For engineers unfamiliar with Citizens Broadband Radio Service (CBRS), the CBRS band is a recently-opened slice of spectrum from 3550 to 3700 MHz which can be used on a licensed (with priority) or unlicensed basis by organizations and enterprises in the U.S. Enterprises are looking to CBRS to quickly spin up private 5G networks for new use cases that require interference-free spectrum and outdoor wide area coverage – think autonomous vehicles or handheld devices in a healthcare setting.
We’re excited about the technology and have acquired a company, Athonet, to deliver an innovative, turnkey solution to our customers. The Athonet network uses standalone 5G, which is technology familiar in the cellular world but new to most enterprise engineers with Wi-Fi backgrounds. As an introduction, here are some of the concepts in CBRS 5G technology.
Key CBRS concepts for Wi-Fi experts
1. Transmission symbols and OFDMA
CBRS uses the 5G New Radio OFDM waveform where multiple subcarriers are modulated independently across the band, in the same way as Wi-Fi, although the subcarrier spacing differs, at 15, 30 or 60 kHz for CBRS against 78 kHz for Wi-Fi 6 and Wi-Fi 7. The modulation schemes are also similar, with QAM used for higher rates, and MIMO techniques multiply data rates where favorable multipath conditions exist.
2. Frame structure and TDD
However, the way transmissions are timed and controlled in CBRS is very different.
Wi-Fi has until recently used a packet-by-packet transmission scheme, where once a node has a transmit opportunity, it sends a full frame of data. A view of transmissions over time will show interleaved frames from different nodes. (Wi-Fi 6 and Wi-Fi 7 have OFDMA options that are similar to CBRS, so an understanding of Wi-Fi 6 helps with CBRS).
CBRS has a fully-framed structure, where base station transmissions define a frame every 10 msec, and client devices synchronize to that frame. The frame is divided into 10 subframes of 1 msec each and each subframe is divided into multiple slots where each slot consists of 14 OFDM symbols. CBRS uses TDD (Time Division Duplex) as uplink and downlink share the same RF channel. In TDD, each symbol, slot or subframe can be designated for downlink or uplink transmissions in a flexible pattern, controlled and advertised by the base station. Typically CBRS networks will use a 50:50 or 60:40 split for downlink to uplink slots, with some time lost at the changeover from downlink to uplink.
3. Resource Elements and Resource Blocks
Because the CBRS frame is structured and predictable, each symbol interval can be identified and referenced by subcarrier and symbol coordinates. This allows engineers to construct a 2-dimensional time-frequency grid divided by subcarriers on the frequency axis and symbols on the time axis. Each cell in the grid, also called a Resource Element (RE), represents one symbol on one subcarrier. REs are grouped into Physical Resource Blocks (PRBs), where a PRB covers 12 subcarriers in frequency and from one to 14 symbols in time.
4. OFDMA and Resource Block allocation
PRBs are predefined in the frame structure, and designated for uplink or downlink, but the process of allocating each PRB to a particular transmitting node is called OFDMA (Orthogonal Frequency Division Multiple Access). This operates like OFDMA in Wi-Fi 6 and Wi-Fi 7, but bear in mind that Wi-Fi allocates subcarriers within a packet duration, whereas CBRS uses PRBs in the frame structure. In both cases, the access point or base station exercises control over both uplink and downlink transmissions, a big difference from traditional Wi-Fi.
5. Control and signaling channels
Wi-Fi does not give special treatment to control traffic: Information is carried in separate packets, interleaved with data packets over the air. The traffic gains some priority from elevated QoS settings.
In CBRS, defined REs in the frame structure are designated for control traffic for uplink and downlink – they are reserved and not available for user data. Sequences of noncontiguous control REs are further structured into different control and signaling channels. For example, control channels schedule each user’s data transmission, link adaptation and HARQ error control. Other channels carry cell-specific broadcast control information to support cell acquisition, uplink synchronization and mobility. The use of designated channels ensures a minimum data rate for control traffic, unaffected by user data.
6. The Scheduler
CBRS has a strong frame structure, a rigid allocation of transmission opportunities to uplink and downlink, and a fraction of symbols reserved for control channels. All this is controlled by the base station, which also identifies the Physical Resource Blocks where each of its client devices should transmit or receive data. But in order to allocate efficiently, it needs to know what traffic each client wants to transmit, its priority and QoS requirements, and the available bit rate for each uplink and downlink connection.
This requires that a good deal of information on data buffer levels, channel conditions and traffic policy is brought together at a single location, the Scheduler function in the base station, which then determines the optimum allocation of PRBs in future frames, and transmits this pattern to its client devices. The Scheduler holds much of the intelligence of a CBRS system, and its algorithms are proprietary and tightly guarded by vendors. A good Scheduler will ensure that each traffic flow attains its required service level under varying network conditions and traffic profiles.
With OFDMA, Wi-Fi 6 and Wi-Fi 7 also have a Scheduler that performs similar functions to that in CBRS, but prior to that, Wi-Fi used distributed mechanisms for bandwidth allocation across the network.
7. Network and cell selection
Whereas a Wi-Fi client scans for networks by listening for beacons and sending probe requests, CBRS embeds network and cell advertisement information in channels in the frame. A client device needs to synchronize to the frame to decode these values, including the operator identifier and cell site ID, and match them to values in its SIM (Subscriber Identification Module) card. Whereas the cellular network uses the PLMN (Public Land Mobile Network) identifier to find a suitable signal, one PLMN ID (315-010) is used across all CBRS networks, so CBRS with 5G distinguishes different networks by using the NID (Network Identifier).
8. Handover
As mobile clients move across the network, they must hand over to new access points – an evolution that is difficult to control, as it often takes place under poor transmission conditions and severe time constraints. Wi-Fi engineers are familiar with the issues of handover and follow many rules of thumb in designing networks for smooth mobility.
The CBRS model has a few important differences from Wi-Fi. The most important is that it is controlled from the base station. As the client device senses a weaker signal, it starts to scan for other radios, and reports this to its base station. When a suitable handover candidate is found, the base stations coordinate to ensure the new one sets up PRBs to accommodate the new traffic before the client is sent a message instructing it to handover.
9. Authentication
Both Wi-Fi and CBRS have very secure authentication and encryption models.
Wi-Fi in enterprises uses WPA3 in 802.1X mode, supporting passwords or X.509 certificates for credentials and authenticating directly to the enterprise. 5G CBRS currently uses 5G-AKA authentication, based on the SIM in the user’s device, where the authentication server may be in the local network’s core or, in the case of a roaming subscriber, in a remote core network. Modern devices have eSIM capabilities, avoiding reliance on physical SIM cards.
More CBRS for Wi-Fi Experts
Frequently Asked Questions: CBRS
About the Author
Office of the CTO, HPE Aruba Networking
Peter Thornycroft is an engineer in the CTO’s group at HPE Aruba Networking with interests in voice-over-WLAN, location and RF technologies. He has over 20 years of industry experience with a variety of wireless, carrier and voice technologies. Peter previously held senior product management and technical marketing positions with Cisco, StrataCom and Northern Telecom.
He is an active participant in the Wi-Fi Alliance and the IEEE 802.11 standards body and holds an MA in electrical sciences engineering from Cambridge University and an MBA from Santa Clara University.
