152 kHz \\ Modulation & 16-/64-/256-QAM \\ Guard time interval & 256 samples \\ Maximum data rate & 1 Gbps \\ Nominal signal bandwidth & 122 MHz \\ OFDM symbol duration & 8.2 μs \\ CP duration & 1.64 μs \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main parameters considered in the design of OFDM system. curve as a function of the selected gain and IF modulations. It was verified that OFDM modulation centered at IF2 has less signal attenuation than the remaining IF frequencies, leading to higher DAC output power. In Table 5, the EVM results are summarized for the maximum digital gain of 7 using four different IF configurations. Additionally, when either I or Q channels are used to transmit and receive data, EVM results are denoted \(EVM\)\(I\) or \(EVM\)\(Q\), respectively. That is, connecting DAC0 to ADC0 is considered as the \(I\)_branch_, and connecting DAC1 to ADC1 channels is the \(Q\)_branch_. Additionally, for a quadrature transmission, both I/Q channel branches are used simultaneously. These results show that a minimum EVM of approximately \(-42\) dB is obtained for all low IF modulations, except for zero-IF, in which synchronization algorithms fail to estimate the beginning of frame, due to the nulls obtained on both DAC/ADC frequency responses. As expected, for other IFs, no OFDM performance degradation is verified, even when an IQ transmission configuration is considered, indicating that very low distortion is present on received signal constellations. Finally, in Figure 19, it can be seen that the 4- and 256-QAM constellations do not present significant scattering distortion on the received symbols, by using IF2 frequency and a maximum digital gain.