# A Baseband Wireless Spectrum Hypervisor for Multiplexing Concurrent OFDM Signals
## 8 Experimental Evaluation
Figure 18: Spectrogram of 12 vPHY’s concurrently transmitting at a center frequency of 2.4 GHz. The following experiments are executed by adding a channel emulator between the Tx and Rx sides of the prototype. At the Tx side, the generated multiplexed signal, instead of being sent to the USRP HW is sent to an abstraction layer that emulates the HW and adds additive white Gaussian noise (AWGN) to the transmitted signal, next, the abstraction layer transfers the noisy signal to the receiving side of the prototype. Figure 20 shows the throughput measurements taken with the proposed architecture prototype for several MCS values and a Duty Cycle (DC) of 95.24%. In this experiment, the prototype works in full-duplex mode (i.e., it is simultaneously transmitting and receiving) with 1 single vPHY (upper part of the figure) and 12 vPHYs (lower part of the figure) working at the same time. We adopt a full-duplex mode to check if this mode somehow impacts the measured throughout, as in full-duplex mode the prototype is being fully used. The measurements were taken with transmissions of 20 ms (i.e., 20 subframes) and a gap of 1 ms between subsequent transmissions, and therefore, a DC of 95.24%. The throughput is calculated as an average over 10 measurement intervals of 10 s each. During one measurement interval (i.e., 10 s) the total number of received bits from all vPHYs is counted and then divided by the interval to produce the throughput measured during that interval. As in this experiment we are only interested in the maximum throughput that can be achieved, the SNR on the link was set to 30 [dB] so that the packet reception rate for all MCS values was equal to 1. For the sake of comparison, the theoretical maximum throughput achieved by the _Streaming_ mode (i.e., transmissions with a DC equal to 100%) is added to the figure. The theoretical maximum throughput is calculated by dividing the size in bits of an LTE transport block for each MCS value [61] by 1 ms. As can be seen, the measured prototype's throughput approaches the theoretical maximum throughput for all MCS values, yielding more than 4.14 Mbps in the single vPHY case and more than 49.2 Mbps in the 12 vPHYs case for MCS 28. Additionally, as can be also noticed, the operation in full-duplex mode has no visible impact on the achieved throughput. This is due to the powerful server, with 12 CPU cores, used to run the prototype. Figure 19: Spectrogram of 12 vPHYs with discontinuous transmissions and independent frequency gains at a center frequency of 2.4 GHz. Next, in Figure 21 we compare the data packet reception ratio (PRR) of a single PHY against the data PRR of 12 vPHYs concurrently transmitting over a range of SNR values. In this experiment, 12 vPHYs concurrently have their signals multiplexed, transmitted and received by the prototype. The experiment uses 3 different MCS values so that all 3 modulation schemes employed by LTE standard (i.e., QPSK, 16 QAM, and 64 QAM) are tested. The PRR is calculated as the average over \(10^{5}\) Monte Carlo trials, where at each trial, the Tx side of the prototype sends either a single PHY signal or the multiplexed signal of 12 vPHYs. In each one of the sub-figures, we compare the PRR of the 12 vPHYs against the PRR obtained with a single PHY, i.e., there is no other signal being transmitted along with that of the PHY under test. As theorized earlier, the PRR curve of the 12 concurrent transmitting vPHYs match the PRR of the single PHY (i.e., the dashed black curve with squares along it). This means that there is no interference between the current vPHY transmissions. These results prove that the prototype of the proposed architecture also, as shown before with the simulation results, provides isolation among all the signals being multiplexed by the prototype. The achieved isolation is due to the orthogonality provided by the IFFT processing implemented by the prototype.