# SCATTER PHY: An Open Source Physical Layer for the DARPA Spectrum Collaboration Challenge
## 4 Experiment Results  
In this section, we demonstrate the effectiveness and usability of SCATTER PHY by presenting the results of several experiments. The experiments were executed with SCATTER PHY running on servers equipped with Intel Xeon E5-2650 v4 CPUs (@2.2 GHz, 30 M cache, 9.60 GT/s QPI, turbo, HT, 12 cores/ 24 threads, 105 Watts), 128 GB of RAM memory connected to x310 USRPs with 10 Gigabit Ethernet links, and equipped with CBX-120 RF daughter-boards [26]. Figure 5 depicts two spectrograms (i.e., the visual representation of the spectrum of frequencies of a signal as it varies over time) saved for 40 ms over a 31.25 MHz bandwidth with both PHYs set to operate concurrently, at each instant, at two of six 4.5 MHz channels. This experiment intends to show that SCATTER PHY is able to generate two concurrent and independent transmit channels per node. For this experiment, each PHY transmits at randomly selected channels a random number of subframes. The number of transmitted subframes, i.e., COT, and the channel number are randomly selected between the ranges of 1-3 and 0-5, respectively. Here, a gap of 1 ms between consecutive transmission is used. The figures were saved with the center frequency of both Tx PHYs set to 2.4 GHz, equal Tx gains of 3 dB, and both USRP Tx outputs connected to the signal-analyzer via an RF combiner and a coaxial cable presenting a 20 dB attenuation. As can be seen in both figures, Figure 5a,b, SCATTER PHY was able to independently transmit at two distinct channels with a different number of subframes. Figure 5a shows the case where no filtering was enabled. As can be noticed, OOBE might cause interference to radios operating at adjacent channels and consequently decrease their throughput. Figure 5b shows the case when the FPGA based FIR filters were enabled. As can be noticed, when filtering was enabled, OOBE was mitigated. Therefore, by enabling filtering, the interference that could be impacting radios operating at adjacent channels was mitigated. Moreover, another direct consequence of using filters was that the channel spacing, i.e., the guard-band between adjacent channels, could be decreased, decreasing the wastage of spectrum band, consequently. Figure 6 shows measurements of the throughput, which were taken with SCATTER PHY working in full-duplex mode, where the two independent PHYs simultaneously and concurrently receive and transmit, for several MCS and PHY BW values. The full-duplex mode was used so that we could check if it  
Figure 5: Comparison of the McF-TDMA scheme with and without filtering. (**a**) FPGA based filters disabled. (**b**) The 128 order FPGA based filters enabled. somehow impacted the throughput measurements, once, in this mode, SCATTER PHY was fully utilized. The throughput measurements were taken for COTs of 20 ms with gaps of 1 ms between subsequent COT transmissions, which resulted in a Duty-Cycle (DC) of 95.24%. The final throughput was calculated as the average over 10 throughput measurement intervals of 10 seconds each. For each measurement interval, the number of bits received during that interval was counted and then divided by the length of the interval, resulting in a throughput measurement. This procedure was repeated 10 times, and then, the individual measurements were averaged. The SNR on the link was set to 30 dB so that the packet reception rate for all MCS values was equal to one. For the sake of comparison, we added to the figure the theoretical maximum throughput achieved when a DC of 100% was used; we called it streaming mode, due to the fact that there was no gap between subsequent subframes. The theoretical maximum throughput was obtained by dividing the size of the TB, given in number of bits for each MCS value, by the subframe time interval, i.e., 1 ms. As expected, the throughput measured with SCATTER PHY was close to that of the streaming mode for all PHY BW and MCS values, achieving more than 84 Mbps for MCS 31 and a PHY BW of 9 MHz. Moreover, full-duplex mode operation had no visible impact on SCATTER PHY's achieved throughput.