# Symbol-Level Selective Channel Estimation in Packet-Based OFDM Systems
## 6 Simulation Results and Discussion  
In this section, we compare the performance of the SLS scheme with that of other channel estimation techniques: the STA, TRFI, DD-TT, and PLS schemes. The PLS scheme is configured to select DD-TT and TRFI on a packet-by-packet basis. The performances were analyzed using a link-level simulator that complies with the IEEE 802.11p standard. The simulation channel environment is assumed to be V2V Expressway Oncoming and V2I Urban Canyon, and the channel model used for the simulation is shown in Table 2.  
Figures 4 and 5 show the BER and PER performance of quadrature phase shift keying (QPSK) with code rate 1/2 in the V2V and V2I channel environments, respectively. It is assumed that a packet consists of 50 symbols. For the V2V simulation in Figure 4, STA and DD-TT show that an error floor occurs in high-SNR regions, and TRFI achieves a lower error performance than the other schemes in low-SNR regions for both the BER and PER performances. The PLS scheme has satisfactory performance as a whole, but the performance degradation occurs between 18 and 24 dB compared with DD-TT. This means that the selected channel estimation technique at the beginning of the packet cannot accurately estimate the channel variation to the end of the packet. Meanwhile, the proposed SLS scheme follows the performance of DD-TT up to 18 dB, can achieve a performance gain of about 5-7 dB at a BER of 2 \(\times\) 10\({}^{-2}\), and offers a gain of about 3-6 dB at a PER of 3 \(\times\) 10\({}^{-1}\) compared with STA, TRFI, and PLS. After 18 dB, the SLS scheme performs better than the other schemes and offers a performance gain of about 2-3 dB at a BER of 5 \(\times\) 10\({}^{-3}\) and a PER of 9 \(\times\) 10\({}^{-2}\) compared with TRFI and PLS. These results demonstrate that the proposed SLS scheme selectively uses DD-TT and TRFI on a symbol-by-symbol basis depending on the channel condition, thereby improving performance. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Schemes** & **Composition** & **Total Complexity** \\ \hline TRFI & 1, 6, 7 & \(O(N)\) \\ DD-TT & 1, 3 & \(O(N)\) \\ Proposed SLS & 1\(\sim\)4 & \(O(N)\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of time complexity. Figure 4: Performance analysis of STA, TRFI, DD-TT, PLS, and proposed SLS schemes (V2V channel, QPSK 1/2, 50 symbols). **(a)** BER performance; **(b)** PER performance. For the V2I simulation in Figure 5, the BER and PER performances of the proposed scheme are similar to those in the V2V simulation. For both BER and PER, STA and DD-TT exhibit good performance, but the BER performance is slightly better than the BER performance. Figure 5: Performance analysis of STA, TRFI, DD-TT, PLS, and proposed SLS schemes (V2I channel, QPSK 1/2, 50 symbols). (**a**) BER performance; (**b**) PER performance. performance at low SNRs, while TRFI performs very well at high SNRs. PLS performs worse than DD-TT between 18 and 30 dB, and the SLS scheme shows the performance of DD-TT up to 18 dB. After 18 dB, the SLS scheme provides the better performance compared with the other schemes, and can achieve a performance advantage of about 4 dB at a BER of 5 \(\times\) 10\({}^{-3}\) and a PER of 8 \(\times\) 10\({}^{-2}\) over the TRFI and PLS schemes. This indicates that the SLS scheme shows good performance in various channel environments. Figure 6 shows the PER performance according to the selection period in the SLS scheme. When the coherence time is defined as a bandwidth with a correlation of 0.9, the V2V and V2I channels according to the scenario in Table 2 have a coherence time of about 118.72 and 102.47 \(\upmu\)s, respectively [30]. Because the symbol duration of one OFDM is 8 \(\upmu\)s in IEEE 802.11p, the selected channel estimation scheme in the \(m\)-th OFDM symbol is valid up to 12\(\sim\)14 symbol intervals. Figure 7 shows the correlation coefficients between the initial channel estimate \(\hat{H}(m,k)\) and two channel estimates for each OFDM data symbol at an SNR of 24 dB.