Understanding CBRS (Citizens Broadband Radio Service)

The Citizens Broadband Radio Service (CBRS) is primarily used in the United States for wireless communication services. It operates on frequencies ranging from 3550 MHz to 3700 MHz.

Initially used by the military, the FCC later recognized its potential for commercial and shared spectrum applications. The CBRS spectrum is divided into three tiers:

• Tier-1: Reserved for incumbent access and primarily allocated to existing federal and non-federal users. These users are protected from interference from other users within the CBRS band.

• Tier-2: Intended for Priority Access License (PAL) holders. PAL users have a higher degree of protection from interference compared to GAA users.

• Tier-3: Employs unlicensed CBRS spectrum, similar to Wi-Fi and other unlicensed bands. GAA users can operate within the CBRS spectrum as long as they do not cause harmful interference to higher-tier users.

CBRS Network Architecture

The CBRS network architecture is designed to accommodate various users within a shared spectrum band. It has numerous use cases and applications, including wireless broadband services, private LTE and 5G networks, fixed wireless access, IoT (Internet of Things), temporary venue connectivity, utility and energy companies, education, and healthcare.

Here are the functions of the network elements within the CBRS architecture:

• Spectrum Access System (SAS): Manages the dynamic sharing of spectrum among different tiers of users. It ensures that incumbent users are protected from interference and assigns available frequencies to PAL and GAA users. In some cases, SAS functionality can be hosted in cloud-based infrastructure.

• Environmental Sensing Capability (ESC): Detects the presence of incumbent radar systems. If ESC detects incumbent activity, it communicates with SAS to adjust spectrum allocation to avoid interference.

• CBRS Compliant APs or Small Cells (CBSD APs): These network devices are equipped with CBRS-capable radio equipment that operates within the CBRS frequency range and covers a specific area to provide wireless coverage. They communicate with user devices, enable their connectivity to the core network (CN), and forward data packets between user devices and the core network of the service provider or network operator. They are also responsible for authenticating user devices and ensuring the security of communication. CBSD APs can prioritize and manage data traffic based on the needs of different applications and services.

• User Devices: Compliant devices communicate directly with APs using CBRS frequencies. Non-compliant devices require a CBRS gateway to communicate with APs. There are different classes of users in the CBRS architecture, namely PAL holders, GAA users, and incumbent users.

• Core Network: This is the central infrastructure that manages and routes data traffic within the network. It includes various networking components such as switches, gateways, routers, and servers. The Core Network (CN) connects on one side with APs and on the other side with external networks such as the internet or private enterprise networks. It performs many functions, such as data routing and forwarding, interconnection of APs with other network elements, authentication and authorization, QoS management, security, handover and mobility management, billing, and accounting. LTE systems use EPC (Evolved Packet Core) as shown in the CBRS network block diagram.

Conclusion

The CBRS network architecture offers several advantages to users and network operators due to its shared spectrum model and interference mitigation approach.

Users will benefit from increased connectivity, improved QoS (Quality of Service), lower costs, and diverse use cases.

Operators can support more users and applications within the same spectrum, avail potential revenue, and expand their coverage and capacity to address the growing demand for wireless services.