# SCATTER PHY: An Open Source Physical Layer for the DARPA Spectrum Collaboration Challenge  
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Footnote 1: email: {[[http://www.mdpi.com/journal/electronics](http://www.mdpi.com/journal/electronics)]([http://www.mdpi.com/journal/electronics](http://www.mdpi.com/journal/electronics))communication networks to compete with other teams in achieving the highest possible throughput, not only for their own team, but also for other teams coexisting in the same spectrum bands [1]. The teams were motivated to set up their networks to be able to not only coexist with other networks, but also use collaboration to ensure that each network maximizes its throughput while facilitating spectrum usage and securing its availability when necessary for other networks and systems. The teams would devise new spectrum access strategies, which would allow radio networks to collaborate autonomously and determine dynamically how the Radio Frequency (RF) spectrum is being used, which enables radios to avoid interference and find better transmission opportunities [2]. The SC2 was run on Colosseum, a testbed with a large number of nodes equipped with Software Defined Radio (SDR) devices, where teams can deploy their networks and use SDRs for their wireless communication. The testbed is able to emulate any desired geographical topology of nodes with diverse physical environments, enabling DARPA to test teams' networks and their performance with and against other networks in a wide variety of scenarios [1]. The SCATTER team has designed and built a multi-layered system, with physical, Medium Access Control (MAC), Artificial Intelligence (AI), and other layers, which were developed and implemented as completely autonomous systems, but interconnected through a publish-subscribe messaging system known as ZeroMQ [3; 4]. ZeroMQ (also known as 0 MQ or ZMQ) is a high-performance asynchronous messaging library, aimed for use in concurrent or distributed applications [5]. Therefore, SCATTER's open-source software defined Physical layer (PHY) was designed as an independent system, able to transmit and receive packets over the air using USRP SDR devices. The design of SCATTER PHY uses srsLTE, an existing software implementation of the LTE protocols, as the basis for its communication and builds on top of it with various modifications to introduce features and mechanisms targeted for the SC2. The ultimate goal of these features is to present an API of the required physical layer functionality to other layers (mainly the MAC and AI layer) which are oblivious to the implementation details of those functions. For example, SCATTER PHY uses custom built FPGA filters to reduce out-of-band emissions and maximize channel utilization. When the MAC layer requests a change of the channel bandwidth, these filters are automatically adapted to accommodate the new bandwidth. Similarly, the physical layer provides spectrum measurement data averaged over time to the AI layer, which uses them to determine the most efficient channel selection algorithms. Spectrum measurement data are obtained by again using the custom built FPGA blocks, which perform FFT and average the obtained power spectral density over multiple measurements to reduce the amount of data sent to AI. In this work, we describe the design and features of SCATTER PHY and experimentally verify the benefits achieved with the outlined design decisions. The SCATTER PHY layer, presented in this work, is an enhancement/extension of the framework presented in [6]. Different from [6], SCATTER PHY presents several enhancements on top of the existing framework. Next, we list and describe the main contributions and differences of this work when compared to [6]. * Introduction of an open-source, highly optimized, and real-time configurable SDR based physical layer for the research and development of novel intelligent spectrum sharing schemes and algorithms.