# Survey on 5G Physical Layer Security Threats and Countermeasures
## 2 Technology Background of Physical Layer
### Parameters and Basic Structure of 5G PHY Layer
#### 2.2.2 PHY Layer Frame Structure  
Depending on whether the deployment type of the 5G network is NSA or SA, the initial access to the network is provided through either 4G or 5G physical channels. 5G technology offers not only faster data rates and lower latency but also a new, flexible frame structure to accommodate a wide range of devices and applications. This structure is more adaptable than its LTE predecessors. It optimizes network utilization by dividing time-frequency resources into blocks and grid elements, ensuring efficient connectivity for various scenarios. The 5G technology implements the so-called time-frequency resource allocation method. The time-frequency resources are divided into resource blocks and further subdivided into resource grid elements. The fundamental scheduling unit in 5G has shifted from a subframe in 4G to a slot [57]. This allows dynamic adjustment of the time slot duration based on the service type. This flexibility enhances network efficiency and responsiveness. Additionally, the concept of mini-slots provides faster response times for certain applications, which are crucial in emergency situations. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Subcarrier** & **Slots per** & **Mean for** & **Min BW [MHz]** & **Max BW [MHz]** \\
**Spacing** & **Subframe** & **Carriers...** & **Min BW [MHz]** & **Max BW [MHz]** \\ \hline
15 kHz & 1 & & 4.32 & 49.5 \\
30 kHz & 2 & \(<6\) GHz & 8.64 & 99 \\
60 kHz & 4 & & 17.28 & 198 \\
120 kHz & 8 & \(>24\) GHz & 34.56 & 396 \\
240 kHz & 16 & & 69.12 & 397.44 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Subcarrier spacing options in 5G NR, reprinted with permissions from [56]. The 5G frame structure incorporates both self-contained and non-self-contained subframes, adding another layer of flexibility. This allows for different data transmission methods depending on the application requirements, maintaining service quality and further reducing latency. The 5G radio frame is 10 ms long and, the same as for the 4G, it is divided into 10 subframes with 1 ms duration. Each subframe is divided into slots, depending on which SCS is considered. Each slot contains 14 Orthogonal Frequency Division Multiplexing (OFDM) symbols in one Resource Block (RB) or 12 symbols for the extended CP. With increased SCS, the number of slots in the subframe increases because of the shorter symbol duration (from the OFDM theory, the OFDM symbol duration is inversely proportional to the subcarrier spacing). A comparison of the frame structures for 4G and 5G is depicted in Figure 3.  
For the duplexing, it is important that any cellular communication systems must be able to transmit in both directions--Uplink (UL) and Downlink (DL)--simultaneously. To provide the highest possible flexibility, 5G supports various duplexing schemes, such as Frequency Division Duplex (FDD),Time Division Duplex (TDD), Semi-static TDD, and Dynamic TDD. TDD operation is set to be the primary duplex arrangement for higher frequencies in 5G, while lower frequencies continues to utilize FDD. This choice is due to the mitigation of interference issues in larger cells, which can be achieved by employing distinct frequencies for the UL and DL directions. Frequency Division Duplex refers to a paired spectrum with separate UL and DL carriers. This allows simultaneous data transfer in both directions due to distinct carrier frequencies. Resource allocation is dynamic and independent for each direction, utilizing paired bands. There are two possibilities in FDD. Half duplex mode is used for frequency bands where it is not possible to have simultaneous transmission and reception in both the UL and DL within the cell. It allows simplified device implementation due to the relaxation or absence of duplex filters. For a certain frequency band, it is possible to have simultaneous transmission and reception in both the UL and DL within a cell.