# Semantic Communication on Digital Wireless Communication Systems
## 1 Introduction
Most of the current research using the JSCC transmission scheme [5; 8; 14; 15; 16; 23; 24] have proposed a transmission framework, as shown in Figure 1b, called the non-bit-conversion JSCC transmission framework. In this framework, at the transmitter, the range of the quantization output needs to be determined based on the modulation order (e.g., QPSK,16 QAM,64 QAM.......). Semantic encoding and quantization are jointly designed to map the encoded quantization output directly to the modulation constellation points. At the receiver, this framework does not require restoring the demodulated constellation points into bits, but instead sends the demodulated constellation points, represented by complex numbers, for decoding in a semantic decoder. The advantage of this framework is that it can avoid the performance loss caused by the bit decision at the receiver while retaining some of the channel state information. However, the disadvantage is that it makes significant changes to the existing wireless communication system protocols and network architecture. For example, at the receiver, the input of the semantic decoder in this scheme is complex numbers, which faces the problem of how to handle complex numbers above the physical layer if the semantic decoding algorithm is not feasible in the base station's physical layer. If the semantic decoding server is deployed in the base station's physical layer, it will face the problem of how to handle the user data in base stations, which conflicts with the existing system specifications. At the transmitter, since this scheme has already been quantized, whether to convert the semantic output to bits will not lead to differences in information loss. However, this framework also brings compiling issues, e.g., where to deploy the encoding algorithm and how to exchange information between nodes in the network architecture. Based on the impact of the non-bit-conversion transmission scheme on the semantic communication system, we believe that bit conversion is indispensable. The reason for this is that existing wireless communication systems, with all layers of the protocols and information exchange between nodes, are designed for bit-oriented transmission. To minimize the impact on existing wireless communication systems, we propose a transmission framework, as shown in Figure 1c, called the bit-conversion-based JSCC transmission framework. In this framework, at the transmitter, the quantization result is converted into bits, and the existing system's bit constellation mapping module is retained. At the receiver, the demodulated constellation points are restored into bits, and then the bits are converted into integers before being sent into the semantic decoder. The advantage of this  
Figure 1: Transmission framework of semantic communication. (**a**) SSCC transmission framework; (**b**) non-bit-conversion JSCC transmission framework; (**c**) bit-conversion-based JSCC transmission framework. transmission framework is that it is convenient to integrate with the existing system and retain the advantages of the JSCC transmission scheme. We will introduce the specific physical layer procedure designed in this framework in the following part of this paper. Based on the above work, the main contributions of this paper mainly include the following:  
1. The specific physical layer procedure of the bit-conversion JSCC transmission framework for semantic communication is designed. Furthermore, a semantic communication simulator is developed to implement and verify this transmission framework. 2. A novel physical layer metric, the IER (Integer Error Rate), is proposed as a physical layer metric for semantic information transmission. And we prove that the IER is more suitable than the BER for semantic communication by simulation. 3. We present a minimum Manhattan distance constellation mapping scheme for m-QAM modulation to optimize the transmission quality in the bit-conversion JSCC transmission framework. 4.