## 3 Signal Acquisition and Preprocessing  
### IEEE 802.11p OFDM PHY Frame  
As mentioned above, the radio frequency fingerprint-based identification method belongs to the research scope of physical layer security. Therefore, the physical layer (PHY) specification in the IEEE 802.11p protocol standard [27], which is used as the underlying technology by DSRC, is first introduced in this section. The IEEE 802.11p standard specifies the PHY entity for an orthogonal frequency division multiplexing (OFDM) system [28], in which the orthogonal subcarriers can be overlapped together without affecting each other, with strong anti-interference ability and high spectrum utilization. In this OFDM system, the frame format of the Physical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU) is defined, which is composed of a preamble field, a signal field, and a data field, including the OFDM PHY preamble, the PHY header, the PLCP Service Data Unit (PSDU), the tail bits and the pad bits. In the PPDU encoding process, it contains multiple steps such as producing the preamble field, adding a guard interval, filling corresponding bit fields, calculating relevant parameters, and inserting pilots. The service bits, whose length is stored in the length field of the PHY header, together with the PSDU, are part of the data field. In addition, the data transmission rate, modulation mode, and coding rate are determined by the rate field of the PHY header in the signal field. Therefore, the content of both the signal field and the data field will change according to the data content actually transferred and transmission rate. However, the data format transmitted in the preamble field is constant, hence it is more feasible to extract stable and invariable RFFs from the PHY preamble field. The OFDM PHY preamble field contains 10 repeated short training symbols and 2 repeated long training symbols, and its main function is to realize signal synchronization. Figure 2 shows the training structure of PHY preamble in the IEEE 802.11p OFDM PPDU frame format, where t1 to t10 represent short training symbols and T1 and T2 represent long training symbols. At 10 MHz channel spacing, the total preamble training duration is 32 us. These cyclic repeated symbols are caused by the periodicity of the inverse Fourier transform, and the period of a Fourier transform or inverse Fourier transform \(T_{\text{FFT}}\) is 6.4 us [29]. The structures of short training symbols and long training symbols are different. A short training symbol consists of 12 subcarriers, which is modulated by the elements of sequence  
\[ \begin{array}{c}S_{-26,26}=\sqrt{\frac{13}{6}}\{0,0,1+j,0,0,0,-1-j,0,0,0,1+j,0, 0,0,-1-j,0,0,0,-1-j,0,0,0,1+j,0,0,\\ 0,0,0,0,0,-1-j,0,0,0,-1-j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0 \}\end{array}  \tag{1}\]  
Since the short training symbol only utilizes 12 of the 52 subcarriers, and its spectrum only has a non-zero spectral amplitude at a multiple of 4, the signal period of the short training symbol is one quarter of that of \(T_{\rm FFT}\), i.e., 1.6 \(\upmu\)s. There are 10 such training symbols in the short preamble field, with a total duration of 16 \(\upmu\)s.  
A long training symbol consists of 53 subcarriers, including a DC zero value, which is modulated by the elements of sequence  
\[ \begin{array}{c}L_{-26,26}=\{1,1,-1,-1,1,1,-1,1,1,1,1,1,1,1,-1,-1,1,1,-1,1,1, 1,1,0,\\ 1,-1,-1,1,1,-1,1,-1,-1,-1,-1,-1,1,1,-1,-1,1,-1,1,1,1,1\}\end{array}