# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 8 Experimental Evaluation  
In this section, we present some experimental results carried out to validate and assess the functionality of the proof of concept implementation of the proposed architecture. All the experiments presented in this section were carried out with the prototype running on a desktop with an Intel Xeon E5-2650 v4 CPU (@2.2 GHz, 30 M cache, 9.60 GT/s QPI, Turbo, HT, 12 Cores/24 Threads, 105 Watts) with 128 GB of RAM memory connected to a x310 USRP with 10 Gigabit Ethernet link, and equipped with CBX-120 RF daughterboards [59]. These RF daughterboards operate from 1200 up to 6000 MHz with a bandwidth of 120 MHz [59]. For all the experiments presented in this section, each vPHY has a useful transmitting BW of 1.08 MHz, which is equivalent to 6 LTE RBs, and a guard-band of 420 KHz at each side of the transmitted spectrum, totaling 1.92 MHz of used BW (i.e., useful-band plus guard-band sections) per vPHY, totaling \(12\times 1.92\) MHz/vPHY \(=23.04\) MHz of occupied BW when we have 12 vPHYs operating concurrently. Due to the lack of full-blown and tested (i.e., operational) open-source projects that implement systems with different but correlated numerologies such as subcarrier frequency spacing, we will not be able to experiment with correlated numerologies. However, as shown in Section 7, working with different but correlated numerologies would not be a problem to the proposed architecture. Figure 17 shows the spectrum of 12 vPHYs transmitting concurrently. This figure was collected with an Anritsu MS2690A Signal Analyzer. The RF front-end center frequency was set to 2.4 GHz and Tx gain set to 3 [dB] with the USRP Tx output connected to the signal analyzer through a cable with 20 [dB] of attenuation. As can be seen, the total transmission BW spans over 23.04 MHz, which is equivalent to having 12 1.92 MHz-wide vPHYs with their center frequencies located at 1.92 MHz apart from each other. Additionally, we can also see that the two vPHYs transmitting at the right and left edges suffer from attenuation, which is caused by cascaded integrator-comb (CIC) filter roll-off. CIC filter are implemented in the USRP to provide decimation by an arbitrary programmable integer decimation factor; however, they present a very significant pass-band roll-off, which are often called _spectral droop_ or _CIC roll-off_[60]. Figure 18 shows the spectrogram (time versus frequency) for the same experiment setup used to capture Figure 17. In this experiment all 12 vPHYs transmit in streaming mode, i.e., each vPHY transmits subframes all the time with no gap between subsequent subframes. The transmitted signal was captured for a period of 100 ms. It can be noticed that all 12 vPHYs transmit at the same time throughout the whole analysis interval, without any gap between consecutive subframes. Figure 16: BER curves for a setup with 3 vPHYs with uncorrelated numerologies. Modulations used are (a) QPSK, (b) 16 QAM and (c) 64 QAM modulation schemes. Figure 19 shows that the proof of concept prototype of the proposed hypervisor is able to handle discontinuous transmissions and to apply independent gains to each vPHY, as described in Section 3. Again, we used the same experiment setup used to capture Figure 17. In this experiment all 12 vPHY's transmit in discontinuous (burst) mode with random number of subframes transmitted in a row, channel number and frequency amplification factor. The number of subframes, channel number and frequency amplification factor of each vPHY are randomly selected between the ranges 0-11, 0-5, and 0-100% respectively. The transmitted signal was captured for a period of 100 ms. As can be seen, the prototype is also able to work on burst mode with independent frequency amplification factor for each vPHY. This result also shows that the prototype also supports runtime configuration of the number of transmitting vPHYs as we see that not all vPHY's might be transmitting during a period. Figure 17: Spectrum of 12 vPHY’s concurrently transmitting at a center frequency of 2.4 GHz.