# Semantic Communication on Digital Wireless Communication Systems
## 2 Bit-Conversion-Based JSCC Transmission Framework and Simulator  
### Bit-Conversion-Based JSCC Transmission Framework  
To better adapt to the existing wireless communication systems, the specific physical layer procedure of the bit-conversion JSCC transmission framework for semantic communication is designed. Since the JSCC approach eliminates traditional channel encoding and decoding, we need to add an adaptation module between the physical layer digital modulation interface of the existing system and the semantic encoder/decoder. This transmission framework does not require changes to the physical layer processing flow after bit mapping at the transmitter or before digital demodulation at the receiver, as shown in Figure 2.  
The specific process design of the adaptation module is illustrated in Figure 3. The functions of the transmitter adaptation module include integer-to-bit conversion, end-of-data indication attachment, rate matching, and data segmentation. The functions of the receiver adaptation module include hard-decision demodulation, data concatenation, end-of-data indication detection, and bit-to-integer conversion.  
Functions in the transmitter adaption module include the following:  
1. **Integer-to-bit conversion**: based on the output range of semantic encoding quantization, determine the minimum number of bits required to represent each integer. Select a specific encoding method, such as natural binary coding, binary complement coding, etc., to convert the integer to then be transmitted into binary.
2. **Adding end of data indication**: for semantic transmission, bit error is allowed when the physical layer sends the received data to the semantic decoder, while the number of bits (or data) should not be changed. For the JSCC scheme, CRC is not required; thus, an end-of-data indication function makes the receiver identify the end of data flow. A special sequence is adopted as the end-of-data indication and is repeated multiple times to improve the robust of detection.
3. **Rate matching and data segmentation**: rate matching and data segmentation are designed with the code rate of channel coding in the traditional system. As there is no channel coding/decoding in the JSCC scheme, the rate matching and data segmentation should be re-designed to adapt the physical layer with no channel coding. Here, we adopt the zero-padding method to make the data fit the scheduled resource. Functions in the receiver adaption module include the following:  
1. **Hard-decision de-mapper**: since there is no channel coding/decoding, the output LLRs of de-modulation should be converted to bits with a simple algorithm. A hard decision de-mapper function is added here to convert the LLRs to bits.
2. **Data concatenation:** the reverse process of data segmentation in the transmitter.
3. **End of data detection:** we employed a simple character comparison algorithm here to identify the special sequence adopted in the transmitter.
4. **Bits-to-integer conversion**: convert the bits to integers with the same binary coding employed in the transmitter.  
Figure 3: Adaption module detail design.  
Figure 2: E2E procedure for bit-conversion-based JSCC transmission framework.  
Comparing this framework with the SSCC transmission framework shown in Figure 4, which has no impact on the existing system, it can be observed that this framework involves minimal modifications to the existing system, while, since the channel coding is removed in the JSCC transmission framework, the computational complexity at the physical layer of this framework is significantly reduced. And the key consideration is whether the proposed transmission framework has an advantage in transmission performance compared with the SSCC transmission framework. The simulation tools and the evaluation will be introduced in the following sections.