# SCATTER PHY: An Open Source Physical Layer for the DARPA Spectrum Collaboration Challenge
## 3 Scatter Phy
As shown in Figure 3, and differently from LTE, SCATTER PHY's frame structure does not carry control data in the first OFDM symbols, which, consequently, gives more room, i.e., subcarriers, for user data transmission. Therefore, it is possible to achieve higher throughput when compared to the LTE standard. The Payload Data Unit (PDU) employed by SCATTER PHY is known as the Transport Block (TB). A TB is the container carrying data that are created at the MAC layer and transferred to the individual PHYs to be encoded and transmitted. One TB is composed of a number of bytes that can be accommodated inside a 1 ms long subframe, given the configured MCS and PHY BW parameters. Therefore, given the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**PHY BW (MHz)** & 1.26 & 2.7 & 4.5 & 9 \\ \hline
**Subframe duration (ms)** & & & 1 & \\ \hline
**subcarrier spacing (kHz)** & & & 15 & \\ \hline
**Sample rate (Msps)** & 1.92 & 3.84 & 5.76 & 11.52 \\ \hline
**FFT size** & 128 & 256 & 384 & 768 \\ \hline
**Useful subcarriers** & 84 & 180 & 300 & 600 \\ \hline
**OFDM symbols per subframe** & & & 14 & \\ \hline
**CP duration (\(\upmu\)s)** & & 4.69 (12 symbols)/5.21 (2 symbols) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: OFDM modulation parameters. PHY BW and the desired MCS, the MAC layer can calculate the number of bytes that can be handled by a 1 ms long subframe. Table 2 presents the coding rate for each one of the 32 defined MCS values. The differences between SCATTER PHY's modulation code scheme and LTE's are (i) 32 different MCS values instead of the 29 defined in the LTE standard, where the three additional MCS values allow SCATTER PHY to reach higher data rates in high SNR scenarios, and (ii) the greater number of subcarriers per subframe, which are used for user data transmission and, consequently, increase the final achieved throughput. However, even though being able to carry more user data per subframe, we kept the same coding rate of the LTE standard for MCS values ranging from zero to 28. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**Code Rate**} \\ \hline
**MCS** & **Modulation** & **1.26 MHz** & **2.7 MHz** & **4.5 MHz** & **9 MHz** \\ \hline
0 & QPSK & 0.0857 & 0.0900 & 0.0940 & 0.0940 \\ \hline
1 & QPSK & 0.1143 & 0.1200 & 0.1220 & 0.1220 \\ \hline
2 & QPSK & 0.1357 & 0.1467 & 0.1480 & 0.1480 \\ \hline
3 & QPSK & 0.1714 & 0.1933 & 0.1920 & 0.1920 \\ \hline
4 & QPSK & 0.2107 & 0.2400 & 0.2400 & 0.2440 \\ \hline
5 & QPSK & 0.2571 & 0.2933 & 0.2960 & 0.2920 \\ \hline
6 & QPSK & 0.3071 & 0.3400 & 0.3440 & 0.3440 \\ \hline
7 & QPSK & 0.3571 & 0.4000 & 0.4160 & 0.4130 \\ \hline
8 & QPSK & 0.4071 & 0.4667 & 0.4640 & 0.4690 \\ \hline
9 & QPSK & 0.4714 & 0.5200 & 0.5360 & 0.5330 \\ \hline
10 & 16 QAM & 0.2357 & 0.2600 & 0.2680 & 0.2665 \\ \hline
11 & 16 QAM & 0.2571 & 0.2933 &  
Communications between SCATTER PHY and the MAC layer are performed through four pre-defined (i.e., protobuf) messages. The first two messages, named Rx and Tx control, are employed, as the name suggests, to control/configure subframe reception and transmission, respectively. The parameters transported by these two control messages are set and sent to the individual PHYs by the MAC layer before the transmission of every new COT, hence allowing on-line parameter configuration. The other two messages, named Rx and Tx statistics messages, are employed to provide the MAC layer with real-time feedback from each independent PHY, providing critical information that is necessary for such a layer to take important actions. Tx control messages transport the TBs (i.e., user data) to be transmitted and Tx parameters regarding that transmission, that is PHY ID, Tx gain, Tx channel, data length, MCS, Tx PHY BW, and transmission timestamp. The transmission timestamp parameter allows time scheduled transmissions, which consequently enable the MAC layer to implement a Multi-Frequency (MF) Time Division Multiple Access (TDMA) medium access scheme.