# SCATTER PHY: An Open Source Physical Layer for the DARPA Spectrum Collaboration Challenge
## 3 Scatter Phy
On the other hand, the fractional frequency offset (i.e., offset values less than one half of the subcarrier spacing) is estimated and corrected by the fine CFO algorithm, which is based on the phase difference of the correlation between the CP and the last part of the OFDM symbol (i.e., the portion used to create the CP). Integer and fractional CFO estimation methods are described in Appendix B. Each PHY Rx demodulation thread is responsible for OFDM demodulation (CP removal and FFT processing), i.e., user data demodulation, channel estimation/equalization, resource de-mapping, symbol demodulation, de-scrambling, de-interleaving/de-rate matching, turbo decoding, de-segmentation, and finally, cyclic redundancy check (CRC) checking. Each one of the independent PHYs receives control and data messages from the 0 MQ Communications Data/Control module. Decoded user data and Tx/Rx statistics are sent directly to the MAC layer through the 0 MQ bus within a protobuf pre-defined message. Figure 2 presents the block-diagram of SCATTER PHY. It shows the blocks making up the receiver (PHY Synch and PHY Demodulation (Demod) threads) and transmitter (PHY Tx thread) sides of each independent PHY. Figure 2: Block-diagram showing the modules making up the transmitter and receiver sides of the SCATTER PHY. When it comes to numerology, SCATTER PHY adopts a subcarrier spacing of 15 kHz, a CP of 5.2 \(\upmu\)s, and supports 1.26, 2.7, 4.5, and 9 MHz bandwidths, which are equivalent to the LTE bandwidths of 6, 15, 25, and 50 resource blocks, respectively [19]. Each one of the PHY's transmission channel Bandwidth (BW) can be configured in real time, through the Tx control messages or via command line at the system's startup. The OFDM modulation parameters adopted by SCATTER PHY are summarized in Table 1.  
The only difference between the OFDM modulation parameters listed in Table 1 and the LTE ones [19] is the FFT size and, consequently, the resulting sampling rate for the PHY BWs of 4.5 and 9 MHz. The reason for employing these non-standard sampling rates is to reduce the computational load of SCATTER PHY and to reduce the data transfer rate required to move base-band IQ (i.e., In-phase/Quadrature) samples between the RF front-end (i.e., the USRP device) and the base-band processor (i.e., the host PC) and vice versa. For example, for a PHY BW of 4.5 MHz, the standard LTE sampling rate would be equal to 512 FFT points \(\times\) a subcarrier spacing of 15 kHz = 7.68 MHz. However, out of the 512 subcarriers, only 300 are useful subcarriers, where the remaining ones are left unused as guard-band subcarriers. Therefore, by employing a smaller FFT size and, consequently, a lower sampling rate, while keeping the same subcarrier spacing, SCATTER PHY can still successfully demodulate all occupied subcarriers. Therefore, the computational load is lower due to the fact that fewer base-band samples have to be moved between the USRP and the host PC. The frame structure employed by SCATTER PHY is presented in Figure 3. As can be seen, signals regarding synchronization and control data are only added to the first subframe of a COT, while the subsequent subframes carry only reference signals and data symbols. With this frame structure, synchronization (i.e., detection of synchronization signal, time-alignment, and CFO estimation/correction) and control data decoding only happen for Subframe #0, i.e., the synchronization subframe. The differences between the proposed frame structure and LTE's are (i) no modulated control data (i.e., LTE's PDCCH channel) being added to first OFDM symbols, (ii) a longer synchronization signal, which improves the subframe detection performance, (iii) control data sent via M-sequences, which makes the control data decoding more robust against interference and noise, and (iv) no division of the subcarriers into resource blocks, once all subcarriers are allocated to a single user only.