# Semantic Communication on Digital Wireless Communication Systems
## 4 Optimization for the Bit-Conversion JSCC Scheme
### Minimum Manhattan Distance Constellation Mapping Scheme
64 QAM modulation, this algorithm can generate the Manhattan distance binary coding table, as Figure 14b illustrates. To evaluate the enhancement of this proposed scheme on the performance of semantic communication, the semantic metric and the physical layer metric introduced in Section 3 are simulated with the following three schemes: SSCC-natural binary coding, JSCC-natural binary coding, and JSCC-Manhattan distance binary coding. These three schemes all transmit with 64 QAM, and the SSCC-natural binary coding scheme uses 0.5 code rate LDPC channel coding. And all three schemes retain the channel estimation and equalization of 3 GPP 5G physical layers to eliminate the impact of different channel types. Simulation configurations are listed in Table 3.  
The simulation results are shown in Figure 15. As shown in Figure 15a, it can be observed that the SSCC-natural binary coding scheme has the best performance in terms of the BER. The BERs of both JSCC cases start to increase when the SNR is lower than 20, while the SSCC cases maintain no bit error until the SNR is lower than 11, which is consistent with the traditional communication performance since the SSCC uses LDPC channel coding. In addition, the JSCC-natural binary coding and JSCC-Manhattan distance binary coding achieve the same BER. However, as shown in Figure 15b, the JSCC-Manhattan distance binary coding scheme is superior in terms of IER performance to the JSCC-natural binary coding scheme. Moreover, as shown in Figure 15c, the JSCC-Manhattan distance binary coding scheme also significantly outperforms the SSCC-natural binary coding scheme within an SNR range of [5; 6; 7; 8; 9; 10]. In terms of semantic performance metrics, within the SNR range of [5; 6; 7; 8; 9; 10], the JSCC-Manhattan distance binary coding scheme is also significantly better than the other two schemes, by 4-6 dB. At the same time, as shown in Figure 15d, as the SSCC scheme adopts LDPC code with a code rate of 0.5, the JSCC-Manhattan distance binary coding scheme has twice the spectral efficiency of this SSCC-natural binary coding scheme. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test Case** & **JSCC–Natural Binary Coding** & **JSCC–Manhattan Distance Binary Coding** & **SSCC–Natural Binary Coding** \\ \hline Semantic transmission & bit-conversion & \
\begin{tabular}{c} bit-conversion \\ JSCC \\ \end{tabular} & bit-conversion SSCC \\ \hline Source file & image & image & image \\ \hline Semantic codec: & LSCI & LSCI & LSCI \\ \hline Quantization range & [0; 1; 2; 3; 4; 5; 6; 7] & [0; 1; 2; 3; 4; 5; 6; 7] & [0; 1; 2; 3; 4; 5; 6; 7] \\ \hline Data-to-binary & natural binary coding &
\begin{tabular}{c} Manhattan distance \\ binary coding \\ \end{tabular} & natural binary coding \\ \hline Channel coding & NO & NO & LDPC CR = 0.5 \\ \hline Bit constellation & 3 GPP 5G & 3 GPP 5G & 3 GPP 5G \\ \hline Modulation & 64 QAM & 64 QAM & 64 QAM \\ \hline Simulation SNR range & [\(-\)5–30] & [\(-\)5–30] & [\(-\)5–30] & [\(-\)5–30] \\ \hline Channel model & AWGN & AWGN & AWGN \\ \hline Channel equalization & LMMSE & LMMSE & LMMSE \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulation configurations for the evaluation of the proposed scheme. Through the above analysis, it can be further seen that a well-designed constellation mapping can be utilized to improve the performance of semantic information transmission, and JSCC is more suitable for leveraging the advantages of semantic communication compared to SSCC. However, this minimum Manhattan distance constellation mapping scheme has a limitation: the quantization output range of semantic encoding has to be bounded to the modulation order. In other words, a lower-order modulation is required when the SNR decreases. Meanwhile, the output range of semantic quantization must also be correspondingly reduced. To address this issue, based on the minimum Manhattan distance constellation mapping scheme, we further propose a hybrid transmission scheme of JSCC and SSCC.