# SCATTER PHY: An Open Source Physical Layer for the DARPA Spectrum Collaboration Challenge
## 3 Scatter Phy
* FPGA based filtered transmissions: Filtering the transmitted signal effectively minimizes OOBE, which allows for a better spectrum utilization, once radios can have their transmissions nearer to each other in the frequency domain, helping to mitigate the wastage of spectrum bands. * Out-of-band full-duplex operation: Both PHYs operate totally independently of each other, meaning that Tx and Rx modules are able to transmit and receive at different channels, set different Tx and Rx gains, and use different PHY BWs. * Timed-commands: This feature allows the configuration in advance of the exact time to (i) start a transmission and (ii) change Tx/Rx frequencies/gains. This allows the MAC layer to implement a TDMA scheme. * Improved synchronization: Through the design and implementation of a novel two-stage synchronization algorithm, it is possible to improve the packet detection ratio when compared to the single-stage approach. * Fine-grained CP based CFO estimation: By generating a complex exponential sequence with smaller sampling steps, it is possible to synthesize the frequency offset more accurately, consequently improving the correction. * Channel emulator: Through the implementation of an abstraction layer between the USRP hardware and the software based PHY, it is possible to create a channel emulator that connects the transmission side to the receiving side without the necessity of any hardware. Through the use of the channel emulator, it is possible to test novel PHY layer algorithms with the addition of white Gaussian noise, multi-paths, frequency offsets, etc. SCATTER PHY was implemented based on the srsLTE library [18]. srsLTE is an open-source and free LTE software based library that was developed by the Software Radio Systems (SRS) Limited company [18]. It implements LTE release 10 and a few narrow-band Internet-of-Things (IoT) and 5G features. As SCATTER PHY was built on an LTE-like library, it consequently absorbed and evolved on top of the existing 4/5G features. We employed OFDM as the SCATTER PHY waveform. OFDM is a full blown technology that is present in a huge number of commercial products due to various advantages it offers such as low implementation complexity, robustness to severe multi-path fading, simple channel estimation, easy integration with Multiple-Input Multiple-Output (MIMO) technology, etc. [19]. User and control data are mapped into subcarriers over 14 OFDM symbols spanning 1 ms. The control data signal carries the used MCS for the current transmission and the number of subsequent subframes with user data modulated with that specific MCS. The control data signal is used by the independent PHY receivers to detect the number of allocated RBs automatically (i.e., control is embedded into the transmitted data signal), the MCS value selected to transmit data of a specific device/radio, and the location of the allocated RBs in the subframe's resource grid. By embedding the control information into the transmitted signal, the Medium Access Control (MAC) layer does not need to know in advance the number of subframes and MCS in a given Channel Occupancy Time (COT). After successful user data decoding, each one of the PHYs independently notifies the MAC layer about the corresponding decoded MCS value, the number of received data bytes, and decoding statistics (e.g., RSSI, SINR, number of detection/decoding errors, etc.). SCATTER PHY works with two types of subframes, namely synchronization and data-only subframes. A synchronization subframe carries the synchronization signal, reference signal, control, and user data. A data-only subframe carries the reference signal and user data. The synchronization signal is a 72 symbol long sequence that is generated using Zadoff-Chu (ZC) sequences [20,21].