## 3 Results and Discussion
### Bit Error Rate and Error Vector Magnitude  
In this section, the performance obtained for the two different mixers is discussed. In Figure 9a,b we present a comparison of the BER and EVM performance across a spectrum of mixers operating at data rates from 10 to 80 Gbps.  
Regarding the GaAs mixer, Figure 9a,b show how the electrically pumped mixers maintained exceptionally low BER and EVM levels across the data rate while remaining below the HD-FEC threshold. Similarly, the InGaAs narrow-band optically pumped mixer demonstrated noteworthy performance and operated within the HD-FEC limits as well. According to Table 1, the InGaAs narrow-band mixers should perform worse than their GaAs counterparts due to the higher conversion loss. Nevertheless, the optically generated signals demonstrated a higher purity than electrical ones. Consequently, the purity of the LO signal compensated for the anticipated performance gap, leading to equivalent final results.  
In contrast, the InGaAs narrow-band mixer, when subjected to electrical pumping, exhibited the least favorable BER and EVM outcomes and stayed close to the SD-FEC  
Figure 8: Constellation diagrams of the 40 and 80 Gbps transmission link using optical LO.  
Figure 9: BER and EVM comparison of different GaAs and InGaAs sub-harmonic mixers. Narrow-band (NB) results under electrical (ELO) and optical (UTC) pumping. (**a**) BER evaluation of the THz link using different sub-harmonic mixers and pumping approaches. (**b**) EVM assessment of the THz link using different sub-harmonic mixers and pumping methods.  
threshold. This issue is a result of the higher conversion loss, but it is also attributed to the LO signal being too high. The AMC employed in the electrically pumped study provided an average output power of 10 mW. To regulate the power level reaching the InGaAs mixer, a 10 dB attenuator was placed before it, reducing the signal to approximately 1 mW at the mixer's input. However, as observed in Figure 3, the NF increased with the input power, indicating that the applied attenuation was not enough to maintain the mixer within its optimal operating range. Consequently, the Schottky diodes experienced overpumping effects, leading to unwanted non-linear effects that degraded performance [44].  
An important figure of merit to compare the different measurements is the signal-to-noise ratio (SNR) achieved in each case. To extract this value, we can use the following theoretical expression for 16-QAM modulation [45]:  
\[SNR(dB)=20\,log_{10}\Big{(}\sqrt{10\cdot erfc^{-1}(8/3\cdot BER)}\Big{)} \tag{1}\]  
where \(erfc^{-1}\) is the inverse error function. With the SNR, we can then calculate the penalty in dB comparing the different mixers. This is depicted in Figure 10.  
Since the obtained BER was very similar for the electrically pumped GaAs and optically pumped InGaAs mixers, the calculated SNR was consistent, with maximum and minimum values of approximately 24.5 dB and 15.5 dB for 10 Gbps and 80 Gbps, respectively. In contrast, the electrically pumped InGaAs mixer exhibited a penalty of approximately 2-3 dB in most cases, with the exception of 7 dB for 10 Gbps. It is important to note that the extracted SNR does not take into account non-linear effects or other impairments that contribute to degrading the BER. Therefore, this figure of merit serves as a simple tool to compare the mixers from a system point of view.  
In summary, the key finding is that GaAs mixers have traditionally been used with electrical pumping due to their high power requirements, making them the preferred option for applications such as communications. Our results demonstrate that an InGaAs mixer under optical pumping, despite exhibiting higher conversion losses, can achieve performance comparable to electrically pumped GaAs mixers. This breakthrough paves the way for future advancements and novel designs in THz receivers, offering new possibilities for improved integration and efficiency in next-generation wireless communication systems.