# 6G Cognitive Information Theory: A Mailbox Perspective
## 2 Related Work
### The Evolution of 5G  
Wireless-communication technologies for different application scenarios can be roughly divided into short-distance communication technologies and wide-area network communication technologies (e.g., cellular networks and low-power WAN). These technologies have promoted the development of mobile edge networks. The explanations of how technologies are fused, as well as the role that data-transmission can play, are elaborated in the following. From the perspective of transmission distance, multi-scale sensor communication can be divided into two types: short distance communication and wide-area network (WAN) communication. The former includes Zigbee, Wi-Fi, Bluetooth, and Z-wave technologies, which are widely used in wireless access networks; whereas the latter include communication technology with low-rate service demand. The low-power WANs (LPWANs) [9] can be divided into two categories: LPWANs, which work in an unauthorized spectrum, such as LoRa and SigFox, and the 2G/3G/4G cellular communication technologies, which are  
Figure 2: Organization of the article. supported by 3 GPP and work in an authorized spectrum, such as EC-GSM, LTE Cat-m, and NB-IoT [10]. The data-transmission rates of these technologies are different, but they are all far lower than that of the current commercial 5G, which makes them suitable for applications with low transmission rates, such as smart electricity meters and large-scale sensor communications. Currently, the emerging fifth generation of mobile communication (5G) network [11] faces the challenging tasks of supporting thousands of users with a transmission rate of nearly 10 Mb/s. More specifically, the linking of hundreds of thousands of mobile devices for the deployment of large-scale sensor networks would require stronger spectrum efficiency, wider coverage, reinforced signaling efficiency, and significantly reduced communication distance compared to 4G networks. Further, 5G applications can be divided into the three following categories:  
* Enhanced broadband mobile communication enables better user experience and access with wider bandwidth in order to support higher quality multimedia services and more living experience than 4G;
* Massive IoT communication provides signaling control with a higher connectivity density and better real-time optimization and supports efficient access and management of massive IoT devices at low cost and power consumption;
* Ultra-latency and ultra-reliability communication provides users with millisecond-level end-to-end latency and nearly a 100% guarantee for business reliability. To satisfy the requirements of 5G for better performance, faster speed, better connection, higher reliability, shorter latency, and specific topological structures in application scenarios, many designs and techniques have been presented. For instance, for the purpose of improving the transmission rate, it has been proposed to spread the spectral range (e.g., millisecond-level communication) to deliver more information while improving the transmission rate. In addition, in order to enhance the spectral efficiency, the upper limit of transmission rate under the unit spectral resource of plots has been recommended using massive MIMO technology [7] and high-order modulation technology. Moreover, in order to enhance data traffic, it has been suggested to adopt a more compact plot arrangement and deploy more plots in each unit area to provide more capacity and more spectrum reuse. In addition, the full-spectrum access adopted by 5G involves the hybrid networking of 6 GHz and lower frequencies with higher frequencies, in which low frequencies are essential to 5G for seamless coverage, while higher ones are supplementary for the enhancement of rate at hot spot areas [12]. These frequencies are combined to fully make use of their advantages and jointly satisfy the needs of 5G for seamless coverage, fast speed, and great capacity.