e 12.5. This also lays the foundation of carrier aggregation: Remote Radio Head (RRH) Band B Master Base Station Small Cell Band A Add on Cells Band A Figure 12.5 - Advanced C-RAN architecture 5G Infrastructure Design Next, let us look at carrier aggregation. Carrier aggregation Carrier Aggregation (CA) allows the co-existence of the 4G and 5G networks. CA allows the mixing of multiple frequency bands, leading to improved cell coverage. Ericsson is considered one of the industry leaders when it comes to CA. To understand CA fully, let us look at the 5G spectrum, consisting of the following elements: Low-band: This provides national-level coverage and can be used indoors below 7 GHz using Frequency Division Duplex (FDD). Mid-band: This is a metro band in the range of 1 GHz to 6 GHz and can penetrate walls and provide fast throughput speeds and enhanced capacity using Time Division Duplex (TDD). High-band: Also referred to as mm Wave for higher-density localized services above 24 GHz. It requires smaller antennas, provides dense coverage, and is more secure. See Figure 12.6 for this concept of the three bands in 5G: Low-band Mid-band mmWave High-band Figure 12.6 - The different bands in 5G Ericsson developed Advanced RAN Coordination for low-latency coordination between the two different RAN compute basebands. (See: https://www.ericsson.com/en/blog/2021/6/ what-why-how-5g-carrier-aggregation) - Figure 12.7 provides a comprehensive view of the C-RAN architecture, built upon the foundations we covered here. Such a comprehensive C-RAN architecture lays the foundation of the RANaaS, which aligns with the public cloud computing concept. Understanding O-RAN Radio C-RAN resources Computing Network resources BBU-pools (RATs 1&2) Fibre optic resources Switch Central Office Data Centre Figure 12.7 - Comprehensive C-RAN architecture C-RAN facilitates rapid deployment of high-density RAN infrastructures. China Mobile first proposed C-RAN architecture in 2009 to move from Distributed RAN (D-RAN) to C-RAN. Both China Mobile and 