# Semantic Communication on Digital Wireless Communication Systems
## 4 Optimization for the Bit-Conversion JSCC Scheme
### Hybrid JSCC/SSCC Transmission Scheme  
In this chapter, a hybrid JSCC/SSCC transmission scheme for semantic communication is proposed. The principle of the hybrid scheme is that, at the transmitter, the quantized data after semantic encoding are converted into bits, and the integer-corresponding bit data are divided into two parts for transmission. The \"higher-order bits\" part is transmitted with a lower-order modulation concatenated channel coding to ensure transmission reliability, for which the SSCC-natural binary coding scheme can be adopted. The \"lower-order bits\" part can be transmitted using the JSCC-Manhattan distance binary coding scheme. The splitting position between the \"higher-order and lower-order\" bits can be adjusted according to the value range of the quantized output and the modulation order, thus solving the limitation that the range of the semantic quantization output must be bound to the modulation. At the receiver, the received bit data are combined and then converted back into integer data, which are sent to the upper layer of the receiver for semantic decoding. The specific data processing flow for transmission and reception are shown in Figure 16.  
Figure 15: Simulation result. (**a**) BER vs. SNR; (**b**) IER vs. SNR; (**c**) spectrum efficiency; (**d**) PSNR vs. SNR. The data procedure step is described as below. It is assumed that the output integer data range of semantic coding and quantization is [0-7]. **The procedure steps in transmitter as following:**  
1. **Step1:** convert the integer data into binary with natural binary coding. 2. **Step2:** split the bit data blocks into two blocks; the first bit of each of the three bits is put into the \"part one\" block, and the last two bits are put into the \"part two\" block. 3. **Step3:** transmit the \"part one\" block with the SSCC scheme (QPSK and 0.5 code rate LDPC coding); transmit the \"part two\" block with the JSCC-Manhattan distance constellation mapping scheme (16 QAM and no channel coding). **The procedure step in receiver as following:**  
1. **Step1:** receive the two bits data blocks and merge them back to a whole bit data block. 2. **Step2:** convert the received bit data into integer values with natural binary coding. This hybrid JSCC/SSCC transmission scheme can fix the problem of the quantization output range of semantic encoding having to be bounded to the modulation order if we want to adopt the dedicated integer constellation mapping scheme. Take an example: assume the quantized output of the semantic encoding is [0-7]; if we adopt the JSCC-Manhattan distance binary coding scheme for transmission, we can only use 64 QAM modulation rather than QPSK or 16 QAM. When adopting the hybrid JSCC/SSCC transmission scheme, a lower-order modulation can be used. In order to evaluate the performance of this hybrid JSCC/SSCC transmission scheme, the bit-conversion SSCC and bit-conversion JSCC transmission schemes are simulated, respectively, as the baselines. In this simulation, the previous LSCI semantic encoding scheme is adopted for image transmission, and the quantized output of the semantic encoding is [0-7]. A detailed configuration of the simulation is included below:  
**Simulation verification-1. **  
The simulation configurations comparing the hybrid JSCC-SSCC transmission scheme with the bit-conversion SSCC scheme are shown in Table 4.  
Figure 16: Hybrid transmission scheme data processing flow. The simulation result is show in Figure 17. It can be observed that the BER of the hybrid JSCC/SSCC transmission scheme is better than the SSCC 16 qam and worse than the SSCC-QPSK. This is consistent with the traditional communication performance, since the hybrid JSCC/SCC transmission scheme employs a portion of QPSK. In terms of the IER, the hybrid JSCC/SSCC transmission scheme shows a significant improvement, approaching the performance of the SSCC-QPSK scheme. Finally, the PSNR of the hybrid JSCC/SSCC transmission scheme is essentially close to that of the SSCC-QPSK scheme.