# Deep Learning for Joint Adapatations of Transmission Rate and Payload Length in Vehicular Networks
## 2 IEEE 802.11p Physical Layer Overview and System Model
### IEEE 802.11p Physical Layer  
The IEEE 802.11p operates on the 5.9 GHz frequency band and utilizes the orthogonal frequency division multiplexing (OFDM) [3]. It offers eight different MCSs ( i.e., the different combinations of quadrature amplitude modulation (QAM) order and channel coding rate) with data rates that range from 3 Mbps to 27 Mbps as summarized in Table 1[13]. The bandwidth of the communication channel is 10 MHz. The OFDM symbol duration is 8 us with inverse fast Fourier transform (IFFT) period of 6.4 us and guard interval of 1.6 us. The total number of OFDM subcarriers, \(U\), is 64, out of them there are only \(U_{D}=48\) data subcarriers and \(U_{p}=4\) pilot subcarriers. The remaining \(U_{N}=12\) subcarriers are null carriers. Here the number of the used subcarriers, \(U_{s}=U_{D}+U_{p}\) is 52 [21].  
The IEEE 802.11p physical layer frame structure is shown in Figure 1. The frame starts with a preamble field that begins with 10 short training symbols (STSs), each of duration 1.6 us. These STSs are used for timing synchronization and coarse frequency offset estimation. Then, the two long training symbols (LTSs), each has a duration of 6.4 us, are added in order to be used by the receiver for fine frequency offset correction and channel estimation. After that, the signal field is used to add signaling information such as the utilized MCS (data rate) and the frame length. After the signal filed, the frame payload is added [13; 21]. For a certain payload data of length \(l\) Bytes, the number of OFDM data symbols, \(K_{D}\), in a frame can be given as [13]  
\[K_{D}=\left\lceil\frac{8l+b_{s}+b_{t}}{N_{DBS}}\right\rceil, \tag{1}\]  
where \(b_{s}\) is the number of service field bits, that equals 16 bits, added at the beginning of the payload data, \(b_{t}\) denotes the number of tail bits, that is six bits, added at the end of the payload data [3], and \(N_{DBS}\) is the number of data bits per OFDM symbol for the utilized MCS as shown in Table 1. Here, operator \(\lceil z\rceil\) is the ceiling function which returns the smallest integer that is larger than \(z\). Note that there are five overhead OFDM symbols that carries no payload data are added in every frame, namely, the two OFDM symbols of the STSs, the two OFDM symbols of the LTSs, and one OFDM symbol of the signal field. Thus, the total number of OFDM symbols in a frame, \(K\), is given by  
\[K=K_{D}+K_{ov}, \tag{2}\]  
where \(K_{ov}\) is the number of overhead OFDM symbols per frame, that equals five for an IEEE 802.11p frame.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**MCS**} & **Data Rate** & \multirow{2}{*}{**Modulation**} & **Channel Coding** & **Coded Bits** & **Data Bits per** \\  & **[Mbps]** & & **Code Rate** & **(N\({}_{BSC}\))** & **(N\({}_{DBS}\))** \\ \hline
0 & 3 & BPSK & 1/2 & 1 & 24 \\ \hline
1 & 4.5 & BPSK & 3/4 & 1 & 36 \\ \hline
2 & 6 & QPSK & 1/2 & 2 & 48 \\ \hline
3 & 9 & QPSK & 3/4 & 2 & 72 \\ \hline
4 & 12 & 16-QAM & 1/2 & 4 & 96 \\ \hline
5 & 18 & 16-QAM & 3/4 & 4 & 144 \\ \hline
6 & 23 & 64-QAM & 2/3 & 6 & 192 \\ \hline
7 & 27 & 64-QAM & 3/4 & 6 & 216 \\ \hline \hline \end{tabular}
\end{table}
Table 1: IEEE 802.11P modulation and coding schemes [13].  
Figure 1: Frame structure of the IEEE 802.11p physical layer.